Liebert Unit not Dehumidifying!!!! HELP

[xnetfly]

We have installed a Liebert unit Mini-mate2, 5ton, MMD92C, 2500CFM, 7.2 FLA, CHW 10 GPM, HW 1.5 GPM. We are having some trouble dehumidifying the air in to the special collections area of our library. When we use a setpoint of 45% at 65F we are getting 60% at 65F. It feels like the unit is undersized. What do we need to do to determine if it is and is there some sequence of operation that the unit works on interally that can be adjusted? Also I was reading the documentation and there was a selection for the Humidity sensitivity from 1 to 10%. What does this selection do? Please help....

[sabre11134]

sensitivity means your window for humidity control. example 5 % sensitivity and 50% setpoint you could control from 52 to 47 % rh. Have you tried to set the unit at 50% rh and 69 degrees. also you should have a sensitivity on your cooling setpoint also. Have you checked to see if your fan speed changes to a slower speed on a call for dehumidify ?

[xnetfly]

This is good information. I am not experienced with this equipment so all this is new information to me. Could you please explain further?? If should the fan slow down on DEHUM mode.

[roc service]

I agree with Sabre11134. See attached.


http://www.librisdesign.org/docs/CollectionPreservation.doc

Try to alter your cooling setpoint to 68 F and RH to 45%. Set the temperature sensitivity at 2 and the RH sensitivity at 5. Do you have a chart recorder for temperature and humidity? If not, then I would recommend you purchase one so you can make adjustments and then see how the recorder tracks those adjustments during a 24 hour period. Does the room see a lot of air changes ie. doors opening, people load etc.

[xnetfly]

I will try those settings. What makes these settings better then others? The space being used has different volume of people entering it at different times. It is on campus at a university so students may use it a lot or a little. We keep the doors closed though.

[viceman]

if the space you are attempting to condition has outside doors opening and closing and isn't sealed from the surrounding spaces (including above ceilings) you will not be able to control humidity. seal it up or look at additional equipment.

[roc service]

You might be fighting a losing battle if the room is not allowed to stabilize. If there are alot of air changes and a poor vapor barrier seal in the room, then how can you expect the room to maintain its temperature and humidity setpoints, especially during the summer months. That's why I would like you to record the temperature and humidity readings at night when the library is closed. This will be an indicator of whether the unit can do the job it was designed for in the first place. What part of the country are you in? Is it a very hot and humid climate where your at?

[xnetfly]

I will trend the room over night to verify temperature and humidity. I should have that information by tomorrow to see if we are in good shape. I also wanted to find out if our unit has been sized correctly. It seems to be a bit small for the space. It is a 92 BTU hence the MMD92C. If you could tell me what size rooms this would be sufficient for that would be great. Thanks so much for taking the time to answer my questions. I understand some of them can be some what tedious.

[Shophound]

Originally posted by xnetfly
We have installed a Liebert unit Mini-mate2, 5ton, MMD92C, 2500CFM, 7.2 FLA, CHW 10 GPM, HW 1.5 GPM. We are having some trouble dehumidifying the air in to the special collections area of our library. When we use a setpoint of 45% at 65F we are getting 60% at 65F. It feels like the unit is undersized. What do we need to do to determine if it is and is there some sequence of operation that the unit works on interally that can be adjusted? Also I was reading the documentation and there was a selection for the Humidity sensitivity from 1 to 10%. What does this selection do? Please help....

Any reason you're trying to keep the temperature at 65 degrees? I work in a museum. We keep the galleries around 72 degrees/50% relative humidity. We have valuable paintings, photographs, and works on paper...the curators are quite satisfied with a 72/50 environment in the galleries and storage facilities.
We use revolving doors to minimize outdoor air intrusion. Our building is pretty large, yet we're able to maintain a pretty consistent environment in the galleries around the clock.

That Liebert may be undersized for 65 degrees/45%RH but not at parameters a bit higher, such as 72/50. Remember the "relative" aspect of humidty to temperature. You can take the same air that at 65 degrees contains 45% relative humidity and warm that air to 72 degrees...the relative humidity will drop to around 38% or so.

Let's do a comparison of setpoints. To reach 65 degrees sensible temperature setpoint and 45%RH setpoint in your space with, let's say, a 70 degree return air temperature and 50% RH, you have to remove 13,700 BTU's. The latent BTU removal would be 24,150. A total of 37,850, or about 3 tons. So, 5 tons should be plenty, right? Well, I gave a rather low return air temperature. Suppose it's actually more like 76 degrees return air with 55%RH. Sensible BTU's needing to be removed jumped to 30,250 and latent to 55,200, giving a total requirement of 85,450, or about 7 tons. Now you're undersized if we assume your return air is always going to be 76 degrees at 55%RH (a hypothetical heat load of the space the unit might see)

At a higher setpoint, say 72/50, with return at 76/55, you need about 3.2 tons total capacity...to maintain that environment with no changes to heat load entering return air inlet. Obviously heat loads change (computers, lights, people entering and leaving room, amount of people, etc), so that's why equipment is sized for worst possible scenarios (more commonly called "design conditions").

Of course these numbers are hypothetical, crunched without knowing your exact heat load calcs. They're just to give you an idea how raising the setpoint for temperature and humidity might achieve your goals with the same machine. Unless the powers that be absolutely mandate a 65/45 environment for your collection, I'd look at bumping up the temp and humidity setpoints.


[Edited by shophound on 07-12-2006 at 11:16 AM]

[roc service]

Liebert Model: MM92C is a 5 ton chilled water system.

At 72F db Total Capacity = 53,900 BTUH

At 50% RH Sensible Capacity = 50,900 BTUH

http://www.liebert.com/assets/products/english/products/env/minimt2/60Hz/bro_12pg/acrobat/sl_10500.pdf

See Page 11

[freddy-b]

Could unit possibly be oversized? Cooling the air quick, but not opening valve long enough to pull out the moisture? CHW temp too high? control enthalpy settings? is it a Wild CHW coil? valve?PID settings for CHW valve control(this could be controlling to tight and maintaining temp fine, but not getting coil cold enough to dehumid worth a damn". water balanced? this is why DX cooling is better for dehumid, by the nature of the beast. you will probably have to set your setpoint low enough to dehumidify, and use your heating coil to reheat, back to temp setpoint.

[Randy S.]

You won't get much dehumidifying unless your chilled water supply temp is below 48 or so. Then you might want to play with the air or water flow if the coil is oversized, so you can keep the coil cold. Pay attention to what the control valve is doing, and try to find a happy medium where there is enough flow to carry the load without the valve slamming open and shut.