Wheeler On HVACR:
The Role Of Refrigerant Lines In Air Conditioning Systems (Part II)
Last month I told you there is a science involved in properly selecting the right type of tube to connect up split-system air conditioners.
There are two types of refrigerant tubing, rigid and soft. The rigid copper type comes in long lengths and it can't be bent or swaged. To make bends and to connect segments all sorts of prefabricated joints, which must be brazed together. Insulation must be purchased and installed on some lines to prevent water from condensing and dripping, and to minimize thermal losses. Also, to prevent oxides from being created inside the tubing, which can damage the compressor and metering orifices, a gentle flow of inert nitrogen gas must be passed through the tube during the brazing process.
The liquid line - the smaller, usually uninsulated tube that connects the output of the condenser coil to the metering device - is often left uninsulated because the cooler the refrigerant (the closer to outdoor ambient), the more efficient the system. However, there are some cases where it must be insulated. This month my column will cover two other types of lines: the suction line and the hot-gas line.
The suction line is the larger, insulated pipe that connects the leaving portion of the evaporator to the compressor return. Its sizing is extremely critical, since system losses are increased four-fold whenever the pressure losses are doubled. So on an R-22 system, the maximum pressure loss from start to finish is limited to about 4-psi. This is one reason why many equipment manufacturers prefer not to see suction filters left in their systems. Adding the extra pressure drop to the existing piping losses may harm the system efficiency and shorten the compressor life.
On the other hand, if suction lines are badly oversized, there may be a problem with returning oil to the compressor. This can cause the compressor to run dry and lock up. Of course, that is only a problem where the compressor is located above the evaporator, or where the oil must come over a loop in the piping.
The reason why the suction line must be insulated is that it is usually operating at about 50 degrees F. It will drip condensed water whenever air can contact it. Any heat gain will cause the system to operate less efficiently. As a result, all missing or gapped insulation should be replaced and gaps covered to prevent water damage and to keep the system operating at peak efficiency.
The hot-gas line is one that we don't normally worry about, since it is usually supplied, sized and installed by the equipment manufacturer. This is the line that connects the compressor discharge to the entering portion of the condenser coil. So, why am I discussing it? Because service technicians usually don't appreciate the critical nature of any pressure losses here.
I've seen all sorts of compressor-killing field innovations at this location in the system, from undersized lines, to bad bends and kinks, to the installation of filter-dryers here. Once again, efficiency losses quadruple with every doubling of pressure losses, and manufacturers have already used a minimal line size in this location. The reason why they can use such a small line diameter here is that it is a very short run.
One of the most common mistakes made by field technicians is when they install filter-dryers to replace manufacturer-installed sieve filters or even mufflers in this line. First of all, the desiccant in filter-dryers doesn't do much drying at the 180 F discharge temperature. And the normal pressure loss makes this an especially bad place to put them. In addition, the pulsing hot gas often damages the integrity of the desiccant, causing it to break into system-damaging powder.
Another important consideration with the hot-gas line is the fact that it's connected to the compressor. The compressor vibrates as it runs, and this vibration will eventually cause the line to fail if it isn't designed with a free-vibrating loop to minimize the stress. Any time this line is shortened, rerouted, or deformed, its life (and the life of the system) is shortened.
So they are just pipes -- copper tubes. No big deal, right? Wrong! They must be properly sized, insulated, piped and routed to make HVAC systems work properly. They must also be properly brazed and connected into the system. But that's another story.