Refrigeration system pipes are sized to offer a low resistance to flow, since this reflects directly on compression ratio, commensurate with economy of pipe cost and minimum flow velocities to ensure oil return.

Pressure losses due to pipe friction can be calculated from the basic for­mulas established by Reynolds and others. However, as with the calculation of heat transfer factors, this would be a time-consuming process and some of the parameters are not known accurately. Recourse is usually made to simplified estimates, tables published in works of reference, or software for this purpose.

Pressure drops on the high-pressure side will usually be small enough to have little effect on the performance of the complete system. Pressure losses in the suction pipe and its fittings, especially if this is long, should be checked, and a correction made for the actual compressor suction pressure. For low — temperature applications, pipe sizes may have to be increased to avoid excessive frictional losses at these low pressures; oil return must always be considered.

Flow control valves, such as evaporator pressure regulators, will not necessarily be the same nominal size as the pipe in which they are fitted. Manufacturers’ data for selection of their products is usually very comprehen­sive, and their guidance should be sought in case of any doubt.

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