Cold Room Humidity
Cold Room Humidity
(OP)
I have a client with a cold room (insulated box), 12'(L)x 18'(W)x 8'(H) that is currently at 38 degrees F, 80% RH. She wants to the room to maintain 38 degrees F and 60% RH.
I don't have much experience with cold rooms so I am really not sure how to get the humidity down. The room is currently served by a 2-ton heatcraft evaporator (refrigeration unit). Because there are hardly any loads in the room, my initial thoughts are that the unit is oversized, however, that is my AC logic talking and I don't really know if that applies to a "cold room" situation.
Any thoughts on how to reduce the humidity in this space?
I don't have much experience with cold rooms so I am really not sure how to get the humidity down. The room is currently served by a 2-ton heatcraft evaporator (refrigeration unit). Because there are hardly any loads in the room, my initial thoughts are that the unit is oversized, however, that is my AC logic talking and I don't really know if that applies to a "cold room" situation.
Any thoughts on how to reduce the humidity in this space?





RE: Cold Room Humidity
RE: Cold Room Humidity
If the room is already kept at 38F then the coil is most likely at a temperature below freezing.
This means that the coild is already removing moisture from the air by frosting onto the coil.
The big thing in these cases is: how much moisture is entering the room ???
Keeping the door shut will help a lot. Every time someone opens the door air with moisture will enter the coldroom.
I don't think cooling down and reheating as suggested by nuhvac is the solution here as you are probably already cooling below freezing thus removing moisture without reheating.
RE: Cold Room Humidity
RE: Cold Room Humidity
The TD is the temperature difference between air in the cold room minus temperature of refrigerant in the unit cooler.
The TD can be approximated by dividing the unit cooler capacity at a 1* TD into the condensing capacity at the desired saturated suction temperature (SST).
I would recommend looking at the ASHRAE Refrigeration Handbook, or going to the Heatcraft website and downloading the Engineering Manual. The Heatcraft manual is free, include estimating tables, and provides a better explanation that I can give.
RE: Cold Room Humidity
RE: Cold Room Humidity
However, in terms of absolute humidity, the difference between 38F/80%RH and 38F/60%RH is not that much. So you probably don't need to do all that much.
Two relatively simple things to try: Slow down the evaporator fan. Turn off the evaporator fan when defrosting.
RE: Cold Room Humidity
RE: Cold Room Humidity
From the ASHRAE Refrigeration, chapter 42, very high RH (about 90%) a TD of 8 to 10 is used. High humidity (approx. 80%) would run a TD of 10-12. Medium humidity (75%) a TD of 12 to 16 is recommended.
The Heatcraft manual continues for low humidity systems, which will dessicate food. The Heatcraft provides a more basic and consolidated approach than the Refrigeration.
Heatcraft recommendation is per above; for 38* F room temperature at 60% RH, a TD of 17 to 22 will be needed.
This is not a chilled water system or coil. The refrigerant is expected to be well below dewpoint, and frost will form if moisture is in the air. That is why door closers, draft curtains, and all the wonderful criteria from the EISA have been inserted specifically for walk-ins.
RE: Cold Room Humidity
RE: Cold Room Humidity
RE: Cold Room Humidity
Also, in the situation you mentioned, humidity is controlled (or reduced) with the cooling coil, not the heater. The heater is simply used to prevent "over-cooling" the space.
RE: Cold Room Humidity
I work for a company that designs and sells dessicant dehumidifiers like you are talking about using and here are some things to consider. You will need to have hot DRY air going through the wheel to reactivate it. You will need to check with Munters to see what frost prvention they have on the wheel, because as the wheel goes from reactivation to the cold air to remove moisture it will most likely cross the frost point in this application. Finally the dehumidifier will be heating the air slightly as it performs this process it is not a pure dessicant such as say silica gel trays. Just some food for thought.
RE: Cold Room Humidity
http://www.heatcraftrpd.com/service/handbooks.asp
RE: Cold Room Humidity
This is not for close control of RH, but if those evaps are oversized, it will be too wet in that room. Reheat won't meet energy code, but would be the only way to have actual control of the humidity, if this were some kind of critical environmental chamber?
Is this a beer cooler or test chamber?
RE: Cold Room Humidity
There are exception to the enrgy code: "Process" loads (such as this applicatrion) requiring simultaneous heating and cooling to maintain space temperatures are exempt form the energy code.
RE: Cold Room Humidity