- Question 4 Part A 20 Marks A Refrigerator Operates Using R134a The Evaporator And Condenser Pressures Are 2 And 8 Bar 1 (38.53 KiB) Viewed 43 times
Question 4 Part A: 20 marks A refrigerator operates using R134a. The evaporator and condenser pressures are 2 and 8 bar respectively. A PH diagram is enclosed on the next page a copy or sketch of which should be included with the hand in i) Draw the ideal cycle for the system on the chart provided (next page), and determine the enthalpy at the 4 main points of the cycle, as well as the dryness fraction at the throttle valve exit [7] In practice, due to additional heat gain from surroundings, the gas exits the compressor at a pressure of 8 bar and a temperature of 50°C. The compressor volumetric capacity remains unchanged. Determine: ii) The enthalpy at compressor outlet, and the degree of superheat. [3] ii) The compressor isentropic efficiency and plant CoP [6] iv) Determine the density of the working fluid at evaporator exit. If the required cooling load is 5 kW's, what is the required compressor capacity (in m®/h) and the mass flowrate (in kg/s) to achieve this desired cooling load? [4]
150 200 250 300 350 400 450 100 20 500 550 600 02 R-134a 10 B 10 8 we 6 6 200 1000 4 4 OM 2 09 2 0.8 0.8 1 0.8 0.8 PRESSURE, MPa 0.4 0.4 0.2 NE LLLLLL ANY 0.2 ER SEIRO YEN 89999999 0.1 0.1 0.08 0.06 0.06 0.04 0.04 ఆయన 0.02 0.02 0.40 100 200 250 450 100 150 Pers.com NSTRE PROP 0.01 500 550 600 Betonomia e Tirane 300 350 400 ENTHALPY, KJ/kg Figure 04.1 8
Part B: 10 marks A Roof mounted solar panel with a surface area of dimensions 5m long and 1m wide is mounted horizontally on a flat roof. It is heated by the sun to a maximum mid-day temperature of 88°C, while the ambient air temperature is only 25°C. Determine the Nusselt number, Rayleigh number and hence the heat transfer co-efficent and total rate of heat transfer from the top of the PV panel at mid-day. You may use the properties below (table 4-1) and the Ra & Nu correlations (table 4-2): Table 4-1: Properties of air at the film temperature Pr ср kg/m3 kJ/kg.K N.s/m2 1.1509 1.007 1.86E-OS v m2/s 1.62E-OS W/m.K 0.0265 m2/s 2.30E-05 0.71 0019 020-201 Table 4-2: Correlations for natural convection Empirical correlations for the average Nusselt number for natural convection over surfaces Geometry length Range of Ra 10410 Na 0.5 10-10 NOIR Na 0.Sara - 0225.10.2017 comple but more accurate Uewertical plate for the per surface of a cold plate and the low surface of a heale Replace to Ra<10 C20-21) Herwall Send pl por surface of the place கோகோ காவி part 104-10 101-10 N0.54 Ne 0.15R! C20-22) (20-231 A. webeples forppare o aceplate 104-10 N0.27 (20-24) Verticale A vertical cylinder can be treated a vertical plate when IU Total Marks [30] END OF EXAM PAPER