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How do Car A/C units work? 

 

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 Post subject: How do Car A/C units work?
Posted: Tue Jan 04, 2005 11:09 pm 
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I have allways wanted to know how a car air conditioner works, does it work the same kinda of way as a home or indoor A/C?

The compresor makes the gas hot, turns it into a liquid gas then that liquid evaporates to make cool air and then cool air gets directed into your car.

I just want to know where the A/C does all this, i know where the compressor is , but i dont know where the coils are , the hot and the cold side.. where does the hot air from the A/C exit and at which point or place does the cold air join to your vents?

If any of your guys have a very good idea about how these systems work, please share with me.
I read how some A/C stuff on www.howthingswork.com, but i want to know more about where things are possitioned in a car.

 

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Posted: Tue Jan 04, 2005 11:19 pm 
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The main reason i wana know is because, my A/c in my car works fine, but once you shut the car of from normal temp, if you turn the car and AC back on, the air is not as cold.. you need to REV the engine hard a few times for cold air to start coming around again, is this a sign of bad compression?

 

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Posted: Tue Jan 04, 2005 11:19 pm 
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ok cold side or the evaparator is in with the heater core inside the car behind the glovebox

hot side or condensor is infront of the engine radiator.

recever dryer is infront of the condensor

it work by liquid gas is pumped into the evaparator the refridgerant boils turning into gas and takes away heat. the compressor then compresses the gas and under pressure in the condensor it condenses back into a liquid then it all starts again.

 

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Posted: Tue Jan 04, 2005 11:35 pm 
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You're car air conditioner is a refrigeration system, not unlike your kitchen fridge, home air conditioner, or cool room.
It consists of four basic parts. The compressor, the condenser, the liquid refrigerant control device and the evaperator.
Basically this is what happens.
Refrigerant enters the compressor as a low pressure, cold vapour. The compressor then compresses the refrigerant into a high pressure, hot vapour.
The high pressure hot vapour then travels to the condenser, located at the front of the car in front of the radiator usually. As the refrigerant travels through the condenser it loses heat to the atmosphere, and as it cools it changes state into a liquid. So leaving the condenser is high pressure, warm liquid refrigerant.
The refrigerant then travels through the receiver/dryer. This device is simply there to filter out any moisture and particles from the refrigerant and also acts as a reservoir for excess refrigerant when the cooling load is light.
Once the refrigerant has passed through the dryer it travels to the LRCD-Liquid Refrigerant Control Device. In your car it is commonly a TX Valve(Thermostatic eXpansion Valve). There are many types of LRCD depending on the capacity of the refrigeration system and the application. It is at the TX Valve that the magic happens. The warm, high pressure refrigerant enters the TX Valve and comes up against a restriction like a needle and seat. So, the flow is restricted. Once the refrigerant passes through the restriction, it reduces pressure dramatically. With the drop in pressure the refrigerant starts to change from a liquid to a vapour and to change states it requires energy, heat energy. So, things start getting cold. The cold, low pressure liquid/vapour mix travels into the evaporator where the air blowing into the car cabin is passed across the evaporator. The heat from the air is absorbed by the refrigerant as the refrigerant continues to change state fron liquid to vapour. The nice cold air now blows into the car and the refrigerant, now a cold vapour returns to the compressor to start the cycle again.

The hotter the car, the more the system needs to work, so the TX opens more and there is less 'spare' refrigerant in the receiver/dryer.
On cooler days when load is light, the TX closes down and excess refrigerant can bank up in the receiver/dryer.

Hope this helps, any more Qs just ask.

Darin.

 

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Posted: Tue Jan 04, 2005 11:44 pm 
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fairmont1998 wrote:
You're car air conditioner is a refrigeration system, not unlike your kitchen fridge, home air conditioner, or cool room.
It consists of four basic parts. The compressor, the condenser, the liquid refrigerant control device and the evaperator.
Basically this is what happens.
Refrigerant enters the compressor as a low pressure, cold vapour. The compressor then compresses the refrigerant into a high pressure, hot vapour.
The high pressure hot vapour then travels to the condenser, located at the front of the car in front of the radiator usually. As the refrigerant travels through the condenser it loses heat to the atmosphere, and as it cools it changes state into a liquid. So leaving the condenser is high pressure, warm liquid refrigerant.
The refrigerant then travels through the receiver/dryer. This device is simply there to filter out any moisture and particles from the refrigerant and also acts as a reservoir for excess refrigerant when the cooling load is light.
Once the refrigerant has passed through the dryer it travels to the LRCD-Liquid Refrigerant Control Device. In your car it is commonly a TX Valve(Thermostatic eXpansion Valve). There are many types of LRCD depending on the capacity of the refrigeration system and the application. It is at the TX Valve that the magic happens. The warm, high pressure refrigerant enters the TX Valve and comes up against a restriction like a needle and seat. So, the flow is restricted. Once the refrigerant passes through the restriction, it reduces pressure dramatically. With the drop in pressure the refrigerant starts to change from a liquid to a vapour and to change states it requires energy, heat energy. So, things start getting cold. The cold, low pressure liquid/vapour mix travels into the evaporator where the air blowing into the car cabin is passed across the evaporator. The heat from the air is absorbed by the refrigerant as the refrigerant continues to change state fron liquid to vapour. The nice cold air now blows into the car and the refrigerant, now a cold vapour returns to the compressor to start the cycle again.

The hotter the car, the more the system needs to work, so the TX opens more and there is less 'spare' refrigerant in the receiver/dryer.
On cooler days when load is light, the TX closes down and excess refrigerant can bank up in the receiver/dryer.

Hope this helps, any more Qs just ask.

Darin.


fwakkk nice,


Yeah 1 more thing, you said that the hotter the car the more the system needs to work, is this 1 of the reasons for poor cooling?

if its a hot day and the air con is going i notice if i REV the engine a few times past 4000rpm , the air gets super cold.... if i continue driving under 2000 rpm ,the air slowly gets warmer.. when i say slowly i mean , driving under 2000 rpm for about 20 min on a hot day, but then i just rev the engine again, and it will get cool again... Does this make sence?

What would be the first thing you would look at if you had my problem.?

 

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Posted: Wed Jan 05, 2005 12:06 am 
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There could possible be nothing wrong. The one short coming of a car air conditioner is that the compressor speed varies with engine revs, so its capacity varies. So when you have a high load on the system, and the car is idling or revving slowly, the compressor can't pump enough refrigerant for the heat load and the cold liquid/vapour mix in the evaporator gets exhausted too early, and the entire eveporator isn't removing heat. Rev the motor up, the flow increases and you feel the air get colder because there is more refrigerant available to remove heat.

Conversly, when a system is short of refrigerant, under high load is when it will first show, as there simply isn't enough refrigerant in the system to work at full capacity.

A quick check you could do is to have the air cond going flat out, fan on high. Have someone hold the revs at around 2000rpm and then after a minute or so feel the large pipe returning from the firewall to the compressor. DON'T grab the other compressor pipe, it will burn you, it's that hot. The metal ends on the 'suction' line should feel cold, like a can of coke from the fridge. If it is cool to warm, you have a problem. Also, if you can see the receiver, it will have a sight glass on top. Again, flat out A/C, 2000 rpm, does the sight glass constantly have a lot of bubbles, or only a small amount. Heaps of bubbles shows the system is short of refrigerant.

 

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