Air Conditioning

So what's all this hype about air conditioning being complicated?

Air conditioning is a rather simple process based on some really simple principles of physics. Here I will describe those principles to you and give you an understanding of the magic so that you can do battle with those who wish to claim that you need to understand vudu in order to service an air conditioning system.

To begin with try a simple experiment. Wet your finger and blow on it. Feel cool? That is the basic principle of air conditioning. Evaporation. There is no magic here. Just physics. Our bodies sweat to cool us down. When that sweat dries we may even feel cooler. And air conditioning is even more effective when we have less body weight on us

Now, an imaginary experiment is in order. We will take a tank of liquid Freon. You used to be able to buy it at a reasonable cost but the powers that be have decided that it is bad stuff that is destroying the ozone layer and causing catastrophic things to happen down here on the earth. Very controversial subject - I am not going to get into that here. Let's suffice it to say that there are still two diametrically opposed views as to the damage that Freon is doing. It will all sort out in the end.

So we have this can of Freon that is in the liquid state. It has been sitting on the table in front of you so it is at room temperature. As I mentioned before it has a very low boiling pressure so we have to have the can sealed with a valve on top. Now if we open the valve on the can the liquid Freon will start to boil (evaporate) as the pressure inside the can drops and reaches atmospheric. The Freon evaporates (boils) and it begins to get cool, like your finger did when the water evaporated. Only difference is that this stuff boils rapidly (evaporates) at atmospheric pressure so it gets VERY cold. If you were to take a fan and blow it across the outside of the can it would cool the air and blow it on your face and you would be cool, not that you aren't a cool person already else you wouldn't be reading this stuff in the first place. Notice that it isn't necessary to blow the Freon in your face, just the cold air surrounding the can. In fact if you blew the cold Freon in your face you would probably pass out for lack of oxygen, so let's put a funnel on top of the can and capture the Freon. We will put a hose on the small end of the funnel and run it to the input side of a pump, OK? So now the Freon is boiling rapidly (evaporating) in the can and getting cold and you are blowing the cool air on your face and the Freon is being captured by the inverted funnel and the pump is sucking all the Freon up and taking it away. So what should we do with it?

Let's hook up the other end of the pump to an empty Freon can and stuff the gaseous Freon back into the can so we can save it (and the ozone layer). But we have a problem - as we capture the Freon gas and compress it, putting it back into the empty can we find that it is getting hot! Ahhhh, isn't physics beautiful?? If we let it expand and evaporate it gets cold - when we compress it it gets hot! There is some balance in nature after all. So, let's take another fan and blow it across the can we are using to capture the Freon gas and blow the hot air somewhere, but where? In our faces? NO, that would be silly. So let's blow it out the window. As a matter of fact, let's get reeeealy smart and move the compressor and the can we are using to capture the "used" Freon outside of the house. That way we can cool the inside of the house with the boiling (evaporating) Freon and reclaim the Freon and cool it outside the house. Hmmmm, beginning to sound familiar?

But now we are faced with a dilemma. Soon we find the can of Freon that we started with becomes empty! We have run out of Freon! Buttt! We have a backup system, don't we? We have been faithfully collecting the gaseous Freon in the can outside the window, and since we have been cooling it by blowing outside air across the can we now have a tank of liquid Freon outside the house, and it is back to a reasonable temperature cuz we have been cooling it outside. So, let's go outside and get the can and bring it inside and begin our process over again, OK??

But wait a minute, this is silly. Suppose I had a hole in the bottom of the outside can so that I could hook up a hose to it and run the hose back inside and connect it to the top of the inside Freon can?? Then the liquid Freon would be pushed back into the house by the force of the compressor and we could have a closed system where we would never run out of Freon, and the ozone layer would be safe!

It is obvious I have been describing a typical air conditioning system, only we use some different terminology to describe the components. The can we had inside the house where the Freon is evaporating is called, are you ready for this? The evaporator! It is not a can, but rather it is a long aluminum tube about 3/8 inch in diameter and about ten feet long formed into a zigzag pattern. In your car, the evaporator unit is buried inside the heater air duct where the fan blows across it moving hot outside air across the coils and cooling it to blow inside the car. Attached to the tubing is a set of thin aluminum fins, all of which are designed to optimize on heat transfer or cool transfer if you will. The gaseous Freon coming out of the evaporator is captured by a hose which is connected to a pump - now called a compressor. The compressor is powered by a serpentine belt which is connected to the engine. So the work of cooling your car comes from the engine. The compressed Freon goes to a device which is similar in construction to the evaporator with tubing and fins. Here is where the Freon is condensed into a liquid again - the name of this part is, ready for this? The condenser coil! In an air conditioning system like you have in your car, the condenser is in front of the car radiator and the forward motion of your car causes air to flow across it cooling the gaseous and pressurized Freon sufficiently to change it back to a liquid state. The liquid Freon now flows back inside the car through a small tubing called the "expansion tube". On some cars this function is controlled by a valve called the expansion valve which can be regulated to control the cooling rate by controlling the amount of liquid Freon flowing back into the evaporator coils, however on most systems it is a tubing with a small inside diameter. This restriction is necessary to maintain a pressure difference between the evaporator coils and the condenser coils.

Now that you understand the operating principles of an air conditioning system, you also know the operating system of your refrigerator, your freezer, and your dehumidifier. They all work on the same principle! Oh, I forgot to mention, this is the same operating principle that is used in a heat pump! All you have to do is put the condenser coil INSIDE the house and the evaporator coil OUTSIDE the house and you can heat the house instead of cooling it! That magic is done with a series of valves that reverses the flow of Freon.

Since this is an automotive page let's get back to automotive air conditioning and see what can go wrong. The main thing that happens to all automotive air conditioning systems is that it verrrry slowly looses Freon. A standard auto air system contains around three pounds of liquid Freon. When the Freon level drops a half pound or so you will notice a decrease in the cooling ability of the system. It's really pretty simple, you want the evaporator to be almost full of liquid Freon so that while it is boiling (evaporating) so that you get maximum cooling without returning liquid Freon to the compressor. If the system leak is small, a half pound in a few years, it is perfectly legal for a mechanic to "top it off" by adding sufficient Freon to fill the system to bring it back to its normal cooling capacity. Don't let an unscrupulous mechanic tell you that you have to have the system "pumped down" before refilling it again. Poppycock! Tell him to "top it off".

Of course if the system has a major leak, like a ruptured hose or a hole in the condenser from an errant stone or an accident, then major repairs are needed and the repaired system will have to "pumped down" to eliminate any air and/or moisture from the system prior to refilling it.

Oh, by the way, when the system looses about a pound of Freon another thing happens that you should know about. Most automotive manufacturers have put in a safety mechanism so that if you lose all of your Freon you will not ruin your compressor by running it on an empty system. You see, there is also lubricating oil mixed in with the Freon - oil which lubricates the compressor's moving parts - oil which will blow out if you have a ruptured hose or other component. It is this safety factor which makes a lot of money for unscrupulous mechanics. They will point to the compressor and say it is broken cuz it ain't turning when it should be. What he didn't tell you is that it won't turn if you have lost enough Freon to cause the "low pressure switch" to disable the compressor clutch! Topping the system off will allow sufficient pressure in the system to allow the safety feature to let the clutch turn on when required! There is nothing wrong with the clutch!

It is worth mentioning here that Freon 12, the kind of Freon we are talking about here, is no longer legal to be produced here in the USA (except in Arizona see the articlehere) so the cost of it has skyrocketed. What used to cost 79 cents for a 14 ounce can now costs close to 20 dollars a pound! And the cost will go higher! So don't let someone tell you you need a "pump-down" and a complete recharge when you can more than likely get away with a "top-off".

If you are into DIY there is a clue in the above discussion about the "safety switch". I mentioned that there is oil mixed in with the Freon, remember? Well, it stands to reason that if there is a Freon leak, there is also an oil leak. That is a tell-tale. Look around the hose fittings for signs of oil leaking. If you have a pressure washer, thoroughly clean around all of the connections and the seals at the compressor shaft. Then run the system for a day or so, assuming it works at all, and look for signs of an oil leak. If you find one, tighten the fitting nearest the oil leak. If it is in the vicinity of the compressor seal you will have to fix the seal or replace the compressor.

Also, if you have access to Freon tank and a hose kit, you can add oil to your system, if needed, by filling up the hose with a half ounce of oil and "shooting" it into the system along with a "top-off" of Freon. You don't need a special "oil injector" kit.

I am frequently asked the question, "How much Freon do I add to 'top it off'"? The answer is you don't want liquid Freon getting back into the compressor cuz you can't compress a liquid and the compressor will object violently, like locking up or worse yet blowing a piston. So, how do you now when to stop? You feel the large cool tubing coming out of the evaporator and going back to the compressor. It will be VERY cold and water will condense on it when there is a small amount of liquid Freon coming out of the evaporator. That's when you stop adding Freon. Just keep feeling the tube and note the temperature drop as you add Freon. I have used this procedure for years and have never blown a compressor. You can go out and buy all the fancy gauges you want, but none of them will tell you when the system is full. Of course you could remove all the Freon from the system with a retrieval system that prevents the escape of Freon into the environment (illegal in most of the world) and then add the correct amount of Freon by specification. But at 20 bucks a pound that can run into big bucks fast. If you choose that route you will have to go to a professional who has the reclaim equipment unless you have a few thousand dollars to blow!

Remember one thing about Freon. It is dangerous stuff. Whenever working with Freon containing components or Freon containers, WEAR SAFETY GLASSES! If one drop of liquid Freon were to get on your cornea (front surface of your eye) it WILL instantly freeze your cornea, causing PERMANENT VISION LOSS! This is serious business! Don't take chances with your vision. WEAR FULL COVERAGE SAFETY GLASSES WITH SIDE SHIELDS!

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