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A general knowledge of piping practices is required to make an intelligent choice. Normally, each of the system components has a stub connection of a specific pipe size. Using the stub connection size as a guide to determine which pipe size to use is generally not a good practice.
Equipment manufacturers design their stub connections for the average or most common pipe size used. The actual pipe size needed may be larger or smaller. Several factors help determine which actual pipe size to use:
The Suction LineOf the major sections of pipe, the suction line is the most critical to size properly. Generally, the suction line is sized for a minimum pressure drop through the line and a minimum velocity in order to ensure good oil return to the compressor. Incorrectly sizing the suction line can lead to oil return problems or a reduction in the system’s capacity.
If the pipe chosen is too large, the velocity of the refrigerant flow will be reduced. Since the refrigerant is in a gaseous state in the suction line, it does not mix well with the refrigerant oil. The velocity of the gaseous refrigerant helps to push the oil along the suction line. If the velocity is too low, the oil will collect in the suction line in the evaporator. This will lead to a shortage of oil at the compressor that can cause damage.
If the suction line is too small, oil return will not be a problem. However, there may be an excessive pressure drop through the suction line. This will reduce the system’s capacity. Deciding which pipe size to use for the suction line is generally a compromise between ensuring good oil return and maintaining a minimum pressure drop through the line.
Discharge and Liquid LinesIt is also important to correctly size the discharge and liquid lines. However, in the discharge line the refrigerant is traveling at a higher velocity, the oil is pushed along, and the pipe size is not as critical. In the liquid line, since the refrigerant is in its liquid form, it tends to mix well with the refrigerant and is carried along easily.
Most manufacturers will include recommended pipe sizes with their installation and service literature. Technicians need to refer to this material when piping in any component. If the original installation and service literature is not available, there are universal tables and charts available to aid in choosing the right pipe size.
Always refer to the appropriate sizing chart rather than simply basing the pipe size on the component’s stub connection. This will always ensure the right size is chosen for the job and that the system will function properly in its design refrigeration capacity.
Marchese is owner of Arctic-Air Refrigeration, Pittsburgh, PA.
Publication date: 09/03/2001