Designing a Data Center Cooling Unit
When we design a data center cooling system, we usually concentrate on its capacity and redundancy. The air management is, most of the time, ignored which is why many data centers have hot spots, problems with cooling and very high PUE.
To design a good air management system in a data center, we need to take the following into consideration:
Keys to designing an efficient air system
- Delta T or CFM/kW: this is the design engineers’ most underused parameter.
In order to decide on a good CFM/kW, the server air flow requirements must be thoroughly checked prior to installation.
For example, if we design a data center with 100CFM/kW, but the server fans draw 130CFM/kW at maximum speed, we will create an imbalance in pressure between the cold aisle, hot aisle and the outside. This means that we will have air infiltrating to the cold aisle from the hot side or from the outside, which would therefore create hot spots.
Decreasing the CFM/kW requirement, and increasing the delta T between the cold aisle and the hot aisle, can help with increasing efficiency and capacity of a cooling unit, but it will create a big imbalance in the room which will cause hot spots. Designing with the inappropriate parameter tends to create complications in the future.
A safe number, based on our experience, is 160CFM/kW or delta T of 20F.
- Containment: there is a flawed impression with designers that a containment in the hot or cold side is required for efficiency purposes only; however, selecting a cooling unit, or designing a cooling system for a data center, with or without good containment can make a significant difference.
For example, if 80F is required in the cold aisle, and your containment is not efficient, then your cooling units need to be oversized in order to compensate the infiltration of hot air between the hot and cold aisles. Designing a perfect containment system is key in any DC cooling design.
We have 3 methods to design a containment system:
A) Hot aisle containment ( HAC): Containing the hot aisles. In this method, air is supplied to the cold side, which means that the room feels like the same temperature as your cooling unit supply temperature.
B) Cold aisle containment (CAC): Containing the cold side. In this method, The room feels like the same temperature as the return temperature of the cooling unit.
C) Chimney system: In this design, the hot air is contained in the cabinet and pushed directly to the return plenum of the cooling unit.
Our preference are methods A and C because, nowadays, we are raising temperatures, therefore it would feel very uncomfortable for any technician or employee to walk into a room that feels 100F. Containing the cold side is more complicated / complex and isn’t worth the added resources.
Whether selecting choice A or C above, depends on usage and application.
Regardless of the type of containment system that is chosen, it is critical that all potential openings are covered. No air should be able to infiltrate between the aisles.
- Speed of supply air fan: most new AHU designs have VFDs or EC fans integrated on the supply fans. We can achieve a good CFM/kW design at maximum speed, however if we reduce the speed of the fan lower than the CFM that is pushed by the servers, we return to the same issue as described in number 1.
The best way is to calculate the capacity of the room. From that, we can use the design CFM/kW and calculate the CFM needed +10%.
If we need a better control, we can install a motorized damper in each cold aisle and modulate it based on that aisle’s capacity.
If the above 3 measures are respected, we will have no issues with hot spots. Unlike what many designers think, the location of the supply air does not matter. The supply air can be far from certain racks, but if the CFM/kW is respected and we have a near perfect containment, we won’t have any hot spots in the data center.
For more information regarding the use of Air2O products in your next data center project, please contact us and an expert will be in contact with you shortly.
About the author: Kevork Kazanjian, M.Eng, ATD.
CEO & Founder of Adag Consultants, specialized in Data Center Design & Construction. Kevork has over 10 years of experience in the field and has designed and built over 200MW of data centers.