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ENGR5003 / Week 3
ENGR266 2022 Q2Past paperOld spec ENGR2662:118 marks22 min

ENGR266 Summer 2022 Q2[PARTIAL] Left out: Parts (a) and (c) rely on the vapour-compression refrigeration cycle and its COP, not in the current slides. Part (b), quality after throttling from R134a tables, is on-syllabus (Week 2 throttling, Week 3 tables).

Answer All parts (a) - (c).

A refrigerator employs the vapour-compression cycle with HFC-134a as its working fluid. The refrigerator maintains the inside at a temperature of 4∘C4^\circ\mathrm{C} and is situated in a kitchen at 20∘C20^\circ\mathrm{C}.

Formulas you may need
  • SFEE per unit mass for each component: q−w=hout−hinq - w = h_{out} - h_{in} (on the formula sheet)
  • Throttle: h4=h3h_4 = h_3 (from the SFEE, on the formula sheet)
  • Refrigerator COP: COPR=h1−h4winCOP_R = \dfrac{h_1 - h_4}{w_{in}} (learn this)
  • Wet-mixture properties: v=(1−x)vf+xvgv = (1 - x)v_f + x v_g, so x=v−vfvg−vfx = \dfrac{v - v_f}{v_g - v_f} (on the formula sheet)
  • Refrigerant mass flow: m˙=Q˙Lh1−h4\dot m = \dfrac{\dot Q_L}{h_1 - h_4} (learn this)
  • Carnot refrigerator COP: COP=TLTH−TLCOP = \dfrac{T_L}{T_H - T_L} (learn this)
  1. (a)
    Calculate the Coefficient of Performance for this refrigerator if the compressor does 40 kJ kg−140\ \mathrm{kJ\,kg^{-1}} of work.
    [4]2:2
  2. (b)
    Calculate the vapour fraction of HFC-134a if the volume after the throttling process is 0.01 m3 kg−10.01\ \mathrm{m^3\,kg^{-1}}.
    [4]2:2
  3. (c)
    Discuss with the help of a P-H diagram what will happen to the coefficient of performance and the required flow rate of the working fluid if the temperature of the kitchen increases.
    [10]