Question

Question 2 A vapour compression cycle acts as both a heat source (radiators and a hot water load) and a cooling load (food preservation). The system requires an operating temperature of 50°C to support the bulk of the heating load and -10°C for the cooling load. a) Determine the operating pressures of the cycle in bar. Also, assuming we require a maximum temperature of 90°C to operate certain higher temperature heat processes (for hygiene related reasons), determine the enthalpy at compressor exit (ha), condenser exit (hu), evaporator entry (hu). as well as the dryness fraction of the liquid vapour mixture after the throttle. [5] b) Assuming we apply 20°C of superheat at the evaporator exit determine: i) The ideal enthalpy (h2s) at compressor exit, & the degree of superheat at both compressor exit. (1) ii) The compressor isentropic efficiency. [1] iii) The actual cooling COP & the heating COP. [2] c) Determine the density of the saturated vapour exiting the evaporator, and hence the refrigerant mass flow rate (kg/s) and the compressor volumetric capacity if the design cooling load is 4.42 kW. [3] d) The system operates for a total of 4500 hrs per year. Assuming electricity costs of 0.14 £/kWh and that the carbon intensity of the UK grid is 0.25kg CO2/kWh, estimate: i) The compressor work, plus the annual cooling & heating demand (2) 11) The annual electric consumption of the cycle [1] iii) The total electricity cost, carbon footprint and break even heating cost V's a condensing gas boiler with an efficiency of 90% and a gas cost of 0.036 £/kWh. [3] iv) Calculate the break even COP below which the heating stage of this cycle would have a higher carbon intensity than a condensing gas boiler at 0.19 kg/CO/kWh. [2] [Total marks 20)

Answer 1

it is spoe hoold zol of super PAGE 1 #solution : Con sidding the ondition with pelemetes from the question oc From p-h chalt Pad (-10°c) 2B0 pol (40%) = 1080 Ethalpy et exist of Corprensos logel, لحلم 80C 5467 K Tips condensar exist lo Bel, 4oC kulkg. At Entory of Exposetor hz- hy Asotropic process) X=0.32 from chelt Byness batin heat of compresso inlel. (06) T, = 10 to=10°c TE=100, Tatual

has 450 kilkg =) Tas =696 PAGE 2 hi= zBS 100 67814513 h= lobed 80'c =467 kJ/kg Degree of spel haat it estropy. 20° = 80-40 40C rhi Isstropic efricht- has Manh, 450 - 415 467-415 = 31 hichy 3.02. Cooling Cop GIS-250 Ach, 967-415 462 258 Heating (op = her ha 4.02. hathi 467-915 Chi-hy) © 46 soling lood = 4k wim, 4 0.025 kg's, ma 415 - 258

PAGE 3 At enit of Cusportos Vi=0/m3lkę M ja Desity p=ta zlokg/m² volumetric expecity For saturated repouo his hoskolky vo 4 m ima 405258 -0.00zakgis m' ùa 0.027 26.721 x103 m315 P te 9.8213 Ing. Dog PLEASE CROTE of It is very much necessly formed