2013-3-6 Chapter 10 Refrigeration and Heat Pump Systems Learning Outcomes Demonstrate understanding of basic vapor- compression refrigeration and heat pump systems Develop and analyze thermodynamic models of vapor-compression systems and their modifications,including sketching schematic and accompanying T-s diagrams evaluating property data at principal states of the systems. determining refrigeration and heat pump system performance,coefficient of performance,and capacity. 1
2013-3-6 1 Chapter 10 Refrigeration and Heat Pump Systems Learning Outcomes ►Demonstrate understanding of basic vaporcompression refrigeration and heat pump systems. ►Develop and analyze thermodynamic models of vapor-compression systems and their modifications, including ►sketching schematic and accompanying T-s diagrams. ►evaluating property data at principal states of the systems. ►applying mass, energy, entropy, and exergy balances for the basic processes. ►determining refrigeration and heat pump system performance, coefficient of performance, and capacity
2013-3-6 Learning Outcomes,cont. Explain the effects on vapor-compression system performance of varying key parameters. Demonstrate understanding of the operating principles of absorption and gas refrigeration systems,and perform thermodynamic analysis of gas systems. Vapor-Compressior Refrigeration Cycle Most common refrigeration cycle in use today There are four principal 50 control volumes involving these components: Evaporator Compressor Condenser Expansion valve s by work and heat are taken as positive in arrows on the schematic and energy palances are written accordingly. 2
2013-3-6 2 Learning Outcomes, cont. ►Explain the effects on vapor-compression system performance of varying key parameters. ►Demonstrate understanding of the operating principles of absorption and gas refrigeration systems, and perform thermodynamic analysis of gas systems. Vapor-Compression Refrigeration Cycle ►There are four principal control volumes involving these components: ►Evaporator ►Compressor ►Condenser ►Expansion valve ►Most common refrigeration cycle in use today All energy transfers by work and heat are taken as positive in the directions of the arrows on the schematic and energy balances are written accordingly. Two-phase liquid-vapor mixture
2013-3-6 The Vapor-Compression Refrigeration Cycle The processes of this cycle are Process 4-1:two-phase liquid-vapor mixture igerant is evaporated through heat transfer from the refrigerated space. Process 1-2:vapor refrigerant is compressed to a relatively high temperature and pressure requiring work input. Process 2-3:vapor refrigerant condenses to liquid through heat transfer to the cooler surroundings. Process 3-4:liquid refrigerant expands to the evaporator pressure. The Vapor-Compression Refrigeration Cycle Engineering model: Each component is analyzed as a control volume at steady state. Dry compression is presumed:the refrigerant is a vapor The compressor operates adiabatically. The refrigerant expanding through the valve undergoes a throttling process. Kinetic and potential energy changes are ignored. 3
2013-3-6 3 The Vapor-Compression Refrigeration Cycle Process 4-1: two-phase liquid-vapor mixture of refrigerant is evaporated through heat transfer from the refrigerated space. Process 1-2: vapor refrigerant is compressed to a relatively high temperature and pressure requiring work input. Process 2-3: vapor refrigerant condenses to liquid through heat transfer to the cooler surroundings. Process 3-4: liquid refrigerant expands to the evaporator pressure. ►The processes of this cycle are Two-phase liquid-vapor mixture The Vapor-Compression Refrigeration Cycle ►Engineering model: ►Each component is analyzed as a control volume at steady state. ►Dry compression is presumed: the refrigerant is a vapor. ►The compressor operates adiabatically. ►The refrigerant expanding through the valve undergoes a throttling process. ►Kinetic and potential energy changes are ignored
2013-3-6 The Vapor-Compression Refrigeration Cycle Applying mass and energy rate balances Evaporator .=h-hs m (Eq.10.3) The term is referred to as the refrigeration capacity,expressed in kW in the SI unit system or Btu/h in the English unit system. A common alternate unit is the ton of refrigeration which equals 200 Btu/min or about 211 kJ/min. The Vapor-Compression Refrigeration Cycle Applying mass and energy rate balances Compressor =h-h(Eq.10.4) Assuming adiabatic m compression Condenser =-4105 Expansion valve Assuming a throttling (Eq.10.6) process 4
2013-3-6 4 Evaporator The Vapor-Compression Refrigeration Cycle (Eq. 10.3) ►Applying mass and energy rate balances ►The term is referred to as the refrigeration capacity, expressed in kW in the SI unit system or Btu/h in the English unit system. ►A common alternate unit is the ton of refrigeration which equals 200 Btu/min or about 211 kJ/min. 1 4 in h h m Q = − & & Qin & Compressor Assuming adiabatic compression Condenser Expansion valve Assuming a throttling process The Vapor-Compression Refrigeration Cycle 2 1 c h h m W = − & & 4 3 h = h (Eq. 10.5) (Eq. 10.6) (Eq. 10.4) ►Applying mass and energy rate balances 2 3 out h h m Q = − & &
2013-3-6 The Vapor-Compression Refrigeration Cycle Performance parameters Coefficient of Performance(COP) h2-h (Eq.10.7 Carnot Coefficient of Performance Tc Bnas=Tu -Tc (Eq.10.1) This equation represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between cold and hot regions at T and respectively. Features of Actual Vapor-Compression Cycle Heat transfers between refrigerant and cold and warm regions are not reversible Refrigerant temperature in evaporator is less than To. Refrigerant temperature in condenser is greater than Tu. Irreversible heat transfers have negative effect on performance. 5
2013-3-6 5 Coefficient of Performance (COP) The Vapor-Compression Refrigeration Cycle (Eq. 10.1) (Eq. 10.7) ►Performance parameters Carnot Coefficient of Performance This equation represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between cold and hot regions at TC and TH, respectively. Features of Actual Vapor-Compression Cycle ►Heat transfers between refrigerant and cold and warm regions are not reversible. ►Refrigerant temperature in evaporator is less than TC. ►Refrigerant temperature in condenser is greater than TH. ►Irreversible heat transfers have negative effect on performance