Explain Subcool/Superheat vs. Ambient Temperature


A residential split AC system is clearly intended to operate over a wide range of ambient outdoor temps. Here in Dallas, we can expect a system to run from 78 degrees to 112 degrees F. I know that excessive subcooling or inadequate superheating can result in liquid refrigerant at the compressor, which is a very bad thing. Would it be true that either or both of these conditions is more likely at lower ambient temps?

If that is the case, I would guess that AC techs would calibrate the charge and temperature expansion valve to ensure that no liquid reached the compressor at the lowest ambient temps at which the unit could be expected to run. Is that true? If it is, what happens if you run the ac when it is 60 degrees outside? Are you sending liquid to the compressor?

Also, if the above is true, does this mean that you have less and less subcooling, and more and more superheating at higher ambient temps, thus making the system less and less efficient as the ambient temps rise? Is the system most efficient at the presumed lowest temperature for which the system was calibrated?


Yes, inadequate superheat at the evaporator exit may result in liquid slugging of the compressor - definitely a bad thing.

Sub-cooling, in and of itself, will not lead to liquid slugging. Generally, greater sub-cooling gives greater performance and efficiency.

The only way for sub-cooling to lead to liquid slugging would be if the throttling device cannot throttle down enough, thus leading to no or insufficient superheat at the evaporator exit. If this condition were to exist, it would most likely happen at low outdoor temperature low indoor load conditions.

Preventing this is mostly a function of selecting the correct expansion valve or other throttling device. An excessive amount of refrigerant charge could mess things up, possibly leading to slugging - and other problems. Too little charge will lead to low suction pressure and evaporator freezing.

As a general rule, lower ambient temperature = increased sub-cooling.

There is a stronger relationship between superheat and evaporator load then there is between superheat and ambient temperature - although obviously evaporator load is a function of ambient.

The relationship between load and superheat is different for different types of throttling devices. A thermal expansion valve is an active control device, and its job is to maintain a constant superheat as the load changes. They do this will, provided you stay within the operating envelope. As noted above, if the valve can’t restrict flow sufficiently, you may get slugging. On the other end, once the valve is wide-open, it can’t do anything more. If full flow isn’t sufficient to absorb the load, then superheat will start to increase.

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