AC-2342 -- Return Bend Pressure Drop in Refrigeration Systems

The design of both evaporator and condenser coils requires a determination of refrigerant heat transfer and pressure loss. Through the use of digital computers, the potential accuracy of design calculations has greatly increased, largely because it is possible to account for variations in both heat transfer and temperature differential within the coils. Obviously, the requirements for detailed information on local variations in heat transfer and pressure drpp have increased significantly.

The two-phase pressure change which occurs within an evaporator or condenser coil can be regarded as composed of three principal components: the friction and momentum components within the straight tubes plus the total loss in the bends. A great amount of research has been conducted, and many empirical or semi-theoretical techniques have been proposed for calculating two-phase pressure loss in straight tubes. However, only one other reference on two-phase pressure loss due to return bends is known to be available in the literature.

Since the contribution of return bends to the total coil pressure drop can approach 50 percent, their influence on the temperature differential within a coil can be very significant. It is clear that there exists a need for an accurate means of determining the pressure drop in these bends.

An experimental study was conducted to determine the pressure drop due to two-phase refrigerant flow in return bends. A principal objective of this study was the development of a correlation of the results for design applications.