Whether you are working on a 1½ ton unit or a 50-ton unit, the principles of the refrigeration cycle is the same. The most important thing to remember about compressors is that they are VAPOR PUMPS. Compressors do not like liquid in them. Liquid will damage valves, wash out oil leading to bearing failure, and break down the windings of a compressor. The suction line should contain vapor and the discharge line puts out vapor. This is accomplished through super-heat.
Let’s look at a normally operating refrigeration cycle. By understanding what is normal, it is easier to detect when there is a problem.
When a compressor is energized, refrigerant in the form of vapor is brought to the intake valves of the compressor through the suction line. The compressor actually compresses this vapor in the cylinders and releases it out the discharge valves of the compressor into the discharge line. This is hot, high-pressure vapor. This vapor then enters the condenser coil where heat is removed causing the vapor to condense to a liquid. Near the end of the condenser coil, there are always extra passes of tubing through the coil. This serves a very important function. When you have your gauges attached to the unit and are reading the liquid pressure, you are reading the saturated liquid pressure of the condenser coil. This point occurs before the liquid leaves the coil. The extra passes are in the coil to take the saturated liquid and sub-cool that liquid. This is essential to the proper operation of the metering device since it relies on a continuous column of liquid at the metering device to function properly. Normal sub-cooling on today’s equipment is between 10 and 15 degrees of required sub cooling.
The sub-cooled liquid leaves the condensing unit and is pumped through the liquid line as high-pressure liquid until it reaches the metering device. The metering device can be an expansion valve, capillary tube, or an orifice. If there is not sufficient sub-cooling of the liquid, the liquid can “flash” off in the liquid line en-route to the metering device. When this happens, the metering device cannot properly control the flow of refrigerant to the coil causing improper cooling or reduced capacity. When there is sufficient sub-cooled liquid present at the metering device, the metering device takes the high pressure liquid, forces it through the device, and releases it as a low pressure liquid. In this state, the refrigerant is now capable of “absorbing” heat in the evaporator coil thereby, cooling the air.
As this low-pressure liquid passes through the evaporator coil, it again changes state. The evaporator coil does just what its name says – it evaporates the liquid and turns it back into a vapor by absorbing heat from the air passing over the coil. Just as there were extra passes in the condenser coil, there are extra passes in the evaporator coil. Again, with your gauges attached to the unit you read the suction pressure. This is the saturated vapor pressure in the evaporator coil. The extra passes in the coil super-heat the vapor assuring that the compressor receives only vapor at the suction valves. This super-heated vapor then returns to the compressor and the cycle starts over again.
So, understanding how the basic refrigeration cycle works should give you some clues as to how super-heat can be used as a diagnostic. Let’s look at it.
If your system has an expansion valve for the metering device, depending on the system, it can have either a fixed super-heat or an adjustable super-heat setting. Most residential expansion valves are of the fixed super-heat variety and are usually set between 10 & 15 degrees of super-heat. Knowing the valve type and setting will help you diagnose possible problems with an expansion valves. If you know the valve should be set for 12 degrees of super-heat and you are out of that range, you should look at the valve. Start with the position of the sensing bulb on the suction line, is it insulated? Is it making good contact? Is it in the correct position in regards to the size of the suction line?. Take the bulb off and put it in a cup of hot water — does the valve open and you see the pressures changing? If the valve was possibly over-feeding, put the bulb in a cup of ice water. The valve should close and the pressures should respond accordingly.
As you can see, super-heat will help diagnose a possible problem with an expansion valve but what about capillary tubes or orifice metering devices? Super-heat is the preferred method of charging units with orifices and cap tubes. To properly do this, though, you need to not only know the indoor dry bulb temperature but you need to know the indoor wet bulb temperature to properly charge by super-heat. The wet bulb temperature really determines the amount of super-heat the system needs to make sure no liquid gets back to the compressor. Most manufacturer’s have charts for this purpose. There is no guessing when it comes to proper super-heat.
If your system shows signs of Low system super-heat ––you need to remember that compressors are vapor pumps and do not like liquid. Not enough super-heat could allow the vapor to condense back to a liquid in the suction line and now you could cause a problem in the compressor. As stated at the start – liquid is the enemy of the compressor. An over-feeding metering device could cause low super-heat. Not enough air flow across the coil with an orifice metering device will keep the evaporator coil from “evaporating” all the refrigerant and allow liquid back to the compressor. This cold be caused by a dirty coil, dirty blower, improper blower speed, etc.. In any case, the heat is not being added to the refrigerant properly allowing the possibility of liquid back to the compressor.
If your system has High system super-heat — this is typically a result of an evaporator being starved of refrigerant. Starving is caused by a TXV or orifice under-feeding, plugged liquid line filter-drier, kinked or restricted liquid line or any other form of refrigerant restriction. Since we are “starving” the coil, the refrigerant picks up more “heat” in the evaporator. The compressor discharge temperature reflects the latent heat absorbed in the evaporator, evaporator super-heat, suction line super-heat, heat of compression, and compressor motor-generated heat. All of this heat is accumulated at the discharge of the compressor and must be removed. Also keep in mind that too much air flow across the evaporator will cause high saturated suction temperature. The increase load on the coil transfers too much heat to the refrigerant. More refrigerant is vaporized, elevating the temperature & pressure.
In both scenarios, the hotter refrigerant entering the compressor needs to be removed. The condenser requires more of the condenser surface to reject the heat. More condenser is needed, less room for sub-cooling. This equates to a lower sub-cooled refrigerant and it becomes a vicious cycle until the compressor fails.
Super-heat is a very important function in the refrigeration cycle. Most importantly, it protects the compressor from liquid but also from the problem of “burning up” due to too much super-heat. Hopefully, this post on SUPER-HEAT as a diagnostic will become part of your “arsenal” on proper diagnostics of air conditioners.