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ENGR5003 / Week 3
ENGR266 2021 Q2Past paperOld spec ENGR2662:215 marks18 min

ENGR266 Summer 2021 Q2[PARTIAL] Left out: The vapour-compression refrigeration cycle and its COP are not in the current slides; the component analysis (R134a tables, isentropic compressor, throttling, evaporator energy balance) is on-syllabus. Part (d) on refrigerant choice is background knowledge.

Answer All parts (a) - (d). You will need HFC-134a (R134a) property tables.

Formulas you may need
  • SFEE per unit mass (each component, ΔKE\Delta KE, ΔPE\Delta PE neglected): q−w=hout−hinq - w = h_{out} - h_{in} (on the formula sheet)
  • Throttle valve (adiabatic, no work): h4=h3h_4 = h_3 (from the SFEE, on the formula sheet)
  • Refrigerator COP: COPR=qLwin=h1−h4h2−h1COP_R = \dfrac{q_L}{w_{in}} = \dfrac{h_1 - h_4}{h_2 - h_1} (learn this)
  • Compressor isentropic efficiency: ηc=h2s−h1h2−h1\eta_c = \dfrac{h_{2s} - h_1}{h_2 - h_1} (learn this)
  • Linear interpolation in the R134a tables (learn this; tables provided)
  1. (a)
    A refrigerator employing the vapour-compression cycle operates with a condenser at 750 kPa and an evaporator at 200 kPa. Assuming that HFC-134a is the working fluid and that the compressor operates isentropically calculate the coefficient of performance.
    [6]
  2. (b)
    For the same refrigeration cycle as in (a) calculate the work done by the compressor and the coefficient of performance if the compressor does not operate isentropically and is only 80% efficient.
    [3]
  3. (c)
    Calculate the rate of heat extraction from the evaporator if the mass flow rate of HFC-134a is 0.4 kg s−10.4\ \mathrm{kg\,s^{-1}}.
    [2]Third
  4. (d)
    Discuss how the choice of coolant may affect operation of the refrigerator and what other factors might limit choice of coolant?
    [4]