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Yesterday Chris Leahy (my Technical Facilities Manager) and I were agonising over why we had low plenum pressure in our Data Center and why we were seeing symptoms of hot air trapped in the roof void. We looked at all the normal stuff:

  • Leaks in the plenum space
  • Badly sealed floor
  • Cable access holes improperly sealed
  • Blockages in the plenum
  • Bad seals between the plenum and the CRAC units

In the end we worked out what the problem was. We have one of our CRAC units switched off as it is under maintenance. CRACS are pretty simple devices, they typically have dust filters, cold coils (water or DX) and an axial fan blower to drive the air down into the raised floor. In our case, because one CRAC was off, it was acting like an open chimney unrestrictedly delivering huge quantities of cold air high into the data center.

In fact it was behaving just like three fully open floor tiles. No wonder the plenum pressure was very low. We asked Stuart Hall at ARUP for his take:

A standby CRAC unit without non-return dampers will allow cold air to back-flow into the hot-zone.

We generally specify CRAC units in our designs with non-return dampers for this reason. The CFD software which I use includes them on all CRAC units by default (though they can be removed).

The amount of air returning would depend on:

  • The amount of floor grilles and other openings within the raised floor
  • The flow rate of air delivered to the floor by the operational CRAC units
  • The resistance to airflow caused by the idle-fan, cooling coil and other geometry within the CRAC

I have witnessed this behaviour whilst surveying data centres. It is also common to see a small quantity of back-flow through CRACs with non-return dampers as they don’t make a perfect seal.

The only arguments that I can think of against installing non-return dampers would be space or cost. Perhaps a business decision was made to accept reduced flow performance in the event of a failure in exchange for cheaper units or physically smaller units.

So here is the lesson of the day – don’t make asumptions that switched off kit is neutral!  If you have CRAC that are not switched on they may be leaking your precious cold air into the roof void.

There Are 2 Responses So Far. »

  1. It has been my experience that air moving back through a CRAC unit due to it being idle is not like open raised floor tiles. This is due to the fact that the subfloor static pressure must push the air up through the fans, coil, and high density filters. Another important factor is that this air (without a CRAC extension into the drop ceiling void) would be emptied out into the ambient air and not a hot-zone. Most data centers mix hot and cold air to reach an homogenized supply temperature thus supply air through the idle AC unit would have a modest affect on the overall cooling. That is, of course, unless the AC unit represents more then 35% of the supply. The best way to fix this would be to employ either hot or cold aisle containment.

  2. Thanks for a great response Larry. A couple of additional points – most high density filters are removable. In our case the filters simply flip over when the airflow runs in reverse. Second issue is that not all technical spaces have a false ceiling.

    The best policy is to use CRACs with dampers.


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