The basic principle of an air conditioner is the transfer of heat from one place to another – in this case from the inside of your vehicle to the outside. This is accomplished by absorbing heat from the inside air into a refrigerant gas flowing through an evaporator coil. The refrigerant is then pumped to a condenser, where the heat is released to the outside air. The refrigerant, a chemical with a low evaporation temperature, flows around a closed loop, driven and pressurized by a compressor. As a part of the cooling process, the air conditioner also removes moisture from the inside air, which makes the area feel more comfortable and keeps the compartment dry and mildew-free.
A belt-driven compressor on the truck’s engine supplies air conditioning whenever the engine is running. To keep the driver comfortable when the engine is shut down, it is necessary to have a separate auxiliary air conditioning system that uses an alternative source of power.
The auxiliary air system runs on 115 Volt AC power, which can be supplied by an onboard diesel genset (often referred to as an auxiliary power unit), from a shorepower connection (when an external AC power source is available), or in some cases from the truck’s batteries using an inverter to convert the 12 Volt battery output into usable 115 Volt AC power.