Relative humidity in a freeze room

[Lars Ekström]

Hi

I have some questions regarding RH in a Freeze room. I have checked that the door and the rest the room is sealed off with extra around the door. When I start the freeze room I have about 35 % RH and 21 degrees. Every eight hour the defroster starts. After a couple of days I turn of the freezer and keeping the door closed and i get 100 % RH. Is the graph below evidence that my freeze room is not sealed off enogh?
The reason I keep the door closed after I turning the freezer off is a project where they are testing pest infected wood and they want to rise the temperature slowly.

So any suggestion on how to keep the RH down in my freezer room.

Regards

Lars



13994

[chemi-cool]

I dont know about 100% but it will be around 95%.

You can do the same outside and see that it is almost the same.

[Lars Ekström]

So it is a normal behaviour that it rises to 95% when i have a completely sealed freez room. Should it not go back to 40% when temperature rises?

[chemi-cool]

No, it will stay like that.
If the room is not sealed, you will notice a lot of ice on the evaporator and frost on the outside where heat enters the room.

If you want low humidity, you should consider using in addition to the cooling unit, molecular sieve drier with dew point of -40C.
That will dry the air very well

[Grizzly]

Hi Lars.
Just out of interest what happens if you disable the defrosts?
Because all that moisture trapped as Ice, then melted so that it flows away as water.
Will re-introduce some Humidity!
As Chemi states if your door is leaking you can turn off the threshold heater and watch the ice build up.

Have you considered changing your defrost cycle and have you looked at your drain pipes and Vapour seal lately?
Grizzly

[mikeref]

External dehumidifier in a closed circuit with your freezer room. It would have to be undersized to allow your refrigeration kit to reach the temperature required, though it must have the capacity to remove moisture with minimal temperature elevation.

The defrost cycle can't be avoided so a thermostat and Humidistat have to be wired in series with this dehumidifier.

Proof reading sorta reminds me of this Gem. Thanks Monkey Spanners and Install Monkey:)
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiG1b6jxo3LAhVG5KYKHdj2DDAQtwIIIjAB&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DrLDgQg6bq7o&usg=AFQjCNHSYbbUm5MdwtnjkYPdzTzMOJnR3Q

[Lars Ekström]

Thanks for the answers.
I will try to change the defrost cycle and see what happends. The freeze room have two pressurerelief valves, could they have something to do with it if i have constant under preussare in the freeze room.

Regards

Lars

[chemi-cool]

Before changing the defrost parameters, you should check if there are any problems with it.

Pressure relief valves are only there to protect the system from over pressure, do not touch them.

[Lars Ekström]

Tested and I have a steady flow of fresh air in to the freezer via the two pressure relief valves. It is a small freeze room, about 15 m3. I cannot find or think of any other way that the fresh air enters the freeze room and gives me the high RH. The door is completely sealed off, no ice buildup what so ever.

What is the worst that can happend.

[chemi-cool]

got you some reading to do:

http://cotes.com/wp-content/uploads/2014/10/Cotes_ColdstoreBooklet.pdf

[Magoo]

hello Lars.
check out.
www.kwangu.com/work/psychrometric.htm
lowering the dry buld temp will ramp up the RH plus air density will change , balance ports manage pressure differences and draw fresh air in, or exhaust air during defrost and density change
With sub zero temps the RH will be high but absolute humidity will be low.
Research a desicant drier to maintain low RH continuosly.

[Nudge]

For strict lab type conditions for constant r/h and temp. control you
really need a second form of dehumidification and cooling for closer
parameters. A second coil could be your problem solver.