Published on September 30, 2016
1. REFRIGERATION SYSTEMS VAPOR REFRIGERATION SYSTEMS The purpose of a refrigeration system is to maintain a system at a temperature below the temperature of its surroundings. CARNOT REFRIGERATION CYCLE Carnot vapor refrigeration cycle The refrigerant enters the evaporator as a twophase liquid–vapor mixture at state 4. In the evaporator some of the refrigerant changes phase from liquid to vapor as a result of heat transfer from the region at temperature TC to the refrigerant. The temperature and pressure of the refrigerant remain constant during the process from state 4 to state 1. The refrigerant is then compressed adiabatically from state 1, where it is a twophase liquid–vapor mixture, to state 2, where it is a saturated vapor. During this process, the temperature of the refrigerant increases from TC to TH, and the pressure also increases.
2. The refrigerant passes from the compressor into the condenser, where it changes phase from saturated vapor to saturated liquid as a result of heat transfer to the region at temperature TH. The temperature and pressure remain constant in the process from state 2 to state 3 The refrigerant returns to the state at the inlet of the evaporator by expanding adiabatically through a turbine. In this process, from state 3 to state 4, the temperature decreases from TH to TC, and there is a decrease in pressure. Area 1–a–b–4–1 is the heat added to the refrigerant from the cold region per unit mass of refrigerant flowing. Area 2–a–b–3–2 is the heat rejected from the refrigerant to the warm region per unit mass of refrigerant flowing. The enclosed area 1–2–3–4–1 is the net heat transfer from the refrigerant. The net heat transfer from the refrigerant equals the net work done on the refrigerant. The net work is the difference between the compressor work input and the turbine work output COP The coefficient of performance β of any refrigeration cycle is the ratio of the refrigeration effect to the net work input required to achieve that effect. DEPARTURES FROM THE CARNOT CYCLE Actual vapor refrigeration systems depart significantly from the Carnot cycle and have coefficients of performance lower than Carnot. The ways actual systems depart from the Carnot cycle are In actual systems, these heat transfers are not accomplished reversibly as presumed above. To achieve a rate of heat transfer sufficient to maintain the temperature of the cold region at TC requires the temperature of the refrigerant in the evaporator T ’C, to be several degrees below TC. Similarly, to obtain a sufficient heat transfer rate from the refrigerant to the warm region requires that the refrigerant temperature in the condenser, T ‘H, be several degrees above TH.
3. Maintaining the refrigerant temperatures in the heat exchangers at T ‘C and T ‘H rather than at TC and TH, respectively, has the effect of reducing the COP. This conclusion about the effect of refrigerant temperature on the COP also applies to other refrigeration cycles. Compression process from state 1 to state 2 occurs with the refrigerant as a two phase liquid–vapor mixture, this is commonly referred to as wet compression. Wet compression is normally avoided because the presence of liquid droplets in the flowing liquid–vapor mixture can damage the compressor and this makes carnot cycle impractical. The expansion process from the saturated liquid state 3’ to the lowquality, two phase liquid–vapor mixture state 4, produces a relatively small amount of work compared to the work input in the compression process. Ton of refrigeration is equal to 200 Btu/min or about 211 kJ/min.
4. VAPOR COMPRESSION REFRIGERATION SYSTEMS Vaporcompression refrigeration systems are the most common refrigeration systems in use today. vapor compression refrigeration system TS CURVE FOR IDEAL VCRS
5. WORK AND HEAT TRANSFERS As the refrigerant passes through the evaporator, heat transfer from the refrigerated space results in the vaporization of the refrigerant. For a control volume, rate of heat transfer per unit mass of refrigerant flowing is the mass flow rate of the refrigerant is referred to as the refrigeration capacity. The refrigerant leaving the evaporator is compressed to a relatively high pressure and temperature by the compressor. Rate of power input per unit mass of refrigerant flowing The refrigerant passes through the condenser, where the refrigerant condenses and there is heat transfer from the refrigerant to the cooler surroundings The rate of heat transfer from the refrigerant per unit mass of refrigerant flowing is Refrigerant at state 3 enters the expansion valve and expands to the evaporator pressure. This process is usually modeled as a throttling process for which
6. The refrigerant pressure decreases in the irreversible adiabatic expansion, and there is an accompanying increase in specific entropy. The refrigerant exits the valve at state 4 as a twophase liquid–vapor mixture. In the vaporcompression system, the net power input is equal to the compressor power, since the expansion valve involves no power input or output Therefore COP of VCRS is Dry compression is presumed Process 1–2s Isentropic compression of the refrigerant from state 1 to the condenser pressure at state 2s. Process 2s–3: Heat transfer from the refrigerant as it flows at constant pressure through the condenser Process 3–4: Throttling process from state 3 to a twophase liquid–vapor mixture at 4. Process 4–1: Heat transfer to the refrigerant as it flows at constant pressure through the evaporator to complete the cycle PERFORMANCE OF ACTUAL VAPORCOMPRESSION SYSTEMS ts curve for actual vcrs
7. ph curve for vcrs Heat transfers between the refrigerant and the warm and cold regions are not accomplished reversibly. The refrigerant temperature in the evaporator is less than the cold region temperature, TC, and the refrigerant temperature in the condenser is greater than the warm region temperature, TH. Coefficient of performance decreases as the average temperature of the refrigerant in the evaporator decreases and as the average temperature of the refrigerant in the condenser increases. The effect of irreversible compression can be accounted for by using the isentropic compressor efficiency Additional departures from ideality results from frictional effects that result in pressure drops as the refrigerant flows through the evaporator, condenser, and piping connecting the various components.
8. ABSORPTION REFRIGERATION These cycles have some features in common with the vaporcompression cycles but differs in following important respects Instead of compressing a vapor between the evaporator and the condenser, the refrigerant of an absorption system is absorbed by a secondary substance, called an absorbent, to form a liquid solution. The liquid solution is then pumped to the higher pressure. Because the average specific volume of the liquid solution is much less than that of the refrigerant vapor, significantly less work is required so, absorption refrigeration systems have the advantage of relatively small work input compared to vaporcompression systems. The other main difference between absorption and vaporcompression systems is that some means must be introduced in absorption systems to retrieve the refrigerant vapor from the liquid solution before the refrigerant enters the condenser. This involves heat transfer from a relatively high temperature source. Natural gas or some other fuel can be burned to provide the heat source, and there have been practical applications of absorption refrigeration using alternative energy sources such as solar and geothermal energy. SIMPLE AMMONIA WATER ARS
9. In this case, ammonia is the refrigerant and water is the absorbent. Ammonia circulates through the condenser, expansion valve, and evaporator as in a vaporcompression system. However, the compressor is replaced by the absorber, pump, generator, and valve shown on the right side of the diagram. In the absorber, ammonia vapor coming from the evaporator at state 1 is absorbed by liquid water. The formation of this liquid solution is exothermic. Since the amount of ammonia that can be dissolved in water increases as the solution temperature decreases, cooling water is circulated around the absorber to remove the energy released as ammonia goes into solution and maintain the temperature in the absorber as low as possible. The strong ammonia–water solution leaves the absorber at point “a” and enters the pump, where its pressure is increased to that of the generator. In the generator, heat transfer from a hightemperature source drives ammonia vapor out of the solution (an endothermic process), leaving a weak ammonia–water solution in the generator. The vapor liberated passes to the condenser at state 2, and the remaining weak solution at c flows back to the absorber through a valve. The only work input is the power required to operate the pump, and this is small in comparison to the work that would be required to compress refrigerant vapor between the same pressure levels. MODIFIED AMMONIA–WATER ABSORPTION SYSTEM In this cycle, a heat exchanger is included between the generator and the absorber that allows the strong water–ammonia solution entering the generator to be preheated by the weak solution returning from the generator to the absorber, thereby reducing the heat transfer to the generator . The other modification shown in the figure is the rectifier placed between the generator and the condenser. The function of the rectifier is to remove any traces of water from the refrigerant before it enters the condenser. This eliminates the possibility of ice formation in the expansion valve and the evaporator.
10. modified ammonia water ars Another type of absorption system uses lithium bromide as the absorbent and water as the refrigerant To achieve refrigeration at lower temperatures than are possible with water as the refrigerant, a lithium bromide–water absorption system may be combined with another cycle using a refrigerant with good lowtemperature characteristics, such as ammonia, to form a cascade refrigeration system.
11. GAS REFRIGERATION SYSTEMS In gas refrigeration systems working fluid remains a gas throughout. They are used to achieve very low temperatures for the liquefaction of air and other gases and for other specialized applications such as aircraft cabin cooling. BRAYTON REFRIGERATION CYCLE The refrigerant gas, which may be air, enters the compressor at state 1, where the temperature is somewhat below the temperature of the cold region, TC, and is compressed to state 2. The gas is then cooled to state 3, where the gas temperature approaches the temperature of the warm region, TH. Next, the gas is expanded to state 4, where the temperature, T4, is well below that of the cold region. Refrigeration is achieved through heat transfer from the cold region to the gas as it passes from state 4 to state 1, completing the cycle.
12. COP is The magnitude of the work developed by the turbine of a Brayton refrigeration cycle is typically significant relative to the compressor work input. To obtain even moderate refrigeration capacities with the Brayton refrigeration cycle, equipment capable of achieving relatively high pressures and volumetric flow rates is needed. For most applications involving air conditioning and for ordinary refrigeration processes, vaporcompression systems can be built more cheaply and can operate with higher coefficients of performance than gas refrigeration systems Gas refrigeration systems can be used to achieve temperatures of about 21508C (22408F), which are well below the temperatures normally obtained with vapor systems. BRAYTON REFRIGERATION CYCLE WITH HEAT EXCHANGER
13. The heat exchanger allows the air exiting the compressor at state 2 to cool below the warm region temperature TH giving a low turbine inlet temperature, T3. Without the heat exchanger, air could be cooled only close to TH, as represented on the figure by state a. In the subsequent expansion through the turbine, the air achieves a much lower temperature at state 4 than would have been possible without the heat exchanger Accordingly, the refrigeration effect, achieved from state 4 to state b, occurs at a correspondingly lower average temperature. SELECTING REFRIGERANTS Refrigerant selection for a wide range of refrigeration and airconditioning applications is generally based on three factors 1) PerformanceIt refers to providing the required cooling or heating capacity reliably and cost effectively 2) Safety It refers to avoiding hazards such as toxicity and flammability 3) Environmental impact It primarily refers to using refrigerants that do not harm the stratospheric ozone layer or contribute significantly to global climate change. The selection of a refrigerant is based partly on the suitability of its pressure– temperature relationship in the range of the particular application. It is generally desirable to avoid excessively low pressures in the evaporator and excessively high pressures in the condenser. Other considerations in refrigerant selection include chemical stability, corrosiveness, and cost. The type of compressor also affects the choice of refrigerant. Centrifugal compressors are best suited for low evaporator pressures and refrigerants with large specific volumes at low pressure. Reciprocating compressors perform better over large pressure ranges and are better able to handle low specific volume refrigerants.
14. Refrigerant Types and Characteristics