Manual Defrost

[Jennifer Leos]

Asits 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.

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.

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 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.

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.

Refrigeration has actively influenced the way we preserve food in both commercial and domestic spaces. Over the years, the refrigerators seen in the market have evolved and progressed to ensure that people get to enjoy the food preservation benefits that come with owning a refrigerator and foodservice businesses can make the most of fresh ingredients.

Despite the differences in branding, there are inherent similarities in all commercial refrigeration. They all have to keep food cold for a long as possible in precise, consistent temperatures conducive to preservation and freshness. The cooling process involves dropping the temperatures inside the refrigerator through a refrigeration cycle.

This cooling of the air inside the refrigerator causes it (the air) to condense into water before freezing and turning into ice crystals. This is called frosting. Over time, this ice build-up can accumulate inside the freezer potentially causing a decrease in performance, reduction in efficiency and a shorter life expectancy of refrigeration components.

This accumulation usually happens in the evaporator, or freezer section of your equipment. As the temperatures get lower, the air around the cooling coils turns into water before crystallising and forming an ice build-up. To prevent this, fridges and freezers include a process called defrosting i.e. the process of removing ice build-up from the refrigerator.

There are two major methods of defrosting. These include: Automatic defrost and Manual defrost. In each, the idea is to prevent the build-up of ice crystals inside the refrigerator.

The manual defrost system is the oldest method in the refrigeration industry and requires a hands-on approach. It is also known as the natural defrost system as no mechanical intervention is required. It simply requires you to unplug the equipment, remove all the food from the refrigerator or freezer, leave the door or lid open and allow ice to melt naturally. Never use sharp implements to chip away at the ice as this may damage refrigeration walls. Placing containers of warm water within the unit may help to speed up ice melt and make the job a little easier. If defrosting an upright unit, you may find placing towels at the front of the door useful to absorb water. Once ice has melted, wipe all surfaces and turn equipment back on. Always ensure units are at correct temperatures before returning stock. It is most common defrost type for static cooled equipment.

Also known as the auto defrost, the automatic defrost was introduced as the need to have a less hands-on system of removing ice build-up increased. Fridges, especially freezers, with an automatic defrost system are also known as frost-free.

The automatic defrost requires no human intervention. It mainly comprises of a fan on the compressor and an electric timer which is factory set to control the temperature for the defrost process to take place.

This system operates through a carefully designed cycle, utilising a combination of sensors and automated controls to monitor ice buildup and execute timely defrost cycles. The defrost cycle mechanisms typically consist of heating elements that activate periodically, melting any accumulated frost on the evaporator coils. Cycles may occur on average twice a day, however this is dependent on the type of automatic defrost. Always refer to user manuals for details about specific equipment.

Automatic defrost functions are supported by all manufacturer's and can be seen in the majority of types of refrigeration including but not limited to, Foster catering storage freezers, display freezers such as the Blizzard BF1SSCR and Artikcold preparation counters.

The automatic defrost system is a newer concept than natural defrost. As such, there are many automatic defrost systems, all different but designed to ensure that there is no ice build-up within the fridge. They include:

This defrosts the refrigerator utilising the naturally occurring hot discharge vapour throughout the defrost cycle. The hot gas defrost system is efficient, reliable, safe and automatic. It also takes a shorter time compared with other defrost methods.

During the defrosting process, the evaporator is turned into a condenser and hot air that would otherwise be directed into the condenser by the compressor in a normal refrigeration cycle is pushed back into the evaporator where the ice has accumulated.

The hot gas defrost system requires additional valves to control the flow of pressure and gas as well as a direct line which bypasses the condenser and the expansion valve. It can be found in a number of types of equipment such as ice machines from Prodis.

The water defrost system basically uses water to get rid of the accumulated ice within the evaporator. It can be done using automatic or manual methods depending on the system and is mainly applied in industrial refrigerators.

The water defrost system is used in low-temperature refrigerators and works by applying heat directly to the accumulated ice. The water, plus melted ice then flows into a drain pan for elimination through evaporation.

The electric defrost system contains electrical heating elements which are then placed on the evaporator coils. This system of defrosting is usually applied in cold rooms where temperatures tend to drop to extreme levels.

Electric defrost systems usually take longer to defrost the ice because most of the heat is not directed to the evaporator. Instead, only about 30% of the heat is used in the defrosting process while the remainder is dissipated into the cold room. This makes this process less efficient than methods such as gas defrost systems.

Off cycle defrost involves the temporary pause or cessation of the refrigeration cycle where refrigerant is prevented from entering the evaporator. This causes the evaporator temperature to increase therefore thawing any build-up of ice. At the end of the defrost cycle, the refrigeration cycle resumes. The length of an off cycle defrost is typically determined by a factory set timer.

Choosing a commercial fridge or freezer with a defrost method that fits your preferences will go a long way in ensuring that you continue to enjoy the benefits of refrigeration equipment in a business setting.

It can be confusing for a laboratory to choose between manual defrost and auto-defrost freezers. A commonly held belief is that manual defrost freezers are the best choice for storing certain sensitive biological materials such as enzymes. Traditional auto-defrost freezers utilize slight intermittent warming of the cooling coils during defrost cycles. The concern is that even this minimal warming has a significant impact on freezer temperature. Ideally, a laboratory freezer used for storing very sensitive, and sometimes irreplaceable, samples would have the best characteristics of both technologies. In this scenario:

We set out to evaluate the impact of both manual and auto-defrost technologies. A study was conducted to evaluate the performance of both technologies by testing sample temperature variability during the auto-defrost process as well as during routine door openings. Door openings were tested to better understand how sample temperature is affected when the door is opened for both auto-defrost and manual defrost units.

Routine door openings are important because they can have a dramatic impact on sample temperatures. Every time a freezer door is opened, warm air and moisture rush into the cabinet, causing the temperature to rise. Manual defrost freezers (also referred to as cold wall) rely on passive movement of cold air and therefore cannot recover quickly from routine door openings. However, auto-defrost units utilize forced-air cooling. Forced-air actively circulates the cool air inside the unit, enabling the temperature to recover more quickly and effectively.

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