How Air Conditioning Works

How Air Conditioning Works

The condensed version

There was a time not all that long ago when automotive air conditioning was considered something of a luxury. Today's drivers don't share that view. Now standard on just about every make and model of car, truck and SUV, air conditioning is as familiar as, well, the steering wheel.

The almost universal acceptance of air conditioning has tracked the gradual advancement of comfort and luxury items in cars. In the early days of the automobile when even a heater was considered a luxury item, the concept of reveling in 70-degree comfort on a sweltering 110-degree Tucson summer afternoon was the stuff of science fiction. Today our "baseline" for comfort and features has ascended to spectacular heights. Gotta have a good sound system, cruise control, anti-lock brakes, and all sorts of air bags too. Air conditioning is practically a given.

Just exactly how does air conditioning make goose-bump-inducing cold air when the outside temperature is hot enough to cook a desert donkey clear through in fifteen minutes? For starters, it doesn't actually make cold air, but instead it removes heat from the car's interior and dumps it outside. The principle is the same used in home air conditioning, refrigerators and most other kinds of refrigeration. But how is that possible if it's flaming hot outside? Glad you asked.

The Ins and Outs of Cool

Boiled down to its essentials, air conditioning is a closed-loop system of tubing filled with a refrigerant that cycles from liquid to gaseous state as it circulates round and round. Automotive air conditioning has components both inside the interior and in the vehicle's engine compartment. Behind the instrument panel, warm, humid interior air is blown over a heat exchanger called an evaporator, which has cold, low-pressure liquid and gas refrigerant flowing through it. The refrigerant is warmed by the interior air flowing over it, and the air is cooled by the refrigerant. With the temperature drop, the air can no longer carry as much moisture, so water droplets condense on the evaporator and the humidity of the air is reduced. That's nice, particularly in hot, humid weather. The liquid water is drained out onto the ground, which explains that harmless puddle of clear liquid you sometimes notice under your car when the air conditioning has been running. To adjust the temperature of the air flowing from the vents, the dashboard-mounted climate controls allow the cold air flowing over the evaporator to be blended with un-chilled air.

As the refrigerant absorbs heat in the evaporator, some of it begins to boil (it has a much lower boiling point than water). This vapor is carried to the compressor unit under the hood that is powered by the engine. The compressor pressurizes the vapor, which raises its temperature at the same time (Basic physics: if you increase the pressure of a gas, its temperature will automatically increase as well.) This hot, high-pressure refrigerant vapor is now much hotter than the outside air around the car (yes, even in Phoenix). It's carried to a heat exchanger called the condenser mounted at the front of the car, where its temperature is reduced below the boiling point and it condenses back into warm liquid form. It is able to revert to liquid form even at a relatively high temperature due to the pressure it's under in this part of the system, which raises its boiling point. Now the refrigerant heads back to the interior where it sprays through a device called the expansion valve and into the evaporator once again. Beyond this valve, the pressure is reduced, and the refrigerant returns to a gaseous state. With the reduction in pressure comes an immediate drop in temperature (remember that pressure/temperature relationship-). The cold refrigerant can be at or below freezing as it flows through the evaporator, where the cycle begins again.

The typical automotive air-conditioning system is a low-maintenance affair. About all you need to is run the system for about 10 minutes once a month (even in winter) to circulate the refrigerant and the small amount of lubricant it carries. The refrigerant (environmentally friendly R134A has been in use since 1996) will last indefinitely barring a leak in the system. Modern systems are electronically controlled and maintain the proper pressure and temperature differentials throughout for efficient operation. The engine driven compressor is part of that control system, and has an electrically operated clutch that allows its drive pulley to freewheel (and not sap engine power) until the compressor is needed.

Automatic climate controls now common on many luxury cars add sophisticated computerized logic to the system to maintain a set temperature. With an array of temperature sensors, the system monitors the temperature inside the car then cycles the air conditioning system, ventilation fans and air-supply mixer to maintain the set temperature. Some of these systems are quite sophisticated, and compensate for sunshine hitting one side of the car, the preferences of the right and left front passengers, and even those of the rear passengers. Is this level of personal comfort really necessary? Of course not. Is it nice? You bet.

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