In automotive systems there are two refrigerants used. One refrigerant, called R-12, developed by Dupont was sold under the brand name Freon. This was used in older model cars. A newer, less polluting refrigerant called R-134a is used in newer cars. It is against the law to produce cars using the older R-12 because it was one of the primary culprits in the erosion of the ozone layer in the upper atmosphere. Many people choose to have their older systems converted to the newer R-134a refrigerant because the dwindling supplies of R-12 are becoming more and more scarce and as a result more and more expensive.
In both cases the refrigerant is colorless and essentially odorless and exists in a gaseous state at room temperature, under the normal atmospheric pressure found at sea level.
Mixed with the refrigerant is oil to lubricate the compressor and in some cases a part called the expansion valve. The oil circulates throughout the inside of all the components along with the refrigerant. Sometimes dye is added to the lubricating oil to facilitate finding small, slow leaks in the system. When refrigerant leaks out, so does the dye. With the help of special goggles and ultra-violet light the stain from the dye can help pinpoint the leak if the car is examined within a very short period of time after the A/C has stopped cooling . Otherwise the dye gets washed away by rain or carwashes.
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Pressure on the refrigerant plays an essential role in how A/C systems work. Here are some more laws of physics that we must accept for our lesson. First, the higher the pressure is on a liquid the higher the boiling point will be and conversely, the lower the pressure, the lower the boiling point. Similar to the boiling point is the condensation point. Boiling happens when the temperature rises past a critical point and condensation happens when the temperature drops below this same point. We will refer to this as the boiling/condensation point. When a liquid boils into a gas it is absorbing heat. When a gas condenses back to liquid it is giving up heat.
Given that if you raise the pressure on a liquid you raise its boiling point, let’s take at look at how it effects water. At sea level water boils at 212 degrees Fahrenheit. Liquid water will never become hotter than 212 degrees. Even if it is on a 500 degree fire the hottest the liquid water can become is 212 degrees. After that it turns to steam. The steam will be 500 degrees or however hot your fire is! However, if we can raise the pressure on the water we can raise its temperature by raising the boiling point. This is the basis of how a pressure cooker works. By raising the pressure in the pot we can heat the water up to 300-400 degrees before it boils, thereby cooking the food faster than if the water could only be kept at 212 degrees before it boils and evaporates away.
Another of our givens is that when you compress anything it gets hotter. This principal is what causes diesel fuel to ignite without a spark plug. Conversely, when you de-compress something it cools. When you hold a spray can, while painting the can cools in your hand due to the drop in pressure in the can as the paint disperses.
A key component in an A/C system is the compressor. It squeezes (or compresses) the refrigerant raising its pressure, it’s temperature, and it’s boiling/condensation point. It also acts as a pump to circulate the refrigerant and lubricating oil around our closed loop.
Refrigerants exist in one of four states in an A/C system while the A/C compressor is cycled on. These are, in order starting from the outlet side of the compressor, hot high pressure gas, warm high pressure liquid, then after a part called the Expansion Valve (or orifice tube in some cars) it is a cold low pressure liquid, and then a cool low pressure gas. The state of the refrigerant, whether it is liquid versus gas, is controlled by pressure which in turn controls its boiling/condensation point. When the compressor is cycled off the pressure differences slowly equalize so that there is one pressure throughout the entire loop. In this state the refrigerant is a gas and its temperature is the same as the ambient temperature wherever you car is sitting.