A CRAC unit is exactly like the air conditioner at your house. It has a direct expansion (DX) refrigeration cycle built into the unit. This means that the compressors required to power the refrigeration cycle are also located within the unit. Cooling is accomplished by blowing air over a cooling coil filled with refrigerant. A CRAC is typically constant volume therefore it can only modulate on and off. Recently, some manufacturers have developed CRAC units that can vary the airflow using multistage compressors, but most existing CRAC units have on/off control only.
CRAH – Computer Room Air Handler
A CRAH unit works exactly like a chilled water air handling unit found in almost all high rise commercial office buildings. Cooling is accomplished by blowing air over a cooling coil filled with chilled water. Typically chilled water is supplied to the CRAHs by a chilled water plant (i.e. chiller). CRAHs can have VFDs that modulate fan speed to maintain a set static pressure either under floor or in overhead ducts.
– Pronunciation –
CRAC sounds just like it looks. Phonetically sounds like: “There is a crack in my windshield”.
I’ve heard CRAH pronounced two ways. The first sounds like ‘craw’, as in it rhymes with “saw”. This is the most common pronunciation. The second sounds like ‘cray’, as if you said the word “crazy” without the “z”. I suppose this mixup is rooted in the southern pronunciations of ‘crawfish’ and ‘crayfish’. How a tiny lobster got all mixed up in the pronunciation of data center air conditioning equipment…I haven’t a clue. Anyways, I prefer the pronunciation ‘craw’.
More Interesting Information
- Introduction to Data Centers – Data Center 101
- EC Plug Fans are a common energy saving retrofit that many data center operators are implementing for both CRAC and CRAH units. Learn more here: What is an EC Plug Fan?
- Variable Frequency Drives (VFDs) are commonly used to reduce fan speeds in order to save energy. Learn the physics behind how VFDs can save so much energy: The Fan Affinity Law.