Hotel Service Unit 11 Refrigeration Q5 – Answer

  1. With reference to refrigeration systems:
    (a) state the THREE basic principles of refrigeration;(3)
    (b) sketch a direct expansion free standing mechanical refrigeration system, labelling the MAIN components and indicating where EACH of the three principles stated in part (a) occurs.(7)

(a) Three Basic Principles of Refrigeration (3 marks)

  1. Evaporation: A liquid absorbs heat when it evaporates.
  2. Compression: Gas temperature and pressure rise when compressed.
  3. Condensation/Expansion: A vapour gives up heat when condensed back to liquid, and liquid temperature falls when expanded.

(b) Direct Expansion Mechanical Refrigeration System (7 marks)

Description for sketch (what to draw & label):

  1. Compressor – draws low-pressure refrigerant vapour from the evaporator and compresses it → high pressure, high temperature gas.
    • Principle: Compression.
  2. Condenser – rejects heat to sea water or air, cooling refrigerant → condenses into high-pressure liquid.
    • Principle: Condensation.
  3. Receiver – stores high-pressure liquid refrigerant before expansion.
  4. Expansion Valve (TXV or capillary tube) – throttles liquid refrigerant, lowering pressure and temperature.
    • Principle: Expansion.
  5. Evaporator – low-pressure refrigerant absorbs heat from the space/cargo, evaporating to vapour.
    • Principle: Evaporation.
  6. Back to Compressor – cycle repeats.

Sketch should show (with arrows for flow):

  • Compressor → Condenser → Receiver → Expansion Valve → Evaporator → back to Compressor.
  • Mark where Evaporation, Compression, Condensation/Expansion occur.