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How does a aircon work,Does it get colder when its hot outside or colder when its cold outside

Blade on Sun August 23, 2009 7:45 AM User is offline

Hello again

A mate of mine was saying the hotter it is outside the car the cooler the air con works,I would of thought if it was cooler outside then the air coming out will be alot colder,If my mate is right how does it work?

knightgang on Sun August 23, 2009 9:14 AM User is offline

Refrigerant gas is compressed into a liquid form in the condesor under high pressure, then this liquid is released under controlled conditions into the evaporator (heat exchanger) there by turning cold as it looses pressure and returns to a gas. This cools of the evaporator which you blow air through and into the cabin to cool the air inside the car. the gas is then carried back to the compressor where it is compressed back into a liquid in the condensor coil (creating heat) and the process starts over again.

Therefore, the cooler it is outside, the less heat the liquid refrigerant has to loose as it turns back into a gas cooling the evaporator and the less the Aircond has to work.

HECAT on Sun August 23, 2009 12:03 PM User is offline

The gassing in the evaporator carries away heat. look up "latent heat of evaporation". The heat is being removed from the air that is being blown over the evap thus making it feel cooler. This heat is then carried to the condenser where the condensation process releases it (sort of the reverse process). These are the two "heat exchangers" and how they absorb or dispel the heat.

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iceman2555 on Sun August 23, 2009 4:01 PM User is offlineView users profile

Quote
Originally posted by: knightgang
This cools of the evaporator which you blow air through and into the cabin to cool the air inside the car. the gas is then carried back to the compressor where it is compressed back into a liquid in the condensor coil (creating heat) and the process starts over again.

The compressor of an automotive AC system is not a gas/liquid pump. The compressor is pump that compresses a 'gas', thereby increasing the pressure/temperature of the refrigerant. The gaseous refrigerant passes thru the condenser... heat is removed from this hp/ht gas, thereby producing a high pressure/high temperature liquid refrigerant. The condenser does not add heat...it removes heat.





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NickD on Sun August 23, 2009 10:15 PM User is offline

AC works on the principle of transferring heat from one point to another. In very basic terms, a gallon of gasoline when combined with oxygen can generate 106 K BTU of heat when burnt. Where if that same gallon of gas could somehow be used to mechanically run a compressor with 100% efficiency, an AC system could transfer as much as 10-15 times that amount of heat. AC systems are most efficient at around 80*F, but that SEER factor decreases as the ambient temperature increases dropping to about unity at around 130*F. Vent temperatures are useless in determining the efficiency or the amount of heat transferred in some unit of heat measurement. Temperature is just the force of the equation, air flow is the other, compounded by humidity that determines the amount of heat transfer.

But in regards to vent temperatures, some AC systems are designed to work much harder at higher ambient temperatures. Fixed orifice systems are the worse, if the low side pressure skyrockets to 80 PSI, maximum vent temperatures could on be as low as 75*F, but in a good TXV POA or variable displacement where the low side is held automatically at 30 psi, still capable for providing near freezing point air temperatures. Provided these systems have the capacity to handle it.

No set answer to your question, too many other variables to consider.

knightgang on Mon August 24, 2009 12:24 AM User is offline

Quote
Originally posted by: iceman2555
Quote
Originally posted by: knightgang

The compressor of an automotive AC system is not a gas/liquid pump.

Did not intend to imply that. I understand that the compressor compresses gas into the condensor, thereby increasing the pressure and when cooled properly converts the gas back into a liquid. All gases have a pressure/temp combination that will convert them into a liquid, common principle of science.

I get what you are saying about the evap and cond are heat exchangers and the evap removes heat fro the cabin and releases it outside off of the condensor. I just cannot get my head around the thought of removing heat. To me it is adding cold. Same difference (to me), just stated in two different ways yet I understand that Cold is the absence of heat. I know this because there is an absolute lowest temperature that you can get to being 0*Kelvin where all molecules stop moving and having no heat at all. Yet there is not absolute Highest temperature. It can continue to increase indefinetly.

Karl Hofmann on Mon August 24, 2009 4:24 PM User is offlineView users profile

Quote
Originally posted by: Blade
Hello again



A mate of mine was saying the hotter it is outside the car the cooler the air con works,I would of thought if it was cooler outside then the air coming out will be alot colder,If my mate is right how does it work?

Blade, You're not known as Slingblade on another forum, are you?

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Never knock on deaths door... Ring the doorbell and run away, death really hates that!

Blade on Mon August 24, 2009 6:21 PM User is offline

Thank you now i know how it works.

No Karl im not Slingblade>>>>Just Blade

Karl Hofmann on Tue August 25, 2009 4:54 PM User is offlineView users profile

That's cool Blade... Just checking as there is a UK refrigeration engineer who uses the handle "Slingblade" and he can be a bit of a wind up merchant

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Never knock on deaths door... Ring the doorbell and run away, death really hates that!

graeme on Thu August 27, 2009 6:37 AM User is offline

The cooler it is outside the car, the more heat that can be transferred from the condensor to the air(Newton's law of cooling), which ultimately will determine how much heat can be absorbed and transferred from the evaporator that feeds the cabin, cooler interior temps.

Sucking warmer air across the evaporator will increase the total amount of heat transferred by the system(Newton's law of cooling again), it will only increase the vent and cabin temps

NickD on Fri August 28, 2009 7:27 AM User is offline

Did you read that in Newton's "Philosophiæ Naturalis Principia Mathematica"? Didn't think he got involved with thermodynamics nor in Boyle's laws. But I am also here to learn.

graeme on Sat September 12, 2009 3:14 AM User is offline

Quote
Originally posted by: NickD
Did you read that in Newton's "Philosophiæ Naturalis Principia Mathematica"? Didn't think he got involved with thermodynamics nor in Boyle's laws. But I am also here to learn.

Yes, Ive read that book and indeed "opticks", but dont remember reading about it there. its covered in high school physics and applied maths here in Aus.

Had a bit of a dig around on the internet, this perhaps was the best I could find in a couple of minutes:

http://www.madsci.org/posts/archives/2000-11/973522810.Ph.r.html

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