Fact 1: Providing adequate air flow capacity and good air flow delivery is as important as cooling capacity.
What does that mean? Cooling requirements for a data center are typically determined by the projected heat load from the IT equipment and other heat generating equipment that will be in the white space. Air flow, on the other hand is just assumed to match the needs of the IT equipment. And if there is adequate cooling capacity then there will be enough air flow. True enough, however where this begins to unravel is in the delivery of the air flow to where it is needed. Success of the cooling systems to meet the needs of the space is dependent on how well the air flow reaches the IT equipment and passes through to extract heat from the components. If certain areas of the room have much higher density rack heat loads and there is not adequate air flow in that area to meet that need, hot spots will arise – leading to the misconception that more cooling is needed. In actual fact better air flow delivery will resolve the problem and save the thousands of dollars in capex for a new and unnecessary cooling system.
Fact 2: Placement
of cooling and IT equipment is crucial to good cooling operation
Another common misconception is that all cooling units will extract the same amount of heat from the data center regardless of their location and proximity to the IT load. Cooling systems operation repsonds to the return air temperature which represents the set point. If cooling systems are close to the highest heat load racks they will pull in more heat and typically operate at a higher percentage of cooling capacity. At the same time, cooling units
further away may be receiving lower temperature return air and therefore will operate at a lower cooling capacity. The result is some cooling units will be operating at 100% capacity while others are operating at 25% capacity or less. We have seen rooms with multiple cooling units of which less than half the units were providing any cooling resulting in the high-density areas experiencing hot spots.
Airflow should be closely matched to the requirements of the IT equipment
In the case of a raised floor data center, supply air is channeled under the floor and enters the white space through perforated tiles. The level of airflow through the perforated tiles is determined by several factors including the pressure differential between the supply plenum and the white space and the location and percentage opening of the tiles.
If there is a high volume and high velocity of air in the supply plenum, the pressure in the supply plenum drops and can, in many cases, result in a negative flow of air through the perforated tiles – meaning the warm air from the room is being drawn down into the supply plenum. This is common when perforated tiles are located too close to the cooling system. When this happens, no cooling is provided to the nearby racks.
The placement of cooling units can result in air pathways crossing in the supply plenum causing vortices, which create dead spots – again, resulting in no air passing upwards through the perforated tiles.
By introducing additional cooling and air flow into a space, the volume of supply air can reach the point where air flow through the perforated tiles decreases causing even more cooling issues.