Sitting comfortably in your cab? Assuming it’s working efficiently then chances are your heating and air-conditioning has a lot to do with that. The human body is very sensitive to heat, so it’s perhaps not surprising that the earliest attempt to develop a mechanical comfort cooling system dates back as early as 1884. Today’s units are thankfully a far cry from ice block trays and fans mounted on horse carriage axles! Set anyone to work in a dust coated glass cab on a hot summer’s day just inches from a diesel engine’s exhaust system and they’ll soon confirm it’s a miserable experience if the AC is malfunctioning.

With vehicle cabs subjected to multiple sources of heat radiation in the form of sunlight and the engine, exhaust and transmission system, it isn’t long before the average human’s preferred temperature range of 21 to 27ºC is compromised. Capable of maintaining an interior temperature 10 to 15ºC lower than the ambient temperature on a vehicle’s exterior, an air-conditioning system grants occupants greater comfortability even on a very hot day.

Commonly measured as Relative Humidity (RH) and defined as a quantity representing the amount of water vapour in the air, humidity has a significant influence on our thermal comfort range. Sweating is the body’s way of cooling itself and maintaining temperature. If, however, the air is at 100% relative humidity and has absorbed as much water as it can for a specific temperature then the moisture won’t evaporate into the air and the retained heat makes us feel warmer than the actual temperature. Likewise, depending on the temperature, low RH can leave us feeling cooler than the actual temperature. To effectively absorb moisture perspiration and breath from passengers, it’s desirable to achieve a level of around 60% relative air humidity in a vehicle’s cabin.

Without an air-conditioning system to control the two comfort variables, temperature and humidity, you definitely wouldn’t be sitting so comfortably!

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