Gas Turbine

14Gas Turbine Power Plants 14·1. Introduction The gas turbine in its simplest form is a heat engine operating by means of a series of processes consisting of compression of air drawn from the atmosphere, increase of air temperature by the combustion of fuel in the air, expansion of hot gases to atmosphere, the who"le being a continuous flow process. It is thus similar to gasoline and diesel engines in its working medium and internal combustion, but is akin to the steam turbine in its aspect of the steady flow of the working medium. The compression and expansion processes are carried out in turbomachines, that is by means of rotating elements in which the energy transfer between fluid and rotor is effected by means of kinetic action, rather than by positive displacement as in reciprocating machinery. Thus in its simplest form a gas turbine consists of a compressor, a combustion chamber, and a turbine unit. Air which acts as a working fluid is compressed in the compressor and energy is added to it in the combustion chamber. The high energy fluid is then expands in the turbine and thus mechanical energy is produced. Part of this energy is used up in driving the compressor, which is usually mounted on the same shaft as that of turbine, and rest of the energy may be utilised for various purposes. Since the compressor is coupled with the turbine shaft, the come pressor absorbs some of power produced, by the turbine and hence lowers the efficiency. The network is therefore the difference between the turbine work and work required by the compressor to drive it. The gas turbine prime mover was first used in 1939 for large central station service. Since then several stations have been built with gas turbines to drive electric generators. Gas turbine generators have been built and electrical outputs upto 100 MW. In some situations gas turbines are the cheapest type of plants available. These situations are when they are used as intermittent or peak load plants in combination with the base load plants. These are particularly useful and economical when the amount of energy required is a small part of the total energy to be supplied by the whole system and the lcpad factor of the plant is less GAS TURBINE POWER PLANTS 589 plants than 15%. In a large system the size of the gas turbine normally employed varies from 10 to 30 MW. These plants are cheaper in capital cost compared with steam stations of the same size. Also the fixed charges of these plants are comparatively lower than those for steam plants. The thermal efficiency of gas turbine plants is however lower compared to that of condensing steam 'plants (20 to 25% compared to 25 to 30%). No doubt a lower thermal efficiency results in increased fuel costs at low load factors, but this is compensated by lower fixed charges as well as lower operating and maintenance charges. A gas turbine plant has the advantage of high reliability, flexibility, low start up time and less space requirements. They are ideally suitable as peak load plants. At some places they are also used as base load plants. In India the 70 MW gas turbine plant at Namrup in Assam works as base load plant with natural gas as fuel. Uran-Gas turbine power plant in Maharastra is the second power plant established in the country. 14·2.Application of gas Turbine Plants Gas turbine plants have the following applications: 1. or hydro 2. 3. To drive generators and supply peak loads to steam, diesel plants. To work as combination plants. To supply mechanical drive for auxiliaries. These plants are suited for peak load purposes as already mentioned because their fuel costs are some what higher while their initial costs are low when these plants are used with conventional boilers they may be used for (a) (b) supercharging or heat recovery from exhaust gases. In supercharging system air is supplied to the boilers under pressure by a compressor mounted on the common shaft with turbine and gases formed as a result of combustion after coming out of the boiler; work in the gas turbine before passing through the economiser and exhausting through the chimney. The turbine drives the compressor and also generators, producing some additional power for the station. 590 POWER PLANT TECHNOLOGY In the exhaust heat recovery cycle the gas turbine plant is fitted with usual combustor and gets the gas supply from the combustor. The gases after expanding in the turbine enter the boiler and transfer part of the heat, to the boiler tubes. In the supercharging system heat transfer in the boiler increases by about 7 to 8%, while in the exhaust heat recovery cycle the heat rates are improved by about 4 to 5%. Also in the later case no mechanical draught is required because due to pressure of exhaust gases the furnace is under positive pressure. The gas turbine is widely used in air craft. There are many installations in ships as propulsion unit. Attempts are also being made to develop it as an engine for automobile use. There is a wide range of industrial applications ranging from petro-chemical, thermal process industries to generate utility industries. 14·3. Types of Gas Turbine Plants On the basis of combustion classified as follows: process the gas turbine may be 1. Continuous combustion of constant pressure type, the cycle working on this principle is called Joule or Brayton cycle. \ 2. The explosion or constant volume cycle; the cycle working on this principle is called Atkinson cycle. Another classification based on the path of the working ~1Jbstance, it is classified as : (i) Open cycle gas turbine in which working fluid enters from atmosphere and exhausts to atmosphere. The working substance air first is compressed in the compressor, and after compression, its temperature is raised by burning fuel in it. The products of combustion along with the excess air are passed through the turbine, developing power and then exhausted into the atmosphere. For next cycle, fresh air is taken in the compressor. (ii) Closed cycle gas turbine, in which working fluid is confined within the plant. The air is heated in an air heater (refer Fig. 14·3·1 (b) by burning fuel externally. The working air does not come in contact with the products of combustion.'The hot air expands in the turbine and then cooled in a precooler and supplied back to the compressor. The same working fluid circulates over and again in the system. GAS TURBINE POWER PLANTS 591 Combustion chamber Compressor Air inta~e (a) Open cycle. Gases. out -'- FUE L c.c. Compressor Gas turbine Heat exchanger Fig. 14·3·1. Schematic diagram of open cycle and closed cycle gas turbines. Gas turbine type. (a) (b) (c) power plants can be anyone of the following Simple cycle Gas turbine power plant. Combined cycle Gas turbine power plant. Co-generation Gas turbine power plant. (a) Simple cycle Gas turbine power plant. It is based on Brayton cycle as stated above in which air is compressed to a higher pressure with the help of compressor and temperature of air firing fuel in the combustion chamber before expanding in the turbine. The difference between work output in expansion process and tl:1e work input in compression process is the net oqtput of Gas turbine .which will be converted into electricity. 592 POWER PLANT TECHNOLOGY (b) Combined cycle Gas turbine Power plant. This type of power plant is combination of simple Brayton cycle gas turbine and Rankine steam cycle as bottoming cycle. Exhaust gases from Gas turbine whose temperature is of the order of 550°C are led the heat recovery steam generator to generate steam which in turn drives steam turbine producing additional power. This cycle derives the advantage of higher temperature achieved in Brayton cycle and lower heat rejection (sink) temperature of Rankine cycle. Gross efficiency of the order of 47% can be achieved in such combined cycle power plant which is higher that super critical pressure conventional power plant. (c) Co-generation Gas turbine power plant: These power plants are similar to combinedcyclepower plants; the basic difference being that the steam generated in the heat recovery steam generator by the gas turbine exhaust gases is used for process application either fully or partially instead of generating electricity only. 14.4. Open and Closed Cycle Gas Turbine (1) Open Cycle Gas Turbine. The arrangement that has proved most successful in the continuous combustion or constant pressure gas turbine which is described as follows: A simple open cycle gas turbine plant consists of the turbine itself, a compressor mounted on the shaft or coupled to the turbine, the combustor, and auxiliaries, such as starting device, auxiliary lubrication system, fuel system, the dust system etc. A modified plant may have in addition to the above, an intercooler, a regenerator and a reheater. The arrangement of a simple gas turbine plant is shown in Fig. 14·4·1. Fuel " Com bustion /chamber 3 Power output c Air from atmosphere (a) Schematic Exhaust gas to atmosphere diagram of gas turbine cycle. resembling a turbine in reverse. \ \ 3 f.or an axial compressor is employed.V. In the centrifugal compressor. air taken in .. the pressure of the gas is increased).(..AS TURBINE POWER PLANTS 593 2 i 4 Constant pressure heat rejection I -----'il. namely. a centrifugal . In traversing the passages between the blades. the kinetic (motion) energy of the gas imparted by the rotation is changed into pressure (internal) energy (i. The quantity of the working fluid and speed required are more. }<'ig.14·4·1. It will be noted that the essential components are three in number.t 4 \ \ \~'..e. An axial-flow compressor consists of sets of moving and fixed blades. The method of operation is as follows: Air enters the air compressor in which it is compressed. --(b) "\r P. through a pressure compression ratio of some 4 or 6 : 1. a combustion chamber (combustor) and a turbine.-=- p \ ~ rTurbine work 'ill. so generally. There are some installations in which the pressure compression ratio is as high as 10 : 1 or even 18 : 1. although these llre not common. Centrifugal compressors are often used in small gas turbines.o~ cp --(e) T-4J •• diagram.. diagram. air compressor. Open cycle gas turbine.. 14·4·1 (a) that the turbine is coupled back to the air compressor by a coupling shaft. 14·4·1 (a) is arranged to develop shaft power. On the other side of tl1e turbine there is a coupling by means of which the turbine can be coupled to drive some external equipment. the high-speed air enters a diffuser. A special shroud is usually built round the burner in order to meter the air to the combustion space.594 POWER PLANT TECHNOLOGY near the shaft of a rotating impeller blade is accelerated outward by centrifugal force. thus protecting both the combustor and the turbine blades from damage. The turbine illustrated in Fig. that is. This is usually dealt with by supplying considerable excess air above the required for complete combustion. From this it will be observed that. The excess air mixes with the very hot combustion products and moderates the temperature of the gas somewhat. It will be noted upon inspecting Fig. steps are taken to ensure that temperatures do not become too high. This ensures that there is good burning of the fuel and that further air is fixed with the very hot combustion products further down to the combustion chamber. In the combustion chamber fuel. which is usually a fuel oil. the gas turbine has a very wide use as ajet . such as gas oil or kerosene is sprayed inform a burner and is burnt continuously. Due to continuous combustion which occurs in the combustion chamber. Thus the air passing through the combustion chamber has its temperature and volume increased while its pressure remains constant. The mass (or weight) of air supplied to the compressor is three to four times the amount required theoretically for complete combustion (about 50 to 60 parts by weight of air to one part of fuel). Due to combustion. If there are several combustion chambers then the take off volute from the air compressor will have ducts feeding the combustion chamber equispaced around it. This brings the final combustion product temperature down to something workable before entry to the turbine. On the other hand. The product of combustion from the combustion chamber are expanded in the turbine from P3 to atmosphere. heat is added to the working fluid from T2 to T3. the turbine would be designed to extract as much energy from the combustion products as possible before they are passed to exhaust. in this case part of the turbine output is used to drive the air compressor and it is the net output which appears for driving external equipment. a nozzle designed to convert kinetic energy into pressure energy. This being the case. At the periphery. The compressed air is passed from the air compressor into the combustion chamber through a duct. There is however. They are then passed rearward of into a nozzle from which they issue with a high velocity and thus they provide the necessary thrust for propulsion of the air craft. It must first be motored up to some minimum speed. The turbine rotor is usually motored upto 'coming in speed' by a starter motor. such as fuel-pump and oil pump are necessary. The basic element of the jet propulsion unit are the same. Gas turbine C. 2. The speed of the gas turbines varies considerably. Turbine of output as high as 20. Gas turbines are not self starting machine as the reciprocating internal combustion engine it is necessary only to turn the engine over one compression. the fuel is turned on ignited and the turbine will then pick up speed of its own.When this speed has been reached. called the 'coming in speed' before the fuel is turned on. Reduction gear boxes are fitted to high-speed turbines for coupling to external equipment in order to reduce this speed. In these circumstances the combustion products will leave the turbine still with a high energy content. Closed Cycle Gas Turbine. and as high as 35. The gas turbine will not start simply turning the burner on.II Combustion chamber AC Genera"tor Exhaust gases Fig. 14·4·2. It can be as low as 3000 rev/min. This can either be electrical or some times it is a small turbine. Simple cycle gliB turbine plant. In an open cycle gas turbine plant. To minimize these superior quality . the fuel is mixed with air in the combustor and combustion gases are expanded in the gas turbine.GAS TURBINE POWER PLANTS 595 propulsion unit for air craft.000 kW have now been built.000 rev/mm. the hot gases cause erosion and corrosion of turbine blades. no power out shaft and the turbine itself its built just large enough to drive the air compressor and auxiliaries.C. the engine will fire and then it will pick up speed on its own. and air consumption as much as 130 kg/s is recorded. The heat conductivity of hydrogen is about 6-8 times that of air and therefore requires smaller heat exchangers. it is not mandatory that it should be air. Xenon and mercury vapour. . Combustion of the fuel takes place in the air heater and is external to the working medium of the system. air or gases of a higlier density than air.596 POWER PLANT TECHNOLOGY of filel has to be used in the combustion chamber.:':. Other working mediums may be helium. argon. The trouble or drawback of an open cycle plant is overcome in a closed cycle plant. Since the close cycle continuously circulates the same working fluid. The density of the working fluid is increased in the closed cycle by placing the system under an initial overall high pressure. 14·4·3. A schematic sketch of a simple closed cycle gas turbine is shown in Fig. The working fluid leaving the turbine is cooled down by the cooling water in the precooler and is recirculated to the compressor. Gases of combustion Air heater Fuel' Accumulators Cooling medium Jo'ig. red4ces the physical size of all components and ducts of the system for the same power output and permits the use of higher temperatures for a given stress limit. This increase in the density. such as the monoatomic gases.medium is not required to support combustion. where the fuel does not mix with the working medium air or gas. hydrogen or neon. the heat added must be supplied through a heat exchanger from an external source and the heat rejected from the system must be through a heat exchanger and a cooling medium. krypton. Also since the working. The advantages of this system over that of the open cycle are: L R.educed size. -r~Jp". Simple closed cycle glIS turbine. It is possible to use a gas of heavier density and higher specific heat than air. 14·4·3. Disadvantages of the closed cycle as compared to open cycle gas turbine engine are: 1. Constant temperatures lead to constant heat drop and constant velocities in the turbine blading and hence the velocity triangles and consequently the turbine and compressor efficiencies remain constant for every power output. The use of high pressure requires a strong heat exchanger. No contamination. The regulation of the closed cycle gas turbine is simpler. 7. In case of an open cycle gas turbine the power control is affected by controlling temperature which affects the efficiency of the turbine at part load. 5. The closed cycle utilizing external heating can use an inexpensive solid fuel. Fuel. The system pressure is proportional to the gas mass flow. There is absence of risk of corrosion and abrasion· of the interior of the turbines. The power output of a closed cycle gas turbine can be controlled by changing the mass flow. Improved part load efficiency. The compressor should remain free of dust and other foreign deposits since the working medium can be cleaned before being put into the systems. Improvement in the rate of heat transmission. Since the system is under an initial high pressure with a working medium other than air. This add~to the cost and increases the engineering problems. Since the working medium does not contain the gases of combustion the turbine and the generator are not subjected to carbon deposits and should remain relatively clean. The control of a closed cycle system is different from the open cycle.AS TURBINE POWER PLANTS 597 2.<. . The complexity and cost of the system particularly in the load control. it is necessary that the system be gas tight. output changes but the temperature drop remains the same. 6. The power output at constant speed can be varied by adding or subtracting the working fluid and thus altering the charge weight. This means that \the periodic cleaning of the component is not necessary and the component efficienciesshould not change appreeiably with continued operation. such as coal. 2. 3. 4. Thus continued operation should not reduce the thermal efficiently. By changing the pressure and mass flow. Fluid friction loss is reduced due to the higher Reyhold's number. is increased. since heat transfer is indirect.••. It has a comparable or better efficiency than steam plants of the same power output with a great saving in weight and space._______ ~~ . Since the compressor is coupled to the turbine then. and temperature of air (or gas) at the end of compression and at the end of expansion are higher than those in the case of an ideal cycle.(14·5·2) But compressor work = rha CPa (Ti where rha = mass of air flow/second CPa = specific heat of air at constant pressure T~= final compression absolute temperature~ Tl = intake absolute temperature. The representation on pressure volume and temperature entropy planes is shown in Fig. . 3. large air heater is required.3. ••••••• JL •• J.l. it is higher in efficiency. It is a dependent system Cooling water must be provided for the precooler. Such a cycle is never possible in practice due to irreversibilities introduced in the operation on account of leakage. A heavy. and is easier to adopt to marine propulsion than the open cycle gas turbine.. smaller in weight and space. 2. Inspite of the disadvantages and complexities of the closed cycle. This eliminates the use of the system as an 'aeronautical' engine. 4' and the ideal by points 1. 4.. turbulence and internal friction. Work Output and Thermal Pressure Gas Turbine Plant Efficiency of Constant The ideal gas turbine cycle using isentropic compression and isentropic expansion is called constant pressure cycle or Brayton or Joule cycle. " . 4. The air heater is relatively inefficient compared to the internal combustion chamber of the open cycle gas turbine engine. _.(14·5·1) ... 2'..l i. The actual processes of compression and expansion are not isentropic. 598 POWER PLANT TECHNOLOGY 3. 14·5·1.IJ._. The actual cycle is represented by points 1. Poor combustion efficiency results due to losses as radiation and other. The provision of cooling water is not a problem in marine propulsion and many land based applications but this is a disadvantages as compared to an open cycle plant. __ . Net work output-Turbine output-compressor T1) work . 14·5. ) where . The frictionless adiabatic temperature is calculated using gas laws and is obtained from the equation.(14... r.Entropy _ . the turbine output is obtained from the equation.' ... by a similar analogy to that used for the air compressor. Volume - Fig.. . 14·5·1.5.\ IsenlroplC\. Turbine output = mt Cpt (T3 .(14·5·5) mt = mass of the combustion products through turbine per second / CPt = specific heat of combustion products ~t constant pressure T3= inlet absolute t~perature of the turbine T'..T1 = 2 Knowing the isentropic efficiency of the compressor. -1 T2 = Tl (~)----r.T'.. . But the final compression temperature is above the normal adiabatic compression temperature du. 14·5·4 T2 () can be Now to consider the turbine.e to turbulence as stated above.3) where Ya = adiabatic index for air. l.GAS TURBINE POWER PLANTS 59f 2 . 2/ --~-_. calculated. is by means of the adiabatic or isentropic efficiency equation. = exhaust absolute temperature of the turbine. T2-T1 lsentrop1c1]comp 'r. Ideal and actual gas turbine cycles on P-V and T-f diagrams. 6 /'. The connection between the frictionless adiabatic compression temperature T2' and the final compression temperature Ti. \ \ 3 T \ I cp.. . T4) .T'. T4 = T3 1 (~. Net Workoutput = mt CPt . which occurs in the turbine.600 POWER PLANT TECHNOLOGY Now the final exhaust temperature from the turbine will be above the frictionless adiabatic exhaust temperature as a result of turbulence.TJ)} ..T4) .ma Cpa (Ti . T4 may be calculated from the gas law equation r.) .m 3 TJturb T3 .T4 T. Now the mass of fuel used is usually small compared with the mass of air.TI) for (T3 - T4) and (Ti - T1) in terms of (T3 . If this is the case. so these two temperatures are connected by the equation .(Ti T1)} Substituting and (T2 . and hence the mass of the fuel is often neglected.)r.. 4 = Isentropic efficiency at the turbine T4 = Frictionless adiabatic absolute exhaust temperature. etc. Net turbine output = mCp (T3 .(T3 . Then from Equation (14·5·8).T1) = mCp {(T3 ..mCp (Ti . In a similar way to that adopted in the case of the air compressor. = ma = m'..T4) . say If the fuel is neglected the it can be considered that CPt = CPa = Cp say..(Ti .T1) .T4) . IsentropIc where Isentropic TJturb ..(14·5· 7) products (14·5·8) where It = adiabatic index for the combustion through the turbine. then m.. then from equation (14·5·9) Net power outPUt of the turbine = mCp {(T3 .(14·5·9) If m: = mass of air in kg/s. energy input Which from equations (14·5·9) and (14·5·10) becomes Thermal 11= mCp {(I:1 . the following relationship is obtained for rr r= (~~)2 (r-· I) ) .(14·5·12) With TIc = 11t ideal conditions.r 1 r~lI-Tk1'1 C -1 ] .(14·5·10) where r is the compression ratio.r. This expression can be differentiated w...r r 1 1. The energy received in the chamber at constant pressure.) .. = 100%.GAS TURBINE POWER PLANTS 601 .1't) . temperature is raised from 1''.1'2) . keeping 1'a and 1'1 as constants.1'2) r = ritCp . and equated to zero to find the value of r for maximum net work. =mCpTJt1'a j 1 p-[r.(1'2 ..1'..t. then energy received at chamber gas turbine is in the combustion In the combustion chamber the 1\ If the fuel mass is neglected as constant pressure in combustion (T:l .(1'2 .1'2) = (T:l ..1'1) (T:l .I'd} --' mCp(T:1 .' to before. and is the ratio of the two specific heats at constant pressure and at constant volume.(14·5·11) = change in enthalpy Now thermal efficiency _ work output . compression ratio r. Ti) T2) = mC p (Ta - approximately 7 = mC p [ Ta - Tl r) r.1 r. Heat supplied to plant = Cp (Ta . This results in the following expression for r. (14·5·14) From equation (14·5·14) it is evident that thermal efficiency of gas turbine plant depends upon the ratio of compression. . the temperature of t)1e gas at the end of compression is the same as the temperature at the end of expansion. The expression (14·5·14) can be differentiated with respect to r and equated to zero to find the compression ratio for maximum thermal efficiency.e. the maximum network from the plant. (14·5·13) = mCpT1 [ ~: - (r)-r-1] r- Thermal efficiency T/th = net work '...C T1 [r-r-. theoretically...1] .---~=~-----------------__ 602 POWER PLANT TECHNOLOGY -IIIIIIIIlnRII i. the efficiencies of compression and expansion.. the turbine inlet temperature and compressor inlet temperature.1 ] . A continuous combustion constant pressure gas turbine takes in air at 0·95 kg I cm2 (93 kN I m2) with a temperature of 20°C. 14·5·1.83 .GAS TURBINE POWER PLANTS 603 ...T1 = 1/c - 464 .1 293·50:4 = 293 x 1·5837 = 464°K Isentropic efficiency for the compressor T2 1/c - T1 = mJ .. For an air flow of 10 kg I s and neglecting the fuel mass as small. i. Cp = 0·24 keall kg°K) (= 1·00 kJ / kg oK) Solution.12 T1 T1 _ T2 - Ti.(14·5·15) If we assume nc = 100%. Take r= 1·4.293 = 206 K 0. (b) the thermal efficiency of the plant. The compressed air is passed to a combustion chamber in which its temperature is raised to 867·C. under ideal conditions y Then ry-1 = ~: . For the compressor (refer Fig.) r-1 T2 = T1 = (~~)-r1·4 .e.. determine : (a) the net power output of the plant if the turbine is coupled to the compressor. with an isentropic efficiency of 83%.16) The equation gives the optimum compression ratio for maximum thermal efficiency. From the combustion chamber the high temperature air passes into a gas turbine in which it is expanded to 0·95 kg I cm2 (93 kN I m2) with an isentropic efficiency of 80%. . (14·5. A rotary air compressor compresses the air to a pressure of 5·70 kglcm2 (558 kN 1m2). Example 14·5·1. ~1-T4 - 73 .206 1140 .1 T4 = T3 (~:)-r.(Ti T1)} = 10 x 0·24 (336·.206) = 312 kcal/s = 312 x 427 :: 1776HP. 75 Ans.. Net power output (81 system) = 10 x 1 x (336 .£= T3 . Methods to Improve Thermal Efficiency of Gas Turbine Plant. 14·6.£).) ..21 (~1.T4) - (Ti T2) T1)} = rhCp (~1"'~ 1.336 = 804 K t" = 531°C Net power out put (MK8) = mCp {(T3 .336 = 1140 .T.500 = 0. Thermal Work output 7J:: Energy input rhCp {(T3 .720) 0·80 = 336 K T.273 = 227°C For the turbine r.T'. .£ Isentropic efficiency of turbine ~1 7J -'-- t.T'. :: (T3 T4)rlt = (1140 . The efficiency and the specific work output of the simple gas turbine cycle is quite low inspite of increased component efficiencies.(Ti :: T1) _ - 3 I - 336 .2031 = 20·31% Aus.206) :: 1300 kW Ans..510~2~~: 7200K = : T. 604 POWER PLANT TECHNOLOGY Ti = 206 + 294 = 500 K. t2 = 500 . Therefore. The exhaust gases from the turbine pass through the regenerator and give their heat to the compressed air. an improvement in efficiency can be attained. . Fig. the turbine exhaust temperature is always greater than the temperature at the outlet of the compressor. This utilization of heat in turbine exhaust can be affected in a heat exchanger called re-generator.. Temperature turbine Generator ) entropy diagram for regenerative cycle. before it enters the combustion chamber.GAS TURBINE POWER PLANl'S 605 Some modifications improve the thermal efficiency of a simple open cycle gas burbine. reducing the heat which must be supplied in the combustion chamber for a given turbine inlet temperature. thereby. before it enters the combustion chamber. The power output will be slightly reduced because of the pressure losses in regenerator and its associated pipework.6. Heat exchanger 2 Gas ----Air intake Fig. if this temperature difference is used to increase the temperature of the compressed air before entering the combustion chamber and. thereby reducing the amount of heat which must be supplied in combustion chamber to get a given turbine inlet temperature T:l' Thus regeneration improves fuel economy. thus there will be improvement in the cycle thermal efficiency. . Due to limitations of maximum turbine inlet temperature ann the pressure ratio which may be used with it. One of the main reasons for the low efficiency of a simple gas turbine plant is the large amount of heat which is rejected in the turbine exhaust. 14·6·1 shows a schematic diagram of such an arrangement. they are: (1) (2) (3) Regeneration Intercooling and Reheating (4) Gas Temperature (5) Pressure ratio (6) Combined cycle and Co-generation (1) Regeneration. by this process there will be a saving in fuel used in the combustion chamber. 14·6·1. if the same final temperature of the combustion gases is to be attained and also there will be a reduction of waste heat. In the regenerator the heat energy from the exhaust gases is transferred to the compressed air.•.. So.. (14·6·2) (T4 . the thermal efficiency is pushed upto the range 20% to 30%.Ti) .T6) = (T5 . drop in H. The thermal efficiency of gas turbines without heat exchanger is usually in the range of 15% to 20%. possible rise _ T. assuming E Tit and Cp constant throughout then . . J ct>Fig. 14·6·2..Ti) = (T4 . They operate most commonly between 70% and 80%. since Ti is the lowest temperature in the heat exchanger. drop in HE l t T Max. 14·6·2. that the maximum exhaust temperature drop available in the exchanger = (T4 .. ·lncrease in combustion t t T JChamber 4' Exhaust temp.Ti) = mCp (T4 - n) = mCp (T..606 POWER PLANT TECHNOLOGY The energy recovered from the exhaust in actual gas turbines varies from 50 to 90%.Ti).Ti) ..(14·6·1) .Ti . recovery. ..T4 - Ti Or it can be written as EmCp (T4 . They percentage recovery of the heat exchanger is called its effectiveness.E. temp. Temperpture entropy diagram for regenerative cycle. With a heat exchanger fitted. The effectiveness of the regeneration is defined as: e = Effectiveness = Rise in air temperature Max. The temperature entropy diagram for the turbine arrangement with heat exchanger is illustrated in Fig.. It will be noted Compressed air temp. (T3_ T5) .T'. T6 to be determined..(14·6·5) and again assuming m and Cp constant throughout.(14·6·6) Example 14-6·1.. The turbine inlet temperature is 800°C... Determine the improvement in the efficiency resulting from the installation of the regenerator. TJ) Thermal 1] = (T3 ..(14·6·3) However the energy required from the fuel is that required to increase the temperature from Ts to T3. the compressor compresses air from 100 kPa and 22°C to 600 kPa. Assume y ]·4 and Cp = ]·03 kJ / kg K.(14·6·4) This is evidently less than that which would be required if no heat exchanger was fitted in which case the temperature increase required from the fuel would be from Ti. . It is lmown that a regenerator with 80% effu:iency is available.. Referring to figure 14·6·2. The nett turbine output will be as before . P2 = TI = 273 + 22 effectiveness = 0·8 1]is turbine = 295 Ie 600 kPa. - TI)} . the isentropic efficiencies of the compressor and the turbine are 0·90 and 0·85 respectively. to Ts.In a gas turbine cycle.. == Solution.GAS TURBINE POWER PLANTS 607 Equation (14·6·2) will -enable the exhaust temperature from the exchanger.. .). = 0·85 First we will determine the thermal efficiency without regenerator and then with regenerator and calculate the improvement in the efficiency due to the regenerator..(Ti. Thermal 11 = mCp {(7:1 . Thus with a heat exchanger the thermal efficiency of the plant is increased. T3 = 273 + 800 = 1073 K e = regenerator 1]is comp = 0·90. . Data given PI = 100 kPa. TI).(Ti. r'<p (7:1 -7:. 1 T/is turbine turbine efficiency is T4 =.. For the isentropic process T2- 1 .4 = 295 (0)(14Similarly • T4 = T:I P2 X = 492 K ( PI.TI 7Jis C = 295 + 492. T4 = T3 - (T3 - 1'.S... 14·6·2 K T. diagram for cxmplc (14·6·2). 1073 Equation for the isentropic 11i".95 = 514 K 2 0·90 The expression for the isentropic 1...4 PI _ TI (~)(7- I)/r 1)/1..e.0·85 (1073 .2 and 3 -.. . )(7.608 POWER PLANT TECHNOLOGY t ~ W 0: ::J ~ <t 0: W (L W 1-' ~ 30 Ok ENTROPY ~ Fig. 'T" compressor efficiency is =- ~--1' 12 -- T2 -TI I on rearrangement T2 12 _ T 1 + ---.643) = 707·5 K.1 - T4 T/isl i.:..I)/r 6."1.) = 1073 .4/14 = 643 K -. 707·5) . A regenerator. if intercooling is used the size of the turbine and compressor can be reduced for the same output or alternatively greater work can be obtained from the plant of the same size.669 = 0·36 or 36% The improvement in the thermal efficiency of the plant. Thus. IntercooJing improves the thermal efficiency.T5) _ (1073 .1 . and this is accomplished by employing multi-stage compression and intercooJing between stages. - T1) Cp (T. ""4 .GAS TURBINE POWER PLANTS 609 = .295) 1073 . One of the ways to achieve this is to cool the air after it has been partiaJJy compressed. due to regenerator instaJJation is = 0·36 . Two possible methods for increasing the work output are: (i) (ii) by reducing the work of compression. - T1) = Cp (T3 - T2) _ (1073 . Usually 2 to 3 stages of compression are used. and by increasing the work done by the turbine.Cp (Ti.0.Ti. .26 70 (2) Intercooling. as discussed above does not change the work output of a gas turbine cycle.707·5) .rn' 12 Also E T5 = 0·80 (707·5 .1- T4) .0·26 = 38m 0.. IntercooJing is used for decreasing the workdone on the compressor.514) + 514 = 669 K For thermal efficiency without regeneration Cp 71th (T3 - T4) - Cp (Ti..(514 .514 = 0·26 or 26% Thermal efficiency with regeneration 71th = Cp (T.295) 1073 . 14·6·3 shows the schematic diagram of a two stage intercooled gas turbine and Fig.80 = T. air rate and work ratio.(514 . 14·6·4 shows the indicator diagram for a two . Fig. Maximum advantage of intercooling occurs when pressure ratio is high. and there should be no loss of pressure in the system. the compression process in the compressor can be made to approach isothermal compression which requires less powr~r than the adiabatic compression.e. 14·6·4. i. T-¢ diagram for t\\O stage intcrcoolcd gas turbine cycle. By employing multistage compression with intercooling between stages. T stage compressor with intercooler. in each'stage . 14·6·3. In the ideal state of intercooling the fluid should be cooled to its ambient temperature. compression efficiency is low and regeneration is employed. .610 POWER PLANT TECHNOLOGY Combustion chamber Compressor 2 3 Exhaust Intercoole r Fig. Schematic diagram of a two-stage intercooled gas turbine. Also the maximum advantage of intercooling is obtained when the pressure ratio for each stage is the same.¢ _ Fig. S i Entropy. the temperature of the fluid before compression. It is arfother method of increasing the specific work output of the cycle. T-~ diagram of reheat cycle. ¢> Reheater 3 Compressor HP Turbine lP Turbine Generator 6" Fig. Fig. Also. T t -4> Fig. reheating improves the output from the turbine due to multiple heating. 14·6·5 shows the schematic diagram of a reheat gas turbine plant and Fig. The gain in work output is obtained be cause of divergence of constant pressure lines on T • diagram. Reheating involves extra equipment of combustors and high temperature resistance material for construction which adds to cost. the complication of spliting the turbine and of producing suitlable controls may offsets much of the gain by use of reheater in m~ny cases. more output will be obtained than that obtained by expansion in a single stage. Schematic diagram of a reheat gas turbine plant.GAS TURBINE POWER PLANTS 611 (3) Reheating. 14·6·6 the corresponding T-¢> diagram. An intercooling improves compressor performance. Thus for the same expansion ratio if the exhaust from one stage is reheated in a separate combustion chamber and expanded. with an increase in temperature. . 14·6·5. 14·6·6. both intercooling and reheat increase the network available from the cycle. However.612 POWER PLANT TECHNOLOGY In order to increase the thermal efficiency of a Brayton cycle. Most of the heat of compression may then be removed by inter-cooling. The effect of i. In a reverse manner. From a practical stand point. in intercooling the compressed air at the exit of one stage is cooled to the inlet temperature of that stage and then compressed in the next stage. The thermal efficiency of the cycle with intercooling and reheat can then be calculated in the usual manner. The mechanical components and T-S diagram for the Brayton cycle with intercooling and reheat is shown in l<'ig. Again. and to cool the air or working fluid between them. The reduction of compressor work achieved in this manner results in an increase in the overall gas turbine output and usually improves the overall plant efficiency. is to have the work of compression approach an isothermal process (i. the analysis becomes a bit more complex but presents no extraordinary difficulty.nter cooling. from a practical stand point. however. compression at a constant temperature).. . If the compressor and the turbines have isentropic efficiencies of less than 100 percent and if there is also regeneration. we can inaease the pressure ratio. intercooling the compressed air at the exit of one stage is cooled to the inlet temperature of that stage and then compressed in the next stage. when carried to the theoretical limits. The theoretical limit of reheating would of course. (b). the t<!mperature at the outlet of the compressor increases. The compressor work required may be reduced by dividing the compression into two or more stages. For the cycle shown in Fig. causing problems with seals and metal fatigue. be an isothermal expansion at the turbine inlet temperature. compression is accomplished in two or more stages. However. As can be seen from the T-S d'iagram in the figure. To minimize such problems. the output of the turbine may be increased by dividing the expansion of the working media into a number of steps and the gas reheated between them. Ideally.(14·6·7). The reheating of the gas or working media back to the limiting turbine-inlet temperature allows a greater portion of the expansion to take place at higher temperatures. the result of reheating is an increase in the output of the turbine through the same expansion pressure range although it has a negligible effect upon the overall efficiency. Ideally. when reheating is properly utilised in conjunction with regeneration. the physical size of a compressor incr~ases with the increase in the pressure ratio. 'i'he total work of compression in the cycle is the sum of the work for each compression stage.e. the increase in overall efficiency is appreciable. the effect of inter-cooling is to reduce the work of compression required to achieve a given pressure. Also. as the pressure ratio is increased. ) .h3) WT = (hn . (a) Mechanical components.) qin = (hn .hi) .h4) + (hz + (h7 + (h7 .N.GAS TURBINE POWER PLANTS 613 (a) t-- 11 :::! w t-- ENTROPY (b) S ~ Fig. 14·6·7. We = (h4 . Brayton cycle with inter cooling and reheater. diagram.N.hS) . (b) T-S. (14·6·8) with T. (a) Mechanical components (b) T-B diagram. and reheater is shown in Fig. intercooler. and rcheater.. Brayton cycle with regenerator.•S (b) Fig. . REGENERATOR 9 GENERATOR T 8 TURBINE INTERCOOLER REHEATER CaOUNG' MEDIAM (a) ENTROPY .614 POWER PLANT TECHNOLOGY The open cycle gas turbine with regenerator. intercooler. diagram. 14·6·S. S. however. and the thermal efficiency of the cycle. and Cp = 1·03 kJ / kg K. this temperature is limited by the potential for blade damage. Alternatively. it would probably be necessary to use special means of cooling the blades.~) h3) + (hs . depends in the first place on the intake gas temperature.[(h2 _ ~) . In practice..h7) + (hs 17th (~ h9)J-=. an increase in the pressure ratio. For still higher temperature.haust gas pressure).e. the increase in thermal efficiency resulting from an increase in gas temperature must be balanced against the greater cost of the turbine. The thermal efficiency of a gas turbine. the pressure in the combustor relative to the ex. By the use of special alloys and protective refractory coatings for the blades. the hot gas might be used to produce process heat i.h7) h3)) .h7) . Upto a point. The thermal efficiency of a gas turbine is related to the pressure ratio (i. However.e. Gas temperatures are commonly in the range from 800 to 900°C. The inlet conditions are 300 K and 100 k pa and the temperature at the inlet to the turbines is 1300 K.. Another approach to increasing the efficiency of fuel utilization would be in a combined cycle or cogeneration system.e.119) h7) qin = (~ + (hs - The thermal efficiency of the cycle is expressed as _ [(~ . Example 14·6. to about 10 at moderate gas intake temperatures or to 20 at high temperatures. is accompanied by an increase in efficiency once again.GAS TURBINE POWER PLANTS 615 We = (112 hi) + (h4 Wr = (~ . the increased cost of the equipment must be taken into account. Determine the compressor work.hi) + (h4 + (hs . The still hot exhaust gas from the turbine provides the heat for generating steam in a waste heat boiler. the turbine work. combinedcycle generation. as defined earlier. cogeneration. Take r = 1·4. Combined cycle and Cogeneration.2. (14·6·7) Gas Temperature. (5) Pressure ratio. The steam is then used to operate a steam turbine i. . the temperature can be increased to about 1250°C or so. which should be as high as possible. In a two stage gas turbine cycle with ideal inter cooling and reheat. the pressure ratio in each stage is 3·5. A regenerator with an efficiency of 70% is used to improve the efficiency. T7)] = 1·03 [(1300 .T1) = 2 x 1·03 (429·10 . T.T4) (Tg .429·10) + 429·10 = 764·90 K. diagram for Two-stage regenerative gas turbine cycle with ideal intercooling and reheat is shown in Fig.908·80) = 805·70 kJ/kg Ans. For isentropic process 1·2 We haveT2 = TI ( = 300 = T4 '7/PI)(r- I)/r = 429·10 K X (3·5)°·4/1.T7) + (T8 - Tg)] = 2 Cp (T6 .764·90) + (1300 . (14·6·7).T4) or T5 = 0·7 (908·80 . The heat supplied is given by qif/t'=(h6h5) - + (h8- h7) + (T8 .616 POWER PLANT TECHNOLOGY Solution. The turbine output is WI = Cp [(T6.7 = (1:<.908·80)] = Cp [(T6 T5) = 954kJ/kg . Likewise T7 = Tg = T6 (3'5f (3·5)-04/14 -r- (r- I) = 1300 = 908·80 K.T7) = 2 x 1·03 (1300 . .300) = 265·95 kJ/kg Ans. The compressor input is We = Cp [(T4 - T3) + (T2 - T1)] = 2 Cp (T2.8.4 also Ideal intercooling and reheating is to be considered. For the regenerator 0. 000 rpm. sliding friction of the piston inside the cylinder. a version of this compressor in the 'free piston' design. such as. the compressor should be such as can be coupled to the turbine shaft which runs at very high speed ranging from about 600 rpm to 40. The impeller converts the mechanical . only a centrifugal or axial compressors can be employed. because it sutTers from a number of disadvantages. of A Gas Turbine Plant 14·7. inertia of moving parts. 14·7·1. A stationary I passage surrounding the impeller diffuser helps to convert most o~ the velocity head into pressure head as the air has a high velocity when it leaves the impeller. etc. The centrifugal compressor consists of a rotor called impeller provided with vanes and moving in a casing or scroll. limitations in speed. Reciprocating compressors can not be used. Compressor. Turbine and Heat exchangers. Combustion chamber. A gas turbine compressor should be able to handle a relatively large volume of air or working media and delivering it at 4 to 6 atmospheric pressure with the highest possible efficiencies. which eliminates use of crank shaft and connecting rods and is at present being developed for use in these plants.GAS TURBINE POWER PLANTS 617 Now thermal efficiency can be calculated 11th = Turbine work . However. Main Components The basic gas turbine components are: (1) (2) (3) (4) Compressor. On the above basic requirements. and are not considered suitable for use in gas turbine plants. moreover. The static pressure of air increases to the tip. called the inducer. A schematic diagram of a radial bladed centrifugal compressor is shown in Fig. Air is given a whirling motion at high velocity by the impeller and is thrown out of it by centrifugal force. is curved to minimize entry losses and is provided with vanes to direct the air to the eye.compressor work Heat input - = WT We 805·70 .265·95 954 = 0·565 or 56·5% Ans. 1. The inlet section at the hub of the impeller on one side. e. Schematic of a centrifugal compressor. 14·7·1. the relative velocity decreases from inlet to outlet due to diverging channel area) and the centrifugal action (i. energy imparted to air by the rotation of the impeller into pressure and kinetic energy. The vane less diffuser converts some part of the kinetic energy into . The pressure rise in the impeller is due to diffusion action (i. the air enters at lower diameter and comes out at higher diameter).e.618 POWER PLANT TECHNOLOGY Diffuser throat (a) Collector Depth of vaned diffuser Vaneless diffuser (b) (e) Fig. The rest of the kinetic energy available at the tip of the impeller is converted into pressure energy in the vaneless and vaned diffuser. The rotor (i. Labyrinth packings provide sealing effect on the air and prevent leakage between the impellers of various stages and from inside the compressor to outside through shaft end connections. In a multistage centrifugal two or more impellers operating in series on a single shaft are provided in a single casing. the diameter greater and it is not as readily adoptable to multistaging. Therefore. The present day practice is to design the centrifugal compressor such that about half the pressure rise occurs in the impeller and half in the diffuser. Some into pressure kinetic energy of a rows of by the rotor is converted part of the energy in the rotor due to diffusion action and the rest is converted in the ( . A pressure ratio of 4·5 : 1 may be obtained in a single stage centrifugal compressor. The effect of multi-staging is to increase the delivery pressure of air. Radial bladed impeller is suitable where low weight and dimension are required.GAS TURBINE POWER PLANTS 619 pressure energy and stabilizes the flow so that it enters the bleded diffuser without shock. The centrifugal compressor is superior to the axial flow compressor in that a high pressure ratio can be obtained in a short rugged single stage machine. whereas the backward turned blade is suitable where higher efficiency is preferred. when high pressure ratios are needed. stationary component) consists imparted to air stationary blades. moving element) consists of rows of moving blades and the stator (i. The compressor discharge can be controlled by varying the speed. a 90° bend is provided to take air to the combustion chambers. However the efficiency is lower. as air compressed in one stage of machine is fed into the next stage for further compression and pressure is multiplied in each stage. In the gas turbine radial bladed impeller is used due to lighter construction and less stressed impeller. The impeller blades are made in two types. For higher pressure ratios multistage centrifugal compressor does not prove to be as useful as an equivalent axial flow compressor. The overall efficiency of a multistage compressor is lower than the efficiency of individual stages. the axial compressor is heavier than the centrifugal compressor but it has higher efficiency than the centrifugal compressor.e. The axil fZow compressor consist of a rotor and a stator as shown in Fig. the radial blades and the backward curved blades. Although. It is relatively insensitive to surface deposits. has a wider stability range and less expensive.e. For the gas turbine (instead of putting the volute casing). 14·7·2. From the vaned diffuser the air is collected in the volute casing and comes out from the outlet pipe. axial compressor is advantageous and it always used for industrial gas turbine installations. e. These latter characteristics limit the part load capabilities. i. Combustion slwuld take place at high efficiency because losses incurred in the combustion process have a direct effect on the thermal efficiency of the gas turbine cycle. Further the pressure losses in the combustion chamber should be low and the combustion chamber should provide sufficient volume and length for complete . and relatively small diameter. The important characteristics of the axial flow compressor are its high peak efficiencies. (3) Combustion Chamber (Combustor) : Generally the air fuel ratio in open gas turbine varies from 50 : 1 to 250 : 1. Therefore. the combustion chamber should provide thorough mixing of fuel and air as well as combustion products and air so that complete combustion and uniform temperature distribution in the combustion gases may be achieved. 14·7·2. the axial flow compressor is sensitive to changes in air flow and rpm. high flow rate capabilities. which is later converted into work by the turbine is supplied. adoptability to multi staging to obtain higher overall pressure ratio. The stat. which results in a rapid drop off in efficiency. The rotor as well as the stator blade channels are of diverging type. the stability range of speeds for good efficiencies is smal1. of this type of compressor and are considered undesirable in some installations.or blades also redirects the air into an angle suitable for entry to the succeeding rows of moving blades. A row of moving blade with a succeeding row of stationery blades is called a stage of axial compressor. However. Arrangement of rotor and stator in axiall10w compressor. Blades are usually made of air foil section.620 Rotating Guide POWER PLANT TECHNOLOGY blades Stat ionary blades blades ROTOR Casing Fig. The combustion process taking place inside the combustion chamber is quite important because it is in this process that energy. to keep the turbine inlet temperature down to permissible limits. stator. (Refer Fig. For these two reasons. Hence requirements of a comhustion chamber are: lower pressure loss. supply the combustion air directly to the fuel and are fitted with vanes to produce a whirling motion of oil and thereby to create turbulence. Also. (a) (b) The types of construction are in general: (1) of the combustion chambers in use (2) (3) tubular or 'can' counter flow.---- ". A typical combustion chamber design employs an outer cylindrical shell with a conical inner sleeve which is provided with ports or slots along its length.~~:~:g ~~:I t Fig. the 'can' type predominates in current practice. Nozzle Outer shell \ \" --I --~UL~-~~~'~~ ~ conic~~::eve . air admitted ahead of the combustion zone serves to cool the combustion chamber and the outlet gases. (c) good flame stability. (e) low weight and frontal area. The rest of thE. for test and for replacement of burned out chambers.GAS TURBINE POWER PLANTS 621 combustion of the fuel. with an igniting device or sparking nearly. . tubular or can straight-through annular parallel flow. At the cone apex is fitted a nozzle through which fuel is sprayed in a conical pattern into the sleeve.) A few air ports provided close to the location of the nozzle. the 'can' type is cheaper and more practical. and Although theoretically the annular chamber possesses advantages over the 'can type'. 14·7·3. (d) low carbon deposit in the combustion chamber. Arrangement of a combustor. 14·7·3. I Gases /T'--'--. and (g) through mixing of cold air with the hot products of combustion. high combustion efficiency. this has not been realized in practice.. (n reliability and serviceability with reasonal1ife. turbine and regenerators. flow. Regenerator and the intercooler are the heat exchangers used in gas turbine plants. heat transfer takes place between exhaust gases and cool incoming air. Generally the blades are made of Nimic 80 alloy (heat resisting). in opposite direction. These include supply of cooling air near the rim or use of different materials for rim and hub sections. the design is called reaction type. The working media is thus not contaminated with the products of combustion. high efficiency. The regenerator is generally shell and tube construction. while if this drop takes place in the moving blades. The main requirements for the gas turbines are light weight. reliability and serviceability. while in the int. the design is impulse type.ercooler the heat transfer occurs between the hot air under compression and cooling water. (3) The gas turbine. ability to operate at high temperatures for long periods. The heat added to the working fluid air or gases of higher density. The combustion chamber of a closed cycle gas turbine engine is actually a heat exchanger. It differs only in the blading material. the surface required for the same amount of heat transfer is much less in the case of the intercooler than for the regenerator. the thermal distortion due to higher temperatures. gas turbines may be irripulse or reaction. However cheaper fuels such as soft coal may be used in the heat exchanger of a closed cycle gas turbine. the fixed blades serving only as deflectors. Efficiencies of between 95 and 98 percent are obtained over a fairly large operating range. The arrangement of the rotor and stator blades in the gas turbine is similar to that of steam turbine. . the means for cooling the bearings and highly stressed parts. Special cooling arrangements for the blades may some times be used in gas turbines. The blade speed is selected on the strength consideration of the wheel. If the entire pressure drop of the turbine occurs across the fixed blades.622 POWER PLANT TECHNOLOGY The combustion chamber in the open cycle gas turbine engine is the most efficient component of the gas turbine. In the heat exchangers. The construction and shape of the gas turbine blades are very similar to that of steam turbines. capable of supplying the heat addition required. The problem has been to design an efficient heat exchanger of a practical size. and high ratio of blade length to wheel diameter to accommodate large gas flows. As in the case of steam turbines. Since water has a much better heat transfer coefficient than do air and gases. with gas flowing inside the tubes and air flowing outside. (4) Heat Exchangers. must be supplied through a heat exchanger from an external source. A reduction in speed (when load increases) opens the fuel valves to restore normal speed. Compressor inlet air usually enters t. The clutch is often made to work under air pressure. for generator drive. A rise in speed (when load decreases) closes the fuel valves to lower the rate of fuel feed and restore speed. and between the combustor and the turbine in the simple cycle plant. The rotation of the turbine-compressor shaft. fuel control system. starting motor or engine. The fuel feed is made responsive to the speed governor. and at about 6000 rpm. Feeding in additional fuel brings the turbine upto rated speed. A failure of the lubricating pump system results in stopping of the unit automatically. air and gas ducts and plant control panel. The starting gear is mounted on the shaft at one end. The starting motor or engine drives the gas turbine ana compressor through a clutch and step up gear. auxiliary lubricating oil pump. The turbine then slowly starts under influence of the gases. Both filter and ducts must be sized to minimize air-pressure drop. gears and driven apparatus. The exhaust duct and stack must also be size to minimize pressure drop because this loss raises the turbine-exhaust pressure and reduces turbine capacity and efficiency. the starting motor is shutdown and clutch disconnected automatically. these include. for about 5 minutes at speed of 500 to 1000 rpm results inelimination of un burnt fuel from the air-gas flow system. In addition there are also automatic devices for alarm and shut down. Often the drive for the lubricating oil pump is taken from the starting-up gear. High pressure oil from the pump is supplied to the hydraulic control system and low-pressure lubricating oil for the gas turbine. The duct system includes the main connection between the compressor and combustor.he gas turbine unit from outdoors through a filter and duct. oil coolers and filters. Exhaust gas from the .GAS TURBINE POWER PLANTS 623 14·8. The filter proves necessary because a slight buildup of solids (fouling) on compressor blading can seriously reduce its efficiency. Auxiliaries and Controls Gas turbine engines need additional equipment to serve the main comP9nents . Speed is then increased to 4000 to 5000 rpm and fuel is allowed to enter the compressor where it is made to ignite and gases produced are passed on to the turbine. Any undue loss in pressure directly reduces the capacity of the unit. A separate motor-driver pump usually acts as stand-by if main pump fails. and an addition of other such connection when additional heat exchangers are employed. inlet and exhaust mumers (silencers). The air capacity through the filter should not increase about 2 mI see. 14·9. Another important point regarding ducts in to support them suitably so that vibrations are reduced to minimum. These fuels are generally costly. They corrode hot metals and build up hard deposits that choke gas passage in the blading. which also raises pressure loss. They may be oil bath type or dry type. in particular. should be capable of standing high temperatures (about 500°C for simple open cycle turbines and 350°C for regenerative cycle). and so adequate expansion joints should be incorporated wherever necessary. The ideal fuel is ofcourse natural gas but this is not always available. boiler fuel oils) are most commonly used for such plants. then mixing it with 5% of water. solid. furnace oils. They dry type filters use glass fibres as the trapping agent. which gives a pressure drop of about 13 to 19 mm of water gauge. liquid and gases. the ducts should be capable of taking up the expansion at joints due to changes in temperature.nozzles and damage valves and plungers of the fuel pumps. Silencers may be used at the inlet and exhaust of air and gas respectively. They should be stift enough to resists vibration caused by the air and gas-Cows. and calcium as part of the ash constituent. then mixing it. . this can be minimized by increasing stack height. Two centrifuges in series receive the mixture to remove the water that takes with it most of the sodi urn originally in oil. When using such fuels one has to be very careful that the fuel used possess proper volatility. Residual oil usually contain sodium. vanadium. The exhaust duct and connections between combustor and turbine.624 POWER PLANT TECHNOLOGY stack must not be allowed to recirculate to the compressor intake. Blast furnace and producer gas can also be used for these plants. The air velocity through the inlet muller may be about 60 mlsec. Furthermore. Filters of various types are used on air compressor inlets. Residual oil may be treated by heating it. viscosity and calorific value. Liquid fuels of petroleum origin such as distillate oils or residual fuels (including fuel oils. as they may clog the small passage ofthe. Moisture and suspended impurities should not be there.Fuels for Gas Turbine Plants Gas turbines can use a wide variety of fuels. Natural gas which is mainly methane has a very high calorific value and is generally used for auxiliary power generation within the oil fields. The viscous types are use a filtering material dipped in oil that catches air-brone particles as they pass through. ~-./ I I H. 14·10·1. . 14·10. Intercooler. Layout of a gas turbine power plant. In cold climate it may be necessary to preheat the residual fuels. Solid fuels (for example pulverised coal) may be used but they create coal handling and ash handling problems. The efficiency of coal fired gas turbine plant is lower than that of oil fired plant. Therefore when starting the unit for cold initially distillate fuels feed into the combustor after which residual fuels may be fed..P Turbine CombusflO chamber --. In many respects it is similar to the steam plant turbine house.. The rotating parts of the plant form a very small part of the total volume of the plant since it is the intercoolers. It is these components which occupy the major portion of the total space.GAS TURBINE POWER PLANTS 625 The increased use of heavy oils has been limited by the effect of vanadium corrosion and deposits build up on blades. " ____ \ \ 1 --' Alternator /" /" // g~~ motor --/1 I / --[. The fuel oil storage tanks are arranged outside but adjoining the turbine house..1. combustion chambers. Present day gas turbine plants use mainly natural gas liquid petroleum fuels.. Heat Air filter exchanger LP Turbine . In some installations even heat exchangers are placed out doors. 14·10.:\ I.-'''0- Fig. Distillate fuels burns more easy than doresiduals fuels. heat exchangers. waste heat boilers and interconnecting ducts work which have to be arranged and accommodated. A typical layout of gas turbine plant is shown in Fig. Plant Layout In the case of a gas turbine plant the main building is the turbine house in which major portion of the plant as well as auxiliaries are installed. . The layout of a gas turbine plant has a very important effect on the overall performance of the plant. the. 8.P. 9. 10. Since there may be a loss of as much as 20% of power developed in the interconnecting ducts with a large number of sharp bends. Space requirement for a gas turbine plant is smaller compared to a condensing steam plant of equal size. turbine. These plants can be readily located in cities and industrial centres very near to the areas of heavy power demand. The capital cost of a gas turbine plant is lower than that of a comparable steam power plant. The gas turbine plant uses fewer auxiliaries compared with steam plant.626 POWER PLANT TECHNOLOG'Y The purpose of the air filter is to clean air. In water scarcity areas they have great application. compressor via intercooler.P.P. 14·11. compressor. 4. 5. turbine and than in L.hot air from there flows to the combustion chamber. Storage of fuel is much smaller and its handling is easy. Products of combustion are first expanded in H. 7.P. The fuel consumption during starting and shutting-down periods is low. The air leaving the H. Number of personnel required for operation is hardly one-third compared with that for a steam plant of same size. 3. Therefore smaller size of the gas turbine components enables complete work tested units to be transported to the site.Comparison of Gas Turbine Plants with Other Plants (A) Comparison with Steam Power Plants 1. The circulating water consumption is less in comparsion to that of a steam turbine plant of the same size. Foundations and buildings are less costly. 6. From there the compressed air enters H. Great care has therefore to be exercised in the design and layout of the air as well as gas circuits. This makes site selection easier. A gas turbine plant can be started quickly and has a short starting time in comparison to steam plant.P. 2. Time for installation required is less 11. compressor enters heat exchanger. From this air filter air flows to the L. As compared to diesel power plants. The operation of turbine is simpler and its capital and maintenance costs are lower than those of steam turbine plant. 14. While the adiabatic expansion of gases in the cylinder of diesel engine is incomplete. 15. (20 kgIkW). is able to operate with lower graaeR of fuel oils than is possible with diesel engines. (112 kglkW) for diesel engine. 6.p. Gas turbine being rotating machine is well balanced at all speeds. combustion and expansion occur independent units unlike diesel plant in which operations occurs in the cylinder of the engine. Specific weight of steam turbine is generally more than twice of the specific weight of gas turbine. The gas turbine is a compact powerful machine and specific weights are low. The gas turbine. Problems of coal and ash handling as encounter in case.GAS TURBINE POWER PLANTS 627 12. 5. 13. 7. 17. 4. (B) Comparison with Diesel Power Plants 1. This is not possible in case of steam power plant. There is greater flexibility in design of a gas turbine plant as the processes of compression. so less vibrations. The components and circuits of a gas turbine plant may be arranged to give the most economic results in any given situation. as compared to 85 kglh.p. of steam plants are eliminated in open cycle gas turbine plants using gas or liquid fuel. 2. Gas turbine plants have lower cost of buildings and smaller site area. . A gas turbine plant becomes more economical for operating below a given load factor as saving on the capital charges outweighs the additional cost of fuel. the gas turbine allows for a more or less complete expansion of gases which increases power output. and reduced 3. Also low grade waste gases may be utiliz~ as fuel. The heat rate of gas turbine is gerierally higher than the heat rate of steam turbine. gas turbine have higher mechanical efficiency due to fewer sliding parts in construction. Gas turbine plants have easier maintenance attendance charges. as 15 kglh. 16. base load. to improve the ov~rall plant efficiency. The oxygen content in this exhaust is around 16% compared with 21% in atmospheric air. The combination gas turbine-steam turbme cycles aims at utilizing the heat of exhaust gases from the gas turbine and thus.ese services the quick starting ability makes the gas turbine plant desirable. When this arrangement is used. 14·12. . bleeding of steam from the steam turbine (for the purpose of fed water heating) is not necessary. A simple cycle gas turbine plant wastes this energy to atmosphere. The full steam supply to the steam turbine is available for expansion and producing mechanical power. Gas turbine exhaust has a temperature of around 500°C. and 3. Fig. However. Gas turbine exhaust gases used for feed water heating. The efficiency of the combined cycle is higher than efficiency of a standard regenerative cycle gas turbine. In some of th. Heat rate of a gas turbine is generally better than the heat rate of a diesel engine. emergency standby. Employing the exhaust gases as combustion air. 14·12·1. as a great deal of its power output is used to run the compressor. The combined steam and gas turbine cycle provides the highest efficiency turbine system available at the present time. the gas turbine plant has lower thermal efficiency as compared with a diesel plant. There are three popular designs of the combination cycles: 1. Water requirements are much less in gas turbine plant in contrast to a diesel plant.628 POWER PLANT TECHNOLOGY 8. hydrostation stand-by etc. Employing the gases from a suppercharged boiler to expand in the gas turbine. while a regenerative gas turbine plant recovers much of this heat to raise overall thermal efficiency. The heat content of gas turbine exhaust is quite substantial. 2. Combination Gas Turbine Cycle Gas turbines after several advantages for Jifferent type of service peak load. in the steam boiler. But instead we can use the gas turbine exhaust as a heat source for a steam plant cycle. shows a combined cycle in which the gas turbine exhaust passes through a heat exchanger to feed water for the boiler of the system plant. 9. The diesel plant is somewhat easier to start and needs less elaborate cooling arrangements. using both exhaust gases and bled steam for the feedwater heating. Use of exhaust gases to heat feed water of steam cycle. 14·12·1. Further the gas inlet temperature to turbine can be increased and this results in an overall increase in efficiency of the plant. About 5% improvement in plant heat rate can be obtained by the use of combined cycle. Fig. rG Generator Gas turbine 12xhoust Air in Fig. and the gas-turbine exhaust. Supplementary fuel and air can be fed to the boiler. If bleeding is also used. The gas turbine exhaust has around 16% oxygen which is enough to support combustion in the boiler. The station capacity is also increased and there is only a slight increase in the cooling water arrangement.T. The heat transfer rate in the boiler are increased due to the high density of air. 14·12·2 shows a combined cycle in which the gas turbine exhaust is used as preheated air for the boiler of the steam plant. So. Fig.GAS TURBINE POWER PLANTS Fuel to combustor To 62fl stacks Feed water heater Air in Turbine exhaust Fig. Arrangement is shown in figure. 14·12·2. To stock Baiter G. . 14·12·3 shows a flow diagram for the supercharged boiler. heats the feedwater before it enters the boiler. Here the combustors of the gas turbine unit are replaced by a steam generator having a supercharged fumace. Combined gas and steam plant (Heat reccvery boiler). the boiler weight get reduced by as much as about 50%. which would be larger than the conventional boiler. the requirement of bled steam is much less than what would be required when no feedwater heating with exhaust gases is employed. Heat rate also gets improved by about 7 to 8 per cent. it is only Rs. 14·13.630 POWER PLANT TECHNOLOGY To stack Feed water Flue gases Feed water Exhaust gases Alternator Star(lng motor tU~brne ~~F=() Fig. 14·12·3. Combined cycle plant efficiency is of the order of 42-47% which are nearly 10-20% more efficient than fossil fuel conventional power plants. Flow diagram of supercharged boiler cycle. Meantime between failure CMTBF) which is mean operating duration between two forced outages. Gas turbine of 100 MW each and a steam turbine of 100 MW. (3) High Reliability/Availability. . Some combined cycle power plants achieved even 95% reliability for years long. As per the North American Electric Reliability Council (NERC) which collects and analyses data of such electricity generating plants. 6 millions in case of combined cycle power plant of capacity 300 MW which comprises of two nos. Presently. Advantages and Power plants. Gasturbine power plant installation costJMW capacity is only Rs 35 millions for a unit of size 100 MW. Disadvantages of the Gas Turbine Advantages (1) Low Installation cost. Combined cycle power plant is highly reliable to the extent of 85% to 90%. whereas. simple cycle Gas turbine power plants achieved a reliability of 95·7%. In case of simple cycle. These figures are very high when compared to reliability figures of the order of 65% generally achieved for conventional power plants. the installation cost / MW capacity for a conventional fossil fuel power plant of unit size 200/500 MW is nearly Rs 10 millions. for Gas turbine power plant is above 1000 hours whereas it is nearly 500 hrs for conventional power plants. (2) Higher Efficiency. Efficiency deterioration due to higher condenser back pressure is not as much as in conventional power plants due to only one-third (1/3) of power output contributed by steam cycle in combined cycle power plants. (7) Less Pollution Problems.2 hrs (one and half-two hrs). this water consumption may also'be eliminated by resorting to air cooling methods. This fast starting characteristics make them favourable to run as peaking power plants or two shifts in a day mode unlike in conventional thermal power plants. These durations are very much on favourable side when compared to installation time of 48 and 60 months for conventional 200 MW and 500 MW plants respectively. two-third (2/3) of full load can be realised within 20-30 minutes by operating gas turbines in simple cycle with the help of by-pass stack. Thermal pollution is also comparatively less due to higher efficiency in case of combined cycle plants. These emissions can be controlled easily to/the acceptable levels by stearil! . Simple cycle gas turbine power plants need neglibrible amount of cooling water for its auxiliaries only. In combined cycle power plants since steam cycle generates only one third (1/3) of the total power output. There is no ground water/water pollution around ash disposal area due to ash dumping in coal fired power plant. Even in combined power plants. However. Gas turbine power plants do not have dust pollution problem unlike in coal fired power plants. Entire combined cycle plant can be brought to full load within. Gas turbine power plants can achieve full load within 20-30 minutes from cold start condition. Pollution due to blow down from cooling water system is also less when compared to conventional thermal power plants. Though gas turbine power plants emit more oxides of nitrogen (Nox) when compared to conventional thermal power plants. 1/2 . air cooled type condenser can be envisaged.(:AS TURBINE POWER PLANTS 631 (4) Low Gestation Time. Water consumption is nearly 40% of the requirement of conventional thermal power plants. (6) Less Water Requirements. Some manufacturers keep gas turbine units as off-shelf items. since gas turbines are of standard equipment. faster returns on the investment and indirectly helps in improving the national economy in promoting the unrestricted growth. The installation time for a simple cycle gas turbine power plant capacity can be installed in 16-18 months and the rest of the capacity which is steam cycle plant can be added in 12-14 months more. This advantage contributes to less interest charges during construction and escalation of power plant cost due to inflation. If availability of water for condenser cooling purpose in difficult even for this quantity. (5) Fast Starting Characteristics. (9) Less man/MW Ratio. There is no requirement of land for disposing ash and storing coal. So human management problems are less comparatively. and maintain than conventional thermal power plants. One of the main factors to decide the location of a thermal power station is availability of sufficient quantity of water for its consumption in the near vicinity. Gas turbine power plants need less man power to erect. Land requirement for water storage is also less unless on perennial water supply facility is existing.. operate. There is no absolute requirement of site to be connected by rail road unlike in coal fired power plant to transport vast quantities of coal to the power plant. which are scarce in our country and their inevitable (or unavoidable) requirement in other industries such as petrochemical industry. fertilizer industry and transport industry. natura] Gasoline Liquid (NGL). naptha. Disadvantages (1) Need of Good Quality Fuels.632 POWER PLANT TECHNOLOGY water injection into gas turbine combustion conventional thermal power plants. Combined cycle power plants can be located even in deserts by envisaging air cooled condenser with marginal loss in its thermal efficiency unlike conventional thermal power plants. Less land requirement and minima] pollution problems favour these p]ants· to be located near load centres like cities. Gas turbine power plants have operating life of around 15-20 years when compared to 25 years for conventional power plants. Gas turbine power plants can be located in areas where minimal infrastructural facilities are available. With heavy residua] fuels such . The requirement ofland for combined cycle power plant is only 10 to 12% of coal fired power plant of equal capacity. heavy residual oils etc. Gas turbine power plants can be operated only with gaseous fuels mainly natura] gas or liquid fuels such as HSD. (10) Clean operating Conditions. Since combined cycle power plants need only 40% of water requirement of conventional thermal power plant. chamber unlike in (8) Flexibility in locating Power plants. Gas turbine power plants life depends mainly on the type of fuel used and the actual combustion temperatures subjected over design combustion temperature. Leser Plant Life. These power plants can be kept in absolutely dean and tidy condition unlike coal fired power plants which adds to the morale of work force. its flexibility with respect to water availability is more. Phase 1 is HBJ pipe line from Hazira (Gujarat) to Babrala (D. efficiency deterioration can be checked upto 80% ofrated load. south western regions western regions and north western region.) which will supply . pipe line networks interconnecting the production and consumption points to meet the requirements of various industries are being contemplated. Power supply situation in our country is becoming worse year by year due to under utilization of existing capacity and faster pace of demand growth. Power shortage can be mitigated on crash programmes by installing combined cycle utility power plants due to their less gestation periods and low installation cost in this age of diminishing capital availability. HSD etc. Our coals have less calorific value and high ash content which cost excessively in transporting to the power plants located in extreme south. 14·14. Though. gas turbine life deteriorates clear fuels such as natural gas.1 of this pipe network is based on the proven reserves while phase II & III are base on the additional proven reserves through conversion of prognosticated resources. At present 40% of the rail freight is mainly due to coal transportation in India. Present gas reserves is of the order of 906·31 billion cubic metres by the end of the seventh plan.. Prospects of Gas Turbine Power Plants in India The application of gas turbine power plants can be foreseen in the following fields in India: (a) (b) (c) (d) (e) base load gas turbine power plants peak load gas turbine power plants Captive power combined cycle plants Retrofitting of old combined cycle and uneconomical power plants. Uneconomical Partial Load Operation. HPS etc. India's gas reserves have increased five folds during the last ten years and these are expected to increase further. Fortunately some of the above regions have fairly good other hydro-carbon deposits such as oil and natural gas. To exploit the natural gas reserves. Base Load Gas Turbine Power Plants. with inlet guide vane system.GAS TURBINE POWER PLANTS 633 faster than with as LSHS. Phase .P. However. Gas turbine power plants efficiency is considerably low when they are operated at partial loads. India has vast coal deposits but these deposits are mainly concentrated in central and eastern parts. Co-generation gas turbine plants. There is further scope of two more combined cycle power plants of total capacity nearly 1000 MW. creating scope for some more combined cycle power plants. Phase 1A is an extension of HBJ pipe line from Auraiya to Kapurthala. can be exploited economically only by underground coal gasification technology.000 MW capacity gas based combined cycle power plants in India by 2000 AD. In phase III. (Natural Gas) to three combined cycle power plants of total capacity 1600 MW being under execution. So. while phase 1 B is a pipe line connecting Bombay south terminal to Banglore which will cater natural gas to combined cycle power plants of capacity 2875 MW. the North East and Eastern part of the gas fields are proposed to be connected to the gas grid. low cost of installation and fast starting characteristics though their thermal efficiency is relatively unfavourable. In 1986-87 nearly 2718 million cubic metres of Natural gas was flared which is colossal wastage of natural resource. All large load centres in India. need this type of power plants to stabilize the grid when frequency is falling either due to overdrawing of power or less feeding to grid due to failure of few operating power plants. G. which will enhance scope for pit head combined cycle power plants further. mainly due to their low gestation periods and least installation cost. since plants to install some gas based fertiliser plants along this pipe line are not materialising. Coal can be gasified to produce lower caloric value gas which can be utilised in pit head combined cycle power plants most economically for power generation. Combined cycle power plants need not depend on the availability of liquid or gaseous fuels in entire future. Uneconomical coal deposits by present technology which are deep in earth and of less seam thickness. it can be visualised that there is a scope of atleast 10. Phase II envisages a pipe line network extension in southern sector upto 'T'rivandrum and its connection to Northern pipe line network. Peak Load Gas Turbine Power Plants. with installed capacity of 672 MW. which will cater to 13 combined cycle power plants of total capacity 3875 MW. These power plants can also rectify the .634 POWER PLANT TECHNOLOGY N. One example of this type of power plant is Uran Gas Turbine Power Station (MSEB). There is scope for simple cycle gas turbine base load power plants to utilize the associated gas from crude oil wells which will be flared otherwise in Bombay High region and north eastern region. Coal gasification technology is on the threshold of commercial utilization. These power plants are mainly simple cycle gas turbine power plants because of their shorter gestation period. fertilizers. such that existing steam cycle facilities can be utilised as bottoming cycle to the gas turbine. This modification can be done by replacing the existing steam generation by HRSG (Heat recovery steam generator) and Gas Turbine. The generator of these gas turbine power plants can also be utili sed as synchrounous condenser to improve power factor of the grid. it is also possible to use Gas turbine exhaust gases as a practical source of heat energy in already existing coal fired steam generator by doing moderate alternations in the steam generator. combined cycle power plants are best suitable to generate electricity at lesser cost and than coal fired conventional thermal power plants even at existing liquid/gaseous fuel prices in India due to their higher efficiency of the order of 4045%. Co-Generation Gas Turbine Power Plants. refl. when gas turbine power plants are not generating power. The primary fuel for these plants will be gas or liquid fuel. Gas turbine units operating on simple cycle will be the ideal solution to act as spinning reserve to cater to peak demand and demand-fluctuations of the grid. Co-generation systems are not only decentralised but also integrated systems of energy based on the total energy concept. food ! processing. Supply to the grid catering to essential load requirements and to restart the tripped power plants. plastic industries etc. The cost of power generated by these plants is less than the larger size utility coal fired power projects. At present. Sometimes. Captive Power Combined Cycle Power Plants. low cost of installationlkW and high reliability. Retrofitting of old and uneconomical power plants. These power plants find its application in process industries like petrochemical. . where large quantities of steam and auxiliary power are required. When unit capacities are below 100 MW. paper industries etc.geration/air-~onditioning plants. In some applications instead of producing steam by the gas turbine exhaust gases. Some of the power plants which can not generate electricity at economical cost due to their less design thermal efficiencycan be converted into combined cycle power plants.GAS TURBINE POWER PLANTS 635 complete grid failures very quickly since they can achieve their full load within 20 minutes restoring partial. unit capacities of the order of 100 MW and below are being mainly installed as captive power plants since most of the regional grids in India can accommodate larger size single unit of 200 MW and more. exhaust gases can be used directly for heating\ requirement such as in centralised adsorption. Efficiencies of the order of 80-85% can be achieved in these power plants. cm. Questions 14·1. From the compressor. (a) (b) 14·3. the pressure ratio is 6. If the turbine and compressor have each an efficiency of 80%. the net power output of the turbine plant if the turbine is coupled to the compressor. and that of turbine is 39·23 N/sq. 688 kW. duel fuel systems etc. back pressure steam turbine in case of the combined cycle. constant pressure gas turbine. 81%. gas turbine. Describe with the help of a suitable sketch. (4 kg/sq. 25·7%) 14·4 In a gas turbine plant. 42·56%) In a gas turbine plant working on the Brayton cycle the air at the inlet is at 27°C. It is compressed through a pressure ratio of 5 : 1 with an isentropic efficiency of 85%. 9·57 kg/sec. and the maximum cycle temperature is 900°C. 14·2. (USA) and National Productivity council for the Department of Non-conventional Energy sources. it has been estimated that the two states of Gujarat and Maharashtra alone have a co-generation potential of more than 2000 MW. . cm) and 700°C. 0·1 Pa. From the combustion chamber. find the percentage incrpase in the cycle efficiency due to regeneration. The above study has further established that Gas Turbines with Co-generation are generally more economical than the other co--generation methods adopted by many of our industries. petrochemicals. paper. where its temperature is raised to 810°C. The pressure ratio is 6·25 and the maximum 14·5.) In a continuous combustion constant pressure. air is take into a rotary compressor at a pressure of 100 kN/m2 and temperature 18°C. (Ans. the compressed air is passed to a combustion chamber. (Ans. working on the Brayton cycle with a regenerator of 75% effectiveness. If the air used is 4·5 kg/s and neglecting the mass of fuel as small. are possible making their application broad based. fertilizers and several other industries which require both process heat and electricity. the thermal efficiency of the plant. determine. the air is passed to exhaust. the air at the inlet to the compressor is at 0·1 MPa.636 POWER PLANT TECHNOLOGY These systems are ideally suited for process industries such as sugar. Calculate the isentropic efficiency of the turbine and the requisite mass flow of air in kg/sec if the compressor efficiency is 85% and overall thermal efficiency is 21%. (Ans. Several variations in gas turbine co-generation systems such as supplementary firing and the waste heat recovery boilers to satisfy the varying or peak energy (either heat or electricity) loads. From the turbine. A gas turbine plant delivers 1712 kW (200 hop) and operates such that inlet pressure and temperature at the compressor is 9·807 N/sq em (1 kg/sq cm) and 15°C. the operation of a continuous combustion. the high temperature air is passed to a gas turbine in which it is expanded down to 100 kN/m' with an isentropic efficiency of 88%. 30°C. In a recent study conducted by Haigler Baily & Co. efficiency of drive to Compressor 75% of the available heat is transferred HP turbine. and then into the high pressure combustion chamber.GAS TURBINE POWER PLANTS temperature is 800°C. are 15°C and 1·03 kg flcm' Atmospheric temperature (0·1 MPa) respectively. What are the main fuels which arc used for gas turbine'~lant? Describe the recent developments introduced in the sim9. the overall efficiency. (Ana.Jlkg (ii) 351·68 kJlkg (iii) 569·43 kJlkg (iv) 16·2% (v) 723 K) 14·6. 14·10. The following data refer to the plant. heat supplied per kg of air cycle efficiency. Describe the controls and auxiliaries necessary in a gas turbine plant. the air from the compressor passes through a heat exchanger heated by the exhaust gases from the low pressure turbine. (Ans. Pressure Isentropic Isentropic Isentropic Mechanical compression ratio in the compressor. and pressure Assuming that the specific heat of air and gas is 0·24(1·03 kJlkg K). The exhaust gases from the high pressure turbine pass through the lowpressure combustion chamber to the low pressure turbine which is coupled to an external load or generator.P. / What are the important considerations to be taken account while deciding about layout of a gas turbine power plant. 14·9. turbine.e gas turbine cyclE)and the result of each on plant heat rate. What are the advantages of closed cycle? . 14·8. The turbine 80%. (i) 259·4 k. efficiency of LP turbine. 4 : 1 0·86 0·84 0·80 0·92 to the efficiency of compressor. Temperature Temperature ofthe gases entering of gases entering LP turbine. The high pressure turbine drives the compressor only. Calculate: (i) 637 and compressor efficiencies are each (ii) (iii) (iv) (v) the the the the the compressor work per kg of air. In the heat exchanger air. How is the plant started and what are the safety devices employed? . and turbine exhaust temperature. turbine work per kg of air. Determine (i) (ii) the pressure of the gases entering the low pressure turbine. efficiency of H. 14·11. Describe briefly a closed cycle gas turbine plant. In a Brayton cycle gas turbine plant. 1·6555 kgllcm' 25·3%) 14·7. Open cycle gas turbine works on (a) (c) Brayton or Atkinson Joule cycle (b) (d) Rankine cycle Erricson cycle. Thermodynamic cycle on which a gas turbine works: (a) (b) (d) Brayton or Atkinson cycle Rankine cycle Erricson cycle. POWER PLANT TECHNOLOGY What are the fuels used in gas turbine plants and what fuel characteristics suit such plants best? Discuss the recent trends to use solid fuels in such plants. 14·14. For starting gas turbine. t . The air fuel ratio in a gas turbine is of the. 14·5. (a) (c) higher same. 14·13. What are combination cycles and why have these been developed? Describe the principal combination cycles using gas turbine-cum-steam plants for power production with advantages of each. The pressure ratio in open cycle gas turbine is of the order of (a) (c) (b) (d) 14·7. Objectives Type Questions 14·1. Gas turbines for power generation are normally used (a) (b) (c) to supply peak load requirements to supply base load requirements both a and b. 14·2.638 14·12. as compared 14·3. 14·4. Compare the gas turbine plants with steam turbine power plants and diesel power plants. (b) lower . order of (a) (c) 7: 1 50: 1 12: 1 18: 1 (b) (d) 15: 1 120: 1 9: 1 6: 1 14·6. the turbine rotor is usually motored upto 'coming in' speed which is equal to (a) (b) (c) rated speed of the gas turbine of the rated speed of the gas turbine no relation with speed of the turbine. The thermal efficiency of gas turbine plants is to condensing steam plants. (c) Joule cycle cycle. (b) (d) blast furnace gas pulverized coal . The thermal efficiency of a simple gas turbine for a given turbine inlet temperature with increase in pressure ratio: (a) (c) increases remains same.. The fuel for gas turbine can be (a) (c) (e) coal gas producer gas anyone of the above. (b) high 14·9.GAS TURBINE POWER PLANTS 639 14·8. (a) (c) open cycle in both the cycles... 14·11. (b) decreases 14·10. Maximum temperature of (a) (c) in a gas turbi~ (b) (d) 700°C 1500°C 1000"C 2000°C. (b) thermal efficiency cycle is increased reheating both a and b is of the order ' 14·14.. a regenerator increases (a) (c) . work output pressure ratio. In a gas turbine plant.. high alloy steel 14·13. (b) closed cycle . Work output employing . (a) (c) low same. (d) 14·15. In gas turbines. high thermal efficiency is obtained in . . (a) of the gas turbine (b) by (c) (e) inter cooling regeneration a.. Efficiency of the gas turbine cycles increased by (a) (c) regeneration reheating (b) (d) intercooling all of the above. The pressure ratio for an closed cycle gas turbine compared to open cycle gas turbine of some power is . 14·16. The blades of the gas turbine rotor are made of (a) (c) (d) carbon steel (b) stainless steel high nickel alloy (Nimic 80).. 14·12. band c. . steam power plant of same capacity (a) .. (a) 80% (b) 90% (c) 14·19. 640 POWER PLANT TECHNOLOGY 14·17. (c) (d) same lower same.. lobe centrifugal using a good 98%.. (b) more (a) (a)5.'.. cost of a gas turbine plant is . (b) (b). .. (d) 15.8. 13.. The combustion efficiency of a gas turbine combustor is of the order of .. 18.. 12. 4. than that of a (d) (a) 2. 14·18. 14. Maximum combuxstion pressure in a gas turbine is compared to diesel engine. 7. (b) (d) reciprocating axial flow cor d any. In a gas turbine the (a) (c) (e) type air compressor can be employed. 19. (c) (c) higher3. as less 20.~. (b) Capital17. 9..(b) (e) (c) (b) 10...