CRAC Unit Reheat 1

[richanton]

I'm an electrical engineer, and I have a mechanical question. I am ordering some Computer room air conditioners, Liebert units. They are units that operate directly off house chilled water, so no compressor. The early discussions called for no humidity control in the room. I'm trying to find out if I need to buy the units with electric reheat. Does the reheat do something other than prevent the humidity from getting too high. From an energy standpoint, it doesn't seem to be very "GREEN".

 

[willard3]

Speak to a mechanical Engineer about this; it's not so simple as you think.

 

[gepman]

You almost never need reheat in computer room conditioners since there is almost always a large heat load from the computers.

Before you apply these units you should verify that chilled water will be available 24 hours a day 7 days a week. Computers (especially servers, mainframes, etc.) are usually left on at all times generating heat. Many times the main HVAC system is shut down during unoccupied hours to save energy due to the low efficiency at extremely small loads. That is why Liebert (and others) make stand alone units (with their own compressor of course) that will work while the main system is off.

Electric (or other reheat) will not remove moisture from the air so it really shouldn't have much affect on the humidity unless you were thinking that it would get to cold and the humidity in the room would increase but the heat load from the computers should take care of that. The only time that I would think that you would need electric reheat in a computer room would be if they were going to be shut down.

 

[gepman]

I would like to comment on my above answer before I get a lot of posts saying that I have neglected something. I come from California where we never worry about anything getting too humid (except for maybe within a mile of the coast). It is possible if you were bringing in a lot of outside air that the room could get to humid. Computer rooms usually have low occupancies and high heat loads and outside air requirements are low. Many times a separate building system brings in all the outside air and takes care of the humidity control requirements.

You could be worried about too low of humidty (for ESD control in a computer room). Reheat won't help there.

I stand by my original statement that it is most likely that you won't need reheat but if you are like a typical electrical engineer and do not know much about psychrometrics and heat load calculations then you probably should take willard3's advice and at least talk to a mechanical engineer.

By the way I don't like piping water over computer rooms so be careful on your location of the Liebert unit.

 

[imok2]

In the County we installed a few chilled water Lieberts and always ran our chilled water lines below the raised floor and installed water sensors in case a leak developed This was the main data center which ran 24/7. Ah to be young again and making things happen :>)

 

[richanton]

Thaanks for the responses.

Well, I have now talked to a mechanical engineer. The mechanical people are saying it's needed because if I don't have it, the way the Liebert works it will keep driving the room temperature down if it can't hit the humidity setpoint. This especially becomes a problem if the Liebert's are somewhat oversized. And that is an issue in my case, since we are building the room first and then gradually populating the area with racks. If the room always had a constant large load, I would be fine, but if my load is low on a high humidity day, there could be a problem either with driving the temperature too low or perhaps causing condensation. I still have my doubts, but heh, I'm electrical. What do I know.

 

[trashcanman]

Reheat is used for comfort, not humidity control. The way it works is:

A humidistat controls the cooling coil. If space humidity is above the set point, the humidistat will call for cooling. The cooling is continued till the humidistat is satisfied, or full cooling capacity is reached, whichever comes first.

A thermostat controls a reheat coil. If the space temperature is below the set point, the thermostat will call for heating. Once the thermostat is satisfied, the heating coil is either shut off or throttled down (depending on what type heating - electric, steam, hydronic).

As stated before, a heating coil WILL NOT remove moisture. It will just lower the RELATIVE humidity. Only a cooling coil or desicant device removes moisture.

I would recommend that for a computer room, the RH be 50%. Any lower, and you could have static electricity problems. Always consult the manufacturer of the computer equipment to see what they recommend.

 

[richanton]

Great explanation. I think I actually understand it now. Thanks.