Get to know Air Blast Freezer of Cold Room: Part 2

From the last article that explained Air Blast freezer and briefly on how it works. In this article, it describes in depth of details of Air Blast Freezer to expand your understanding on how this type of freezer work.

An Important Part of Freezer’s Mechanism

Important thing in principle of froze freezer is air circulating system design to fit the needs of frozen products. If air circulating system is well designed, quality products can be expected and as well energy efficiency and cost control.

Good freezer design means cold air is blown evenly to every product in every area, arrangement of goods must have appropriate space to allow cold air to flow through. In addition, gaps for cold air to flow between trolleys are also important.

What will Happen if Products are not Properly Loaded?

Every freezer is designed to match requirement to freeze goods such as weight, temperature, and type of products, however, in practical used may not be as state in requirement if there is too much goods loaded, more energy will be required cooling goods down. This can lead to more time and money spend.

When situation as mentioned above happen, safety system will stop compressor from working. System may have misunderstood that the compressor is too small when a compressor was actually designed with right size and specification.

If system is often load inappropriately, it can cause damage to compressor. On the other hand, if freezer is under-load then it is not worth to run the machine.

Cold Air Circulating in Freezer

In theory, air is a very bad heat conductor since air have low specific heat capacity, high velocity of air is requiring absorbing heat from the goods. Higher velocity of air flow mean fans must work at higher work rate; this leads to exothermic heat exchanger to cool down system and this is a burden to cooling system. However, if it is already calculated and make sure that fan cooling system work efficient enough, it will not affect cooling of goods.

Example 1 pre-calculation of cooling load in a large room, air flow velocity is set to be around 5.08 m/s. This room have a large contact surface area and cooling fan release high amount of heat.

Example 2 pre-calculation is made for small room, air flow velocity is set to be around 5.08 m/s. This room have a small contact surface area and cooling fan release low amount of heat.

Temperature of Cold Air in Freezer

Temperature of cold air circulated by the fan should be sufficient to froze product over instantaneously. The temperature at the centre of the product must be below -18 Celsius and temperature of cold air is usually as low as -35 Celsius. This shorten freezing time however, it should not forget about goods arrangement since it can affect flow of cold air.

Choosing a very low temperature for cold air result in higher cost. It is crucial to consider relationship between wind speed and temperature the maximum freezing effect on the goods. For Air Blast Freezer both continuous and batch, should have cold air velocity around 1.5-6 m/s and temperature from -30 to -40 degree Celsius and display in the table below.

This table represent relationship between Air Velocity temperature of air after it pass through the goods.

Freezing Time

For Air Blast Freezer design, quantity and time require to freeze products will determine the size of the room. Size of cooling coil determines and control temperature of the whole refrigeration system. User must understand the importance of time require to freeze product and factor that can affect this, for example, quantity, arrangement, air velocity and temperature of goods before enter freezer.

There is no clear formula to finding a right amount of time since there’s too many factors that can affect time. However, it can experiment with some controlled factor e.g. quantity of products, arrangement of product to know freezing time for the freezer.

Defrost in Freezer

When Air Blast Freezer is used for a period, there will be ice form on the wall, floor, ceiling, door knob and even on the cooling coil. These ices should be melted from time to time to prevent excessive accumulation of ice, hazard and problem can be cause by accumulated ice. Moreover, ice have direct effect to the cooling coil where it can reduce ability to do heat transfer of the coil.

There are 3 main causes of ice accumulation in the column:

1. Water evaporate from product before freezing most product contain 1-3/100 of water ratio to its weight. In the absence of packaging, these waters will become ice.

2. Steam from outside flow into the room when the door is opened or through the air pressure valve in the room.

3. Air that leaks into the room through wall.

Tips for Preventing and Reducing Ice Forming in the Cold Room

It can be easily achieved by building Ante Room in front of freezing room. Ante room usually have temperature around 0-5 Celsius. If there is no Ante Room cold air in freezing room will directly exposed to warm air around 25 – 30 Celsius and this will create a lot of steam which leads to ice in freezing room.

Air Blast Freezer space design to reduce ice in freezer room

Conclusion

From information about Air Blast Freezer design, there are many operating instructions and details. However, if the company can meet all these requirements, it can help the freezing business achieve its objective. Moreover, the company can also save energy, reduce cost and produce quality product for consumers.

If you need more information or instruction about Air Blast Freezer, do not hesitate to contact us. We have specialist that can help increase quality and efficiency of your business.

Written by Dr. Sukit Litikorn
Engineering Support Director, Refrigeration System
Harn Engineering Solutions Pcl.

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References :

– HANDBOOK OF POULTRY SCIENCE AND TECHNOLOGY

– Proceeding of ENCIT 2010 “AIR BLAST FREEZERS AND THEIR SIGNIFICANCE TO FOOD FREEZING : A REVIEW”

– FRITERM TERMIK CIHAZLAR SANAYI VE TICARET A.S. “BLAST FREEZING APPLICATIONS IN CONVENTIONAL ROOMS”