Obvious question? Or is there more to it than different kind of gas that compressor handles?

Of course refrigerant gas has something to do with design of air-conditioning compressor. For one thing, refrigerant gas has a unique property of being a liquid at normal room temperatures, and it can become a gas at a slightly higher temperature.

Furthermore, lubricant oil can mix freely with refrigerant gas! And lubricant have to be recovered and returned back to compressor.

A bit complicated isn't it?

For big buildings, we need big cooling capacities. Central air-conditioning systems using chillers are used. Chilled water is most convenient way to collect heat from indoor spaces. We just need pipes. Miles and miles of them.

Chilled water from chillers is about 44 degree Fahrenheit (about 7 deg C). In order for room heat to be transfered to water efficiently, air in room is circulated through tubes containing flowing chilled water.

That is main function of air handling unit in a central air-conditioning chilled water system.

Another function of an air handling unit is to clean air. It does this by filtration.

A most basic air handling unit, or AHU, contains a fan, a cooling coil, and an air filter enclosed in a box-like structure.