CRAC selection
 

Hi,

I wanted to know what the basic selection criteria for a CRAC system in a data center. does it only depend on the cooling required by the room?
i am confused about the room cooling with regards to the height of the room. i have come to understand that if the height of the room is more, the cooling needs are less. but doesn't a high ceiling mean that there is more air to cool?

We will revert to some fundamentals about computer room cooling the first being that the cooling system is there to cool the IT equipment and not the room. So what does this mean, it means that we shall try to get the air form the CRAC unit through the servers and back to the CRAC in as short a route as possible and not to mix with the air in the rest of the room. Hence the room volume is irrelevant.This is why the containment systems are so effective. In a perfect system we would have cold aisle containment and hot aisle containment and the air which pass through this system and back to the CRAC units would not enter the rest of the room hence we have the utmost efficiency. In most cases we will have one or the other and in a non contained room we still attempt to ensure that all cold air from the CRAC unit goes through the servers and then back to the CRAC units at as high a temperature as possible. (The higher the return air temperature entering the CRACs the higher the COP of the CRAC unit = less power usage)
So what is the relevance of the high ceiling.
With a higher ceiling we can attempt to utilise the space a higher level to bring the hot air back to the CRAC units. This can be done by installing a ceiling and containing the hot aisle and the CRAC units upto that ceiling hence creating a sealed hot chamber.
If we dont hace a false ceiling then we can still extend a containment system form the hot aisle and the CRAC unit to within say 800mm of the ceiling slab and rely on the fact that hot air rises and the stratification effect to ensure that the hot air will travel across the underside of the slab and get sucked into the CRAC unit intake which is also at that level.
The higher the ceiling the less likely there will be mixing between the different bands of air temperature ie hot and cold.

a high ceiling just ensures a return path for the warm air to the CRAC. but in most data center these days, they prefer aisle management rather than going thru the hassle of room cooling. in aisle management, it does not matter how high the ceiling is.

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