For one reason or another, (not consulting with an engineer) they improvised by creating a "hot pod" that contained the hot aisle of two rows without a top chimney (connection to a return source to the CRAC units). Image 2. The end result was a hi tech-looking pod that looked like a duck, walked like a duck.... but it didn't quack like a duck.
When my friend described the situation to me, my conclusion was that he somehow managed to positively pressurize the hot aisle; but how? The answers can be shown using CFD simulation. The model was created by my colleague Eric Fournier (an expert in CFD modeling!!) for purposes of this blog only. We exclusively use 6Sigma Room from Future Facilities for all our CFD modeling. We assumed densities across the rows as that information was not shared with me. The largest density 10 kW/rack, the smallest 3 kW/rack.
Complaints were flowing in from the leasing client that the face of the idle servers are too warm (standing in the cold aisle). The facility operator also noticed that they are constantly having to over-cool the cold aisles to compensate for rising temperatures along the front of the idle racks (total waste of energy) and avoid breaching of Service Level Agreements (SLA's). Image 3 shows the same effect.
The concept of containment and releasing heat works when you create a differential pressure. In household chimney or furnace design, you have to create a draft using a long vertical stack with an intake at the low point and vent at the high point. This is a natural vent concept. A pressurized vent is similar but you have to physically connect it to a negative pressure source (return fan). In a contained system, you need that physical connection and a negative pressure source: a duct, return plenum, direct connection to the CRAC unit return fans. All these components are missing in this installation.
Video 1. Pressure Across Isometric View
In the open hot aisles, pressure is more "even" with higher pressure logically in the center and towards the top of the hot aisle as the air exits the servers and leaves the aisle horizontally and vertically back to the CRAC unit returns; hence minimal re-circulation back into the servers.
Shown below are two video which animate variations in temperature through a section and an isometric view. The CFD shows a plume veering to the left side of the pod perhaps as a result of the location of one the CRAH units. This could be adding to the effect of the inefficiency of the air leaving the top of the pod and putting higher pressure on the idle servers on the left row. If this pod had a chimney, air would be leaving evenly in the center of the hot aisle.
- Pressure can play a number on you and you need to understand how to manage it.
- Properly designed containment systems work great; it is well worth to involve a qualified person to design one.
- Improvising designs is dangerous. A poor design can lead to breaching of Service Level Agreements (SLA's) and often ends up with a client packing up their servers and walking away to a more reliable site.