CRAC vs CRAH comparisonCRAC – Computer Room Air Conditioner

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.

24 thoughts on “CRAC vs CRAH

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    1. Thanks for your feedback and I totally agree. I try to add as many pictures/videos that I can but they are sometimes hard to gather.

  3. Great post. I was checking constantly this blog and I’m impressed! Very useful info particularly the last part 🙂 I care for such information a lot. I was looking for this particular info for a long time. Thank you and good luck.

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  5. Pingback: What is an EC Plug Fan? — Data Center Huddle

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  7. Dear,

    1) Can use the CRAC machine into office area ?

    2) Is no raise floor, can I use the CRAC in office ?

    3) Base on energy considersation, CRAC and AHU is good energy saving ?


    1. Hi Allen,
      1) Technically it is possible can but it is not ideal as CRAC units are designed for data center environments. There are probably better solutions you can find for office areas.
      2) Again, there are probably better solutions for office spaces.
      3) This is hard question to answer because it depends on the specifics and the environment. You’d have to do some calculations to determine which is better for energy savings.

  8. Dear Dan
    why I should use CRAC for computer room , fan coil is not enough?
    Has CRAC two compressor one in the unit itself and the othe on the roof?

    1. Hi Khaled – A Fan coil unit is almost the same thing as a CRAH (where the cooling source is chilled water). CRAHs are designed specifically for Data Centers where fan coils are more common in commercial office buildings. However both provide cooling in more or less the same way.

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  10. Hello Dan,

    Great article,however, the diagram does not look right. The compressor compresses the refrigerant after it has picked up heat from the data center. It’s that compression that raises the temperature of the refrigerant (without adding heat) that makes it possible for the refrigerant to dump it’s heat at the condenser.


    1. Hi Tom – Thanks for the clarification. And yes you’re right. I was just showing the key components of the CRAC unit. I believe the same is true for the CHW valves. They come after after the coil.

  11. Could it be generally stated that a CRAC unit is used in smaller applications which would often include skid/container type systems as opposed to CRAH units which would be more suited to large central plant chilled water systems?

    1. CRAH units by design require a chilled water system to work. So yes, in smaller stand-alone data centers and/or data closets, CRAC units are preferred because they don’t require the expense of a chilled water plant. For larger data centers, chilled water plants are more economical due to higher efficiency than CRAC units.

  12. Hi Dan ,
    Could it possible that we also Use this CRAH Air handling Unit as Hot & cold regarding the weather Conditions ? please clarify and to which Minimum (cooling Load )extent we can use CRAH instead CRAC.

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