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Q2 (16 Marks) Auxiliary Machinery 🔥 Repeated 6x in exams
MEKG • Written Exam

(a) Sketch and describe a high pressure cut-out in a refrigeration system. (6)

(b) The refrigeration compressor has stopped due to operation of the h. p. cut-out. Explain:

(i) The possible causes. (3)

(ii) How these causes would be found and possible remedies. (3)

(c) What steps are taken if the compressor "short-cycle" on low pressure cut-out? (4)

Appeared In: Jul 2026Feb 2026Jul 2025Feb 2024Jul 2019Apr 2019

✓ Verified Model Answer (Text Solution)

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Part (a)

A high-pressure cut-out in a refrigeration system is a safety device that protects the system from operating at dangerously high pressures. It consists of a bellows connected to the compressor discharge, a spring, an adjustment screw, and a switch arm. Under normal conditions, the switch arm is held up, maintaining electrical contact. When pressure exceeds the set limit, the bellows expands, releasing the switch arm, and the compressor cuts out, preventing further damage. The cut-out needs manual reset after troubleshooting and pressure returns to safe levels. It ensures system safety and prevents over-pressurization risks.

Part (b)

(i) The possible cause of HP cut out could be due to:

  • Dirty condenser
  • Overcharge of refrigerant
  • Condenser coolant failure
  • Clogged filter drier
  • Malfunctioning expansion valve
  • Faulty pressure switch

(ii)

  • Dirty condenser - Visual inspection of condenser, clean the condenser
  • Overcharge of refrigerant - check the refrigerant level in sight glass, reduce the refrigerant charge.
  • Condenser coolant failure - check in/out pressures, clean the condenser.
  • Clogged filter drier - visual inspection of drier, change the drier
  • Malfunctioning expansion valve - inspect expansion valve, repair or replace the valve
  • Faulty pressure switch - inspect the switch, repair or replace the pressure switch
Part (c)

The steps are taken if the compressor "short-cycle" on low pressure cut-out are:

  • To provide sufficient suction pressure control difference according to the system loading and frequency of room inspection
  • Refrigerant charges should be adequate, the system should be without leaks. The suction line filter is to be kept clean with no obstruction in suction line.
  • The leaky solenoid valve is to be replaced. The evaporator coil is to be defrosted regularly and ensure the inner surface is clean.
  • Piston rings, cylinder liner, discharge valve, by-pass valve and safety valve are to be maintained in good condition. Compressor capacity is to be selected according to the system requirement and nature of loading.
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