Basic principles of refrigeration piping design

Basic principles of refrigeration piping design

The desing of refrigeration piping systems is a continuous series of compremises. It is desirable to have maximum capacity, minimum cost, proper oil return, minimum-power consumption, minimum refrigerant charge, low noise level, proper liquid refrigerant charge, low noise level, proper liquid refrigetant control, and perfect flexibility of sistem operation from 0 to 100% of system capacity without lubrication problems. obviously all of these goals cannot be satisfied, since some are in direct conflict. In order to make an intelligent decision as to just what type of compromise is desirable, it is essential that the piping designer clearly understand the basic effects on system perfonmance of the piping design in the different parts of the system.

In general, pressure drop in refrigerant lines tends to decrease capacity and increase power requirements, and excessive pressure drops should be avoided. the magnitude of the pressure drop allowable varies depending on the particular segment of piping involved, and each part of the system must be considered separately. There are probably more tables and charts available covering line pressure drop and refrigerant line capacities at a given pressure drop than on any other single subject in the field on refrigeration.

It is most important, however, that the piping designer realize that pressure drop is not the only criteria that must be considered in sizing refrigerant lines, and that often refrigerant velocities rather thanpressure drop must be the determining factor in system desing. In addition to the critical nature of oil return, there is no better invitation to system difficulties than an excessive refrigerant charge.  A reasonable pressure drop is far more preferable than oversizer lines which can contain refrigerant far in excess of the system's needs. An excessive refrigerant charge can result in serious problems of liquid refrigerant contro, and the flywheel effect of large quantities of liquid refrigerant in the low pressure side of the system can result in erratic operation of the refrigerant contron divices.

The size of the service valve supplied on a compressor, or the size of the connection on a condenser, evaporator, accumulator, or other accessory does not determine the size of line to be used. Manufacturs select a valve size or connection fitting on the basis of its application to an evarge system, and such factors as the type of system control, variation in load, and other factors can be major factors in determining the poper line size may be either smaller or larger than the fittings on various system components. In such cases, reducing fittings must be used.

Since oil must pass throug the compressor cylinders to provide lubrication, a small amount of oil is always circulating with the refrigerant. Refrigeration oils are soluble in liquid refrigerant, and at normal room temperatures they will mix completely. Oil and refrigerant vapor, however, do not mix readily, and the oil can be properly circulated through the system only if the mass velocity of the refrigerant velocities must be maintained not only in the suction and discharge lines, but in the evaporator circuits as well.

Several factors combine to make oil return most critical at low evaporating temperatures. As the suction pressure decreases and the refrigerant vapor becomes less dense, the more difficult it becomes to sweep the oil along. At the same time as the suction pressure falls, the compression ratio increases, and as a result compressor capacity is reduced, and the weight of refrigerant circulated decreases. Refrigerantion oil alone becomes the consistency of melasses at temperatures below 0° F., but so long as it is mixed with sufficient liquid refrigerant, it flows freely. As the percentage of oil in the mixture increases, the viscosity increases.

At low temperature conditions all of these factors start to converge, and can create a critical condition. The density of the gas decreases, the mass velocity flow decreases, the as a result more oil starts accumulating in the evaporator. As the oil and refrigerant mixture becomes more viscous, at some point oil may start logging in the evaporator rather than returning to the compressor, resulting in wide variations in the compressor crankcase oil level in poorly designed systems.

Oil logging can be minimized adequate velocities and properly designed evaporators even at extremely low evaporating temperatures, but normally oil separators are necessary for operation at evaporating temperatures below -50° F. in order to minimize the amount of oil in circulation.