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David PS
29-12-2003, 10:30 PM
We have a customers who operates a freezer room for long term storage of fish. To keep the product in top condition he wants to eliminate the defrost cycle.

Our initial proposal was a wheel dehumidifer however someone has raised the prospect of filling the room with dry nitorgen to acheive the same result.

To explore the dry nitrogen option, discounting any health and safety issures due to low oxygen levels, has anyone any opinions relating to the control of such an application?

Would it be pressure controlled, what pressure would be required, how practical is it to achieve a leak tight room.

Regards

David

Peter_1
30-12-2003, 10:35 AM
Once installed a Munters dehumidfier wheel in a freezer with poor results.

And what about sublimation of the fish? (correct English word for th phase change from solid to vapor?)
You will even have vaporisation at freezing temperatures. Water wells vaporise outside even if everything is frozen.

Why he wants to eliminate the defrost cycle? I think that despite all the argument he will give to you - which will I beth no technical aruments - you always will come back to the basic solution: defrosting.

We did twice a small ULO room but it was for vegetables. everything must be sealed or taped.
I think you will encounter problems but who knows.

Gary
30-12-2003, 11:41 AM
You might consider a niagara system:

http://www.niagarablower.com/index2.htm

The evaporator is continuously sprayed with glycol to eliminate frost, then the glycol is heated to remove the moisture.

David PS
30-12-2003, 03:49 PM
The customer is storing whole tuna. Because it is being stored unwrapped over a long period of time the skin will change colour if a defrost cycle is present, this is caused by the air temperature change during the defrost cycle.

The skin changing colour obviously lowers the price when they sell the tuna as fresh at a later date.

In a -50'C storage room we are allowed a maximum air temperature of -44'C at all times.

The dry nitrogen idea is based on a similar system that has been used on test chambers to eliminate the defrost cycle, the only difference is that the test chambers stored mechanical parts.

Peter_1
30-12-2003, 05:08 PM
Just for my own interest: how will you reach -50°C? Is it a tailor-made unit?

Peter

David PS
30-12-2003, 05:47 PM
Custom built cascade, long term we want to lower the temperature to -60'C.

Dan
30-12-2003, 07:16 PM
The Niagra system, as Gary suggests, has great benefits, but I am not sure it is designed for such low temperatures. I seem to recall alternating refrigeration systems which damper off the evaporator during defrost to create the effect of no pause in refrigeration.

Peter_1
30-12-2003, 07:21 PM
I just wanted to post almost the same suggestion: I once saw on an IQF tunnel evaporators defrosting on hot gas and there where tailor made baffles on suction and pulsion side which closed before a defrost started.

Jasper
30-12-2003, 08:14 PM
The skin changing colour obviously lowers the price when they sell the tuna as fresh at a later date.

surely this is illegal

RogGoetsch
02-01-2004, 02:15 AM
I would be surprised if the color change is not due to sublimation of frozen moisture from the skin of the fish. Drying the air will only increase the rate of sublimation.

I worked in a cold storage plant of a fish processor in Seattle in my youth: salmon, tuna, halibut, etc. We quick froze the fish, then to prevent "freezer burn" we dipped them in a prepared water solution (sugar and something else added to the tank) a couple of times, then into plastic lined bins. (I was told the small amount of sugar was crucial to keep the ice coating from shattering when the whole frozen fish were knocked around.)

Even so, storage was limited to a few months and the top fish would get "freezer burn" if the lining was disturbed.