Equilibrium…we all try to achieve it in our lives. An argument can go on forever if both sides maintain a high energy level and refuse to cool things down. An argument can end when both sides take it down a notch and each reaches a happy place that they can both accept…a state of emotional equilibrium.
Odd as it may seem your air conditioning system is trying to do the same thing…reach a happy state of equilibrium…a balance between the high energy state and the low energy state. Fortunately the system won’t get there under most circumstances because when the high and low energy states are equal then the unit stops providing cooling.
To simplify our thinking about this, substitute the word “temperature” for the word “energy”. Now remember back to your days in physics class and remember that energy flows from a high state to a low state until the two states match and then the flow stops. An air conditioning system takes advantage of that basic law of physics by absorbing the warm energy in a room and sending it to a lower energy place where the warmth is released and the cycle can start all over again.
One problem in this description of an air conditioning unit is that we usually don’t want the high energy released back into our rooms so we have to send that energy outside the room, or building, to get rid of it. We do that by using refrigerants or water to transport the heat energy. We also have to be sure that when we send it outside that it is at a higher energy level than the outdoors. That is why we have compressors (and chillers which are just really big compressors) in our systems. The compressors act as both a pump and as a device to actually add energy to the fluid that is pumped through the cooling coil in the room. If you grab the side of a pipe entering a cooling coil it will feel relatively cold. If you grab the pipe between the coil and the compressor it will feel a bit warmer. If you grab the pipe on the leaving side of the compressor you might burn your hand. The system has added enough energy to make sure that when the refrigerant or water reaches the outdoors it is at a higher energy state than the air outside the building. That can be quite a challenge in a place like Phoenix or Dubai.
Most manufacturers realize that their equipment might be installed in those climates so they pick components in their systems that can operate under those circumstances. But there are limits to what can be done.
The most widely available commercial cooling systems on the market are DX packaged units. In order to satisfy the largest market (and sell the most equipment) these units are intended to be used for comfort cooling of people. Since most people are “comfortable” when their office is around 75 degrees these units are designed around that operating point. That is their happy point and that is the temperature of the air that is being returned to the cooling coil where heat energy can be absorbed into refrigerant or water and then sent outside to be removed. The units will continue to operate at higher temperatures but remember that we need to be sure we send the heat outside at a higher level than the air outside.
If you read the technical manuals for virtually every rooftop unit on the market you will see that, for a lot of really esoteric reasons, that rooftop unit is designed to operate at no more than 90 degrees returning to the cooling coil. At that point the combination of components in the rooftop unit will be “maxed out” if the outside air temperature is in the 120 to 130 degree range. When the outside air temperature is 135 degrees on a roof in Phoenix then the high energy state and low energy state are so close together that almost no work is done and the temperature of the air coming out of the air conditioning unit starts to go up because the system is no longer rejecting much heat. The system has quit working as intended and the situation usually spirals out of control as more heat builds up in the refrigerant or water. Eventually so much heat has built up, and is compounded by the compressor, that the system shuts itself down to protect itself.
So, what does all this rambling have to do with anything? Most of my blogs lately have been about mission critical/data center energy issues. It is a big deal, and lots of folks are working on solutions, but economics sometimes trumps clear thinking or limits what can be achieved.
We are seeing more and more data centers specified with DX rooftop packaged units. While these are normally high quality products they were originally designed to be at a happy place with, at most, 90 degree air being returned to the coil. In a data center that is being designed to the latest ASHRAE standards the cold aisle can be anywhere between 80.6 and 113 degrees F. When you allow a 20 degree F temperature rise across the servers before you return the air to the unit then I think you can see the problem. The rooftop unit is being asked to operate well above its built in safety circuit limits. Thus you end up with a self-limiting factor on how effective you can be in reducing the operating expense in the data center. Even if you believe that your servers will be fine at 80.6 degrees F your HVAC unit probably will not be so fine. And, to be honest, this same logic applies to CRAC units as they are nothing more than split DX systems. So you have self-limited your options to cold aisle temperatures of no more than about 70 degrees F and your data center costs more to run than it could.
There is a class of rooftop unit that is better equipped to handle these situations and that class of equipment is commonly known as a DOAS, or Dedicated Outdoor Air System. These systems, like our Applied Air FAP product, have been designed to expect Phoenix type temperatures across the cooling coil. Returning hot aisle air at 105 or 110 degrees F is well within their “normal” operating ranges. These systems are more expensive than a conventional rooftop packaged unit because of the components that are selected but they also provide the operating range that will allow the designer and operator to take advantage of the elevated temperatures that ASHRAE and the IT equipment people recommend for reducing data center operating expenses.