Week 9. Refrigeration Cycles I
Objectives 1. Introduce the concepts of refrigerators and heat pumps and the measure of their performance. 2. Analyze the ideal vapor-compression refrigeration cycle. 3. Analyze the actual vapor-compression refrigeration cycle. 4. Review the factors involved in selecting the right refrigerant for an application. 5. Discuss the operation of refrigeration and heat pump systems. 6. Evaluate the performance of innovative vapor-compression refrigeration system. 7. Analyze gas refrigeration systems. 8. Introduce the concepts of absorption-refrigeration systems. 9. Review the concepts of thermoelectric power generation and refrigeration
Refrigerators And Heat Pumps The transfer of heat from a low-temperature region to a high-temperature one The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance () R HP HP Desired output Cooling effect Q Required input Work input W Desired output Heating effect Q = Required input Work input W R 1 L net,in H net,in
The Reversed Carnot Cycle Carnot Refrigerator 1 R, Carnot T H T L Carnot Heat Pump HP, Carnot 1 1 T 1 L T H The reversed Carnot cycle is not a suitable model for refrigeration cycles Process 1 2, 3 4 : achievable Process 2 3 the compression of a liquid-vapor mixture Process 4 1 the expansion of high- Moisture-content refrigerant in a turbine Carnot refrigerator and T-s diagram of the reversed Carnot cycle
The Ideal Vapor-Compression Refrigeration Cycle Many of the impracticalities associated with the reversed Carnot cycle can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as an expansion valve or capillary Four processes: 1-2 Isentropic compression in a compressor 2-3 Constant-pressure heat rejection in a condenser 3-4 Throttling in an expansion device 4-1 Constant-pressure heat absorption in an evaporator T-s diagram for the ideal vapor-compression refrigeration cycle
The Ideal Vapor-Compression Refrigeration Cycle P-h diagram -Process 3-4 is isenthalpic process (expansion valve) -Processes 2-3 and 4-1: Q is determined as deviation between h s -Process 1-2: W is determined as h 2 -h 1 The s of refrigerators and heat pumps operating on the vapor-compression refrigeration cycle can be expressed as R HP q w L net,in q w H net,in h1 h4 h h 2 2 1 h2 h3 h h 1 The P-h diagram of an ideal vapor-compression refrigeration cycle
Ex 1) The Ideal Vapor-Compression Refrigeration Cycle
Ex 1-1) The Ideal Vapor-Compression Refrigeration Cycle Consider an ideal refrigeration cycle that uses R-134a as the working fluid. The temperature of the refrigerant in the evaporator is -20 o C, and in the condenser exit, it is 40 o C. The refrigerant is circulated at the rate of 0.03 kg/s. Determine the and the capacity of the plant in rate of refrigeration.
Actual Vapor-Compression Refrigeration Cycle An actual vapor-compression refrigeration cycle differs from the ideal one owing to the irreversibilities (e.g. fluid friction causing pressure drops and heat transfer to or from the surroundings) that occur in various components. The compression process in the actual cycle is not isentropic The entropy of the refrigerant may increase (process 1-2) or decrease (process 1-2 ) due to cooling effect The refrigerant is subcooled somewhat before it enters the throttling valve and superheated before it enters the compressor
Ex 2) The Actual Vapor-Compression Refrigeration Cycle
Ex 2-1) The Actual Vapor-Compression Refrigeration Cycle A refrigeration cycle utilizes R-134a as the working fluid. The following are the properties at various points of the cycle designated in Figure. P 1 =125 kpa, T 1 =-10 o C P 2 =1200 kpa, T 2 =100 o C P 3 =1190 kpa, T 3 =80 o C P 4 =1160 kpa, T 4 =45 o C P 5 =1150 kpa, T 5 =40 o C P 6 =P 7 =140 kpa, x6=x7 P 8 =130 kpa, T 8 =-20 o C The heat transfer from R-134a during the compression process is 4 kj/kg. Determine the of this cycle.
Innovative Vapor-Compression Refrigeration Systems The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free However, for large industrial applications efficiency, the major concern is not simplicity. Modifications and refinements are necessary - Cascade Refrigeration Systems - Multistage Compression Refrigeration Systems - Multipurpose Refrigeration Systems with a Single Compressor - Liquefaction of Gases
Cascade Refrigeration Systems Need to operate a large temperature range -Way of dealing with a large pressure range in the cycle and a poor performance for reciprocating compressor Solution - Two or more refrigeration cycles that operate in series -Refrigerants in both cycles can be the same or different, but a certain refrigerants with more desirable characteristics can be used on each cycle Result -The compressor work decreases and the amount of heat absorbed from the refrige- rated space increases - The ratio of mass flow rates is m A h m Ah5 h8 m B h2 h3 m h B 2 5 h h 3 8 R,cascade Q W L net,in m A m B h1 h4 h h m h h 6 5 B 2 1