Traditional Computer Room Cooling Approach

In the days of large mainframe computers, there was a need for careful environmental controls within limited temperature and humidity ranges. While computer equipment has changed notably and is a bit less environmentally sensitive, the fundamental concern of environmental control remains. Recognizing the need for change in 2008, the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) relaxed the recommended environmental conditions for “Class 1 Electronics” which includes the type of equipment found in data centers and computer rooms. Room temperatures are now targeted to remain between 18 °C – 27 °C (64.4 °F – 80.6 °F) compared to the previous 20 °C – 25 °C (68 °C – 77 °F). This change in standards by itself permits more flexibility in the design of cooling systems and energy efficiency by allowing a larger temperature swing and a higher operating temperature within the room (i.e., less cooling demand).  Similar relaxing was done for the range of low end and high end moisture content in the air which now allows for up to 60% relative humidity in computer rooms under the right conditions. This is also a significant change that helps with overall energy efficiency but provides an appropriate interior environment for most computer equipment.

The typical approach to meeting the ASHRAE targets in computer rooms has been to provide free standing cooling units located inside the room. Referred to as Computer Room Air Conditioning (CRAC), these cooling units are commonly very sensitive to temperature and humidity changes, include correspondingly sensitive controls to maintain design conditions, and commonly carry a high price tag to go along with their high level of sophistication. Nonetheless, their common means of operation would be to cool the air in the room by setting the controls to call for air conditioning once the return air temperature reached a specified level, typically 72 °F. At that point, the unit would kick on and provide cool air typically set for 55 °F as it enters the room. This cooled air is intended to flow freely around open racks of equipment and theoretically provide a fully conditioned, uniformly cooled, and optimized environment both for the computer equipment to operate in and for people to work in when needed. In practice, however, a number of common problems have emerged from this traditional approach.

Uneven cooling / hot spots
The first and most common issue is that the cooling is in fact not uniform throughout the room. The supply and return air mix in a more or less random pattern influenced thermal dynamics, equipment use, and occupants. As a result, unwanted hot spots can be created that don’t receive the proper cooling, causing adverse effects on both the room and the equipment in that location. This condition is exasperated by cabinet or rack conditions that interrupt or short circuit intended air flow creating a condition referred to as cooling bypass. This commonly occurs when cooling air intended to reach the warm aisles is inadvertently channeled or directed elsewhere due to openings or blockages in cabinetry or air passages that prevent the equipment from being properly cooled, hence the resulting “hot spot.”

Mixing supply air and return air in typical data centers using traditional cooling methods with Computer Room Air Conditioning (CRAC) systems can produce unwanted hot spots and require equipment to operate at lower set points for cooling. Images courtesy of Chatsworth Products, Inc.

 

High energy costs
The second issue with this traditional approach is that energy usage and the associated costs are high. The equipment in these rooms typically runs 24/7, 365 days a year. Keeping the entire room continuously cooled within the design levels can be a huge, ongoing, demand in all seasons, under all utility pricing structures, and sometimes more critically than other occupied spaces in the building. 

Change constraints
Traditionally cooled rooms are subject to the capacity of the cooling equipment that is connected to that room. Over time, owners or users of the room commonly seek to upgrade the computer equipment or add to it either because of changes in their needs or due to normally scheduled technology refreshing. The typical result in any of these conditions is both higher density of computer equipment and higher total cooling load. The concern of course is that the existing cooling system may be too inefficient to accommodate those changes, thus constraining the amount of change or growth that is feasible after the initial design and construction of the room or data center.

Now, to be fair, all of these problems have potential solutions in the traditional way of thinking. The issue is that the solutions can be rather complex and costly to implement, making them less attractive than some increasingly more popular alternatives.