Refrigeration systems that circulate an oversupply of liquid to the evaporator have been used in practice for more than 40 years. The most common usage is in large tonnage applications having multiple evaporators. The main advantage is reduced operating expenses, which result from higher system efficiency. Principles and design guidelines for liquid overfeed syste~s are presented in the 1973 Systems Volume of ASHRAE Handbook & Product Directoryl. The refrigerants commonly used are R-12, R-22,R-502, and R-717 (ammonia). Recommended minimum rates of recirculation, i.e., ratio of total refrigerant flow rate to evaporation rate, are from 2 to 1 for R-12 and R-502 and to Gor 7to 1 for ammonia in large diameter downfeed tubes.

Liquid overfeed systems employ a low pressure receiverorl;iccumulator to separate the vapor and liquid leaving the evaporator. The inte~connecting piping is referred to as a wet return and carries a mixture of· liquid and gas in cocurrent flow. An improved method of sizing these lines for a given pressure drop was the objective of this investigation. In the first phase of the study a detailed literaturestirvey on gasliquid flow in pipes was conducted, with an emphasis- on horizontal flow. A study of the literature resulted in the seiection of six methods of two-phase pressure drop calculation. These methods were due to the following investigators: Lockhart-Martinelli , Martinelli-Nelson, Hughmark , Dukler, and Baroczy.