CRAC UNITS | What are they?

May 22, 2023 |

A CRAC unit is a critical piece of cooling equipment, consisting of filters, fans, coils, and an external direct expansion [dx] condenser, that is connected to a range of refrigerant pipework to precisely manage and control the temperature and humidity of a space within specified design tolerances. They can usually be found within a mission-critical space, data center, data hall, IT, or UPS Room.

What does ‘CRAC Unit’ stand for?

The full form of CRAC Unit is – Computer Room Air Conditioning Unit.

Locations and Rooms where they are found

The units can be found in multiple locations, commonly:

  • Data Centers / Mission Critical Spaces
  • Data Halls
  • CRAC Corridors
  • IT Rooms
  • Electrical Switch Rooms
  • UPS [uninteruptable power supply] Rooms

Different CRAC Unit Design Configurations

There are a few configurations that will be seen, depending on the room construction, as noted below:

Down flow configuration

A ‘Down flow’ unit is one where the air is removed from the space through the top of the equipment, drawn downwards, where it is conditioned prior to being blown back into the space via the lower frontal section of the unit.

This configuration is quite common in Data Halls where a ‘fan wall is used’

Up flow configuration

An ‘Upflow’ unit is one where the air is removed from the space through the lower frontal section of the equipment, drawn upwards, where it is conditioned prior to being blown back into the space via the top of the unit.

Frontal flow configuration

A ‘Frontal Flow’ unit is one where the air is removed from the space through the lower frontal section of the equipment, drawn upwards, where it is conditioned prior to being blown back into the space via the front top section of the unit.

This is common in Electrical Switch / UPS / IT Rooms, ducting may be connected to direct the air better as usually there is other equipment that can hinder the airflow.

[this can be reversed for a frontal flow from the lower section of the unit, as per a downflow configuration].

Under floor supply

The CRAC unit will sit on a frame, usually a plenum, that is installed within the floor void to allow the conditioned air to pass through. Using a ‘down flow’ configuration.

The hot air [return] will be drawn through the top of the unit, conditioned, and then discharged into the floor, which will act as a plenum.

The conditioned air in the floor will be allowed into the space, where cooling is required, by the use of floor grilles, positioned in the locations needed.

Cooling medium used

A CRAC Unit, computer room air conditioning unit uses refrigerant to manage the cooling process., this is by Direct Expansion [Dx] where an independent external condenser unit is connected to the indoor CRAC unit via refrigeration pipework.

A CRAH unit (computer room air handling unit), uses chilled water. See our article 'CRAH UNITS | What are they' for more detailed information on that type of equipment.

Refrigerant used

The most common refrigerant used in these systems is R407A, a non-ozone depleting, stable, reliable refrigerant used in low/medium refrigeration systems.

What are the main components of a CRAC Unit?

The main component of a CRAC unit/system consists of:

  • Unit Frame
  • Panels [sides, bottom and top]
  • Sound attenuation [sound attenuation/absorbing foam/fiber glass insulation sandwiched between the panel skin]
  • Access doors to various compartments
  • Inspection windows
  • Base frame
  • Supply air plenum
  • Return air plenum
  • Filters
  • Fan section
  • Energy efficient fans
  • Fan guards
  • Variable speed drive
  • Motors
  • Vibration isolators
  • Cooling coil
  • Condensate drip pan
  • Condensate pipework
  • Condensate pump [if required]
  • Reheat coil [if required]
  • Humidifier [if required]
  • Control/expansion valves
  • Control panel
  • Control wiring
  • Network switch [if using multiple units]
  • Compressor
  • Range of refrigerant pipework
  • Refrigerant [R407A]
  • External air cooled condenser, consisting of,
  • Condenser unit frame
  • Condenser unit panels
  • Condeser fans
  • Condenser coils

CRAC vs AHU: If we took a CRAC Unit and laid it horizontally a lot of the layout and components would have a similarity to an Air Handling Unit or even the more basic Fan Coil Unit.

If you are wondering what the difference is between a CRAC Unit and CRAH Unit see our article 'CRAC Unit vs CRAH Unit | What's the difference'

How does a CRAC Unit Work?

A CRAC Unit works by recirculating air from a space needing to be conditioned by using its fans to draw the hot air in and then passing it over a cooling coil where it is cooled before being blown back into the space.

The cooling process of the coil uses a direct expansion system and a refrigerant vapor compression cycle to produce the cooling capacity necessary.

The refrigeration liquid in the cooling coil pipework of the CRAC unit absorbs and transfers the heat from the hot air passing over it, cooling it.

[Below is a simple CRAC Unit Piping Diagram]

The heat is then taken via the refrigerant pipework and refrigerant as a gas through the compressor where it will be compressed before passing through the external condenser unit which will use fans to pass ambient air across the coil removing the heat from the refrigerant prior to passing through the expansion valve and the cooling coil.

CRAC Unit Manufacturers

There are lots and lots of manufacturers of CRAC Units worldwide, the main ones that we have come across are:

  • Airedale
  • Stulz
  • Trane
  • Vertiv / Liebert [Emerson]
  • York

CRAC Unit and Condenser System Checklist

To cover what is needed on site for the Pre-Functional Checks prior to commissioning see our article ‘CRAC UNIT | Pre-Functional Test Checklist’


Frequently asked questions

Is a CRAC Unit a Split System?

It is a type of split system, in that there is an indoor unit [CRAC] and an outdoor unit [Condenser], with cooling via a refrigerant process.

Do CRAC Units have humidifiers?

The supply of a humidifier in a CRAC unit will depend upon the design of the overall system. Sometimes they have humidifiers, other times they do not.


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