One problem we have seen with curtains on cold aisles is the flapping or drawing in of the curtains due to the airflow through the adjacent perforated tiles. Sometimes people call this the “dancing curtain problem.” The Bernoulli effect (air plane wing effect) of lower pressure on the side of the higher air velocity causes this. This flapping can push the curtains close to the server intakes and then the suction of the intakes can suck the curtains onto them, blocking and essentially starving the servers. We caught this blocking three times before we adopted a policy of avoiding curtains on cold aisles.
After examining this and having the good fortune of a customer doing some testing we concluded that flapping curtains are not a fuction of air balance in the contained zone. Rather, it is a function of turbulence caused by the airflow through the nearby tiles. So, even if one has an oversupply of air, the curtains just don’t push out gently as one might expect.
We tried adding weights to the curtains to prevent their movement, but this only resulted in heavy flying objects banging into the servers. Ultimately, we added two solid panels to the sides of the strip door. By the time we did this, the cost was about the same as a sliding door like the one pictured below.
Sliding Door Preferred Over Curtains for CAC
The design approaches to combat the flapping curtains include:
Hot aisle containment – there is no air flow from the floor
Sliding doors – they don’t move
Solid floor tiles at ends of rows – no nearby air movement
Extended containment perimeter – basically place doors away from aisle end in some manner
Existing data centers protected with FM200 or other clean agent systems can be a challenge to contain because the cost of adding or moving nozzles is very high. One alternative has been the use of electronic fuse links that trigger off the FM200 system. Though the links are pricey (about $250 each) and need testing, it is a workable solution that can still offer an ROI that meets project goals.
Electronic Fuse Links
The electronic links pictured above where installed on a hot aisle containment system pictured below. The site has a ceiling with two levels and the FM200 nozzles are installed in the lower section of the ceiling. The curtains were installed with a gap above them to allow some air movement (so not total isolation) in order to assure that air flow to the existing smoke detectors were not obstructed.
Hot Aisle Containment
These links have an electronic trigger off a signal from the FM200 system as well as a thermally activated mechanism. The wiring requirements are similar to those for smoke detectors and is through conduit that you can see in the above picture. The other wires are tethers to stop the fall of the curtain after it drops about two feet to clear the FM200 nozzles.
A customer did a burn test on Polargy’s PolarDAM Air Dam Foam and posted it on Youtube. Of course, we were happy to see that the foam performs as specified and is self extinguishing.
Interestingly, I visited another customer who had put a few pieces of our air dam foam in an environmental test chamber to see how well it withstands high temperatures and varying temperatures. I had no idea that they were doing this until my visit. We examined the sample pieces and could see no indication of the foam decaying. I was surprised to learn he had this test running for the past three months.
Today I talked to a facilities engineer for a lab data center. They have already added strip curtain doors to the ends of the cold aisles and the top of the aisle remains uncapped. Next, they plan to install VFDs and put temperature sensors at the rack tops in the cold aisle. They plan to drive the VFD’s off the temperature at the top of the cold aisle.
This is an interesting approach because it seems to assure that the air flow is sufficient to the cold zone. And, with an open top, it is especially important to assure sufficient cold air into the aisle. Without that, or in an under-supply situation, the aisle would take hot exhaust air from over the tops of the racks.