How does an a/c system work?
 

So, I am trying to wrap my head around the fundamentals of the a/c system. I don't have any car related question, but I think if I, and others understand the basic operation, it would help in diagnosing any future problems.

This is what I understand so far:

1. Start at the compressor: refrigerant is compressed to the point that it becomes a liquid, it gets pumped through the condenser to cool the liquid refrigerant.

Q: Why exactly does it get hot? Is it mainly because of the compression?

2: The now cooler (still liquid) refrigerant passes through the receiver/dryer where it is basically filtered of any dirt/moisture.

3: Goes via "high side" line to expansion valve,

Q: From what I understand, the expansion valve is kinda like a garden hose sprayer. When the refrigerant is "sprayed" it boils and evaporates,

Q2: Is the refrigerant already at boiling point before it actually gets there? Does it not boil because of the pressure and lack of having anywhere to boil out?

4: The now "boiled" out refrigerant is now a vapor. At this point, it absorbs heat from the cabin, and transfers it back to the compressor via the low side line.

Q: What specifically does the accumulator do in this process?


I might be way off, but I was hoping one of the a/c gurus might be able to chime in and correct me.

Not enough time to do a full writeup, but I can point out a few fast facts.

Refrigerant exits the compressor as a gas, not liquid. A hot gas, yes, but gas nonetheless. Traversing the condensor, the gas gives up heat to the airflow over the condensor, and in the process condenses into a liquid.

The expansion valve is the dividing line between the high side and the low side. Remember, the low side of the expansion valve is plumbed to the suction side of the compressor. Recall that liquids boil at a lower temperature under lower pressures. So the liquid refrigerant boils after passing through the expansion valve, in the process absorbing heat from the airflow passing over the evaporator.

Basically, what you have is a heat pump. You boil the liquid in the airflow through the cabin, hence absorbing heat. Then you condense the vapor in the airflow outside the car, hence giving off heat. In the process, you move heat from the cabin to the air outside the car.

I've not looked, but bet google could find you some decent descriptions.

1. The compressor does not compress it to a liquid, it makes pressure and pumps it to the condensor where it gives up heat which CONDENSES it into a liquid. It got hot because it removed heat from the evaporator.

2. The filter drier does filter and dry but it also acts as a buffer for liquid refrigerant in case there's too much refrigerant between the compressor and expansion device.

3. The fact that the refrigerant makes it's way into a LOW PRESSURE area allows it to evaporate or boil into a gas. The boiling point is lower at lower pressure, thus the lower pressure in the evaporator allows it to boil or evaporate.

4. You got it right in the evaporating substance drawing heat from the cabin. Your mixing system types with the filter/drier and accumulator. Pressure cycling systems have an accumulator and not a filter/drier. It is in a different position in the system serving a different purpose.

Ok, the bulb is dimly lit at this point. Just so im clear: the condenser cools the refrigerant thus condensing in to liquid.

The reciever/drier acts as a liquid buffer? In a r12 system, that would explain the bubbles (or lack of) in the sight glass?

I found this helpful:
http://www.rayeveritt.com/Parts/Hoses/HoseLayout.html

AC Controls '95 E420
 

Now that I vaguely grasp how the AC system works, how does the temp get controlled in the cabin? I know that there is a temp sensor near the rear view mirror. When I dial 70F into the temp wheel, and the push button is on auto what happens?
Thanks,
Anziani

Temperature regulation in the cabin is accomplished primarily through control of the fan speed. Higher fan speeds deliver more cooling, lower fan speeds deliver less cooling. As the temperature of the cabin approaches the setpoint, the fan speed is reduced. As the fan speed is reduced, the TXV will reduce refrigerant flow through the evaporator. Eventually the evaporator temperature will approach the freezing point of water. The car will then begin cycling the compressor off&on to avoid freezing condensate on the evaporator. This control is performed by the CCU, using input from an evaporator temperature sensor in the HVAC case.

A secondary method of controlling cabin temperature is to start pulsing the monovalve to warm up the heater core. Generally this only happens at night, when there is no radiant heat from the sun and minimum fan speed still supplies too much cooling. In this situation the air is cooled - too much for comfort - by the evap, then warmed back up a little bit by the heater core. This sounds wasteful, but actually supplies the best comfort. Cooling the air as much as possible reduces the humidity to a comfortable level, then reheating prevents the cabin from becoming uncomfortably cold.

Hope this helps.

Thanks so much. Now I have a handle on it. This car has always throttled right down and worked perfectly. However, last weekend I sensed that it was not backing off and it was getting right cold in the cabin. Will have to pay more attention next time and do a little testing.
Anziani
'93 300CE 188K
'95 E420 118K