Aux 2 Unit 8 Refrigeration General overview Q2 Answer

  1. With reference to refrigeration systems:
    (a) state the THREE basic laws of refrigeration;(6)
    (b) state the location and method of re-setting of EACH of the following:
    (i) the High Pressure cut out;(2)
    (ii) the Low Pressure cut out.(2)

(a) The THREE Basic Laws of Refrigeration (6 marks)

  1. Heat flows from a hotter body to a colder body
    • Heat transfer is always from high temperature to low temperature regions, never the reverse without external work.
  2. Heat can only be transferred from a colder body to a hotter body by doing work
    • This is the principle of the vapour compression cycle: the compressor does the work to move heat “uphill.”
  3. The refrigerating effect is produced by the evaporation of a liquid at low pressure and temperature
    • When a liquid evaporates, it absorbs latent heat from its surroundings, creating cooling.

(b) Pressure Cut-outs

(i) High Pressure Cut-out (2 marks)

  • Location: On the compressor discharge line or condenser outlet side, where discharge pressure is monitored.
  • Method of Re-setting:
    • Usually a manual reset push button on the control panel or directly on the cut-out unit.
    • This ensures the operator investigates the cause (e.g. cooling water failure, blocked condenser) before restart.

(ii) Low Pressure Cut-out (2 marks)

  • Location: On the compressor suction line, sensing the evaporator pressure.
  • Method of Re-setting:
    • Normally automatic reset once suction pressure rises back to a safe level.
    • Some designs may also incorporate a manual reset for safety.