Facebook Mist Cooling?

HI there ParFour and thank you for your question.

Yes, Facebook's Prineville data center uses mist cooling, previously known as evaporative cooling. For those in refrigeration, this is nothing new. Water cooled chillers have been using this technique since, well, since they were invented!

The way mist cooling works is the warm outside air enters the building's HVAC system where it is sprayed/misted with water. This water evaporates and in doing so, takes some of the heat from the air. This is how the air is cooled.

We find it interesting because in recent years, we have seen a decline in the use of water cooled chillers in many areas due to their high use of water. To have a feasible suppy of water, your site located near a water source such as a river, lake or (recently as seen with HP & Google) a sea or ocean.

The Facebook design uses a sort of double-decker design that we have seen with other data centers. The servers are on one level, and the air handling is on the other.

It seems to work well for Facebook, although they have had some issues with excessive moisture in the data center:

One challenge we encountered was keeping our air handler lineups from “fighting” with each other as they dealt with the rapid changes in the temperature and humidity of the outside air between day and night. For example, if outside air dampers of one lineup are at 70%, the adjacent lineups would have their outside air dampers at 20-30%. This alternate modulation, or fighting, often led to stratification of air streams.

This fighting is a simple controls issue that, we feel, should have been worked out before going live in a production data center.

Another, more significant, issue was an error in the sequence of operation controls that led to complete closure of the outside air dampers, causing the one-pass airflow system to function like a recirculatory system. The problem began to manifest in late June as outside air conditions started changing rapidly. The economizer demand signal began responding to the changes; that’s when the erroneous control sequence drove economizer demand to 0, leading to complete closure of the outside air dampers. Thus the data center was recirculating the hot exhaust air at high temperature and low humidity. The evaporative cooling system reacted to this high temperature and low humidity, spraying at 100% to maintain the maximum allowed supply temperature and dew point temperature. This resulted in cold aisle supply temperature exceeding 80°F and relative humidity exceeding 95%.

This seems like a controls issue that arose from inexperience with a new type of data center cooling system. Our take is that this should have been caught during test and balance and/or commissioning. (Although that may have been impossible since Facebook only experienced the condensation problem with certain ambient conditions, which are nearly impossible to replicate for commissioning purposes).

Any idea who the engineer was for that Facebook data center? Who is responsible for the calculations of outdoor air vs humidity vs temperature? That would be the engineer right? Unless they could blame it on the equipment malfunctioning. It's surprising that a company as huge as Facebook would build such an important data center with "experimental" cooling methods like mist cooling.

I heard that this new Facebook Data center has misters to cool the air instead of chillers or CRAC units. I know they have custom servers for reduced heat load but I do not understand the process of spraying water mist onto the concrete floor and that cools the servers...

They had to shut down too from what I understand because of condensate issue...." WELL DUH".

I am not closed minded but I have never ever heard of this before, of course that is what every one said about the chilled doors for server racks too....

chilled doors for servers?

Shane: When water evaporates, it does so by changing phase, which absorbs heat. Evaporate Cooling.

So the "mist" cooling used in Facebook's data center is just a new version of an old scene?

Yes, more or less. When water evaporates, it releases latent heat.

If you know anything about cooling towers, this is how they work. Warm/Hot water is sprayed above fans, and the moving air causes the water to evaporate. That's how nuclear power plants are cooled. (well, they have very complex, redundant cooling systems with multiple stages and heat exchangers). But ultimately, the heat generated by the nuclear reaction passes through water, which is sprayed inside the cooling tower (those big, hourglass/curved things) and evaporates. That's why you see "smoke" or a "cloud" coming out of a reactor. It's actually just water vapor that was used to cool the reactor.

Evaporative cooling, or "mist" cooling as it is in this case, is fairly efficient, but it's best used on a large scale. Also, you can't get the cold temperatures needed for comfort cooling (people) and some data centers.

You don't need cold temperatures for data ceners anymore!

Heh. I had a feeling I'd get called out on that. True, you can probably run servers at higher temperatures like 80°F but I'm just saying that most people still cool down to 72°F or so.