Mobile air conditioning system workings explained

Mobile air conditioning system workings explained

Having clean filtered air delivered to your cabin that can be easily adjusted to your preferred temperature, regardless of the season, is an operator essential. Helping to ensure our comfortability and even safety, we can’t fail to notice when the air conditioning isn’t performing, but how many understand how the system actually works.   

Developing a basic knowledge of how an air conditioning circuit operates can be invaluable when it comes to system maintenance and service procedures.


First let’s consider heat movement, this occurs in three main ways:

§  Conduction – This is the method by which heat is transferred through solid material. Particles heat up at varying rates depending on a material’s thermal conductivity.

§  Convection - Convection is the heat transfer means by which the molecules or atoms of a liquid or gas move in a circular motion and carry heat. If for example air that is hotter than the air in a vehicle’s cab is blown into the compartment it will move upwards, forcing down cooler, dense air. This circulation of air, which will eventually distribute the warmer heated air evenly around the vehicle, is known as convection.

§  Radiant – Radiant heat is created by the rays of the sun hitting a solid object and causing it to heat up.


Stop to consider these types of heat movement and you start to appreciate the challenge your HVAC system faces in delivering a constant regulated temperature. For example, the large glass windscreen in your cab which is so necessary for visibility is usually poorly insulated. It doesn’t stop there, the vehicle itself is subjected to factors such as radiant heat from the sun’s rays and cold weather, which both impact on a cab’s temperature.


If you visualize a person using a sponge to first soak up, then wring out a spillage, this process is not altogether dissimilar to what’s going on beneath your bonnet. As demonstrated in the expansion valve system pictured, the mobile air conditioning system extracts heat from a vehicle’s passenger compartment and releases it into the atmosphere by using refrigerant which changes state as it repeatedly cycles through a closed (sealed) system.


AP Air Europe Ltd is a leading supplier of air conditioning parts for agricultural and construction vehicles, delivering to 50 different countries. Search over 50,000 products by vehicle or part/OEM number and read more informative blogs at

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