Cold Room Humidity 2

[dblyle]

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?

 

[nuhvac]

You have to remove the moisture... That means cooling it down to a point where you can reheat it back up to 38F.

 

[Zesti]

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.

 

[jakejas]

I'm sorry, I am a new graduate working in the HVAC field and enjoy reading the forums to learn from other's practical experience, so this is probably an obvious question, but when you say evaporator, are you talking about a swamp cooler, or the evaporator part of the cooling coil? The only other help I can offer is in college we had to design a system for a pool room and we had to cool the air below the temperature we wanted in order to reduce the humidity ratio and then use a heater to heat it back up to reduce the relative humidity.

 

[mauricestoker]

I think what you are looking for is the temperature difference, or TD. Sounds like a prep room or cold warehouse. You would be looking at a TD of 17-22* F.

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.

 

[nuhvac]

zesti is right your coil will already be having frost issues on your evaporator coil, and is right about the amount relative humidity entering the unit, but I must say that actually doesnt matter. If you are cooling air down to 38F you are going to be at the dew point unless you reheat it. Simple psychrometrics show this. The only way to lower humidity is to cool below the desired temperature and reheat, or to use a dessicant drier. I dont believe a dessicant would be the way to go in this enviroment.

 

[MintJulep]

It's hard to make any firm suggestions without more system and load specifics.

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.

 

[nuhvac]

I agree with MintJulep, but i would warn against slowing down your evaporator fan. I design and sell equipment like this and we are always getting calls about broken coils or undersupplied air because someone has decided to slow down the supply fan without consulting us. Also if the fan curve does not match that lower cfm you can get surging on the fan which will destroy it and possibly the coil (had that happen a few weeks ago). If you want to slow down the fan be sure to contact the manufacturer of the equipment prior to any action.

 

[mauricestoker]

The coil will have a defrost cycle, which will drain into the condensate pan. At 38*F, enclosure air, gas defrosting or electric could be used.

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.

 

[dblyle]

Thanks for all of your suggestions on this. I am cautious about dropping the temperature in the room too much due to the tight constraints on the temperature (38 degrees +/- 1 degree). A large temperature drop will require reheat, which I really don't want to do. I have found a dehumidifier (munters HC-150) that will handle the moisture (as MintJulep stated, the difference is not that much). My thoughts are that by adding this to the room, it will handle the additional moisture and drop the humidity levels while maintaining the temperature.

 

[imok2]

For a 12x18x8 walkin at 38*F for a normal load you need about 13,000 btu/hr and for a heavy load you need about 17,000 btu/hr, you have about 24,000 btu with a light load which means that your system is too big. So you will have to do as was recommended that is run it longer and then reheat the air. Now the problem may require a defrost which would reheat and defrost the coil in the room but when you reheat the room the humidity may go lower then 60% so you can see that there are a lot of veriables. You may want to install an EPR valve in the suction line and adjust it to maintain a lower back pressure (colder evaporator) to remove more moisture. Be that as it may you will need a TD of about 20-22*F (air conditioning anyone?)

 

[dblyle]

Jakejas, I am referring to the evaporator coil that is physically in the room. While the method you mentioned is typically used in this area (Houston) to control humidity, newer energy codes are preventing both cooling and heating (reheat) of the same air, so make sure you check the codes before you design such a unit.

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.

 

[nuhvac]

dblyle,
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.

 

[mauricestoker]

I think you need to add just a little more information. Is the door going to be open frequently, what are you storing, how much, where the unit is, and what is the insulation (R value). For general usage, it would seem the unit is oversized, and a TD of 20-22 would be needed as IMOK has estimated. I think the engineering manual would help you with any estimate needed, as well as basic principles.

http://www.heatcraftrpd.com/service/handbooks.asp

 

[coldmaster]

Definately agree with mauricestoker...in a refrigerated room, humidity is determined by the coil TD, the match between the nominal capacity of the evap and the capacity of the condensing unit. Higher TD, colder coil, more frost on the coil, and out the drain during defrost. Definately make sure that you fans are off during defrost cycle. Also make sure that if you have multiple evaporators, that they go into defrost at the same time.

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?

 

[cry22]

"newer energy codes are preventing both cooling and heating (reheat) of the same air"
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.

 

[mauricestoker]

So this is a process load and not a beer cooler or prep room; could not divine them from what was stated. As a process load, then the requiremens of EISA, which differ from 90.1-2007, would still need to be addressed. In that case I would recommend a digital scroll. HGB would not apply, as 90.1-2007 would not apply.