Refrigeration Cycle Reading Problems - -7, -9 -, -46, -49, -03 Definitions the st law of thermodynamics tells us that heat flow occurs from a hot source to a cooler sink, therefore, energy in the form of work must be added to the process to get heat to flow from a low temperature region to a hot temperature region. refrigeration cycles may be classified as vapour compression gas compression refrigerators and heat pumps have a great deal in common. The primary difference is in the manner in which heat is utilized. Refrigerator C }{{} takes heat from H }{{} transfers to Heat Pump C }{{} takes heat from H }{{} transfers to
The coefficient of performance (COP) is given by COP benefit cost where the benefit for a refrigeration process is the cooling load given as Q L. This is the net benefit, i.e. heat is removed from the cold space. For a heat pump, the benefit is the heat added to the hot space, i.e. Q H. COP refrig Q L W in Q L Q H Q L Q H Q L T H (s 4 s ) T L (s 3 s 2 ) T H T L T L T H T L COP heat pump Q H W in Q H Q H Q L Q L Q H T L T H T H T H T L 2
Vapour Compression Refrigeration Cycle Room Air sat. liquid Q H Condenser superheated vapour Expansion Valve h 4 h 3 compressor gas Evaporator 2 phase sat. vapour Q Food L 3
Refrigeration Process Process -2s: 2s-3: Description A reversible, adiabatic (isentropic) compression of the refrigerant. The saturated vapour at state is superheated to state 2. w c h 2s h An internally, reversible, constant pressure heat rejection in which the working substance is desuperheated and then condensed to a saturated liquid at 3. During his process, the working substance rejects most of its energy to the condenser cooling water. q H h 2s h 3 3-4 An irreversible throttling process in which the temperature and pressure decrease at constant enthalpy. h 3 h 4 4- An internally, reversible, constant pressure heat interaction in which the working fluid is evaporated to a saturated vapour at state point. The latent enthalpy necessary for evaporation is supplied by the refrigerated space surrounding the evaporator. The amount of heat transferred to the working fluid in the evaporator is called the refrigeration load. q L h h 4 4
Common Refrigerants There are several fluorocarbon refrigerants that have been developed for use in VCRC. R R2 CCl 2 F 2 dichlorofluoromethane - used for refrigeration systems at higher temperature levels - typically, water chillers and air conditioning R22 CHClF 2 has less chlorine, a little better for the environment than R2 - used for lower temperature applications R34a CF H 2 CF 3 tetrafluorethane - no chlorine - went into production in 99 - replacement for R2 R4b C 2 H 3 F Cl 2 dichlorofluoroethane Ammonia NH 3 corrosive and toxic - used in absorption systems R744 CO 2 behaves in the supercritical region - low efficiency R290 propane combustible 5
Designation Chemical Ozone Depletion Global Warming Formula Potential Potential 2 Ozone Depleting & Global Warming Chemicals CFC- CCl 3 F 3,400 CFC-2 CCl 2 F 2 0.89 7,00 CFC-3 CClF 3 3,000 CFC-3 C 2 F 3 Cl 3 0.8 4,500 CFC-4 C 2 F 4 Cl 2 0.69 7,000 CFC-5 C 2 F 5 Cl 0.32 7,000 Halon-2 CF 2 ClBr 2.2-3.5 Halon-30 CF 3 Br 8-6 4,900 Halon-2402 C 2 F 4 Br 2 5-6.2 carbon tetrachloride CCl 4.3,300 methyl chloroform CH 3 CCl 3 0.4 nitrous oxide N 2 O 270 Ozone Depleting & Global Warming Chemicals - Class 2 HCFC-22 CHF 2 Cl 0.048,600 HCFC-23 C 2 HF 3 Cl 2 0.07 90 HCFC-24 C 2 HF 4 Cl 0.09 440 HCFC-25 C 2 HF 5 0.000 3,400 HCFC-4b C 2 H 3 F Cl 2 0.090 580 HCFC-42b C 2 H 3 F 2 Cl 0.054 800 Global Warming, non-ozone Depleting Chemicals carbon dioxide CO 2 0 methane CH 4 0 HFC-25 CHF 2 CF 3 0 90 HFC-34a CF H 2 CF 3 0,000 HFC-52a CH 3 CHF 2 0 2,400 perfluorobutane C 4 F 0 0 5,500 perfluoropentane C 5 F 2 0 5,500 perfluorohexane C 6 F 4 0 5,00 perfluorotributylamine N(C 4 F 9 ) 3 0 4,300 - relative to R 2 - relative to CO 2 6
Cascade Refrigeration System two or more vapour compression refrigeration cycles are combined used where a very wide range of temperature between T L and T H is required Advantages the refrigerants can be selected to have reasonable evaporator and condenser pressures in the two or more temperature ranges COP Q L( ) W net ( ) overall( ) 7
Absorption Refrigeration System Differences between an absorption refrigeration system and a VCRC VCRC vapour is compressed between the evaporator and the condenser process is driven by work Absorption RS the refrigerant is absorbed by an absorbent material to form a liquid solution heat is added to the process to retrieve the refrigerant vapour from the liquid solution process is driven by heat 8
Process liquid ammonia Room Air Q H Condenser ammonia vapour only Source of Heat Generator Q * H Expansion Valve weak ammonia water solution cold regenerator Evaporator Absorber 2 phase dry vapour QL Q* L Food strong ammonia cold water solution sink pump 9