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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. In this booklet we will focus on the APU-based solutions. In the next chapter we will provide an
overview of your air conditioning choices.
THE BASICS