Removing Heat, Not Adding Cold
The air conditioning system in your car is one of those things that really has nothing to do with how well the vehicle runs, or how well the vehicle performs the tasks for which it was designed. However, it is a system that when not working can make the car more undrivable than something like an engine misfire. In the middle of July many people will put off getting new tires, or fixing that check engine light, in favor of getting the A/C system working.
So what is going on with this magical system of super coolness? Honestly it works the same way as the refrigerator in your kitchen. The principles at work are no different, and the first thing to understand is the fact that A/C systems do not add cold to the interior of your car, they take away heat.
Heat is a form of energy, and when it is taken away you are left with the absence of heat, or cold. Since heat is energy it can be stored and moved like many other forms of energy. The air conditioning system takes heat out of the air inside the car, moves that heat to the outside of the car, and dissipates it into the surrounding air. When heat is removed we are left with the absence of heat which leaves us feeling much more comfortable.
In order to do this, a refrigerant is used to transfer the heat around in the system. This refrigerant possesses specific properties that allow it to be pumped through the system in both liquid form under high and low pressure, and vapor form under high and low pressure.
The key to the refrigeration process is the diabatic expansion of the refrigerant in the evaporator core of the A/C system. This expansion causes heat to be drawn into the refrigerant. The evaporator is a small radiator type unit usually located under the dash. Instead of being a place where heat is released, the evaporator is a place where heat is absorbed. The refrigerant enters the evaporator as a high pressure liquid through a tiny orifice. As it passes through the orifice the pressure drops dramatically and the high pressure liquid can then expand and evaporate into a gas. This evaporative process causes the refrigerant to pick up heat. Air is blown through the fin-like structure of the evaporator so the refrigerant can take the heat from the air.
All of this newly evaporated refrigerant is then pumped out of the evaporator under the dash to the front of the vehicle. As it moves, it passes through the compressor in the system that compresses the gas making it very hot. The hot gaseous refrigerant then passes through the condenser at the front of the car and condenses back to a liquid state as it gives off heat into the surrounding air. Think of the moisture that accumulates on the side of a cold soda can as condensation. At this point the process starts all over again.
If any of the components are not functioning properly, this cycle will not work to remove heat. The most common problem is a leak in the system causing loss of refrigerant. If this happens then the system lacks the capacity to remove the BTUs necessary to make the passengers feel cool and comfy.
Sometimes when ambient conditions are especially hot, the A/C system will struggle to keep you cool. Some things to remember about your A/C system: You can’t have instant cold. In order for the system to keep you cool it must first remove all of the heat from itself. This might take a few minutes.
Also, make sure to set your fresh/recirculate controls to recirculate. This means the system will draw air from inside the vehicle instead of the outside. If the system is drawing in air that is 100° it might not cool enough on one pass through the evaporator, but if it recirculates air that has already been cooled once, it can cool it a little more at it goes back through the system.
The last tip is to set your mode selector to discharge air from the dash only, do not split the discharge air between dash and floor. You need to move the heat away from around your head and shoulders in order to be comfortable. Nobody ever complained of feeling comfortable up top but feeling too hot around the feet. Not to mention the fact that the cold air coming out higher up on the dash is going to sink to the floor anyway. This flow will also help the recirculation mentioned earlier.