First of all, there are the two different 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 requires all sorts of prefabricated joints, which must be brazed together. Then 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.

So, why would anyone want to use rigid tubing for air-conditioning lines? Because, any time there are long exposed runs of lines, rigid tube makes the job look clean and professionally done.

On the other hand, soft copper-tube lines can be flared, formed to round corners, and connections can be formed by "swaging" (enlarging) the end of one copper tube so connecting fittings aren't required. Prefabricated linesets come already insulated, and with a small holding charge and rubber plugs at each end to keep it clean and dry. In addition, they may already be fitted with connectors that screw right on to the equipment manufacturer's fittings at either end.

So, the decision of whether to go through all the extra installation steps of using rigid tubing or to take the easy route of using pre-assembled soft-copper tubing comes down to the final appearance of the job. If nobody can see it, use the flexible stuff.

Sizing is especially important. There are charts that show the proper tube size to use, depending on its application (suction, liquid, or hot-gas line), the refrigerant type, the tonnage of the air-conditioning equipment, and how far it will be run. The longer the lines, the larger the line size. And there are also length limitations. To protect the compressor, it is usually recommended that the (suction and liquid) line length should be no shorter than 25-ft. and no longer than 80-ft. In addition, there are height restrictions. So it's always best to check with the equipment manufacturer whenever there is an unusual piping application.

The Liquid Line

The liquid line is the smaller, usually uninsulated tube, which connects the output of the condenser coil to the metering device. If this line is sized too large, it requires extra liquid refrigerant to fill it, which can cause a bad refrigerant-to-oil ratio in the compressor, shortening its life. If it is sized too small, or kinked, or used in high vertical runs, there is too much pressure drop. And since a pressure drop changes the evaporating point of the refrigerant, this causes flash gas to form, which greatly reduces the air-conditioning system's capacity and efficiency.

The reason why the liquid line is left uninsulated is because the cooler the refrigerant (the closer to outdoor ambient it is), the more efficient the system. However, there are applications where the liquid line should be insulated. For example, whenever it must make a long run through a very hot environment (attic?), it's best insulated. And in some Japanese mini-split designs, the metering device is in the outdoor unit, so the connecting line must be insulated.

TO BE CONTINUED: Read more about refrigerant lines, including the suction line and the hot-gas line, in June Supply House Times.