Liquid overfeed systems are commonly used on large tonnage applications of industrial refrigeration. The principles of operation for these systems and design guidelines are provided in ASHRAE HANDBOOK & Product Directory, 1973 Systems Volume l. The piping which connects the evaporator outlet to the low pressure receiver or accumulator is referred to as a wet return. It carries a mixture of liquid and vapor, the proportion being determined by the recirculation ratio, R. Recirculation ratios generally used are in the range of from 2 to 1 to 5 to 1. An improved method for sizing the piping which carries this two-phase gas-liquid mixture was the object of this investigation. A previous publication presented the results of a comprehensive literature survey made on experimental measurements and proposed calculation methods, for two-phase pressure drop in horizontal lines. A statistical analysis of 795 data points from the AGA/API 'Dukler data bank, together with additional data on refrigerant two-phase flow, showed that Hughmark's correlations3 and 4 produced the most accurate calculations for pressure drop. Design tables for four Sefrigerants, namely, R-12, R-22, R-502, and R-7l7 (ammonia) were prepared , based Qn the Hughmark correlations. The design table for ammonia, covering evaporating temperatures from -40F to +20F (-40°C to -67°C) and recirculation ratios of 2/1, 3/1, 4/1, and 5/1 are reproduced in the publication2 summarizing that study.

The present investigation was undertaken because of the serious lack of accurate data from refrigeration systems on two-phase pressure drop, particularly under the conditions encountered in liquid overfeed systems. The desire of the sponsor was for measurements of pressure drop in straight pipe flow and in fittings and valves typical for large-scale industrial refrigeration applications. Limitations of space and budget made it necessary to conduct the ~nvestigation on a one in. nominal piping circuit. Ammonia was selected as the refrigerant because of its predominance in application of liquid overfeed systems.