Refrigeration and Air conditioning

Basic Mechanical Engineering1 REFRIGERATION & AIR CONDITIONING Rejeesh C R Asst. Professor, ME Dept Federal Institute of Science & Technology 3-Jan-14 Definition and Scope of Refrigeration 2  Literally, the word “refrigerate” is to make or keep cool or cold; to preserve (food, biologicals, etc) by keeping cold and freezing, as in Webster’s New World Dictionary.  According to ASRE (American Society of Refrigeration Engineering), refrigeration is defined as the science of providing and maintaining temperatures below that of surroundings.  Refrigeration may also be defined as the artificial withdrawal of heat, producing in a substance or within a space a temperature lower than that which would exist under the natural influence of surroundings. How Does It Work? 3 High Temperature Reservoir Heat Rejected R Work Input Heat Absorbed Low Temperature Reservoir Main Applications of Refrigeration 4 1. 2. 3. 4. 5. 7. 8. Comfort air conditioning Industrial air conditioning Food and Pharmaceutical cold preservation Industry and Construction processes Air liquefaction and separation Rocket propellant and Cryogenics Superconductivity and low temperature physics UNIT OF REFRIGERATION 5  The standard unit of refrigeration is ton refrigeration or simply ton denoted by TR.  It is equivalent to the rate of heat transfer needed to produce 1 ton (2000 lbs) of ice at 00C from water at 00C in one day, i.e., 24 hours.  This unit of refrigeration is currently in use in the USA, the UK and India. In many countries, the standard MKS unit of kcal/hr is used.  expressed into SI system, it is found that 1 TR is 210 kJ/min or 3.5 kW. Coefficient of Performance (COP) 6 Refrigeration effect is an important term in refrigeration that defines the amount of cooling produced by a system(i.e. the rate of heat absorbed from a body to be cooled). This cooling is obtained at the expense of some form of energy. Therefore, a term called coefficient of performance (COP) is defined as the ratio of the refrigeration effect to energy input. COP = Refrigeration effect Energy input (work spent) COP of a refrigerator will be greater than 1. While calculating COP, both refrigeration effect and energy input should be in the same unit. Types of Refrigeration Systems 7  The two principle types of refrigeration plants found in industrial use are: 1. Vapour Compression Refrigeration (VCR) and 2. Vapour Absorption Refrigeration (VAR).  VCR uses mechanical energy as the driving force for refrigeration, while VAR uses thermal energy as the driving force for refrigeration. Vapour Compression Refrigeration 8  Compression refrigeration cycles take advantage of the fact that highly compressed fluids at a certain temperature tend to get colder when they are allowed to expand.  If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature.  In this case, fluid is used to cool a low temperature environment and reject the heat to a high temperature environment. Simple Vapour Compression System 9 Simple Vapour Compression System 10 Vapour Compression Refrigeration 11 Vapour compression refrigeration cycles have two advantages.  First, a large amount of thermal energy is required to change a liquid to a vapor, and therefore a lot of heat can be removed from the air-conditioned space.  Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid. This means that the heat transfer rate remains high, because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. Simple Vapour Absorption System 12 Comparison between Vapour Absorption system 13 Vapour Compression System a) Uses low grade energy like heat. a) Using high-grade energy like Therefore, may be worked on exhaust mechanical work. systems from I.C engines, etc. b) Moving parts are only in the pump, which b) Moving parts are in the is a small element of the system. Hence compressor. Therefore, more wear, operation is smooth. tear and noise. c) The system can work on lower evaporator c) The COP decreases considerably pressures also without affecting the COP. with decrease in evaporator pressure. d) No effect of reducing the load on d) Performance is adversely affected performance. at partial loads. e) Liquid traces of refrigerant present in e) Liquid traces in suction line may piping at the exit of evaporator constitute no damage the compressor. danger. f) Automatic operation for controlling the f) It is difficult. capacity is easy. Domestic Refrigerator 14 A domestic refrigerator uses the cooling effect of an evaporating liquid. A refrigerant liquid (such as Freon, a compound of carbon, fluorine and chlorine) is pumped through cooling coils (the evaporator) in which it expands (evaporates) and absorbs heat from the surroundings. The evaporator is formed into the ice-making compartment of the refrigerator. Domestic Refrigerator 15 Domestic Refrigerator 16  After passing through the cooling coils in the evaporator, the vapour is compressed by a compressor (usually driven by an electric motor) and condensed back to liquid when the absorbed heat is released. The cycle of events is then repeated over and over again.  The refrigerator is really a heat engine working in reverse. In order to take heat out of the low-temperature interior of the refrigerator and transfer it to the higher temperature of the surrounding air, work must be done.  If it is to work continuously, a refrigerator must be supplied with energy from outside. This external energy is usually electricity, which operates the electric motor driving the compressor.  In the food chamber of a domestic refrigerator the temperature is just above the freezing point of water, about 1° or 2°C: in the ice-maker and in the deep-freeze it is usually around -15°C. Ice Plants 17 Fig. A brine type block ice plant In the manufacture of block ice, tapered rectangular cans filled with water are immersed in a tank of refrigerated brine as shown in Fig. Ice Plants 18 The brine which is cooled to about -5ºC by a refrigeration process extracts the heat from the water and produces block ice within the can. The cans are then removed from the tank and thawed for a short time in a tank of water to release the block from the can. The blocks are then stored in a cold room and can be crushed on demand. The freezing period is typically between 16 and 24 hrs although plants known as rapid-block are available that have freezing periods of only a few hours. Refrigerants 19 The working fluid used in a refrigerating system is called as a refrigerant. Features:   It carries heat from a cold place to hot place. It changes from vapour state during heat absorption and condenses to liquid while heat rejection liquid to. Common refrigerants are Fluorinated Hydrocarbons (Freon), CO2, SO2, Air, Ammonia, H2O etc. Desirable Properties of Refrigerants 20  Thermodynamic Properties  Chemical Properties  Physical Properties  Economic Criteria Thermodynamic Properties 21 Condensing and Evaporating Pressure The Condensing and Evaporating pressures should be moderate and above atmospheric pressure to avoid leakage of air into the system. A very high pressure will make the system heavy and bulky whereas in case of very low pressures, there is a possibility of air leaking into the refrigerating system. 22 Critical Temperature The critical temperature of the refrigerant should be as high as possible above the condensing temperature in order to have a greater heat transfer at a constant temperature. If this is not taken care of, then we will have excessive power consumption by the refrigeration system. Latent heat of Vaporization This should be as large as possible to minimize the quantity of refrigerant to be circulated. 23 Freezing Point It should be lower than the operating temperature to prevent solidification and choking of passages during flow of fluid through evaporator. Specific Heat The specific heat of the Refrigerant should be low. This ensures that the amount of vapour formed during throttling process is minimum. Chemical Properties 24 Chemical Stability and Inertness It should be chemically stable for the operating ranges of temperature. Also, it should not react with the materials of the refrigeration system or with which it comes into contact. Flammability The refrigerant should be inert and should not catch fire when subjected to high temperatures. Ethane, butane, isobutene etc are highly undesirable as they catch fire quickly. CO2 is suitable as it is non-flammable, and also acts as a fire-extinguisher. Toxicity The refrigerants used in air conditioning, food preservation etc. should not be toxic. Physical Properties 25 Leakage and Detection It is desirable that the refrigerant has a pungent smell so that its leakage can be detected immediately. Miscibility with Oil The refrigerant should not react with the oil else the lubricating strength will be reduced. Viscosity It should be as small as possible to ensure that the pressure drop in the system is as small as possible. A low viscosity refrigerant will require less energy for its circulation through the refrigeration system. Economic Criteria 26 Cost of Refrigerant The cost of the refrigerant has a big impact on the overall cost of the refrigeration system. Hence, its cost should be as low as possible. From this viewpoint, ammonia and water are ideally suited, but their low thermodynamic and chemical properties restrict their use in all types of refrigeration systems. Availability and Supply The refrigerant should be easily available in the market and in abundant quantity. This ensures that the cost of the refrigerant is not prohibitive. Air Conditioning 27 The science that deals with supplying and maintaining desired atmospheric conditions of a system irrespective of its surrounding conditions. Types:  Comfort Air Conditioning Primary intention is human health and comfort.  Industrial Air Conditioning If primary intention is not human comfort. 28 Physical Processes involved in Air Conditioning  Air Purification  Temperature Control  Humidity Control  Air distribution Evaporative Cooling There are occasions where air conditioning, which stipulates control of 29 humidity up to 50 % for human comfort or for process, can be replaced by a much cheaper and less energy intensive evaporative cooling. Evaporative cooling is an extremely efficient means of cooling at very low cost. The concept is very simple and is the same as that used in a cooling tower. Air is brought in close contact with water to cool it to a temperature close to the wet bulb temperature. The cool air can be used for comfort or process cooling. Sprinkling Water •The disadvantage is that the air is rich in moisture. •The possibility of evaporative cooling is especially attractive for comfort cooling in dry regions. This principle is practiced in textile industries for certain processes. Hot Air Cold Air Winter Air Conditioning 30 Summer Air Conditioning 31 Psychrometry 32 Psychro – meaning ‘cold’ metrics – meaning ‘measure of’ But Psychrometry is more than the measurement of cold.   It is the subject which deals with the behaviour (properties) of moist air. The properties of moist air are called psychrometric properties. Psychrometric Properties 33 1. Dry Air Mixture of Oxygen, Nitrogen, CO2, Argon, Neon, Helium etc… 2. Moist Air Ordinary atmospheric air i.e., A mixture of dry air and water vapour 3. Saturated Air Air which contains maximum amount of water vapour which the air can hold at a given temperature and pressure. The maximum quantity of water vapour that can be present in air depends upon temperature and pressure of air. AIR DRY 78% Nitrogen  20.9% Oxygen  1% Argon  .1% Other Gases   WET 78% Nitrogen  20.9% Oxygen  1% Argon  .1% Other Gases  PLUS Water Vapor Psychrometric Properties 35 4. Dry Bulb Temperature A psychrometer comprises of a dry bulb thermometer and a wet bulb thermometer. The dry bulb thermometer is directly exposed to the air and measures the actual temperature of air and is called dry bulb temperature. 5. Wet Bulb Temperature Temperature recorded by a thermometer, When the thermometer bulb is surrounded by a wet cloth exposed to the air. The difference between the DBT and WBT is called wet bulb depression (WBD) and it depends on RH of air . Psychrometric Properties 36 6. Specific humidity / Absolute humidity/ humidity ratio It is defined as the ratio of mass of water vapour to the mass of dry air in a given volume of moist air. 7. Relative Humidity It is defined as the ratio of the mass of water vapour in a given volume of moist air at a given temperature to the mass of water vapour in equal volume of saturated air at same temperature. 8. Sensible heat of air It is the enthalpy of dry air which can be measured from its DBT. Psychrometric Properties 37 9. Dew Point Temperature It is the temperature at which the condensation of moisture begins when air is cooled at constant pressure. The difference between the DBT and Dew Point Temperature is known as Dew Point depression. 10. Total heat of moist air It is the sum of Sensible heat of dry air and Sensible plus latent heat of water vapour present in it. Psychrometric Chart 38 It is the graphical representation of the various thermodynamic properties of moist air. All data essential for the complete thermodynamic and psychrometric analysis of air-conditioning processes can be summarised in a psychrometric chart. Chart has specific humidity or water vapour pressure along the ordinate and the dry bulb temperature along the abscissa. The chart is normally constructed for a standard atmospheric pressure of 760 mm Hg or 1.01325 bar, corresponding to the pressure at the mean sea level. Psychrometric Chart 39 Psychrometric Chart © American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC001-EN THE END