Spring is coming. Will your data center be a spring chicken or an old bird? As the weather begins to warm (hopefully) this is a great opportunity to take stock of your data center and it’s economizer system and operation.
We have this sensor, we call it the million dollar sensor. Now how can a single sensor be worth a million dollars? The answer is simple, really, it’s the outside air temperature and humidity sensor, and we have selected it such that many of the problems with other sensors, particularly humidity, are eliminated. By having a highly accurate sensor, we can reduce the margins for the economizer to engage ( for instance, return air temperature plus 2 degrees instead of 4) and have knowledge that we won’t be what I call “anti-economizing” where the air we bring in has more enthalpy than the air exiting the building.
By having accurate humidity measurement, we also have accurate enthalpy measurements. Depending on the region of the country, enthalpy economizer control can add a few hundred hours, but in many regions a comparative dry bulb will work very well with a simpler control scheme and less component failures.
So make sure your outside air sensors, return air sensors, and check that economizer sequence of operation.
But let’s talk about that economizer logic. Now, hopefully everyone is using the ASHRAE temperature bands, from what I have seen the recommended band has the most universal acceptance, and with those bands you have set the economizer aggressively. The important thing about the recommended band is that it is an operating band and not a limit. You should use every corner of the band to maximize energy savings: let humidity ride up to the upper limit rh and dew point and allow temperature at the server inlet to approach 81 degrees. The energy savings which can be realized are very substantial. If you can satisfy your environment with 70 degree air in the supply plenum, then any time the outside air is less than 70 degrees you can be fully economizing. In Chicago, that is 7000 hours a year with no mechanical cooling. And if you have your server inlet at 75-77 degrees, and your air volume is set up for a nice 25 degree delta across the servers, than you should have a partial economizer available whenever it’s less than 98 degrees out which in Chicago is about 2 hours a year (in reality it’s less but that’s for the 95-99 degree temperature bin). The only real lockouts for partial economizer should be high humidity in the data center environment. Using the warmer air at 75-77 at server inlet or more, you won’t be worried about relative humidity, you will be approaching the max dew point ASHRAE rating first. So you just need to protect your raised floor from getting to humid. One method is monitoring the dew point at the server inlet and as the upper limit of the ASHRAE band is met, bet reduce the maximum economizer percent, and if dew point goes outside of the and we can disable the economizer. There’s lots of ways to do it, but really pushing those savings requires aggressive use of the ASHRAE bands.
Be aware of the word of caution on Enthalpy economizers. Enthalpy Economizer Issues
You can push things further, especially in dry climates, with direct evaporative cooling which cools the air without mechanical cooling. But a lot of climates can get a great portion of the economizer hours with the aggressive use above.
And you may even be able to re-evaluate your bus loading, alluded to in previous posts, if you never need to engage the full capacity of the cooling system (as well as the fact that the cooling will work more efficiently when the return air is warmer).
A word of caution: R-22 is not a high temperature and pressure refrigerant. That is to say, when that return air starts passing 85 degrees, R-22 based machines can start to experience reliability issues as they stop cooling the compressor properly and lots of other effects can happen. So you may want to reach out to your vendors to discuss. Conversely, by using the expanded band, you may so reduce compressor hours though that this impact is overshadowed by low run hours and your equipment actually ends up lasting longer. But, it is a consideration to keep in mind.
Anyway, let’s all be spring chickens! Don’t leave these gobs of money on the table.