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Differences Between Auto and Manual Defrost and Why They Are Important

By Aimee O'Driscoll, 23 August 2018

Laboratory freezers come with auto- or manual-defrost functions. Some manufacturers let you choose which system you’d prefer in certain models while others even come with both systems in a single unit. Depending on the stability needs of your samples, one type of defrost might be a better fit for your lab than the other.

In this post, we explore the practical differences between the two and why they’re important, to help you with your decision.

Auto-Defrost Freezers

As its name suggests, an auto-defrost freezer defrosts automatically. Auto-defrost freezers contain evaporating coils that are located in the ceiling and have fans that circulate cold air when the freezer is closed.

These fans allow for an even air flow and provide consistent cooling all throughout the freezer. They shut off as soon as the door opens, limiting the circulation of warm, outside air.

 

Automatic defrost freezers.

The 24" All Freezer on the left has an automatic defrost system. With the Premier Countertop Freestanding Freezer on the right, you can choose which system (auto- or manual-defrost) you would like.

The cold air that is circulated has a lower humidity level causing all ice or condensation that forms to evaporate and deposit on the coils. At regular intervals -- or when frost is detected -- the evaporating coils are heated. This heating melts and evaporates frost, thus defrosting the freezer.

During the defrost cycle, the temperature in the freezer fluctuates slightly due to the fact that the cooling coils are heated. The fans are shut off when the door is opened and re-activation of the fans is delayed after defrosting, to prevent the circulation of warm air. You can also minimize temperature fluctuations by keeping the freezer filled with your material and limiting the number of door openings.

Practical Advantages of Auto-Defrost Freezers

  • These units require less maintenance than a manual defrost freezer. Defrosting happens automatically.
  • An auto-defrost freezer will have a more uniform temperature than a manual-defrost freezer due to the forced circulation of air by the internal fans.
  • Samples can be stored inside the freezer during defrosting, so you  don’t need to expose samples to temperature deviations upon transfer.

The major drawback of an automatic defrost freezer is that a small increase in temperature cannot be avoided during the defrost cycle. Therefore, these units may not suitable for critical temperature storage applications, such as housing vaccines. Additionally, samples of very small volume will be most sensitive to temperature spikes, so such samples may be better off stored in a manual-defrost freezer.

Manual-Defrost Freezers

Manual-defrost freezers cool by circulating refrigerant through coils in the walls (and sometimes in the shelves as well). The internal temperature differential causes the cold air to circulate, resulting in a more constant internal temperature as compared to an auto-defrost freezer. Although, places closer to the wall and towards the bottom can be slightly colder than the rest of the freezer.

 

Manual defrost freezers.
A Premier Manual Defrost Freezer by American BioTech Supply and a Standard Manual Defrost Chest Freezer.

When condensation and ice build up on the walls, they limit the air flow and the cooling power of the coils, making it necessary to defrost the freezer.

Practical Advantages of Manual-Defrost Freezers

  • Since there is no air circulation in these types of freezers, they are preferred for open container storage as your material will not dry out as quickly.
  • Manual-defrost freezers are typically cheaper. For example, the Premier Countertop Freestanding Freezer mentioned earlier has the option of auto- or manual-defrost, the latter being over $100 cheaper.
  • A manual freezer should be more energy-efficient because it doesn’t have to heat coils or run fans.
  • Manual-defrost freezers have a more constant internal temperature because there is no heating of coils.

The major drawback of a manual-defrost freezer is the impracticality of having to manually defrost it. Samples need to be relocated, the unit needs to be turned off, and the frost removed. Then it needs to return to temperature before the samples can be put back. Aside from the impracticality of this, it also means samples being exposed to ambient temperature while being relocated, albeit for a very short period of time.

Freezers With Both Auto- and Manual-Defrost Systems

Some freezers come with dual defrost functionality, giving you the best of both worlds. For example the upright lab freezers by Jeio Tech shown below come with both systems.

 

An upright lab freezer.

 

The auto-defrost system automatically activates on detecting frost accumulation in the evaporator. An additional manual defrost system is in place for immediate frost removal.