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 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.