Refrigeration Cycle Heat flows in direction of decreasing temperature, i.e., from ig-temperature to low temperature regions. Te transfer of eat from a low-temperature to ig-temperature requires a refrigerator and/or eat pump. Refrigerators and eat pumps are essentially te same device; tey only differ in teir objectives. Te performance of refrigerators and eat pumps is expressed in terms of coefficient of performance (): R HP QL W Q W net, in H net, in Te Reversed Carnot Cycle Reversing te Carnot cycle does reverse te directions of eat and work interactions. refrigerator or eat pump tat operates on te reversed Carnot cycle is called a Carnot refrigerator or a Carnot eat pump. Warm medium, T H T Condenser T H T L Turbine Evaporator Compressor s Cold medium, T L Fig. -: T-s diagram and major components for Carnot refrigerator. Te reversed Carnot cycle is te most efficient refrigeration cycle operating between two specified temperature levels. It sets te igest teoretical. Te coefficient of performance for Carnot refrigerators and eat pumps are: Re f, Carnot T H / T L HP, Carnot T / T L H M. arami ENSC 6 (S ) Refrigeration Cycle
Te Carnot cycle cannot be approximated in an actual cycle, because: - executing Carnot cycle requires a compressor tat can andle two-pases - also process - involves expansion of two-pase flow in a turbine. Te Ideal Vapor Compression Refrigeration Cycle Te vapor-compression refrigeration is te most widely used cycle for refrigerators, airconditioners, and eat pumps. Saturated liquid Expansion valve Condenser Supereated vapor = Saturated liquid + vapor Evaporator Saturated vapor Compressor Fig. -: Scematic for ideal vapor-compression refrigeration cycle. ssumptions for ideal vapor-compression cycle: irreversibilities witin te evaporator, condenser and compressor are ignored no frictional pressure drops refrigerant flows at constant pressure troug te two eat excangers (evaporator and condenser) eat losses to te surroundings are ignored compression process is isentropic M. arami ENSC 6 (S ) Refrigeration Cycle
T P q H w in q H w in s q L s q L Fig. -: T-s and P- diagrams for an ideal vapor-compression refrigeration cycle. -: reversible, adiabatic (isentropic) compression of te refrigerant. Te saturated vapor at state is supereated to state. w c = -: n internally, reversible, constant pressure eat rejection in wic te working substance is de-supereated and ten condensed to a saturated liquid at. During tis process, te working substance rejects most of its energy to te condenser cooling water. q H = -: n irreversible trottling process in wic te temperature and pressure decrease at constant entalpy. Te refrigerant enters te evaporator at state as a low-quality saturated mixture. = -: n internally, reversible, constant pressure eat interaction in wic te refrigerant (two-pase mixture) is evaporated to a saturated vapor at state point. Te latent entalpy necessary for evaporation is supplied by te refrigerated space surrounding te evaporator. Te amount of eat transferred to te working fluid in te evaporator is called te refrigeration load. q L = Notes: Te ideal compression refrigeration cycle is not an internally reversible cycle, since it involves trottling wic is an irreversible process. If te expansion valve (trottling device) were replaced by an isentropic turbine, te refrigerant would enter te evaporator at state s. s a result te refrigeration capacity would increase (area under -s) and te net work input would decrease (turbine will M. arami ENSC 6 (S ) Refrigeration Cycle
produce some work). However; replacing te expansion valve by a turbine is not practical due to te added cost and complexity. Te improves by to % for eac C te evaporating temperature is raised or te condensing temperature is lowered. ctual Vapor Compression Refrigeration Cycle T 6 s Fig. -: T-s diagram for actual vapor-compression cycle. Most of te differences between te ideal and te actual cycles are because of te irreversibilities in various components wic are: -In practice, te refrigerant enters te compressor at state, sligtly supereated vapor, instead of saturated vapor in te ideal cycle. - Te suction line (te line connecting te evaporator to te compressor) is very long. Tus pressure drop and eat transfer to te surroundings can be significant, process 6-. - Te compressor is not internally reversible in practice, wic increase entropy. However, using a multi-stage compressor wit intercooler, or cooling te refrigerant during te compression process, will result in lower entropy, state. - In reality, te refrigerant leaves condenser as sub-cooled liquid. Te sub-cooling process is sown by - in Fig. -. Sub-cooling increases te cooling capacity and will prevent entering any vapor (bubbles) to te expansion valve. - Heat rejection and addition in te condenser and evaporator do not occur in constant pressure (and temperature) as a result of pressure drop in te refrigerant. Selecting te Rigt Refrigerant Wen designing a refrigeration system, tere are several refrigerants from wic to coose. Te rigt coice of refrigerant depends on te situation at and. Te most common refrigerants are: R-, R-, R-, R-a, and R-0. R: CCl F diclorofluorometane, used for refrigeration systems at iger temperature levels- typically, water cillers and air conditioning (banned due to ozone layer effects) M. arami ENSC 6 (S ) Refrigeration Cycle
R: CHClF as less clorine, a little better for te environment tan R - used for lower temperature applications Ra: CFH CF tetrafluoretane - no clorine- went into production in 99- replacement for R mmonia NH : corrosive and toxic- used in absorption systems-ceap- ig R7: CO beaves in te supercritical region- low efficiency R90: propane combustible. Many factors need to be considered wen coosing a refrigerant: ozone depletion potential,global warming potential: combustibility: leak detectability: termal factors: termal factors cont d.: CFC (clorofluorocarbons) refrigerants allow more ultraviolet radiation into te eart s atmospere by destroying te protective ozone layer and tus contributing to te greenouse effect tat causes te global warming. s a result te use of some CFCs (e.g. R-, R-, and R-) are banned by international treaties. all ydro-carbon fuels, suc as propane te refrigerant saturated pressure at te evaporator sould be above P atm. - te eat of vaporization of te refrigerant sould be ig. Te iger fg, te greater te refrigerating effect per kg of fluid circulated. - te specific eat of te refrigerant sould be low. Te lower te specific eat, te less eat it will pick up for a given cange in temperature during te trottling or in flow troug te piping, and consequently te greater te refrigerating effect per kg of refrigerant - te specific volume of te refrigerant sould be low to minimize te work required per kg of refrigerant circulated since evaporation and condenser temperatures are fixed by te temperatures of te surroundings - selection is based on operating pressures in te evaporator and te condenser. Oter desirable caracteristics of refrigerant are: non-flammable, being cemically stable, and be available at low cost. M. arami ENSC 6 (S ) Refrigeration Cycle
Cascade Refrigeration Cycle Systems tat ave (or more) refrigeration cycles operating in series. T H Expansion valve Condenser 7 6 8 Evaporator Compressor Expansion valve Condenser Heat excanger Compressor Evaporator T L Fig.-: -stage cascade refrigeration cycle. Cascade cycle is used were a very wide range of temperature between T L and T H is required. s sown in Fig. -, te condenser for te low temperature refrigerator is used as te evaporator for te ig temperature refrigerator. Cascading improves te of a refrigeration cycle. Moreover, te refrigerants can be selected to ave reasonable evaporator and condenser pressures in te two or more temperature ranges. M. arami ENSC 6 (S ) Refrigeration Cycle 6
T 7 6 Decrease in compressor work input 8 Increase in refrigeration capacity 7 s Fig. -6: T-s diagram for -stage cascade system. Te two cycles are connected troug te eat excanger in te middle, wic serves as evaporator (cycle ) and condenser (cycle ). One can write: m 8 m m R, cascade m m 6 Figure -6 sows te increase in refrigeration capacity (area under -7 ) and decrease in compressor work (- -6-). Example -: two-stage Refrigeration cycle Consider a two-stage cascade refrigeration system operating between pressure limits of 0.8 and 0. MPa. Eac stage operates on an ideal vapor-compression refrigeration cycle wit refrigerant R-a as working fluid. Heat rejection from te lower cycle to te upper cycle takes place in an adiabatic counter flow eat excanger were bot streams enter at about 0. MPa. If te mass flow rate of te refrigerant troug te upper cycle is 0.0 kg/s, determine: a) te mass flow rate of te refrigerant troug te lower cycle b) te rate of eat removal from te refrigerated space and te power input to te compressor c). ssumptions: M. arami ENSC 6 (S ) Refrigeration Cycle 7
) Steady operation ) KE= PE=0 ) diabatic eat excanger. Figure -6 sows te T-s diagram for te cascade cycle. Entalpies for all 8 states of refrigerant R-a can be read off Tables - and -. Writing energy balance for te eat excanger, te mass flow rate of te refrigerant troug te lower cycle can be found: m m m 0.0 m m 8 m kg / s.88 9.7kJ / kg m.9.6 0.09kg / s 8 m kj / kg b) te eat removal by te cascade cycle can be determined from: Q W L in m W 0.090kg / s9.6.6kj / kg 7.8 Comp, in W Comp, in m m.6kw c) te of te cycle will be: 6 kw R Q W L net, in 7.8kW.6kW.6 M. arami ENSC 6 (S ) Refrigeration Cycle 8