building cooling to support electronics equipment cooling – part 2 computer room air conditioning (crac) units

introduction

continuing on with this series (cooling zone august 2004 – december 2004), the next component of building cooling equipment we will introduce is a computer room air conditioning unit or as it is more commonly known, a crac unit. crac units are typically used to cool large scale (20kw or more) installations of electronic equipment. they are sometimes called by other names such as cahu, cacu, etc. but for the purposes of this article, we will only use the crac unit acronym.
although these units all look very similar from the outside (figure 1), the components within the crac unit can be configured to integrate with a wide variety of building cooling systems including chilled water, direct expansion (dx), glycol, air-cooled, water-cooled, etc. and the units themselves are available in capacities ranging from 6 to 65 “nominal” tons (or 20 to 115kw of electronic equipment cooling).
for the majority of electronic equipment installations, the heat generated by the equipment is discharged into the computer room environment in the form of hot air through small fans that are integral to the electronic equipment packaging. this hot air from the electronic equipment is known as return air (i.e. air that is being returned to the cooling equipment to be cooled). the return air is transported to the crac units where it is cooled and redelivered to the room environment adjacent to the air inlets of the electronic equipment.
the cold air delivered to the electronic equipment room environment from the crac unit is known as supply air or conditioned air. in the electronic equipment industry, this air is sometimes referred to as chilled air but the term chilled air is not used in the building cooling industry.

figure 1 – typical crac unit

location

more often than not, the crac units themselves are located in the same room as the electronic equipment. the crac unit itself has a fairly large physical footprint (the larger crac units are 8 to 10 feet wide by 3 feet deep) and also requires unobstructed maintenance clearance areas to the front and the two sides of the unit making the overall impact to the floor area of an electronic equipment room significant.
where spatial constraints result in the electronic equipment room to not be able to accommodate any crac units, they may be located in adjacent rooms with ducted connections to supply and return air from the electronic equipment room (figure 2). however, the crac units cannot be located too far from the computer equipment that they are meant to cool due to limitations of the crac unit fans.

figure 2 – crac unit located outside the electronic equipment room

airflow paths

the airflow paths to and from a crac unit can be configured a number of different ways. the cold supply air from the unit can be discharged through the bottom of the unit, through the front of the unit or through the top of the unit. the warmer return air can also be configured to enter the crac unit from the front, the rear or the top.
the most common airflow path used by crac units is referred to as “downflow” (figure 3). a downflow crac unit has the combination of return air entering the top of the unit and supply air discharged from the bottom of the unit, typically into a raised floor plenum below the electronic equipment racks. the supply air pressurizes the raised floor plenum and is reintroduced into the electronic equipment room proper through perforated floor tiles that are located adjacent to the air inlets of the rack mounted electronic equipment.

figure 3 – downflow crac unit airflow path

upflow crac units have return air entering the crac unit in the front of the unit, but towards the bottom. the supply air is either out of the top or out the front but near the top of the unit. a frontal supply air discharge is accomplished through the used of a plenum box which is basically a metal box with no bottom panel that is placed over and entirely covers the top of the unit. the plenum box has a large grille opening in its front face to divert the air out in that direction (figure 4).

figure 4 – upflow crac unit airflow path

instead of using a plenum box, an alternate airflow distribution method for an upflow crac unit is to connect the top discharge to conventional ductwork. the ductwork can then be extended to deliver supply air to one or more specific locations within the electronic equipment room (figure 5).

figure 5 – ducted upflow crac unit airflow path

upflow crac units are typically used in areas that either do not have raised floors or areas that have shallow raised floors used only for power / data cable distribution.

configurations
the common components within all cooling configurations of crac units are a fan and a coil. other optional components that are available to all crac unit configurations include a humidifier and a heating element, the combination of which help control the humidity of the electronic equipment room environment. the specific type of building cooling system that is being utilized determines and additional components located within the unit as well as determining the type of liquid media that is circulated in the coil.
in addition to the upflow and downflow approach to classifying the crac units, the units can also be classified by whether or not the refrigeration process occurs within their physical enclosure. these fall into two main categories called compressorized (e.g. air cooled, water cooled, glycol, etc.) and chilled water systems.
a crac unit is said to be compressorized or self-contained if it contains the compressors within its housing. compressorized crac units require a remote piece of equipment to perform the heat rejection and a liquid (condenser water, glycol, etc.) is piped between the crac unit and the heat rejection device.
in a chilled water crac unit, the mechanical refrigeration takes place outside of the physical extents of the crac unit enclosure in a remotely located chiller (see august 2004 cooling zone article for chiller description). no compressors are required in the crac unit and only the chilled water piping from the chiller is required to provide chilled water to the coil. chilled water crac units have a greater capacity (up to 60 nominal tons or 115kw) than compressorized systems (up to 30 nominal tons or 88kw).
some of the manufacturers making crac units used in “building cooling to support electronics equipment cooling” include liebert, data-aire, and stultz. the specific data quoted above reflects the liebert deluxe system 3 family but these manufacturers as well as the others should be considered based on the many parameters such as local service track record, price, energy, acoustics, size, availability, quality, features, etc.

about the author: don beaty, pe is president of dlb associates consulting engineers, pc and chair of ashrae technical committee tc 9.9. 732-774-2000 [email protected]
about dlb associates: a consulting engineering firm specializing in building cooling worldwide including electronic equipment environments. dlb’s projects have been as large as 36,000 tons; 84,000 kw; and over 1 billion dollars in build-out programs. www.dlbassociates.com 
about ashrae: a 50,000 member nonprofit engineering society specializing in cooling. tc 9.9 is focused on providing critical information for the industry including high density cooling as well as promoting convergence of the various segments of the industry including both building and equipment cooling, equipment manufacturing, it, operation, & management. http://tc99.ashraetcs.org/
© 2004 by don beaty. all rights reserved.