Refrigeration CyclesRefrigeration Cycles

Power UsersPower Users

Refrigerators and Heat PumpsRefrigerators and Heat Pumps

Refrigerators: Refrigerators: move heat move heat fromfrom colder spacecolder space

Heat pump: move Heat pump: move heat heat toto warmer warmer spacespace

Both require work Both require work inputinput

Refrigerators and Heat PumpsRefrigerators and Heat Pumps

Performance is Performance is expressed in term expressed in term of coefficients of of coefficients of performanceperformance

Carnot Cycle ModelCarnot Cycle Model

Reverse Carnot CycleReverse Carnot Cycle

Reverse Carnot CycleReverse Carnot Cycle

Coefficients of performance for Coefficients of performance for Carnot cycles can be expressed in Carnot cycles can be expressed in terms of temperature terms of temperature

Ideal Vapor-Compression Ideal Vapor-Compression Refrigeration CycleRefrigeration Cycle

1-2 Isentropic 1-2 Isentropic compression in compression in compressorcompressor

2-3 Constant-pressure 2-3 Constant-pressure heat rejection in a heat rejection in a condensercondenser

3-4 Throttling in an 3-4 Throttling in an expansion valve expansion valve (irreversible process)(irreversible process)

4-1 Constant-pressure 4-1 Constant-pressure heat absorption in an heat absorption in an evaporatorevaporator

Ideal Vapor-Compression Ideal Vapor-Compression Refrigeration Cycle AnalysisRefrigeration Cycle Analysis

Steady-flowSteady-flow

Where hWhere h11 = h = hg@Pg@P11 and h and h33 = h = hf@Pf@P33

Actual Vapor-Compression CycleActual Vapor-Compression Cycle

DifferencesDifferences Evaporator Evaporator

dischargedischarge Higher specific Higher specific

volume in volume in compressorcompressor

Compressor lossesCompressor losses Condenser pressure Condenser pressure

losseslosses

What Refrigerant?What Refrigerant?

Types:Types: Chlorofluorocarbons (CFCs)Chlorofluorocarbons (CFCs) AmmoniaAmmonia Hydrocarbons (propane, ethane, Hydrocarbons (propane, ethane,

ethylene, etc.)ethylene, etc.) Carbon dioxideCarbon dioxide

What Refrigerant?What Refrigerant?

Driving criteria:Driving criteria: TemperaturesTemperatures of of

where heat is where heat is removed removed fromfrom and and where it is moved where it is moved toto..

Heat PumpsHeat Pumps

Reverse of Reverse of refrigeratorsrefrigerators

Bring heat from Bring heat from lower temperature lower temperature space to higher space to higher temperature space.temperature space.

Energy sourceEnergy source AirAir Water/soilWater/soil

Gas Refrigeration CycleGas Refrigeration Cycle

Gas Refrigeration CycleGas Refrigeration Cycle

Fluid always a gas, no phase changeFluid always a gas, no phase change

Lower COP than vapor-compression Lower COP than vapor-compression systemssystems

Used in aircraft and gas liquefactionUsed in aircraft and gas liquefaction