Quick, easy and guaranteed? Can those terms all be used to describe anything that actually proves to be true? They can be if you’re talking about trying to determine if an A/C system is working at peak performance. It’s really an old trick that (surprisingly) isn’t used or understood by a lot of today’s technicians. But rest assured that once you understand it, you will use it every time. After all, it’s the laws of physics at work in your A/C system, and you just can’t fight the laws of physics.
What’s the test? For lack of a better term, it’s just referred to as the “feel test.” What’s nice about it is that you already have the diagnostic tools required. It’s cheap. It’s accurate. It works every time…without fail.
The principle is this. The cooling of the A/C system revolves around the evaporator. Yes, you have to have the complete system working properly; the condenser has to be cooling efficiently; the compressor has to be pumping and circulating the refrigerant; the expansion device must also be working properly. But most importantly, the evaporator has to be working efficiently to absorb the maximum amount of heat from the hot air passing over it. When that’s working, the job is finished. So, how do you determine when the A/C system is cooling to maximum performance? Do the “feel test.”
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To understand the test, you have to understand what’s happening in the evaporator. Under proper conditions, high pressure liquid refrigerant, returning from the condenser, passes through the expansion device which in fact is just an “engineered” restriction in the system. Because refrigerant is pressure/temperature related, whatever we do to one, we automatically do to the other, correct? That means, simply reducing the pressure of the liquid refrigerant will cause the temperature to be reduced. The expansion valve (or orifice tube) does just that. It reduces the pressure of the high pressure liquid refrigerant making it low pressure, low temperature liquid refrigerant.
The cold liquid refrigerant enters the bottom of the evaporator. Hot ambient air passes over the evaporator (forced by the blower motor). Because heat always transfers or moves from areas of high temperature to areas of low temperature (another law of physics), the heat from the hot air is absorbed by the cold liquid refrigerant.
Now, before we get too involved in what’s happening in the evaporator, let’s address one “side note” that is bound to come up. For the purpose diagnosing the A/C system, it’s really not important to understand exactly what the temperature of the cold liquid refrigerant is. Simply understand that it is much colder than the hot air passing over the evaporator but not so cold that it would cause the condensation forming on the evaporator to freeze. Therefore, for arguments sake, let’s just nail it down to being a temperature just above freezing: 32ºF (0ºC). In actual fact, the temperature is really a result of the refrigerant pressures, but that could turn out to be an entire different article.
So with cold liquid refrigerant in the evaporator absorbing heat from the hot air passing over it, what would you expect to happen to the liquid refrigerant? What happens is exactly the same as what would happen to a kettle of water on a stove top. Put enough heat into the water and it boils, correct? The exact same thing happens to the liquid refrigerant. Once it has absorbed enough heat from the hot air passing over the evaporator, the liquid refrigerant boils. It simply turns into a vapor and is then removed from the evaporator by compressor suction. What is really important to understand now is how the temperature of the liquid refrigerant (evaporator inlet) relates to the temperature of the vapor refrigerant (evaporator outlet). Note that we’re not talking about the actual temperature of the refrigerant—we’re talking about how the temperature of the vapor relates to the temperature of the liquid. In other words, how do the temperatures compare to one another, or, what’s the difference in the temperature of the vapor and the temperature of the liquid? The short answer is this, if the system is working properly and at peak performance, there is no difference in the temperature of the vapor and the liquid. At least none that you can feel.