How Stuff Works

Fuzz Town­shend ex­plains how air­con keeps you cool

Classic Car Weekly (UK) - - This Week - fuzz town­shend CCW’s mas­ter me­chanic

Air con­di­tion­ing

Air con­di­tion­ing thor­oughly wove its way into the car scene in the 1940s, when swel­ter­ing Amer­i­cans be­came ac­cus­tomed to re­frig­er­ated air be­ing pumped around their homes and shop­ping malls.

Climb­ing into a sun-drenched sedan was an un­pleas­ant change of cli­mate and so, with the in­tro­duc­tion of en­gine-driven air con­di­tion­ing com­pres­sors, an al­most seam­less tran­si­tion be­came de rigueur.

Nowa­days, air con­di­tion­ing is ubiq­ui­tous on all but the low­est spec­i­fi­ca­tion cars and has re­mained ba­si­cally the same since its in­tro­duc­tion, al­though un­til the 1990s a dif­fer­ent re­frig­er­ant was used. This old re­frig­er­ant, R-12, was a chlo­roflu­o­ro­car­bon (CFC) and was found to be dam­ag­ing to the planet’s ozone layer. It was re­placed by the non-CFC, R134a re­frig­er­ant.

Es­sen­tially, the process is sim­i­lar to a house­hold re­frig­er­a­tor. The com­pres­sor draws in the re­frig­er­ant in the form of a gas and then com­presses it.

In a sim­i­lar fash­ion to when air is com­pressed, such as by a bi­cy­cle pump, the re­frig­er­ant gas heats up due to the prox­im­ity and con­se­quent fric­tion of its con­stituent com­pressed mol­e­cules. It then passes through the con­denser, which looks and acts in a sim­i­lar fash­ion to a ra­di­a­tor, in that air is passed be­tween the tubes, cool­ing the re­frig­er­ant therein to a now pres­surised liq­uid form, like steam cool­ing back to wa­ter.

Be­fore the re­frig­er­ant can be used to cool the car’s in­te­rior, any wa­ter con­tained within it must be elim­i­nated, as ice crys­tals would dam­age the com­pres­sor and block the sys­tem. So it’s de­liv­ered to the re­ceiver-dryer, which uses a gran­u­lar des­ic­cant to re­move the wa­ter.

The wa­ter-free re­frig­er­ant then passes through ei­ther a ther­mo­static ex­pan­sion valve, or an ori­fice tube. The job of these de­vices is to re­duce the pres­sure of the re­frig­er­ant be­fore it en­ters the evap­o­ra­tor which, in turn, cools it down to around zero de­grees Cel­sius.

The cooled re­frig­er­ant en­ters the evap­o­ra­tor, which re­sem­bles a ra­di­a­tor and is lo­cated within the car’s pas­sen­ger com­part­ment. Warm air from the pas­sen­ger com­part­ment is blown through the evap­o­ra­tor’s tubes and fins and the heat is trans­ferred to the re­frig­er­ant.

As a re­sult of this heat trans­fer, the re­frig­er­ant be­comes a gas again and passes from the evap­o­ra­tor back to the com­pres­sor, and so the cy­cle be­gins again.

If an ori­fice tube is used in the sys­tem, an ac­cu­mu­la­tor placed be­tween the evap­o­ra­tor and the com­pres­sor pre­vents any re­frig­er­ant that hasn’t turned back to a gas from en­ter­ing the com­pres­sor. The rea­son for this is that com­pres­sors can only com­press gas.

Con­di­tion­ing the air also re­duces its mois­ture con­tent. This mois­ture is col­lected and al­lowed to drain and this can of­ten be seen drib­bling harm­lessly from the car, which of­ten alarms its owner.

One dis­ad­van­tage of hav­ing an air con­di­tion­ing sys­tem fit­ted to a car is that the com­pres­sor saps some of the en­ergy from the en­gine when the sys­tem is in use. This can be de­tected most ob­vi­ously when you switch an air con­di­tion­ing sys­tem on with the en­gine idling. The en­gine note dips no­tice­ably, in­di­cat­ing that it is hav­ing to work harder, which in turn has a detri­men­tal ef­fect on the over­all fuel con­sump­tion of the car.

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