Ian Seaton, CPI’s Technology Marketing Manager, recently received the following question.
Question: My data center is a limited floor space (1600 sq) and running at 18 kW per cabinet. The cooling experts are indicating that a load of 18 kW would require about 2700 CFM to cool the equipment. They also indicate that max let through on a "Tate" floor tile is 1300 CFM. In this scenario we designed a wider cold aisle to accommodate a (2:1 - 2.5:1) floor tiles to enclosure ratio to meet the CFM required by the equipment.
This scenario leads to the potential of high density exceeding traditional cooling capabilities whether using CPI's Vertical Exhaust Duct or not, if there is not the floor space for additional perf tiles then your equipment cold air demand can potentially exceed the let through ratio. If true, would there be a watt per square foot - line/threshold, based on traditional cooling, raised floor height etc.
Answer: First, it is theoretically possible that you could get to a density where you just don’t have enough floor space to fit in enough flow tiles. Additionally, it can also become problematic on where to fit all the air handlers. For those reasons, we typically try to guide designers for very high density applications away from raised floors and computer room air handlers. Very large chilled water central air handlers feeding the data center through the ceiling or through a wall can be a much more effective design alternative for higher densities, and if they are roof mounted or mounted between the roof and the data center, the architectural concept is closer to being able to accommodate air-side economization. All that notwithstanding, it should not necessarily start being a problem at 18 kW per cabinet or even up into the 20s, at least in a 100% isolation data center such as cabinets deployed with CPI's Vertical Exhaust Ducts. Read more about CPI Passive Cooling Solutions.
Also, you want to think about what kind of airflow you really need. In your example, the 2700 CFM for 18 kW works out mathematically, but not necessarily practically. That might take a little explaining. Your typical 1-2 RMU servers will consume about 150+ CFM per kW, which falls right into your example. However, blade centers will range from 60-90 CFM per kW. These figures come from the equation describing sensible cooling: CFM = 3.1W ÷ ΔT, where W = watts and ΔT = the temperature rise of the air through the server between inlet and exhaust. As you can see, the heat transfer in blades is much more efficient than in standard and legacy servers.
Then, consider what you can fit into a cabinet. For example, 21 2 RMU HP DL-380’s will consume 42 RMU of cabinet space and produce about 10 kW of heat. Therefore, you will find that it is not likely that you can get 18 kW of low ΔT (ergo high airflow) equipment into a single rack. It is not impossible, but it is not likely in most applications. Therefore, the higher densities are more typically going to be comprised of blades or larger equipment that is going to be designed to use air more efficiently, with a resultant higher ΔT.
Remember also with the Vertical Exhaust Duct, there is no traditional hot aisle or cold aisle so you can stick those perforated floor tiles anywhere in the room just to assure the space is adequately replenished with replacement chilled air.
I don’t think that you can get much more definitive than that without actually building a computational fluid dynamics model of the space in question. Our Technical Support group can help with that.Give them a call or send them an email at firstname.lastname@example.org. If you have a question for me, leave a comment or email me at email@example.com. Ian Seaton, CPI Technology Marketing Manager