Question

Problem #1 (40 Marks) A refrigeration system is designed to maintain a cool space at -20°C in arn environment of +50°C. The wide temperature range is difficult to achieve with a single refrigerant, because the refrigerant will generally be at excessively high pressures at the top of the temperature range, and sub-atmospheric pressures at the bottom end The problem is solved with a 2-stage cascaded system that uses refrigerant 134a (R134a) and refrigerant 22 (R22) respectively for the high temperature and low temperature parts of the process -with an intermediate heat exchanger that transfers heat from the condenser of the bottom cycle to the evaporator of the top cycle. The two refrigeration cycles are both standard vapour-compression cycles with the compressor inlet state a saturated vapour, and state 3 a saturated liquid. Both compressors have isentropic efficiencies of nc0.84 The saturation temperatures of the two cycles are given in the table below Lo R134a60°C 0°C +10°C30°C R22 Note that a minimum 10°C difference is maintained between the refrigeration system and the external physical systems it is interacting with. This difference is also maintained in the heat exchanger between the two cycles. a) b) Represent the top cycle only on a T-s diagram. Determine the temperature, pressure and specific enthalpy of each of the eight states and summarise them in table form. c) If the mass flow rate in the top cycle is m1340.4 kg/s, calculate the mass flow rate in the bottom cycle, assuming the intermediate heat exchanger is adiabatic d) Determine all the relevant heat flow rates in this system, and the electrical power inputs to the two compressors. Determine the coefficient of performance (COP) for a Carnot cycle operating between these two environmental temperatures. Determine the COP for this system, and identify the reasons for the differences between the values. e)

Answer 1

су R 22-c 10°C Ct 273+60

Now 万-T, っ 339.oss-2 73 ·o.ry T-2 3 At30 C

296-o36 K = 2 3636. C T 276 036-(273-30 -(273-30 306.13大 3 excharg bek R-22 and eraporatr of R-131a. u 306.13- 23

Power n one -134 се 8D 5 372 63 hs -A (e) (Co P 392.63-21( 89 )χ о.У 29 6IS ン2-4416