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Understanding Defrost Cycles on Refrigerators and Freezers

By Doran Amos, 04 July 2022

Why is defrosting important?

Defrosting is an essential step in maintaining your laboratory refrigerator or freezer to ensure that it continues to provide adequate and stable cooling for your stored items.

Defrosting offers a number of key benefits:

  • It keeps the evaporator coils frost-free, enabling them to cool the interior of the refrigerator or freezer effectively;
  • It maintains good airflow in the refrigerator or freezer, so that the air temperature is stable and uniform throughout the inner compartment;
  • It prevents frost from taking up valuable space within the inner compartment;
  • It stops damage to sensitive items or components by frost.

Defrosting is essential for ensuring that your stored items are properly refrigerated

Depending on the model of refrigerator or freezer, defrosting can be accomplished using either Manual Defrost or Auto Defrost methods, or in some cases, both. 

Put simply, Manual Defrost requires you to empty and switch off the appliance to allow the ice to melt naturally, while Auto Defrost uses an automatically controlled defrost cycle to periodically remove built-up frost. If you’d like to learn more about the differences between Auto and Manual Defrost, check out our in-depth article on this topic.

Auto Defrost offers several potential advantages over Manual Defrost, such as: 

  • Convenience, as there is no need to manually schedule defrosting times, nor to organize how to relocate items temporarily to another refrigerator or freezer;
  • Time-saving, as the frost can removed more quickly than by hand;
  • Protection of sensitive items, as there is no risk of them being exposed to larger variations in temperature during relocation.

In this article, we’ll be looking in detail at what the defrost cycle is, how it’s controlled, and common methods of automatic defrosting.

How does the refrigeration cycle work?

To get started in understanding the defrost cycle, let’s first take a brief look at how a refrigerator or freezer works during its normal refrigeration cycle. Feel free to skip this information and continue to the section on What is the defrost cycle? if you are already familiar with the basics of how refrigeration works.

Evaporator and condenser coils

The basic cooling function of the refrigerator or freezer involves the absorption of heat from the inner compartment by refrigerant fluid flowing through evaporator coils, which is then released to the outside environment as the refrigerant flows through condenser coils (typically mounted on the back or base of the unit). 

As suggested by the names of the two coils, this cooling makes use of the phase transition of the refrigerant from a liquid to gas as it absorbs heat in the evaporator, and from a gas to liquid in the condenser. These phase transitions allow the refrigerant to absorb and then release large amounts of heat, respectively.

Ai DeepFreeze -40°C Upright Medical Freezers use the refrigerant R507, which transitions from liquid to gas above -40°C at 1 atm

Compressor

During active refrigeration, the refrigerant is pumped around the circuit connecting the evaporator and condenser coils by the compressor

As well as pumping the refrigerant, the compressor increases the pressure and temperature of the refrigerant gas arriving from the evaporator before it reaches the condenser. This ensures that the refrigerant in the condenser coils is above room temperature, enabling heat transfer to the surrounding environment.

Expansion valve

Finally, as the cooled refrigerant re-enters the inner compartment of the refrigerator, it passes through an expansion valve, which controls the amount of refrigerant injected into the evaporator coils based on their temperature and pressure. 

As the injected refrigerant rapidly expands into the evaporator coils, it loses pressure, causing its temperature to drop substantially. The rapid cooling of the refrigerant ensures that it is below the target temperature of the inner compartment, allowing it to maintain the cooling of stored items, despite heating from the outside.

Evaporator and condenser fans

Refrigerators and freezers also typically use fans to support refrigeration — the evaporator fan drives air through the compressor and around the inner compartment, improving temperature uniformity within the compartment. Meanwhile, the condenser fan drives airflow through the condenser coils, facilitating heat transfer to the outside environment.

What is the defrost cycle?

During the refrigeration cycle, frost can build up on the evaporator coils within the inner compartment. This can occur even in refrigerators with a target temperature above freezing, as the temperature of the evaporator coils themselves may need to be below 0°C to maintain the temperature of the inner compartment. 

Due to the humidity of the air within the inner compartment, ice crystals can begin to form and accumulate on the evaporator coils — thereby reducing their cooling efficiency, and potentially disrupting the stability and uniformity of the temperature within the inner compartment.

American Biotech Supply Premier Laboratory Refrigerator with Auto Defrost functionality

In appliances with Auto Defrost functionality, the normal refrigeration cycle is temporarily paused and an alternative defrost cycle is automatically activated to reduce ice build-up on the evaporator coils. 

This defrost cycle involves active or passive heating of the evaporator coils to melt the accumulated ice (see the section on Common automatic defrosting methods below for more details). 

The melted ice then drips off the coils and is removed from the inner compartment through a drain line into a drip pan, typically located at the base of the appliance, from which it evaporates into the outside environment.

Defrost cycle control

The defrost cycle used by a particular refrigerator or freezer can be classified based on the criteria that the control board in the appliance uses to determine when to turn the cycle ON or OFF:

  • Time–temperature: The defrost cycle is turned ON based on a timer (e.g. every 6 hours) and turned OFF when a thermostat attached to the evaporator coils signals that a pre-set temperature threshold (e.g. 5°C) has been exceeded;
  • Time–pressure: The defrost cycle is turned ON by a timer, and OFF when a pressure switch signals that the refrigerant pressure in the evaporator coils has dropped below a pre-set value (this approach is equivalent to the temperature threshold, given the relationship between temperature and pressure in the refrigerant);
  • Time–time: The defrost cycle is turned ON and OFF by a timer (e.g. the cycle activates every 6 hours for 30 mins).

In some models of refrigerator or freezer, the defrost cycle can be set to switch ON and OFF using more complex methods of control. For example, Jeio Tech Laboratory Freezers use an innovative auto-defrost method that triggers the defrost cycle based on detecting the amount of ice that has accumulated on the evaporator.

 

FDG/FCG/FMG/MHG Freezer models by Jeio Tech, which detect the amount of accumulated ice to activate the Auto Defrost cycle

Regardless of the precise method of control, these appliances typically have a fail-safe mechanism that terminates the cycle if the normal mechanism fails (e.g. a timer that terminates the cycle after 35–45 mins if the thermostat or pressure switch has not signaled the end of the cycle already).

Common automatic defrosting methods

As described above, Auto Defrost methods may rely on either passive or active heating to remove frost and ice from the evaporator coils. In this section, we discuss three of the most common methods of automatic defrosting — off-cycle defrost, electric defrost, and hot-gas defrost — and their relative pros and cons.

Off-cycle defrost

The off-cycle defrost method involves simply pausing the refrigeration cycle to allow the temperature of the evaporator coils to passively rise, melting any accumulated ice. As this method relies on the air temperature within the inner compartment being above 0°C, it is only possible in refrigerators, not freezers.

During the off-cycle defrost, the compressor is turned off to temporarily stop the flow of refrigerant. In addition, the expansion valve closes to prevent the refrigerant from entering and cooling the evaporator coils, allowing them to defrost naturally.

Meanwhile, the evaporator fan remains on to circulate the warmer air within the inner compartment and defrost the evaporator coils.

This method is slow, but offers the advantage of only leading to small variations in the interior temperature of the refrigerator, making it potentially suitable for refrigerating sensitive samples.

Electric defrost

The electric defrost method is the most common approach to automatic defrosting. It uses an electric heater attached to the evaporator coils to actively melt the ice during a defrost cycle. 

In addition to the compressor being turned off and the expansion valve being closed, the evaporator fan is turned off to limit the increase in air temperature in the inner compartment during the evaporator coil heating.

American Biotech Supply Premier Stainless Steel Pharmacy Freezers use an electric heater to automatically defrost the freezer compartment

As described in the Defrost cycle control section, the heater is then automatically turned off by a timer (e.g. after 30 mins), or once a pre-set temperature or pressure threshold is reached for the evaporator. 

Typically, there is a short delay period after the heater is turned off to allow the water to drip off the evaporator coils into the drain line, ensuring that the ice is not re-frozen on the evaporator once the refrigeration cycle restarts.

Electric defrost is a tried and tested method that is generally affordable and simple to service. However, because of inefficiencies in transferring the heat to the evaporator and the prolonged defrosting times, this approach can lead to spikes in the interior temperature. Therefore, refrigerators or freezers with electric defrost are generally not recommended for storing highly temperature-sensitive samples.

Hot-gas defrost

Finally, rather than defrosting the evaporator using an externally powered heating source (e.g. an electric heater), the hot-gas defrost method uses the heated refrigerant that comes from the compressor itself to defrost the evaporator coils. 

This involves a solenoid valve that redirects the flow of heated, high-pressure refrigerant gas from the compressor via a “shortcut” line that feeds directly into the expansion valve. 

This approach has the advantage of being more energy efficient, as it re-uses existing heat within the system. It also heats the evaporator coils from within, minimizing heat loss to the inner compartment of the refrigerator or freezer that can also cause larger variations in the interior temperature.

In addition, it is faster than other methods of automatic defrosting, with hot-gas defrosting taking approximately 8 min for a refrigerator and 12 min for a freezer, compared with around 30 min using electric (heater) defrosting.

However, the increased complexity of manufacturing and servicing refrigerators and freezers with hot-gas defrost functionality has limited the production and uptake of these appliances.

Conclusion

Defrosting is an important step in ensuring that the contents of your laboratory refrigerator or freezer are properly cooled and protected from temperature variations and frost damage. In this article, we’ve dived into the details of the defrost cycle, how it’s controlled, and common automatic defrosting methods

We hope that this knowledge can help you to better understand how the defrost cycle works, and to make wise purchasing decisions when the time comes to invest in a new refrigerator or freezer! We’re always happy to respond to your questions if you have any further queries about any of the refrigerators or freezers we sell.

How to Defrost a Manual Defrost Freezer

By Aimee O'Driscoll, 08 June 2018

Defrosting a freezer can be a dreaded task in many labs. But if you have a manual defrost freezer, then it’s a job that needs to be done. We explain the steps you can take to make the process as painless as possible.

Read more ...