Chapter 11: Refrigeration Cycles 11.5. Selecting the Right Refrigerants When designing a refrigeration system, there are several refrigerants from which to choose, such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons, and even water. The right choose of the refrigerants are based on various factors. The most two parameters that need to be considered in the selection of the refrigerants are the temperatures of the two media and the heat transfer between the two media. Refrigerants Type Expensive Coefficient of Performance Toxic R134a Lowest Higher than R22 and R12 Friendly to the ozone cost R22 Higher cost Lower than ammonia and Most damage the ozone than R134a R134a R12 (Freon) Higher cost Lower than ammonia Most damage the ozone than R134a Ammonia Low cost High High toxicity 11.6 Heat Pump A heat pump can be used to heat a house in the winter and cool it in the summer. Heat pumps are generally more expensive to purchase and install than other heating systems, but they save money in the long run in some areas because they lower the heating bill. The most common energy source for heat pumps is atmospheric air. The soil and water could be another source. Fig. 11.5 Heat Pump in Heating and Cooling Mode 1
11.7 Innovative Refrigeration Systems 11.7.1. Cascade Refrigeration systems Reason: A large temperature range causes low COP which means large range of the pressure. The two cycles operate in series are a good way to provide more efficient cycle which is called Cascade Refrigeration Cycles. The two cycles are connected through the heat exchanger in the middle as shown in Fig.11.6. Fig.11.6 Cascade Refrigeration cycle m A(h 5 h 8 ) = m B(h 2 h 3 ) m A = h 2 h 3 m B h 5 h 8 COP R,cascade = Q L W net = m B(h 1 h 4 ) m A(h 6 h 5 ) + m B(h 2 h 1 ) 2
11.7.2 Multistage Comparison Refrigeration Systems Instead of using the middle evaporator (indirect heat exchanger), the Flash Chamber is employed. The fash chamber is used since it has better heat transfer charactersitics. Fig.11.7. A Two-Stage Refrigeration Cycles Energy Balance on the two compressor yields: W in = m 1(h 2 h 1 ) + m 9(h 4 h 9 ) Energy Balance on Condenser: H = m 4 (h 4 h 5 ) Energy Balance on Evaporator: L = m 1(h 1 h 8 ) Mass Balance on Flash Chamber: m 6 = m 3 + m 7 Energy Balance on Flash Chamber: m 6h 6 = m 3h 3 + m 7h 7 3
11.7.3 Multipurpose Refrigeration Systems with a Single Compressor This is the ordinary Refrigerator-Freezer unit which is economical unit. Fig.11.8. Refrigerator-Freezer Unit with a Single Compressor. Energy Balance on the compressor yields: W in = m 1 (h 2 h 1 ) Energy balance on the Refrigerator yields: L, R = m 5 (h 5 h 4 ) Energy balance on the Freezer yields: L, F = m 6 (h 1 h 6 ) The Coefficient of Performance is COP Ref = L, F + L, R W in 11.7.4 Liquefaction Gases The liquefaction of gases has always been important area of refrigeration since many important scientific and engineering process at cryogenic temperatures (i.e. temperature below about -100 o C) depend on liquefied gases. How can we lower a gas temperature below its critical-point value? The liquefaction can be achieved using the regenerator heat exchanger. 4
Fig.11.9. Linde-Hampson for liquefying gases. 11.8. Gas Refrigeration Cycles (Reversed Brayton Cycle) It is known as a reversed Brayton cycle. Fig.11.10. Closed Reversed Brayton Cycle (Gas Refrigeration Cycle) 5
COP R = Q L = W net m 1(h 1 h 4 ) m 2(h 2 h 3 ) m 1(h 1 h 2 ) Fig.10.11. Open Reversed Brayton Cycle. The reversed Brayton cycle or Gas Refrigeration cycle have two desirable characteristics: 1. They have simple and light component (they are suitable for aircraft cooling). 2. They are suitable for liquefaction of gases and cryogenic applications. Fig.10.12. Regenerative Gas Refrigeration Cycle. 6
11.8. Absorption Refrigeration Systems To get inexpensive energy source, such as geothermal, solar, and waste heat energy, the absorption refrigeration cycle can be employed as shown in Fig.10.13.This cycle utilizes NH3 as a refrigerant. Fig.10.13. Ammonia Absorption Refrigeration Cycle. 7