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Datacenter Chilled Water Crac performance

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angryjohnny

Electrical
Mar 28, 2005
2
I have 6 downflow cracs that are being supplied with 45deg cw. I am seeing a varing delta T's across the coil averaging 12deg with one as high as 27deg. The total available name plate tonage is 140 Tons with an IT load of 150Kw. The cracs are all set with 67deg set point but the room has an ambient 76 deg. Based on available tonage and the electrical load I would have expected the room to be much closer to the set point.

I was wondering about the circuit setters on each crac because they are all wide open with one 10% closed, which is also the crac with the 27deg delta across the coil. Would slowing the cw flow across the coil allow me to get the space temp colder.
 
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For a start, there is nothing wrong with 76degF, and you are probably chasing an unreasonable goal trying to attain a 67degF RAT. Your crac's were probably rated at 75 for the nameplate capacity. The crac's like the high return air temp.
 
I agree there is nothing wrong with the space temp. My reason for being greedy in trying to obtain a lower ambient air temp is that my IT load is supported by the same central plant that supports the comfort cooling for over 375,000sq ft of office space. When summer rolls around all chillers are fully loaded. These are old centrifigruals with a built in 15 min delay on power failure. I have no redunancy in the system.
 
we were in the same position as we finally installed a new chiller on the roof and tied it into the existing piping to the crac units with shut off valves so we could use the central plant system in an emergency. Consider that virtually all data centers have more cooling equipment than is needed for two reasons: First, the need for reliability; and second, the need for oversized equipment. Most facilities have N+1 for every five units installed, and many data centers are designed for future loads. The result is 25% to 50% more air handling capacity than is needed. This sounds good, but it's not. For example, by combining excess capacity with return T & H control, the result is elevated variable supply temperatures. It's bound to happen. The only ways to modulate cooling capacity are to vary the volume of air or the temperature. In a data center with a room set point of 72-F and a 54DEG.F design supply temperature, and 50% excess cooling capacity, the result will be an average supply temperature of 62?F.
The final result is loss of humidity control,what is the room humidity, and an increase in localized hot spots. The single biggest contributor to the inefficiency of CT is by-pass air. We define by-pass air as cooling that returns to the ACU without doing any work or air that does not pass through electronic equipment and gets warmer by extracting heat.

Consider that most computers and other electronics equipment use front to back air flow to dissipate the heat generated in the box and 100% of the power into the computer is turned into heat. A typical computer will elevate the air temperature between 30?F and 40?F from the intake to the exhaust.

If the return air is only 10?F to 20?F warmer than the supply, then a significant amount of the supply air is mixing with the hot air coming from the back of the cabinets. This will "dilute the hot air" before it returns the ACU and not do any work to extract heat. In a data center for every five ACUs, the by-pass airflow rate is an astounding 60%.
 
Angryjohnny, the delta T you mention is on the water side or air side? Do you have a cfm value for these units? Also with circuit setters all fully open, if they were actually water balanced, shows we didn't have enough DP...

Imok2, are your units capable of a dehumidification mode? All you should need is for one of the units to be in that mode. There's not a lot of air exchange in these rooms; sometimes near zero. If one CRAC coil is cool enough to wring out the moisture, the ramaining ones can chug along at their 75° return set point, no worries. Just periodically rotate the CRAC unit set for dehumidification to equalize drainage, coil corrosion, etc. If they don't have a dehumidification feature, just set one unit for a return temp of 60° and the rest for whatever you want room dry bulb to be...
 
Have you considered chilled water storage? Storage for 15min and circ pumps can be on a separate gen backed source, as well.

Will help in both normal power loss and dropping the plant.
 
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