An Air Conditioning Primer
| 01 May 2005
I’m sure some of you need no introduction to the principals behind automotive Air Conditioning. But if you’re not familiar with how the system functions, it can be somewhat of a mystery. With that thought in mind, the Pony Tricks column this month will try to describe some of the fundamentals. Information comes from such sources as service manuals and a recognized authority on air conditioning for classic cars, Classic Auto Air located in Tampa, FL. A typical A/C system is shown in the schematic on page 3.
You probably recognize most of the parts. For instance you recognize the compressor which is belt-driven by the engine, you know that the condenser sits in front of the radiator where outside air can flow through the fins, and you know that the evaporator is inside the heater assembly in the passenger’s compartment where air from the blower blows through its fins.
From that starting point, lets take a look at how this thing works. Classic Auto Air breaks the system down into five major parts:
1. The compressor.
2. The compressor mount and drive arrangement attached to the front of the engine.
3. The evaporator/blower assembly integral with the heater.
4. The condenser mounted in front of the radiator.
5. The hose set including the filter/dryer or the receiver, as it’s sometimes called.
I would add another component to the list and that would be the expansion valve. And although the diagram shows the expansion valve inside the heater assembly, it’s actually just outside the firewall in the engine compartment of our early Mustangs.
Start by taking a look at the diagram and in particular, take a look at the arrows inside the hoses. Let’s follow a small quantity of refrigerant as it flows through the hoses and through the other components in the direction of the arrows. We’ll begin at the firewall on the exit side of the evaporator where the refrigerant is headed toward the compressor.
The refrigerant leaves the evaporator as a low pressure vapor and it’s laden with heat absorbed from the passenger compartment. The compressor sucks-in the vapor and, as it’s name implies, compresses it. Then it pumps it to the condenser. Since the temperature of the vapor is much hotter than the temperature of the air passing through the condenser, cooling takes place in the condenser and the compressed vapor changes (condenses) into a liquid. Leaving the condenser, the refrigerant, now mostly in liquid form, goes to the filter/dryer where it enters the top of the unit and is filtered, dried, and stored. The pickup tube that carries the refrigerant out of the filter/dryer extends to very near the bottom of the device to make absolutely sure that liquid (and not residual vapor) is sent through the pressure hose to the expansion valve as the flow continues.
The expansion valve has the very important job of controlling the flow of refrigerant to the evaporator. Pressure of the liquid refrigerant on the incoming side of the valve is high but pressure on the side feeding the evaporator is low, as much as 10 times lower than the incoming pressure. The sudden drop in pressure allows the refrigerant to expand rapidly. Rapid expansion at low pressure causes boiling as the refrigerant goes into the evaporator tubes. Boiling turns the refrigerant back into a vapor and at the low pressure, this takes place at about 35º F. At 35º, the vapor can absorb large amounts of heat so as warm passenger compartment air is blown through the evaporator fins, much of the heat is transferred to the vapor. This cools the air with a net result of cold air being returned to the cabin.
The small quantity of refrigerant that we followed through the system has done its job so it heads back to the compressor to start all over again. Actually, the various steps in the process take place in a closed loop so picture the cumulative effect of an infinite supply of small quantities of refrigerant going through the system in a continuous fashion. The evaporator stays cold and the cool air returned to the cabin is what we feel when we have the air conditioner turned on.
Without getting too technical, let’s take a closer look at the expansion valve. It’s a critical part of the system but what’s it really doing? In simple terms, it’s regulating the amount of refrigerant that can pass into the evaporator. A capillary tube senses the temperature of the vapor leaving the evaporator and signals the valve to vary its opening to optimize cooling: less flow at impending freeze-up and more flow as the temperature rises. At optimum and as noted above, the evaporator temperature would be approximately 35º F under stable conditions. Don’t confuse operation of the expansion valve with temperature control in the passenger compartment. That’s handled separately.
With 35º refrigerant and no other regulation, the temperature of the air returning to the passenger compartment would be about 45º F. That’s pretty cold so temperature control for the comfort of occupants depends on a thermostat setting. The thermostat setpoint is varied by one of the controls for the heating/cooling system. The control is attached to a temperature sensitive switch which in turn is attached to a capillary tube mounted on the evaporator fins. The thermostat switch turns the compressor on and off to maintain your setpoint.
So when everything is stable and working correctly, the A/C system operates at maximum efficiency and you control how much cold air you need to use to make the cabin comfortable.
Freon Refrigerant vs. the Environment
Refrigerant for first generation A/C systems is a DuPont product called R-12 Freon (dichlorodifluoromethane). If it escapes into the air, Freon has a negative impact on the atmosphere’s ozone layer so it’s use is being phased out. As a result, it’s very expensive but it’s required in original equipment A/C systems where design appearance and design efficiency are important.
There are options where originality is less important. The newer and much more environmentally friendly R-134a refrigerant will work but with reduced efficiency. An environmentally safe refrigerant called ES-12a is more efficient than either R-12 or R-134a. It can be used if the service valve ports are upgraded to R-134a specification.