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Is air circulation a problem with a rear door heat exchanger?

We use rear door heat exchanger cooling. We have the hoses for the doors and electrical trays mounted in the 24-inch raised-floor plenum. We use ladder trays above the racks for network connectivity. We have no CRAC units, only an air handler to maintain positive pressure and humidity.

What needs to be addressed regarding air flow inside the room and under the floor?

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I have Thermal Guidelines for Data Processing Environmentsthird edition from ASHRAE, and it doesn't seem to deal with a room populated with only RDHX and no CRAC.

There are really two major considerations with rear door heat exchangers (RDHXs), which are among the most energy-efficient cooling approaches available. The first is maintaining clear above-floor paths for air recirculation. The second is providing external humidity control.

RDHX cooling should need no additional air flow. If the room is properly designed and the cooling capacities of the RDHX units match or exceed the heat loads generated by the computing equipment in each cabinet, then the air discharged from any cabinet through the RDHXs will be at room temperature. The computing equipment will then draw the input air it requires from the room environment.

The discharge air from the RDHXs needs to be unimpeded so the room is maintained at a stable temperature. Air also has to be free to recirculate back to the fronts of the cabinets to provide equipment intake air. This is the antithesis of conventional modern data center cooling design, so it can confuse those not fully familiar with RDHX systems. You can even revert to legacy cabinet arrangements if you are using entirely RDHX cooling, placing cabinets face to face and back to back, but this is not necessary.

Building codes require that a certain amount of outside air be brought into a room so that people are not breathing stale air. This air should be filtered and may even be preconditioned before being released into the room to minimize contaminants and stabilize its temperature and humidity before it mixes with the data center air. It does not matter whether this air is delivered from under-floor or overhead. Since the quantity is small and it is not needed for actual cooling, and if it goes through a raised-floor plenum, the space doesn't need to be as high or as clear of obstructions as is necessary for full under-floor cooling. Further, a well-designed data center should be essentially a "sealed room," so the "makeup air" will tend to create a positive pressure that will also act to avoid air-born contaminant infiltration whenever a door is opened.

I am going to assume that the air handler you are using is a conventional office unit running off the building's chilled water system and that you are not differentiating between the two types of precision cooling units -- computer room air conditioners (CRACs), which contain internal refrigeration compressors and computer room air handlers (CRAHs), which utilize chilled water from an external chiller plant. I would certainly prefer a precision air conditioner in a data center, but the distinction is not critical to answering your question about airflow. The RDHXs and server fans should provide all the actual airflow needed for the cabinets on which they are mounted. Nothing additional should be needed to assist it.

Remember, however, that RDHXs provide only sensible cooling. They do nothing for humidity control; the water running through them is usually maintained at aboutfour degrees Fahrenheit above the room's dew point to ensure there is no chance of getting condensation on the coils. So if the humidity in the room is higher than it should be, the RDHXs won't remove it. Neither will they add humidity if it is needed.

Humidity control with RDHX cooling is usually provided by some amount of base cooling using conventional CRACs or CRAHs. This also removes heat from equipment for which RDHX cooling is impractical, such as network switches. Alternatively, humidity control can be provided by standalone humidifiers and dehumidifiers. But remember that, per the ASHRAE Thermal Guidelines book, humidity control should be via dew-point sensing, not by relative humidity.

The ASHRAE Thermal Guidelines book doesn't address these issues directly because rear door coolers are just one of many ways to maintain the thermal envelope that the book sets forth. The ASHRAE Liquid Cooling Guidelines book provides more information regarding devices that circulate coolant through the cabinets. Likewise, the Thermal Guidelines book sets the humidity envelope, but neither it nor the Liquid Cooling book tells you how to achieve it. That is the responsibility of your design consultant.

About the author
Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom and Wilke LLC. McFarlane has spent more than 35 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professional program, is a data center power and cooling expert, is widely published, speaks at many industry seminars and is a corresponding member of ASHRAE TC9.9, which publishes a wide range of industry guidelines.

This was first published in March 2013

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