同济大学：《工程热力学》课程电子教案（讲稿）Chapter 10 Refrigeration and Heat Pump Systems

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 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. ►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

2013-3-6 Features of Actual Vapor-Compression Cycle The COP decreases-primarily due to increasing compressor work input-as the temperature of the refrigerant passing thro ough the ator is reduced relative to the temperature of the cold region,Tc. temperature of the refrigerant passing through the condenser is increased relative to the temperature of the warm region,Tu Features of Actual Vapor-Compression Cycle Irreversibilities during the compression process are suggested by dashed line from state 1 to state 2. An increase in specific ompani compres on process work input r compre ess 1-2 is greater than or the counterpart is compression process 1-2s. Since process 4-1,and thus the refrigeration capacity is the same for cycles 1-2-3 4-1and1-2s-3-4-1,cycle 1-2-3-4-1 has the lower COP 6

2013-3-6 6 Features of Actual Vapor-Compression Cycle ►The COP decreases – primarily due to increasing compressor work input – as the ►temperature of the refrigerant passing through the evaporator is reduced relative to the temperature of the cold region, TC. ►temperature of the refrigerant passing through the condenser is increased relative to the temperature of the warm region, TH. Trefrigerant ↓ Trefrigerant ↑ Features of Actual Vapor-Compression Cycle ►Irreversibilities during the compression process are suggested by dashed line from state 1 to state 2. ►An increase in specific entropy accompanies an adiabatic irreversible compression process. The work input for compression process 1-2 is greater than for the counterpart isentropic compression process 1-2s. ►Since process 4-1, and thus the refrigeration capacity, is the same for cycles 1-2-3-4-1 and 1-2s-3-4-1, cycle 1-2-3-4-1 has the lower COP

2013-3-6 Isentropic Compressor Efficiency The isentropic compressor efficiency is the ratio of the minimum theoretical work input to the actual work input,each per unit of mass flowing: work required in an isentropic n from r inlet (-Wev/rit)s h2s m (Eq.6.48 （-W/m)h-h red in an actual Actual Vapor-Compression Cycle Example:The table provides steady-state operating data for a vapor-compression refrigeration cycle using R-134a as the working fluid.For a refrigerant mass flow rate of 0.08 kg/s,determine the (a)compressor power,in kW, (b)refrigeration capacity,in tons, (c)coefficient of performance. (d)isentropic compressor efficiency 岛 7

2013-3-6 7 Isentropic Compressor Efficiency ►The isentropic compressor efficiency is the ratio of the minimum theoretical work input to the actual work input, each per unit of mass flowing: (Eq. 6.48) work required in an actual compression from compressor inlet state to exit pressure work required in an isentropic compression from compressor inlet state to the exit pressure Actual Vapor-Compression Cycle (a) compressor power, in kW, (b) refrigeration capacity, in tons, (c) coefficient of performance, (d) isentropic compressor efficiency. Example: The table provides steady-state operating data for a vapor-compression refrigeration cycle using R-134a as the working fluid. For a refrigerant mass flow rate of 0.08 kg/s, determine the State h (kJ/kg) 1 241.35 2s 272.39 2 280.15 3 91.49 4 91.49

2013-3-6 Actual Vapor-Compression Cycle m (a)The compressor power is m。=imh2-h) 底-0o8g28015-24135kW /s=3.1 kw (b)The refrigeration capacity is Cin=rin(h-hs) n-0084324135-9149Ln1om60s kg 211kJ/minmin 3.41 tons Actual Vapor-Compression Cycle a品岛 (c)The coefficient of performance is B (h-h4) (h2-h) 月=24135-91490e=3.36 (280.15-241.35)kJkg 8

2013-3-6 8 Actual Vapor-Compression Cycle (a) The compressor power is ( ) c 2 1 W = m& h − h & ⎟ − = ⎠ ⎞ ⎜ ⎝ ⎛ = 1kJ/s 1kW kg kJ (280.15 241.35) s kg Wc 0.08 & 3.1 kW (b) The refrigeration capacity is ( ) in 1 4 Q = m& h − h & ⎟ − = ⎠ ⎞ ⎜ ⎝ ⎛ = min 60s 211kJ/min 1 ton kg kJ (241.35 91.49) s kg 0.08 Qin & 3.41 tons State h (kJ/kg) 1 241.35 2s 272.39 2 280.15 3 91.49 4 91.49 Actual Vapor-Compression Cycle (c) The coefficient of performance is ( ) ( ) 2 1 1 4 h h h h − − β = = − − = (280.15 241.35)kJ/kg (241.35 91.49)kJ/kg β 3.86 State h (kJ/kg) 1 241.35 2s 272.39 2 280.15 3 91.49 4 91.49

2013-3-6 Actual Vapor-Compression Cycle State 1 2s 234 hu回2413522.39280.1591.4991.49 %-低园，- 巾。/m (h-h) %=27239-2413kg- (280.15-241.35kJkg 0.8=80% p-Diagram The pressure-enthalpy(p-/)diagram is a thermodynamic property diagram commonly used in the refrigeration field. ant Co Condense /2s2 Evaporato 9

2013-3-6 9 Actual Vapor-Compression Cycle (d) The isentropic compressor ( ) ( ) ( ) / / 2 1 2 1 c c s c h h h h W m W m s − − = = & & & & η = − − = (280.15 241.35)kJ/kg (272.39 241.35)kJ/kg ηc 0.8 = 80% State h (kJ/kg) 1 241.35 2s 272.39 2 280.15 3 91.49 4 91.49 efficiency is p-h Diagram ►The pressure-enthalpy (p-h) diagram is a thermodynamic property diagram commonly used in the refrigeration field

2013-3-6 Selecting Refrigerants Refrigerant selection is based on several factors: Performance:provides adequate cooling capacity cost-effectively. Safety:avoids hazards(i.e.,toxicity). Environmental impact:minimizes harm to stratospheric ozone layer and reduces negative impact to global climate change. Refrigerant Types and Characteristics Refrigerant Data Including Global Warming Potential (GWP) HFC blend 2282 25 R-407C HFC blend (23/5/52 Weisht 1526 to the atmosphere 10

2013-3-6 10 Selecting Refrigerants ►Refrigerant selection is based on several factors: ►Performance: provides adequate cooling capacity cost-effectively. ►Safety: avoids hazards (i.e., toxicity). ►Environmental impact: minimizes harm to stratospheric ozone layer and reduces negative impact to global climate change. Refrigerant Types and Characteristics Global Warming Potential (GWP) is a simplified index that estimates the potential future influence on global warming associated with different gases when released to the atmosphere