CCU Question

[acgibo]

Hello all

I visited an IT server room today and renewed the supply fan on 1 of 2 CCU units. Set both units to 20 degrees and 50% humidity.
Before I left the customer asked me if we could recalibrate the 2nd unit.
the reason, well he said they seem to be fighting each other. One will be cooling while the other is cooling and heating. one humidifying one dehumidifying.
the room is roughly 18ft by 10ft, with most of the equipment at one end.- would this make a difference??
I cant understand why the units were not linked at time of install- perhaps this is not the done thing!!
would it be best to link the two???
is recalibration of controls needed??

I would appreciate your input.

Thanks Gibo

[winfred.dela]

Did i understand right: 2 sets cooling/humidifier/dehumidifier units for an 18ft x 10ft room?
What is the equipment size?

It sounds like the equipment is oversized for the room temp & humidity requirement.

Will you be able to just run one set of equipment & have the other as back up?
You may then schedule each unit for a day or two.

Hope to have more details on the room load & eqpt capacity so will be able to understand the whole problem.

[NoNickName]

equipment at one end.- would this make a difference??

It would make an ocean of difference.
Please follow the request of your customer. Dozens of plants with CCUs around europe have the problem you mentioned just because the maintenance person is too lazy to do it.

I cant understand why the units were not linked at time of install- perhaps this is not the done thing!!
would it be best to link the two???
is recalibration of controls needed??

Not all types of microprocessor can be linked and use a master unit sensors. If it was not done (sometimes they don't even know it's possible), then the two units were supplied with their own sensors.
In our manuals calibration is declared as compulsory 6-monthly maintenance.

[Andy W]

We used to call it piggybacking and wire them master and slave instead of 2 independant systems as they will fight one another.

[acgibo]

I did think piggybacking would be the obvious answer, just seemed too obvious, if you see what i mean , so wanted to check. Thanks Andy

As for recalibration being compulsory 6 monthly, thats VERY interesting, wouldnt have thought it has been done yet. I will check the data left on site and get that done. Thanks Nonick.

[Makeit go Right]

After recalibrating the units,

...how about setting the second system @ 21 degrees. That way, the second system will only come in if the room conditions escapes away from the set 20 degrees, ie if No-1 unit is not coping.

[NoNickName]

No, if the two units are independent, they have to have the same setpoint, otherwise there is a gray area between cooling band of n.1 and heating band of n.2 (or vicerversa). In otherwords, the dead zone can overlap the cooling band, or the cooling band can overlap the heating band (depending how large dead zone, cooling band and heating band are).
Besides, 19° is way too cold for an IT server room, and it will end up with a high rh%, at the same time incapable of reducing it. Also, the energy consumption is unnecessarily high.
A typical setpoint for server rooms is 24°50% or even 26°45%. Some prefer 22°60%, but not 19°C which is a temperature for seasoning room or wine cellars.

[frank]

You will also end up with one unit doing all the work with no duty sharing.

[Abby Normal]

A typical setpoint for server rooms is 24°50% or even 26°45%. Some prefer 22°60%, but not 19°C which is a temperature for seasoning room or wine cellars.

Not arguing at all nonick, its just that IT guys are a funny sort.

I have a redundant pair of 40 ton precision AC (leiberts) and fought tooth and nail to get the client to agree to a set point of 21C in writing.

Their old system maintained 67F (19C) as measured at the unit and because of poor air distribution they had hotspots . If they set it higher than 19C, the hotspots were too warm and they had problems.

We did quite an upgrade with the pair of 40 tonners and a complete new air distribution systems. It works great at 21C and I think they would not have hotspots if they set it at 26C either. Only about 10 CFM of air flow per square foot of floor space. System pushes the heat out of the room.

But as soon as you leave, it gets set down to 19C. So far its holding at 50%.

They have not froze up yet, been in operation more than a year. A redundant pair though so when they finally freeze one, it will set off an alarm and switch over to the other system. I will get called to investigate and again remind them of the 21C minimum set point that I stuck my neck out for.