ME-403-F REFRIGERATION & AIR CONDITIONINGBy: DESH DEEPAK Asst. Prof. Dept. of Mechanical Engg. WCTM Gurgaon Note: Examiner will set 9 questions in total, two questions from each section and one question covering all sections which will be Q.1. This Q.1 is compulsory and of short answers type. Each question carries equal marks (20 marks). Students have to attempt 5 questions in total at least one question from each section. SECTION A Introduction: Definition of refrigeration & air conditioning; Necessity; Methods of refrigeration; Unit of refrigeration; Coefficient of performance (COP), Fundamentals of air-conditioning system; Refrigerants- Definition, Classification, Nomenclature, Desirable properties, Comparative study, secondary refrigerants, Introduction to eco-friendly Refrigerants; Introduction to Cryogenics. Air Refrigeration System: Carnot refrigeration cycle. Temperature. Limitations; Brayton refrigeration or the Bell Coleman air refrigeration cycle; Necessity of cooling the aero plane; Air craft refrigeration systems, Simple cooling and Simple evaporative types, Boot strap and Boot strap evaporative types, Regenerative type and Reduced Ambient type system, Comparison of different systems, problems. Introduction to Refrigeration and Air Conditioning: Refrigeration is defined as “the process of cooling of bodies or fluids to temperatures lower than those available in the surroundings at a particular time and place”. It should be kept in mind that refrigeration is not same as “cooling”, even though both the terms imply a decrease in temperature. In general, cooling is a heat transfer process down a temperature gradient, it can be a natural, spontaneous process or an artificial process. However, refrigeration is not a spontaneous process, as it requires expenditure of exergy (or availability). Thus cooling of a hot cup of coffee is a spontaneous cooling process (not a refrigeration process), while converting a glass of water from room temperature to say, a block of ice, is a refrigeration process (nonspontaneous). “All refrigeration processes involve cooling, but all cooling processes need not involve refrigeration”. Refrigeration systems are also used extensively for providing thermal comfort to human beings by means of air conditioning. Air Conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odour and circulation, as required by occupants, a process, or products in the space. The subject of refrigeration and air conditioning has evolved out of human need for food and comfort, and its history dates back to centuries. Applications of refrigeration in: a) Food processing, preservation and distribution b) Chemical and process industries c) Special Applications such as cold treatment of metals, medical, construction, ice skating etc. d) Comfort air-conditioning Applications of air conditioning : a) Industrial, such as in textiles, printing, manufacturing, photographic, computer rooms, power plants, vehicular etc. b) Comfort – commercial, residential etc. Methods of Refrigeration: a) Natural Method: The natural method includes the utilization of ice or snow obtained naturally in cold climate. Ice melts at 00C. So when it is placed in space or system warmer than 00C, heat is absorbed by the ice and the space is cooled. The ice then melts into water by absorbing its latent heat at the rate of 324 kJ/kg. But, now-a-days, refrigeration requirements have become so high that the natural methods are inadequate and therefore obsoleting. b) Mechanical or Artificial Refrigeration: A mechanical refrigeration system works on the principle of reversed Carnot cycle as shown in Fig.6.2. Work δw is delivered to the refrigerating system, causing it to remove heat δQ2 from the body or system (at lower temperature T 3) and to deliver it along with work, δw, to another body at higher temperature, T 1, so that, δQ1 = δw + δQ2 There can be two methods by which the temperature T 2 < T3 may be attained within the refrigerating system. i) By lowering the temperature of the working substance in the refrigerating system to the level of T2. In this case, the heat will be absorbed due to temperature difference and T 3 will decrease as heat δQ2 flows out. ii) By evaporating some fluid at an appropriate pressure. In this case, a constant temperature T2 will be maintained and latent heat of fluid will be absorbed as δQ2. Depending upon the above method used, there are two types of mechanical refrigerating systems: i) Air systems: Uses air as a working fluid. Air does not undergo any change of phase, but absorbs heat due to temperature difference. ii) Chemical Agent Systems: The working fluid changing its phase while boiling from liquid to vapor state, thereby it absorbs the latent heat. W is the work consumed by the heat pump.5 kW) or 210 kJ/min. it is customary to define a term called coefficient of performance (COP) as the ratio of the refrigeration effect to energy input. The COP for heating and cooling are thus different.5 kJ/s (3. Q is the heat supplied to or removed from the reservoir. QC is the heat removed from the cold reservoir. Coefficient of performance: Refrigeration effect is an important term in refrigeration that defines the amount of cooling produced by a system. from liquid water at 00C. the COP is the ratio of the heat removed from the cold reservoir to input work. Therefore. A ton of refrigeration is defined as the quantity of heat to be removed in order to form one ton (1000 kg) of ice at 00C in 24 hrs. The COP provides a measure of performance for heat pumps that is analogous to thermal efficiency for power cycles. This cooling is obtained at the expense of some form of energy. because the heat reservoir of interest is different. The coefficient of performance or COP (sometimes CP) of a heat pump is the ratio of the heating or cooling provided over the electrical energy consumed. the COP is the ratio to input work of the heat removed from the cold reservoir plus the heat added to the hot reservoir by the input work: COPheat pump = QH W COPrefrigerator = QC W Where. COP = Q W Where. This is equivalent to 3. for heating. . When one is interested in how well a machine cools.Unit of Refrigeration: Capacity of refrigeration unit is generally defined in ton of refrigeration. QH is the heat supplied to the hot reservoir. However. . Replacement of an existing refrigerant by a completely new refrigerant. for example in large air conditioning systems. the selection of suitable refrigerant has become one of the most important issues in recent times. However. R-134 etc. Unlike primary refrigerants. R-11. the secondary refrigerants do not undergo phase change as they transport energy from one location to other. and serviceability etc. When used in compression or absorption systems. Refrigerant selection criteria: Selection of refrigerant for a particular application is based on the following requirements: i. is an expensive proposition as it may call for several changes in the design and manufacturing of refrigeration systems. The thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures. then pure water can also be used as secondary refrigerant. The temperature at which freezing of a brine takes place its depends on its concentration. As the name implies. in this lecture the attention is mainly focused on those fluids that can be used as refrigerants in vapour compression refrigeration systems only. Air used in an air cycle refrigeration system can also be considered as a refrigerant. An important property of a secondary refrigerant is its freezing point. size. which are used for transporting thermal energy from one location to other. Primary and secondary refrigerants: Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants. propylene glycol or calcium chloride. for enhanced efficiency. Due to several environmental issues such as ozone layer depletion and global warming and their relation to the various refrigerants used. There is Sensible Heat change by the secondary refrigerant. R-22. However. Of course. Ex: NH3. these fluids provide refrigeration by undergoing a phase change process in the evaporator So there is Latent heat absorption by the refrigerant. The concentration at which a lowest temperature can be reached without solidification is called as eutectic point. Primary refrigerants are those fluids. Hence it is very important to understand the issues related to the selection and use of refrigerants. These solutions are known under the general name of brines. Generally. which are used directly as working fluids. In this lecture attention is focused on primary refrigerants used mainly in vapour compression refrigeration systems. for whatever reason. the freezing point of a brine will be lower than the freezing point of its constituents. The commonly used secondary refrigerants are the solutions of water and ethylene glycol. Thermodynamic and thermo-physical properties . important practical issues such as the system design. Secondary refrigerants are also known under the name brines or antifreezes.REFRIGERANT: A refrigerant is a substance used in a heat cycle usually including. In principle. any fluid can be used as a refrigerant. initial and operating costs. for example in vapour compression and vapour absorption refrigeration systems. Antifreezes or brines are used when refrigeration is required at sub-zero temperatures. reliability. secondary refrigerants are those liquids. a reversible phase transition from a liquid to a gas. depend very much on the type of refrigerant selected for a given application. safety. if the operating temperatures are o above 0 C. the saturation pressure should be above atmospheric for prevention of air or moisture ingress into the system and ease of leak detection.7. critical temperature. Thus the normal boiling point is a good indicator of the critical temperature of the refrigerant. Trouton’s rule shows that the latent heat of vaporization will be high for refrigerants having lower molecular weight. This once again shows a need for trade-off between high COP and high volumetric capacity. The specific heat of refrigerant is related to the structure of the molecule. Thermodynamic and thermo-physical properties: The requirements are: a) Evaporator (Suction) pressure: At a given evaporator temperature. and iii. molecular weight and structure.C (P2/P1)^(1/n) d) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small. Cp = T (ds/dt)p h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops The thermodynamic properties are interrelated and mainly depend on normal boiling point. the volumetric capacity will be lower for refrigerants with high critical temperatures. On the other hand since the vapour pressure will be low when critical temperature is high. Environmental and safety properties. c) Compression ratio: Should be as small as possible for high volumetric efficiency and low power consumption.6 to 0. the discharge pressure should be as small as possible to allow light-weight construction of compressor. The important properties such as latent heat of vaporization and specific heat depend on the molecular weight and structure of the molecule. *NH3 has largest latent heat of vaporization( 1369KJ/kg K) amongst common refrigerants. Economics i. If specific heat of refrigerant vapour is low . It is observed that for most of the refrigerants the ratio of normal boiling point to critical temperature is in the range of 0. ηvolumetric = 1+ C.ii. Higher suction pressure is better as it leads to smaller compressor displacement b) Condenser (Discharge) pressure: At a given condenser temperature. A high critical temperature yields higher COP due to smaller compressor superheat and smaller flash gas losses. the following properties are also important: e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small f) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet g) Vapour specific heat: Should be large so that the degree of superheating will be small. The normal boiling point indicates the useful temperature levels as it is directly related to the operating pressures. In addition to the above properties. condenser etc. . R 11. However. phosgene-COCl ).e. R134a) are likely to be regulated in future.K. R22). Some fluids are toxic even in small concentrations. Since vapour and liquid specific heats are also related.g. d) Toxicity: Ideally. However. the ODP of refrigerants should be zero. all fluids other than air can be called as toxic as they will cause suffocation when their concentration is large enough.Maintenance condition . they are dangerous only when the concentration is large and duration of exposure is long. at present the environment friendliness of the refrigerant is a major factor in deciding the usefulness of a particular refrigerant.Presence of open flames . i. In fact. Since ODP depends mainly on the presence of chlorine or bromine in the molecules. The freezing point of the refrigerant should be lower than the lowest operating temperature of the cycle to prevent blockage of refrigerant pipelines.. leading to higher flash gas losses. Thus toxicity is a relative term. R 12) or will be phasedout in near-future(e. refrigerants with as a low a value of TEWI are preferable from global warming point of view.Type of occupancy . a small value of vapour specific heat indicates higher degree of superheat. Refrigerants with zero ODP but a high value of GWP (e. ii. Some refrigerants such as CFCs and HCFCs are non-toxic when mixed with air in normal condition.. CFCs and HCFCs) or bromine cannot be used under the new regulations b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming. which becomes meaningful only when the degree of concentration and time of exposure required to produce harmful effects are specified.g.e.Odor of refrigerant. Environmental and safety properties: Next to thermodynamic and thermophysical properties. the environmental and safety properties are very important. refrigerants having either chlorine (i. when they come in contact with an open flame or an electrical heating element.e.Amount of refrigerant used vs total space . c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming. Some fluids are mildly toxic. a large value of vapour specific heat results in a higher value of liquid specific heat. they should be non-ozone depleting substances. However. they decompose forming highly toxic elements (e. Naturally. compression process will be dry).e. Refrigerants having non-zero ODP have either already been phased-out (e. In general the degree of 2 hazard depends on: . and . Studies show that in general the optimum value of molar vapour specific heat lies in the range of 40 to 100 kJ/kmol.g. i.then the shape of the vapour dome will be such that the compression process starting with a saturated point terminates in the superheated zone (i. The important environmental and safety properties are: a) Ozone Depletion Potential (ODP): According to the Montreal protocol.g. refrigerants used in a refrigeration system should be non-toxic. Other important properties are: f) Chemical stability: The refrigerants should be chemically stable as long as they are inside the refrigeration system. For these systems the refrigerants should have as high a dielectric strength as possible j) Ease of leak detection: In the event of leakage of refrigerant from the system.g. R 22) special precautions should be taken while designing the system to ensure oil return to the compressor i) Dilelectric strength: This is an important property for systems using hermetic compressors.Thus from toxicity point-of-view. R1140) are most hazardous. g) Compatibility with common materials of construction (both metals and non-metals) h) Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. These refrigerants are designated by R XYZ. it should be easy to detect the leaks. i) Fully saturated Hydro-carbon. its chemical composition. for refrigerants with limited solubility (e. R744. Refrigerants belonging to Group A1 (e. R12. of Fluorine atoms = 2 4 4 . R22. ASHRAE has divided refrigerants into six safety groups (A1 to A3 and B1 to B3). molecular weight etc. From the number one can get some useful information about the type of refrigerant. R11. iii.g. R718) are least hazardous. halogenated compounds: These refrigerants are derivatives of alkanes (C H ) such as methane (CH ). ammonia). ethane n 2n+2 (C H ). However.g. R134a. Based on the above criteria. Refrigerants that are completely miscible with oils are easier to handle (e. Ex: R 22 X = 0 ⇒ No. where: 2 6 X+1 indicates the number of Carbon (C) atoms Y-1 indicates number of Hydrogen (H) atoms. All the refrigerants are designated by R followed by a unique number. Designation of refrigerants: Since a large number of refrigerants have been developed over the years for a wide variety of applications. while refrigerants belonging to Group B3 (e. Economic properties: The refrigerant used should preferably be inexpensive and easily available. For flammable refrigerants special precautions should be taken to avoid accidents. a numbering system has been adopted to designate various refrigerants.g. R12). e) Flammability: The refrigerants should preferably be non-flammable and non-explosive. Only 2 digits indicates that the value of X is zero. and Z indicates number of Fluorine (F) atoms The balance (2xX+5-Y-Z) indicates the number of Chlorine atoms. of Hydrogen atoms = 2-1 = 1 Z = 2 ⇒ No. of Carbon atoms = 0+1 = 1 ⇒ derivative of methane (CH ) Y = 2 ⇒ No. the usefulness of a particular refrigerant depends on the specific application. which somewhat restricts its application. where: X+1 indicates the number of Carbon (C) atoms Y-1 indicates number of Hydrogen (H) atoms. ∴ the designation is R 717 Carbon dioxide: Molecular weight is 44.The balance = (2x0+5-2-2)= 1 ⇒ No. these refrigerants have to be phased out due to their Ozone Depletion Potential (ODP). the synthetic refrigerants face an uncertain future. R 12. e. while the other refrigerants were used in large systems such as large air conditioning plants or cold storages. of Chlorine atoms = 1 ∴The chemical formula of R 22 = CHClF Similarly it can be shown that the chemical formula of: R12 = CCl F 2 2 2 R134a = C H F (derivative of ethane) 2 2 4 (letter a stands for isomer. The synthetic replacements for the older refrigerants are: R-134a (HFC-134a) and blends of HFCs. However. and Z indicates number of Fluorine (F) atoms The balance (2xX+3-Y-Z) indicates the number of Chlorine atoms. R 12 was used primarily in small capacity refrigeration and cold storage applications. Ex. they are being studied widely and are likely to play a prominent role in future. R 22. Generally. Prior to the environmental issues of ozone layer depletion and global warming. synthetic refrigerants are non-toxic and nonflammable. R134 and R134a) ii) Un. ∴ the designation is R 744 Water: Molecular weight is 18. As a result. the most widely used refrigerants were: R 11. R 502 (CFC 12+HCFC 22) etc. Among the refrigerants used.g. Other natural refrigerants that are being suggested are hydrocarbons (HCs) and carbon di-oxide (R-744). R 11 was primarily used with centrifugal compressors in air conditioning applications. R 22 (HCFC 22). e. it is toxic and is not compatible with some of the common materials of construction such as copper. molecules having same chemical composition but different atomic arrangement. all the other refrigerants are synthetic refrigerants . R 12 (CFC 12). compared to the natural refrigerants the synthetic refrigerants offer lower performance and they also have higher Global Warming Potential (GWP). Ammonia has good thermodynamic. R 502 and ammonia. e. The most commonly used natural refrigerant is ammonia. except ammonia. thermophysical and environmental properties. ∴ the designation is R 718 Comparison between different refrigerants: Synthetic refrigerants that were commonly used for refrigeration.g.g. R1150 = C2H4 iii) Inorganic compoundrefrigerants: These are designated by number 7 followed by the molecular weight of the refrigerant (rounded-off).: Ammonia: Molecular weight is 17. Though these refrigerants have some specific problems owing to their eco-friendliness. This is also one of the oldest known refrigerants. These refrigerants are n 2n 2 4 designated by R 1XYZ.saturated Hydro-carbon: These refrigerants are represented as (C H ) such as C H ). However. Of these. However. cold storage and air conditioning applications are: R 11 (CFC 11). most of these blends are non-azeotropic in nature. . At the same time. it has been agreed by the global community to phase out the ozone depleting substances (ODS). it has been very widely used due to its excellent thermodynamic and thermophysical properties. The depletion of stratospheric ozone layer was attributed to chlorine and bromine containing chemicals such as Halons. It is also possible to use blends of various HFCs to obtain new refrigerant mixtures with required properties to suit specific applications. CFCs. Though ammonia is toxic. unlike the HFC based refrigerants. In view of the environmental problems caused by the synthetic refrigerants. as this will change the composition of the mixture. HCFCs etc. The scenario changed completely after the discovery of ozone layer depletion in 1974. it was also observed that in addition to ozone layer depletion. Since the natural refrigerants are essentially making a comeback. opinions differed on replacements for conventional refrigerants. hydrocarbons is not a new phenomena. as a result there could be significant temperature glides during evaporation and condensation. Since ozone layer depletion could lead to catastrophe on a global level. which do have considerable global warming potential.and are non-toxic and non-flammable. The alternate synthetic refrigerants are normally non-toxic and non-flammable. hydrocarbons and their blends It should be noted that the use of natural refrigerants such as carbon dioxide. The alternate refrigerants can be classified into two broad groups: i) Non-ODS. carbon dioxide. synthetic refrigerants based on Hydro-Fluoro-Carbons (HFCs) and their blends ii) Natural refrigerants including ammonia. all the other refrigerants used in cold storages had to be phased-out and a search for suitable replacements began in earnest. but is a revival of the once-used-and-discarded technologies in a much better form. one advantage of using them is that they are familiar in terms of their strengths and weaknesses. However. As a result except ammonia. Another important advantage is that they are completely environment friendly. most of the conventional synthetic refrigerants also cause significant global warming. and it is also important take precautions to prevent leakage. expanding against a piston.e. the temperature of the gas also drops during the process. If the expansion is reversible and adiabatic. by using the ideal gas equation Pv = RT and the equation for isentropic process P1v1γ = P2v2γ the final temperature (T2 ) is related to the initial temperature (T ) and initial and final pressures (P and P ) by the equation: 1 1 2 where γ is the coefficient of isentropic expansion given by: Isentropic expansion of the gas can also be carried out in a steady flow in a turbine which gives a net work output. As shown. isothermal heat rejection in a compressor . i. When the gas does adiabatic work in a closed system by say.. consequently. its internal energy drops. In the present chapter gas cycle refrigeration systems based on air are discussed. Gas cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases. where m is the mass of the gas. The temperature of an ideal gas can be reduced either by making the gas to do work in an isentropic process or by sensible heat exchange with a cooler environment. the cycle consists of the following four processes: Process 1-2: Reversible. Neglecting potential and kinetic energy changes. u and u are the initial and final internal energies of the 1 2 gas. in which a gas is used as the working fluid. Since the internal energy of the ideal gas depends only on its temperature. all the internal heat transfer processes are sensible heat transfer processes. Figure 9. The gas does not undergo any phase change during the cycle.1(a) shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-v coordinates.AIR REFRIGERATION SYSTEM Introduction: Air refrigeration systems belong to the general class of gas cycle refrigeration systems. T and T are the initial and final temperatures and c is the specific heat at constant 1 2 v volume. the work output of the turbine is given by: Reversed Carnot cycle (Carnot Refrigeration Cycle): Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks. adiabatic compression in a compressor Process 2-3: Reversible. This is difficult to achieve in practice. w 3-4 exactly matches the work of isentropic compression w . For a gas to have heat transfer isothermally. which gives rise to large frictional effects. l h Limitations of Carnot cycle: Carnot cycle is an idealization and it suffers from several practical limitations. adiabatic expansion in a turbine Process 4-1: Reversible. 1-2 the COP of the Carnot system is given by: Thus the COP of the Carnot system depends only on the refrigeration (T ) and heat rejection (T ) temperatures only. . the work of isentropic expansion.Process 3-4: Reversible. All actual processes are irreversible. it is essential to carry out work transfer from or to the system when heat is transferred to the system (process 4-1) or from the system (process 2-3). One of the main difficulties with Carnot cycle employing a gas is the difficulty of achieving isothermal heat transfer during processes 2-3 and 4-1. hence completely reversible cycles are idealizations only. the volumetric refrigeration capacity of the Carnot system is very small leading to large compressor displacement. In addition. isothermal heat absorption in a turbine The heat transferred during isothermal processes 2-3 and 4-1 are given by: Applying first law of thermodynamics to the closed cycle. isobaric heat absorption in a heat exchanger Process 1-2: Gas at low pressure is compressed isentropically from state 1 to state 2. isobaric heat rejection in a heat exchanger Process 3-4: Reversible. we can write: where r = (P /P ) = pressure ratio p 2 1 Process 2-3: Hot and high pressure gas flows through a heat exchanger and rejects heat sensibly and isobarically to a heat sink. This may be thought of as a modification of reversed Carnot cycle. reverse Brayton cycle and also the cycle on T-s diagram. This cycle is also called as Joule or BellColeman cycle. Applying steady flow energy equation and neglecting changes in kinetic and potential energy. no work transfer takes place and the entropy of the gas decreases. adiabatic expansion in a turbine Process 4-1: Reversible. The enthalpy and temperature of the gas drop during the process due to heat exchange. the ideal cycle consists of the following four processes: Process 1-2: Reversible. As shown in the figure. Again applying steady flow energy equation and second T ds equation: . adiabatic compression in a compressor Process 2-3: Reversible. as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes. Figure shows the schematic of a closed.Ideal reverse Brayton cycle: This is an important cycle frequently employed in gas cycle refrigeration systems. The enthalpy and temperature of the gas rise during the process due to heat exchange. Again applying steady flow energy equation and second T ds equation: From the above equations. that is 1 . From steady flow energy equation: 4 where r = (P /P ) = pressure ratio p 3 4 Process 4-1: Cold and low pressure gas from turbine flows through the low temperature heat exchanger and extracts heat sensibly and isobarically from a heat source. no work transfer takes place and the entropy of the gas increases. the following observations can be made: a) For fixed heat rejection temperature (T ) and fixed refrigeration temperature (T ).Process 3-4: High pressure gas from the heat exchanger flows through a turbine. providing a useful refrigeration effect. the COP of 3 reverse Brayton cycle is always lower than the COP of reverse Carnot cycle . undergoes isentropic expansion and delivers net work output. The temperature of the gas drops during the process from T 3 to T . the COP can also be written as: From the above expression for COP. it can be easily shown that: st Applying 1 law of thermodynamics to the entire cycle: The COP of the reverse Brayton cycle is given by: using the relation between temperatures and pressures. the specific refrigeration effect [c (T -T )] also reduces simultaneously.b) COP of Brayton cycle approaches COP of Carnot cycle as T approaches T (thin cycle). p 1 4 c) COP of reverse Brayton cycle decreases as the pressure ratio r increases p Actual reverse Brayton cycle: The actual reverse Brayton cycle differs from the ideal cycle due to: i. Non-isentropic compression and expansion processes ii. Pressure drops in cold and hot heat exchangers . 1 4 however. At high altitudes. a cooling system is a must. Even though the COP of air cycle refrigeration is very low compared to vapour compression refrigeration systems. safe. Typically. the outside pressure will be sub-atmospheric. for high speed aircraft flying at high altitudes. Design of the complete system is much simpler due to low pressures. its temperature increases to about 353 K. cooling systems are required to keep the cabin temperatures at a comfortable level. For example. The aircraft engine already consists of a high speed turbo-compressor. ii. non-toxic and non-flammable. Even though the outside temperatures are very low at high altitudes. the temperature increases significantly.8 bar. Air is cheap. it is still found to be most suitable for aircraft refrigeration systems as: i. cooling system may not be required. Cold air can directly be used for cooling thus eliminating the low temperature heat exchanger (open systems) leading to lower weight iii. the temperature will be about 332 K. Large internal heat generation due to occupants. still cooling of cabin is required due to: i. This effect is called as ram effect. iv. less than 50% of an equivalent vapour compression system iv. when outside air at a pressure of 0. Maintenance required is also less. equipment etc. hence separate compressor for cooling system is not required. When air at this low pressure is compressed and supplied to the cabin at pressures close to atmospheric. Heat generation due to skin friction caused by the fast moving aircraft iii. Simple aircraft refrigeration cycle: . Leakage of air is not a problem ii. If the cabin is maintained at 0. This reduces the weight per kW cooling considerably.Aircraft cooling systems: In an aircraft. This effect adds heat to the cabin. Solar radiation For low speed aircraft flying at low altitudes.2 bar and temperature of 223 K (at 10000 m altitude) is compressed to 1 bar. which needs to be taken out by the cooling system. however. i. The cold air at state 5 is supplied to the cabin. η . and is cooled to state 4 in the air cooler.. Bootstrap system: . Due to irreversibilities. As shown in the T-s diagram. the actual pressure at the end of ramming will be less than the pressure resulting from isentropic compression. as a result its temperature drops from 4 to 5. This air is compressed in the main compressor to state 3. During this process its temperature increases from 1 to 2. which maintains the required air flow over the air cooler.Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram. Ram The refrigeration capacity of the simple aircraft cycle discussed. This is an open system. Q is given by: where m is the mass flow rate of air through the turbine. This simple system is good for ground cooling (when the aircraft is not moving) as fan can continue to maintain airflow over the air cooler. The ratio of actual pressure rise to the isentropic pressure rise is called as ram efficiency. It picks up heat as it flows through the cabin providing useful cooling effect. Its pressure is reduced to cabin pressure in the turbine (state 5).e. The power output of the turbine is used to drive the fan. the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2). The regenerative system is good for both ground cooling as well as high speed aircrafts. The heat rejected in the air cooler is absorbed by the ram air at state 2. The cooling turbine drives a fan similar to the simple system. It also incorporates a secondary compressor. As shown in the figure. this is at the expense of additional weight and design complexity. Other modifications over the simple system are: regenerative system and reduced ambient system. this system consists of two heat exchangers (air cooler and aftercooler). The air is then cooled to state 4 in the air cooler. as a result a separate fan is not required. As shown in the cycle diagram. it is not suitable for ground cooling. Since the system does not consist of a separate fan for driving the air through the 7 heat exchangers. a part of the cold air from the cooling turbine is used for precooling the air entering the turbine. in stead of one air cooler of the simple system. The reduced ambient system is well-suited for supersonic aircrafts and rockets. where in the velocity of the aircraft provides the necessary airflow for the heat exchangers. which is a modification of the simple system. expanded to cabin pressure in the cooling turbine and is supplied to the cabin at a low temperature T . . It is then cooled to state 6 in the after cooler. This system is suitable for high speed aircraft. In a regenerative system. However.Figure shows the schematic of a bootstrap system. As a result much lower temperatures are obtained at the exit of the cooling turbine. which is driven by the turbine of the cooling system. This air is further compressed to state 3 in the main compressor. ambient air state 1 is pressurized to state 2 due to the ram effect. The air from the air cooler is further compressed from state 4 to state 5 in the secondary compressor. in general ground cooling is normally done by an external air conditioning system as it is not efficient to run the aircraft engine just to provide cooling when it is grounded. however. Properties of aqua ammonia. Performance Applications.Introduction. Effects of operating conditions on COP. Electrolux Refrigeration. Compound VC cycle . Individual compression system with individual or multiple expansion valves.Necessity of compound compression. Multistage Ref. Actual COP of the System. Problems. Individual compression systems with individual or multiple expansion valves but with and without intercoolers. Steam Jet Refrigerating System. Comparison of VC cycle with Air Refrigeration cycle. Analysis of VC cycle considering degrees of sub cooling and superheating. Intercooling with liquid sub –cooling and / or water inter cooler: Multistage compression with flash inter-cooling and / or water inter-cooling. Problems. VC cycle on p-v. Performance. systems with individual or multiple expansion valves.SECTION B Vapour Compression (VC) Refrigeration Systems: (A) Simple Vapour Compression (VC) Refrigeration systems-Limitations of Reversed Carnot cycle with vapour as the refrigerant. Systems. Relative merits and demerits. Other Refrigeration Systems: (A) Vapour Absorption Refrigeration Systems – Basic Systems. Analysis. . Relative merits and demerits. t-s and p-h diagrams. refrigeration is obtained as the refrigerant evaporates at low temperatures. Carnot Refrigeration System Carnot Refrigeration Cycle on T-S diagram . The input to the system is in the form of mechanical energy required to run the compressor. It is used as reference against which the real cycles are compared.VAPOUR COMPRESSION REFRIGERATION vapour compression refrigeration systems are the most commonly used among all refrigeration systems. The basic Carnot refrigeration system for pure vapour consists of four components: compressor. The Carnot refrigeration cycle: Carnot refrigeration cycle is a completely reversible cycle. wherein the working fluid (refrigerant) undergoes phase change at least during one process. Hence these systems are also called as mechanical refrigeration systems. condenser. which is also called as reverse Rankine cycle. As the name implies. turbine and evaporator. The Carnot cycle involves two isothermal heat transfer processes (processes 4-1 and 2-3) and two isentropic work transfer processes (processes 1-2 and 3-4). hence is used as a model of perfection for a refrigeration cycle operating between a constant temperature heat source and sink. The actual vapour compression cycle is based on Evans-Perkins cycle. In a vapour compression refrigeration system. Refrigeration effect (q = q ) is obtained at the 4-1 e evaporator as the refrigerant undergoes the process of vaporization (process 4-1) and extracts the latent heat from the low temperature heat source. these systems belong to the general class of vapour cycles. wet compression is very difficult especially with reciprocating compressors. Practical difficulties with Carnot refrigeration system: It is difficult to build and operate a Carnot refrigeration system due to the following practical difficulties: i. a mixture consisting of liquid and vapour have to be compressed isentropically in the compressor. the work output from the turbine in case of the liquid will be small. In practice. This can be explained very easily with the help of the above T-s diagram . Even though some types of compressors can tolerate the presence of liquid in ii. Such a compression is known as wet compression due to the presence of liquid. The second practical difficulty with Carnot cycle is that using a turbine and extracting work from the system during the isentropic expansion of liquid refrigerant is not economically feasible. which get damaged due to the presence of liquid droplets in the vapour. This is the reason why exactly the same expression was obtained for air cycle refrigeration systems operating on Carnot cycle. During process 1-2.Coefficient of Performance (COP) is given by: The COP of Carnot refrigeration cycle is a function of evaporator and condenser temperatures only and is independent of the nature of the working substance. This is due to the fact that the specific work output (per kilogram of refrigerant) from the turbine is given by: since the specific volume of liquid is much smaller compared to the specific volume of a vapour/gas. . This problem is particularly severe in case of high speed reciprocating compressors. particularly in case of small capacity systems. It can be seen from the above expression that the COP of a Carnot refrigeration system increases as the evaporator temperature increases and condenser temperature decreases. saturated. respectively. saturated vapour compression refrigeration system consists of the following four processes: Process 1-2: Isentropic compression of saturated vapour in compressor Process 2-3: Isobaric heat rejection in condenser Process 3-4: Isenthalpic expansion of saturated liquid in expansion device Process 4-1: Isobaric heat extraction in the evaporator By comparing with Carnot cycle. As shown in the figure the standard single stage. which incorporates these two changes is known as Evans-Perkins or reverse Rankine cycle. Above figure shows the schematic of a standard. . This is the theoretical cycle on which the actual vapour compression refrigeration systems are based. the isentropic expansion process can be replaced by an isenthalpic throttling process. As a result. h1 and h4 are the specific enthalpies (kJ/kg) at the exit and inlet to the evaporator. it can be seen that the standard vapour compression refrigeration cycle introduces two irreversibilities: 1) Irreversibility due to non-isothermal heat rejection (process 2-3) and 2) Irreversibility due to isenthalpic throttling (process 3-4). b) negligible kinetic and potential energy changes across each component. Qe is given by: Qe = mr (h1 – h4) Where mr is the refrigerant mass flow rate in kg/s. single stage (SSS) vapour compression refrigeration system and the operating cycle on a T s diagram. one would expect the theoretical COP of standard cycle to be smaller than that of a Carnot system for the same heat source and sink temperatures. and c) no heat transfer in connecting pipe lines. *A simple analysis of standard VCR system can be carried out by assuming a) Steady flow.Vapour Compression Refrigeration Systems (VCR cycle): Dry compression with a single compressor is possible if the isothermal heat rejection process is replaced by isobaric heat rejection process. A refrigeration system. Evaporator: Heat transfer rate at evaporator or refrigeration capacity. The steady flow energy equation is applied to each of the four components. Similarly. respectively. Wc is given by: Wc = mr (h2 – h1) Where h2 and h1 are the specific enthalpies (kJ/kg) at the exit and inlet to the compressor. Condenser: Heat transfer rate at condenser. Qc is given by: Qc = mr (h2 – h3) Where h3 and h2 are the specific enthalpies (kJ/kg) at the exit and inlet to the condenser. hfg are the saturated liquid enthalpy. hence applying the definition of quality ( or dryness fraction) Where x4 is the quality of refrigerant at point 4. hg. the exit condition of the condenser will be in .e . saturated vapour enthalpy and latent heat of vaporization at evaporator pressure.e . Expension devices: For the isenthalpic expension process h3 = h4 The exit condition of the expension device lies in the two-phase region. respectively. Hence it is possible to cool the refrigerant liquid in the condenser to a few degrees lower than the condensing temperature by adding extra area for heat transfer. the temperature of the heat sink will be several degrees lower than the condensing temperature to facilitate heat transfer.Compressor: Power input to the compressor. hf. respectively. The COP of the system is given by: Use of Pressure-enthalpy (P-h) charts: Standard VCR cycle on p-h chart Subcooling and superheating: In actual refrigeration cycles. In such a case. Subcooling is beneficial as it increases the refrigeration effect by reducing the throttling loss at no additional specific work input. Another practical advantage of subcooling is that there is less 4 4’ 3 3’ vapour at the inlet to the evaporator which leads to lower pressure drop in the evaporator. Even though useful superheating may or may not increase the COP of the system. In gas cycles. Useful superheating increases both the refrigeration effect as well as the work of compression. It can be seen from the T-s diagram that without subcooling the throttling loss is equal to the hatched area b-4’-4-c.the subcooled liquid region. The temperature of refrigerant at the exit of the compressor increases with superheat as the isentropes in the vapour region gradually diverge. Figure 11. heat rejection and refrigeration take place as the gas undergoes sensible cooling and heating. it is possible for the refrigerant vapour to become superheated by exchanging heat with the surroundings as it flows through the connecting pipelines.4 shows the VCRS cycle without and with subcooling on P-h and T-s coordinates. If the superheating of refrigerant takes place due to heat transfer with the refrigerated space (low temperature heat source) then it is called as useful superheating as it increases the refrigeration effect. hence the vapour at the exit of the evaporator can be superheated by a few degrees. a minimum amount of superheat is desirable as it prevents the entry of liquid droplets into the compressor. in a typical gas cycle. Such a superheating is called as useless superheating as it does not increase refrigeration effect. On the other hand. whereas with subcooling the throttling loss is given by the area a-4”-4’-b. the working fluid (a gas) does not undergo phase change. Thus the refrigeration effect increases by an amount equal to (h -h ) = (h -h ). If the refrigerant is a pure substance then its . In a vapour cycle the working fluid undergoes phase change and refrigeration effect is due to the vaporization of refrigerant liquid. Similarly. consequently the operating cycle will be away from the vapour dome. Comparison between gas cycles and vapor cycles : Thermodynamic cycles can be categorized into gas cycles and vapour cycles. Hence this process is known as subcooling. depending mainly upon the nature of the working fluid. the temperature of heat source will be a few degrees higher than the evaporator temperature. Hence the COP (ratio of refrigeration effect and work of compression) may or may not increase with superheat. Also subcooling ensures that only liquid enters into the throttling device leading to its efficient operation. As mentioned in the previous chapter. Among these the vapour compression refrigeration systems are predominant. then there will be a temperature glide during vaporization and condensation. vapour jet systems etc.temperature remains constant during the phase change processes. However. it is possible to subcool the refrigerant liquid and superheat the refrigerant vapour by exchanging heat between them. Since the refrigeration effect is produced during phase change. vapour absorption systems. the required mass flow rates for a given refrigeration capacity will be much smaller compared to a gas cycle. large amount of heat (latent heat) can be transferred per kilogram of refrigerant at a near constant temperature. One way of achieving the required amount of subcooling and superheating is by the use of a liquid-suction heat exchanger (LSHX). which is detrimental to system performance.Single stage VCR with Heat Exchanger on T-s and p-h diagram . if one were to rely only on heat transfer between the refrigerant and external heat source and sink. then it may get superheated by exchanging heat with the surroundings as it flows through the connecting pipelines (useless superheating). Use of liquid-suction heat exchanger: Required degree of subcooling and superheating may not be possible. Vapour cycles can be subdivided into vapour compression systems. Also.Single stage VCR with Heat Exchanger Fig B-2. A LSHX is a counterflow heat exchanger in which the warm refrigerant liquid from the condenser exchanges heat with the cool refrigerant vapour from the evaporator. Figure B-1 shows the schematic of a single stage VCRS with a liquid-suction heat exchanger. As shown in the T-s diagram. Fig B-1. since the temperature of the refrigerant liquid at the exit of condenser is considerably higher than the temperature of refrigerant vapour at the exit of the evaporator. Figure B-2 shows the modified cycle on T-s and P-h diagrams. Hence. if a zeotropic mixture is used as a refrigerant. if the temperature of refrigerant at the exit of the evaporator is not sufficiently superheated. Compressor discharge temperature increases iv.v (T1-T6) Cp. for fluorocarbon and ammonia based refrigeration systems a single stage system is used upto an evaporator . Due to these drawbacks.B-3 shows the effect of decreasing evaporator temperatures on T s and P h diagrams. though not in the same proportion when the condenser temperature increases for a given evaporator temperature. Throttling losses increase ii. It can be seen from the T s diagrams that for a given condenser temperature. The performance of single stage systems shows that these systems are adequate as long as the temperature difference between evaporator and condenser (temperature lift) is small. Similar effects will occur. as evaporator temperature decreases: i. Superheat losses increase iii.v (T3-T4) < (T1-T6) Degree of subcool < Degree of superheat ϵLSHX is the effectiveness of heat exchanger. Fig. in frozen food industries the required o o evaporator can be as low as –40 C. However. while in chemical industries temperatures as low as –150 C may be required for liquefaction of gases. The temperature lift can become large either due to the requirement of very low evaporator temperatures and/or due to the requirement of very high condensing temperatures. Multi-stage vapour compression refrigeration system: A single stage vapour compression refrigeration system has one low side pressure (evaporator pressure) and one high side pressure (condenser pressure). drying etc. On the high temperature side the required condensing temperatures can be very high if the refrigeration system is used as a heat pump for heating applications such as process heating. For example. there are many applications where the temperature lift can be quite high.The heat transferred between the refrigerant liquid and vapour in the HX.l > Cp. In such cases multi-stage systems are used in practice. The volumic refrigeration effect also decreases rapidly as the specific volume increases with decreasing evaporator temperature. single stage systems are not recommended when the evaporator temperature becomes very low and/or when the condenser temperature becomes high.l (T3-T4) = Cp. Quality of the vapour at the inlet to the evaporator increases v. However. Q Q LSHX And LSHX is given by: = mr (h3 –h4) = mr (h1 –h6) (h3 –h4) = (h1 –h6) Cp. Generally. as the temperature lift increases the single stage systems become inefficient and impractical. For example. the refrigeration effect decreases and work of compression increases as shown in the P h diagram. Specific volume at the inlet to the compressor increases As a result of this. . The performance of this system can be obtained easily by applying mass and energy balance equations to the individual components. etc. effect of evaporator temperature on cycle performance on T-s and p-h diagram Multi-stage Compression with flash gas removal and intercooling : Figure B-4. For example. A two-stage system is used upto –60 C and a three-stage system is used o for temperatures below –60 C. As mentioned the use of combination of water cooling with flash tank for intercooling reduces the vapour generated in the flash tank. It is assumed that in this process the superheated refrigerant vapour gets completely de-superheated and emerges out as a saturated vapour at state 4. in practice complete de-superheating may not be possible. A multi-stage system is a refrigeration system with two or more low-side pressures. The superheated vapour from the water cooled heat exchanger bubbles through the refrigerant liquid in the flash tank. Multi-stage systems can be classified into: a) Multi-compression systems b) Multi-evaporator systems c) Cascade systems. multi-stage systems are also used in applications requiring refrigeration at different temperatures. However.o o temperature of –30 C. in a dairy plant refrigeration may o o be required at –30 C for making ice cream and at 2 C for chilling milk.(a) and (b) show a two-stage vapour compression refrigeration system with flash gas removal using a flash tank. and intercooling of refrigerant vapour by a water-cooled heat exchanger and flash tank. It is assumed that the flash tank is perfectly insulated and the potential and kinetic energy changes of refrigerant across each component are negligible. Fig B-3. In such cases it may be advantageous to use a multi-evaporator system with the low temperature evaporator operating at o o –30 C and the high temperature evaporator operating at 2 C. Apart from high temperature lift applications. lower pressure drop and better heat transfer in the evaporator b) Throttling losses are reduced as vapour generated during throttling from P to P is separated in c i the flash tank and recompressed by Compressor-II. a) Quality of refrigerant entering the evaporator reduces thus giving rise to higher refrigerating effect. c) Volumetric efficiency of compressors will be high due to reduced pressure ratios d) Compressor discharge temperature is reduced considerably.Fig.h7) where is mI the mass flow rate of refrigerant through Compressor-I.B-4. where is mII the mass flow rate of refrigerant through Compressor-II.h8) (h4.a.b two stage VCR system with flash gas and inter-cooler From mass and energy balance of the flash tank: m7 + m3 = m8 + m4 m7h7 + m3 h3 = m8h8 + m4h4 mI = m1 = m2 = m9 = m3 mII = mI (h3. For the above system. . the COP is given by: The above system offers several advantages. The condensed refrigerant liquid is then throttled in o the expansion device and is then fed to the evaporator to complete the refrigerant cycle.B-5. the required input to absorption systems is in the form of heat.VAPOUR ABSORPTION REFRIGERATION SYSTEMS Vapour Absorption Refrigeration Systems (VARS) belong to the class of vapour cycles similar to vapour compression refrigeration systems. Since conventional absorption systems use liquids for absorption of refrigerant. Since these systems run on low-grade thermal energy. Similar to vapour compression refrigeration systems. low e pressure refrigerant vapour enters the absorber where it comes in contact with a solution that is weak in refrigerant. However. The solution that is now rich in o refrigerant is pumped to high pressure using a solution pump and fed to the generator. The weak solution absorbs the refrigerant and becomes strong in refrigerant. they are preferred when low-grade energy such as waste heat or solar energy is available. low temperature and low pressure refrigerant with low quality enters the evaporator and vaporizes by producing useful refrigeration Q . In the generator heat at high temperature T is supplied. On the . From the evaporator. Hence these systems are also called as heat operated or thermal energy driven systems. the low temperature. This high pressure vapour is then condensed in the condenser by rejecting heat of condensation to the external heat sink at T . Since conventional absorption systems use natural refrigerants such as water or ammonia they are environment friendly. Fig. The heat of absorption is rejected to the external heat sink at T . these are also sometimes called as wet absorption systems. unlike vapour compression refrigeration systems. As shown in the figure in a continuous absorption system. as a result refrigerant vapour is generated at g high pressure. vapour absorption refrigeration systems have also been commercialized and are widely used in various refrigeration and air conditioning applications. VCR and VAR systems Figure B-5 show a continuous output vapour compression refrigeration system and a continuous output vapour absorption refrigeration system. A small amount of mechanical energy is required to run the solution pump. However. Various energy transfers in VARS COP of VAR System: Neglecting pump work Wp COP = ≥0 . high-pressure solution that is weak in refrigerant is throttled to the absorber pressure in the solution expansion valve and fed to the absorber where it comes in contact with the refrigerant vapour from evaporator. that as far as the condenser. then the absorption system operates between the condenser and evaporator pressures. It can be seen from Fig. work input required to pump a liquid (solution) is much less than the work required for compressing a vapour due to very small specific volume of liquid (W = -vdp). If we neglect pressure drops. In vapour compression refrigeration systems the vapour is compressed mechanically using the compressor. Thus continuous refrigeration is produced at evaporator. Since for the same pressure difference. Heat rejection to the external heat sink takes place at absorber and condenser. while heat at high temperature is continuously supplied to the generator.solution side. the hot. Fig. Pressure in absorber is same as the pressure in evaporator and pressure in generator is same as the pressure in condenser. the mechanical energy required to operate vapour absorption refrigeration system is much less than that required to operate a compression system. the difference lies in the way the refrigerant is compressed to condenser pressure.B-6.B-5. expansion valve and evaporators are concerned both compression and absorption systems are identical. where as in absorption system the vapour is first converted into a liquid and then the liquid is pumped to condenser pressure using the solution pump. Qa+Qc = Qe+Qg = COP COPVARS = ηcarnot X COPreversed carnot cycle .but .