Carnot Refrigeration: CO2 Offers Big Pressure Differential in Data Center Cooling — Within Just one Degree

Oct 9, 2017
by Josh Anderson
Ann-Sophie Hamel-Boisvert, Account Manager at Carnot Refrigeration

TORONTO, CANADA — Ann-Sophie Hamel-Boisvert is an Account Manager at Carnot Refrigeration, a Canadian firm whose vision is to create and develop refrigeration throughout the world that is respectful of the environment, in order to make it a global standard for all sectors. At our Second Annual Canadian Data Center Summit earlier this summer, Ms. Hamel-Boisvert presented a special breakfast workshop address titled Innovations in Cooling with CO2. Below is the second article in a two-part transcript of some of her remarks. The first article, published previously, provided a brief historical interview of cooling with CO2, while the below remarks outline the different processes that comprise CO2 cooling as well as the benefits therein for the data center industry.

“People often assume that CO2 isn’t efficient when outside temperature is higher,” continued Hamel-Boisvert. “But one thing that came from the renewed interest (see our previous article) is a better understanding of systems. There are three systems for CO2 cooling. One is trans-critical, which is when it’s hotter outside, and sub-critical, when it’s cooler outside. In England, in the early 1900s, they only did sub-critical cooling, but when the temperature is higher, you loosen capacity. With technological advances of modern times, we kind handle these effects by going into trans-critical mode.”

Next, Hamel-Boisvert explained that for cooling data centers in particular, CO2 cooling also has a third mode of operation, which is the free cooling mode. “In this mode, because of thermal properties of CO2 and the higher pressures, we can do cooling by thermocycling, without ever having to use a pump or a compressor.”

“So basically, you have your CRAC unit and you evaporator in your room, and you have your condenser on the roof. We call it a recycle because it’s very similar to what happens with thermos,” she explained. “So you have your evaporator, which is heated by indoor air. It becomes vapor, then because of the high pressure and the thermal properties of CO2, you can build enough pressure in that evaporator to get your CO2 to your condenser without using anything. So you’re going to build enough pressure to combat all of the pressure losses in your tubing to your condenser.”

According to Hamel-Boisvert, a big difference between CO2 and traditional HFCs is the pressure differential that you get — with one degree. “So if you have, let’s say 18 degrees in your evaporator, and 17 degree temperature outside, you will be able to build about 10 PSI of pressure differential to pump your gas up to your condenser. Without having to use any external forces. Then once my evaporator is in your condenser, it’s going to condense, and through gravity, you’ll be able to get your liquid back to your evaporator, and start the cycle all over again. So as you can imagine, this is a really efficient way of doing some cooling.”


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