Question

For a fully automatic provisions refrigeration system incorporating a number of rooms:

(a) Explain how each room temperature is set. (4)

(b) Describe the sequence of events following a demand for increased refrigerant flow from one room. (4)

(c) State with reasons the devices incorporated into the system to protect the machinery and equipment against malfunction. (4)

(d) State how satisfactory operation of the plant can be established? (4)

Answer 1

Part (a)

Each refrigerated room has its dedicated:

Solenoid valve
Thermostatic expansion valve (TEV)
Evaporator coil
Thermostat

For example:

Meat Room: -16°C to -11°C
Vegetable Room: +5°C

When the temperature in a room rises above its set point, the thermostat senses the change and opens the solenoid valve. This allows refrigerant to flow to the evaporator via the TEV, cooling the room. Once the room's desired temperature is achieved, the thermostat shuts the solenoid valve, stopping refrigerant flow.

The compressor operates based on the overall demand across all rooms. It will run when at least one solenoid valve is open and cut off when all are closed, reducing suction pressure.

Part (b)

Sequence of events following increased refrigerant demand in a room:

The thermostat detects an increase in room temperature and energizes the solenoid valve for that specific room.
The solenoid valve opens, allowing refrigerant to flow to the evaporator via the TEV, which reduces the refrigerant's pressure and temperature.
The refrigerant absorbs heat from the room, evaporating as it passes through the evaporator coil.
As refrigerant vapor returns to the compressor, suction pressure increases.
The LP cut-out resets, and the compressor starts, compressing the refrigerant into a high-pressure, high-temperature gas.
The refrigerant is condensed back to a liquid and recirculated. The cycle continues until the desired temperature is achieved, at which point the thermostat closes the solenoid valve.

Back pressure valves are installed in higher-temperature rooms (e.g., vegetable room) to prioritize refrigerant flow to colder rooms during heavy cooling demands.

Part (c)

Safety Devices incorporated in refrigeration system:

High Pressure (HP) Cut-out:

Trips the compressor when discharge pressure exceeds safe limits, protecting the system from overpressure.
A manual reset is required.

Oil Differential Pressure Cut-out:

Cuts off the compressor if oil pressure drops below the safe differential.
Prevents damage due to inadequate lubrication.

Low Pressure (LP) Cut-out:

Stops the compressor if suction pressure falls too low, preventing operation under low refrigerant conditions.
Automatic reset.

Condenser Relief Valve:

Relieves pressure from the condenser to prevent rupture or damage.

Safety Head:

Lifts if liquid refrigerant enters the compressor to prevent mechanical damage.

Oil Heater:

Prevents crankcase oil from becoming excessively cold and losing viscosity.

Part (d)

Ensuring satisfactory operation of the plant:

Monitor running parameters daily.
Perform maintenance as per the manufacturer’s guidelines.
Test HP, LP, and oil differential cut-outs at regular intervals.
Clean condenser coils and renew silica gel periodically.
Check for correct oil levels.
Conduct frequent checks to identify and rectify refrigerant leaks.
Ensure proper defrosting of ice buildup on evaporator coils to maintain efficiency.
Overhaul major components as recommended by the manufacturer to ensure reliability.
Maintain proper logs and follow standard operating procedures for refrigeration system operation.

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