4.(a) State THREE reasons for the operation of a High Pressure Cut-out in a vapour compression refrigeration system.(3)
(b) Describe the operation of the High Pressure Cut-Out, employed in a vapour compression refrigeration system shown in the figure.
(a) THREE reasons for the operation of a High Pressure Cut-out in a vapour compression refrigeration system (3 marks)
The High Pressure Cut-out protects the system against excessive condenser/discharge pressure. It may operate due to:
- Failure or loss of condenser cooling medium (e.g. seawater flow failure, air-cooled condenser fan failure).
- Overcharge of refrigerant → too much liquid in the condenser raising pressure.
- Blocked or fouled condenser tubes → reduced heat transfer surface, causing pressure build-up.
(b) Operation of the High Pressure Cut-Out (7 marks)
Principle of operation (based on the figure):
- The cut-out is a pressure-actuated switch connected to the compressor discharge line.
- Diaphragm/Bellows element:
- Discharge gas pressure acts on the diaphragm/bellows.
- Opposed by a pre-set calibrated spring force (set to the maximum safe pressure).
- Normal operation:
- Pressure remains below the set limit → spring holds contacts in the closed position, allowing compressor motor to run.
- Overpressure condition:
- When discharge pressure rises above the set point, the force on the diaphragm overcomes the spring.
- This movement operates a toggle linkage that trips the electrical contacts.
- Contacts open → compressor motor is stopped, preventing damage.
- Reset:
- Typically requires manual reset (push-button) to restart, ensuring the engineer checks the fault before recommissioning.