SUBCOOLING IN A/C

To simplify my description of subcooling, I’ll use the air conditioning mode as an example. As you know, when the liquid refrigerant in the evaporator absorbs the heat from the indoor air, it boils off to a gaseous form. This refrigerant gas is pulled out of the evaporator, taking the heat energy that it absorbed with it.

The compressor compresses the refrigerant into a high-pressure gas, which increases its temperature at the same time. This high-temperature, high-pressure gas enters the condenser and begins dissipating the heat that it absorbed to the outdoor air.

As heat is removed from the refrigerant, it eventually turns back into a liquid. This liquid stacks up in the bottom of the condenser circuits, waiting to exit the condenser and return to the evaporator via the liquid line. There must be liquid in the liquid line or the system will not function. If there is any gas in the liquid line (bubbling sight glass), the system will behave as if it is restricted, and the evaporator will be starved of refrigerant.

Having the proper amount of liquid stacked up in the bottom of the condenser is critical because as the load on the air conditioning system increases, the amount of liquid required in the evaporator increases. The liquid stored in the condenser supplies the increase in liquid needed for the evaporator to operate properly and prevents the condenser from running out of liquid refrigerant at higher load conditions.

When charging an air conditioner, it is not only critical to ensure the proper refrigerant level in the evaporator (with superheat), but also a proper level in the condenser. To do this, we check how much subcooling the system is creating.

Subcooling is a measurement of how much the liquid in the condenser cools down before exiting. When the hot gas in the condenser first turns to liquid, its temperature is at the saturation point. This means that the temperature of the liquid as it first forms is at the same temperature at which the refrigerant is condensing — the saturation temperature.

SATURATION TEMPERATURE

As an example, look at a refrigerant temperature-pressure chart. Let’s use R-22 for this exercise. As its pressure changes, its saturation temperature also changes.

Saturation temperature is the temperature at which the refrigerant will either condense or boil, depending on whether it is gas or liquid, respectively. What this means to you is that if you know the pressure of the liquid in the liquid line as it exits the condenser coil, you also know the temperature of the liquid as it was created.

Let’s say that the liquid pressure in a system you are working on is 242 pounds (Figure 1). The temperature-pressure chart tells you that R-22 will turn into a liquid at 115 degrees F. That 115 degree liquid gets blown down to the bottom of the coil and must wait its turn to exit. While it continues to drain through the coil, it gives up more heat before it can leave. The general rule of thumb is that it should give up about 10 degrees before it leaves the condenser coil. If your system is working properly, the liquid temperature leaving the coil should be 105 degrees.

The more refrigerant is stacked in the condenser, the more time it has to travel through the condenser, so it gives up more heat or becomes more subcooled (lower temperature). If there is less refrigerant in the bottom of the coil, it has less time to travel through the coil, so it gives up less heat (becomes less subcooled).

The convenient thing about this is that by checking the subcooling, you can tell how much refrigerant is in the condenser.

A reminder why subcooling is important: Among other things, if the subcooling is too low, the condenser will “run out of” refrigerant prematurely at higher load conditions, overheating the compressor and reducing performance and efficiency. If the subcooling is too high, the system will be overcharged, reducing performance, efficiency, and ultimately damaging compressor valves and start components.

SUBCOOLING IN DIAGNOSTICS

So far I’ve talked about subcooling as a charging tool, but it is also an invaluable tool for diagnosing troublesome systems. In fact, if you diagnose a refrigerant system without using subcooling, you have only been guessing.

Imagine you have two systems with orifice metering devices that have refrigerant system problems. One has a liquid restriction with a normal charge; the other one is undercharged. They both have abnormally low high- and low-side pressures and abnormally high superheat. The only difference between the two systems will be their subcooling. The restricted machine will have abnormally high subcooling and the undercharged machine will have abnormally low subcooling.

(By the way, many techs seem to think that, contrary to the previous example, a system with a liquid restriction exhibits low suction and high head pressures. This is not the case — although a liquid-restricted system will have excessively high head pressure after several techs misdiagnose it as undercharged and add refrigerant to get the suction pressure up.)

SUBCOOLING CONSIDERATIONS

Here are some things to consider when you use subcooling as a charging or diagnosis tool:

The 10 degrees subcooling rule of thumb is just a rule of thumb. Many machines out there were designed to operate at 15 degrees subcooling. In addition, many variables besides refrigerant charge affect subcooling, like airflow, coil matching, metering device type, and sizing, as well as coil cleanliness.

A low airflow or dirty evaporator can raise subcooling. A dirty condenser can lower subcooling. Too large an orifice will also lower subcooling (and visa versa).

To calculate subcooling properly, you must use liquid pressure, not discharge pressure.

When measuring the liquid line temperature, ensure that your temperature probe is very tight to the liquid line and insulated to get the most accurate reading.

Ensure that your high-side pressure gauge is accurate. Small inaccuracies can make your calculations worthless.

Some manufacturers are now including subcooling charts with their systems, but, for the most part, you are on your own when it comes to using subcooling to service machines. The best way to deal with this is to measure subcooling on every system you put your gauges on.

Do this for a couple of weeks without necessarily taking any action as a result of your subcooling measurements. If you are observant enough, as time goes on, you will start to see the interrelationships between a system’s subcooling and any problems it might have.

By giving yourself some time to get acquainted with subcooling, you will avoid over-diagnosing systems during your learning curve. Once you get it, you’ll never go back.

Howard Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

Publication date: 08/19/2002