Here it is March already, spring is just around the corner, and summer will be upon us before we know it. With this in mind, it is a good time to think about air conditioning again and how it all works, especially the refrigerant side of the unit. For the next few postings, I will be discussing the refrigerant cycle and its diagnostics.
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.
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 or in the scroll 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 should always be 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 condenser 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. This is easily calculated by taking your saturated liquid pressure and converting it to temperature (right on your gauge set). Now measure the actual liquid line temperature, and subtract it from your saturated temperature and you know how much sub-cooling you have in the system.
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 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. You can, again, convert the saturated vapor pressure to temperature on your gauges. then measure the actual suction line temperature and you can now calculate how much super-heat is present in the suction line. Super-heat is essential to make sure the vapor does not condense back to a liquid in the suction line assuring only vapor returns to the compressor. This super-heated vapor then returns to the compressor and the cycle starts over again.
When a system has an expansion valve for the metering device, the proper method of checking the charge is with sub-cooling. You do need to check the manufacturer’s specs for the proper or required sub-cooling for the equipment you are working on. Super-heat for most Expansion Valves is “fixed” between 12 and 15 degrees. This can be used to diagnose a poorly operating expansion valve.
Super-heat is very critical when diagnosing a system where the metering device is an orifice. When the system has an orifice, the charge must meet the manufacturer’s specifications based on indoor wet bulb temperature so you know the load on the coil, and your outdoor ambient. All manufacturers have their required super-heat charts published in their install guides or on the unit itself.
In my next posting, I’ll discuss some of the refrigerant cycle diagnostics that can be used to assure a proper operating system.