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Q: Can you point us to any documentation on what sorts of requirements we should be asking for in terms of selecting the exact transformer to use? This application will be single-sourced HPC systems, so we'll expect fairly high loading on the transformer most of the time. We need to understand the characteristics/attributes that we want and find them in a USA supplier or justify why we need them and that no USA supplier offers them. Thanks for any pointers you may have.

A: The DOE 2016 requirement for low-voltage distribution transformer efficiency is specified at 35% load, which is probably a good assumption for a commercial building, but a bad one for a data center striving to make the best use of limited power distribution resources. That standard requires no less than 98.83% efficiency (again, at 35% load). Overall transformer losses are the total of a fixed no-load loss and a load loss that scales with the square of the load, thus the DOE standard numbers are dominated by the no-load loss and the high loads that you are striving for will be dominated by the load loss and there are trade-offs in transformer design between the two.

I'd suggest getting the efficiency curve from Powersmiths for their ESAVER-50H (optimized for high loading) and use that to specify the efficiency requirement at e.g. 75% load, which is a standard rating point. Other parameters, especially impedance, need to be managed to optimize inrush current, fault level, and arc-flash to make sure it will integrate with the existing installation. We learned this the hard way at FLEXLAB. Hope this helps!


Q: A quick question about CRAH VFD fan control. At some WHS data centers, there are some CRAH units with VFDs controlling to an underfloor static pressure setpoint of 0.1" wc. The static pressure in the room varies from 0.02" to 0.04" wc, so the VFDs are always running at 100% trying to reach the setpoint. I am planning to propose controlling the VFDs to cold aisle temperature setpoints instead of the UF static pressure setpoints. The goal would be to reduce CRAC VFD fan speed. In you experience, do you think this is the right approach? 

A: Your approach to CRAH fan control should work well in your application, given that with your cold-aisle isolation and blanking panels, you should have good air management. So the chilled water valves in the CRAHs would be controlled on supply air temperature (correct?), and the fans would be controlled on cold-aisle temperature; we recommend using a sample of the top-of-rack temperatures for control, and monitoring the rest of the inlet temperature sensors to alarm any hot spots. This scheme is a relatively direct way to ensure that the IT equipment is getting the recommended inlet air temperature. Even better than rack inlet temperatures, is to get the inlet temperatures directly from the IT equipment, but often this access is made difficult by security protocols.

Do check for inadvertent recirculation paths for hot air, including above and below the IT equipment in each rack, around the sides of the IT equipment, and networking equipment that often has airflow from back to front (i.e. the inlet is on the cable-connection side of the equipment, such that if it is installed with the cables on the back, it will be a air recirculator). Sometimes other equipment is installed backwards from an airflow point of view. Below and between the racks should also be sealed off to prevent recirculation.
Other fan control schemes that have been used successfully, although they are less direct that the above, include differential pressure between underfloor and cold aisle, and differential pressure between the cold aisle and the hot aisle (since a slight positive pressure in the cold aisle helps ensure minimal recirculation of hot air).  Your setpoint for underfloor air pressure is at the high end of where most perforated tiles are rated, so a lower setpoint might work better, depending on whether your center is actually designed for the higher pressure, and whether the design flows are needed for your loads. But go with your plan to use temperatures per the above. And please let us know how it goes!

Q: How do systems that are outside air economizers with evaporative cooling work?

A: Increasingly indirect economizer/evaporative cooling schemes available. These are air-handling units outside the data center that use air-to-air heat exchangers and indirect evaporative cooling, usually with a compressor-based backup. Ducts connect these AHUs to the hot and cold air plenums in the data centers. All provide most of the energy savings of an air-side economizer without the (overblown, in most cases) concern about bringing outside air into the data center. And they are likely to be a system with a higher capital cost than a conventional ASE.