Data centers and IT rooms are where an organization’s IT equipment is housed, operated and managed.  IT equipment generates heat and will malfunction or fail when the heat is not removed.  This is where effective computer room cooling comes into play.


CRAC and CRAH Units

Computer room air conditioners (CRAC) or computer room air handlers (CRAH) are two types of units commonly used to handle the heat generation load. Both units consist of a fan, motor and cooling coils.  The motor drives the fans, which will pull the return air across the cooling coil and removes heat from the air before it is supplied back to the space. The difference between CRAC and CRAH units is the method used to cool the liquid in the cooling coil.

CRAC units operate using the DX refrigeration cycle.  These units utilize refrigerant as the working fluid and require a condenser and compressor. CRAH units operate using chilled water. This requires a chiller or chiller plant, which produces the chilled water and a chilled water valve.

As with any HVAC unit, air can be dispersed in a variety of ways.  One preferred method for distributing supply air to IT equipment requires a raised floor, drop ceiling and hot/cold aisle server rack layout.


The CRAC/CRAH unit, assuming a downflow configuration, will blow the supply air under the raised floor. Perforated floor tiles placed in the cold aisles will allow the supply air to reach the front or inlet side of the cooling system in the servers.  The outlet side of the server cooling system will blow the now heated air into the hot aisle.  Lay-in grilles placed above the hot aisles allow the hot air to rise into the drop ceiling plenum and return to the CRAC/CRAH unit.  Additional design options, like CRAC/CRAH redundancy in a mission critical operation or air containment/enclosures, should be considered based on the parameters of the project.

Allegheny Design Services

Proper mechanical, electrical and plumbing (MEP) design is critical for moving air, water and electricity to the areas they need to go. Data centers and IT rooms are prime examples of this.  Allegheny Design Services provides complete MEP engineering services to our clients across the country.

To see more of our MEP projects and to learn more about our team, click here.

Thanks for reading,


Written by Alex Clarkson, EIT, Junior Mechanical Engineer

Alex-ClarksonAlex Clarkson joined the Allegheny Design Services team in March of 2015 as an engineering Intern. He graduated magna cum laude from West Virginia University with his bachelor’s degree in mechanical engineering in May 2015. While at WVU, Alex served as the treasurer of the student branch of ASHRAE during the fall 2014 semester and president during the spring 2015 semester. Prior to joining ADS, Alex worked as an MEP engineering Intern at Miller Engineering. He is now a junior mechanical engineer at ADS, primarily focusing on the design of mechanical and plumbing building systems.

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