Why do we need a defrost cycle?

As we know, air contains moisture (humidity) and whenever someone enters the freezer room, the humidity in the air will start to collect and freeze on the evaporator coil. Moisture in food or other products can also contribute to this. The defrost cycle is important for the freezer room to prevent the evaporator coil from freezing into a solid block of ice. If this were to happen, then it prevents airflow from moving across the coil and heat will not be added to the refrigerant to be able to boil it off into a vapour as it leaves the evaporator coil. The main risk when this happens, is that liquid refrigerant can travel down the suction line and therefore cause damage to the compressor. The heat exchanger also loses cooling capacity and the room can struggle to get down to temperature.

Its worth noting that the defrost cycle is only required on freezers and not in coolrooms. This is because the refrigerant temperature in the evaporator of a freezer is -25’C, whereas in a coolroom it is much warmer at only -5’C.

How does the electric defrost cycle work?

Normal Cooling Operation

Basically, the room thermostat controls the liquid line solenoid, so that when the room temperature reaches setpoint, the solenoid valve closes and the system pumps down until the compressor cycles off on its low pressure switch. This is called the ‘pumpdown cycle’. Its worth remembering that we can absorb 31 times more heat into a liquid, than we can into a vapour, and hence why we flood the evaporator coil with liquid refrigerant to make it as efficient as possible during normal cooling operation.

Defrost Cycle

So when it comes to the defrost cycle, its important that we pump all the liquid refrigerant out of the evaporator before switching on the electric defrost heaters. Otherwise, we would need to use 31 times more heat to boil off the liquid refrigerant before we could even start to melt the ice. Switching on the defrost heaters when the evaporator coil is full of liquid is not only inefficient and consumes more electrical energy, it will also cause high suction pressures when the compressor is called to restart again.

So the correct operation of the defrost cycle is that first, the unit pumps down and clears all the liquid refrigerant out of the evaporator coil. Then the defrost heaters come on to defrost the coil. We usually request the defrost cycle to start 4 times per day ie every 6 hours and it will generally be switched off (terminated) by one of two methods.

Timed on/Timed Off – This uses a timeclock (analogue or digital type available) to start and also stop the defrost cycle

Timed On/Temperature off – This uses a timeclock to start the defrost cycle, but then has a temperature sensor pushed into the evaporator coil to stop the cycle when the coil temperature reaches +8’C.

Types of defrost

Off cycle – Simply turns the compressor off and lets the frost on the coil melt naturally.

Electric Defrost – Electrical heating elements are used to melt any ice build up on the coil.

Hot Gas Defrost – Uses hot gas from the high side of the system to defrost the evaporator coil

Defrost settings to check

Below are some of the important settings to check on the controller:

Frequency: Every 6 Hours

A defrost cycle should be run every 6 hours

Duration: 30 Minutes

If there is no coil temperature sensor, then the duration should be set to 30 minutes

Temperature Off: +8’C

This is the temperature of the evaporator coil at which the defrost cycle should be stopped.

Drip Time: 2 Minutes

This allows for 2 minutes drip time so that any moisture that is on the coil will be allowed to drip off before the refrigeration cycle starts again.

Fan operation during defrost: Off

The fans should be off during the defrost to prevent moisture being blown into the freezer room and freezing on the walls or ceiling.

Fan off time after defrost: 1 Minute

The fans should remain off when the refrigeration cycle starts again as this will allow the coil to cool down before air is circulated throughout the room.

Troubleshooting frozen evaporator coil faults

• TXV Freezes up aswell as the top row of the coil but not the rest of the coil – This is usually caused by system under charge. ie the liquid line isnt 100% full of liquid refrigerant and causes a lower pressure at the valve. This causes it to freeze up and essentially starve the evaporator. Other causes include: pressure drop in the liquid line, over condensing, incorrect liquid line pipe sizing, blocked filter drier.
• Evaporator Fan not running – Fan motor seized or electrical failure, no run signal from controller
• Blocked Evaporator Coil – Dirty coil, plastic or rubbish blocking the coil
• Evaporator Pressure Regulator (EPR Valve) set incorrectly on coolrooms (refrigerant is too cold and evaporating at a low temperature)
• Not enough defrosts – Ice build up becomes too thick and doesnt defrost completely before restarting the system (timed off defrosts)
• Duration of defrost not long enough – System restarts before the coil has fully defrosted
• Defrost termination sensor incorrectly located in coil – Poor location of the sensor means that the coil doesn’t defrost fully