Cooling server rooms with room air conditioners? That sounds enticing to many a data center operator. Room air conditioners were not originally designed for use in equipment rooms (precision air-conditioning systems capable of regulating room temperature much more accurately were developed for that job), but they have become much cheaper recently, and in tests they now deliver similar results to precision systems. So what is wrong with the idea of saving on an expensive precision air-conditioning system and instead investing in a room air conditioning system?
The answer is in fact: everything. Because the concept underlying room air conditioners is completely different from that of precision systems. Whereas roughly 90 % of a precision air-conditioning system's function is sensitive cooling – meaning it actually lowers the room temperature – about 40 % of a room air conditioner's function is so-called latent cooling. In this mode, the air is dehumidified rather than the room temperature being lowered – so in purely physical terms no cooling actually takes place. That seems absurd at first hearing, but it is nevertheless quite logical. Because the purpose of room air conditioners is to create a room climate which people find comfortable. But humans' temperature perception is linked to the humidity of the air. If it decreases, people will perceive what is actually the same temperature as being cooler. Room air conditioning systems utilize this difference between the actual and perceived room temperature: They first remove humidity from the air, and only switch to physically detectable sensitive cooling when the dehumidification effect alone is no longer sufficient.