1. Additional first cost due to:
• Larger line sizes. The liquid line to the evaporators conveys the liquid destined for recirculation as well as that for evaporation. The return lines from the evaporators to the separating vessels carry liquid as well as vapor. Even though there is less flash gas in this suction line (Advantage 3), designers usually select a larger size than they would for conveying only the vapor of evaporation.
• The liquid supply lines to the evaporators are cold, so they must be insulated.
• Additional cost of mechanical or gas-pumping equipment.
2. Greater refrigerant charge. The evaporator and suction line contain a greater fraction of liquid than is the case with direct expansion.
3. There is an operating cost associated with the pumping equipment.
The advantages of liquid recirculation generally dominate over the disadvantages in low-temperature, multiple-evaporator installations. When the plant serves low-temperature assignments, achieving good heat transfer in the evaporators is crucial, and the plant operates with high compression ratios where the quantitites of flash gas are appreciable and high suction superheat could be a problem. For moderate-temperature evaporators, such as those on docks, direct expansion should be considered. For a system with a small number of low-temperature evaporators, flooded coils may be the best choice. As the number of evaporators increases, a breakeven point is passed where the liquid recirculation system becomes the most economical choice. The critical number of evaporators is a function of the systems, but usually when the system includes more than three to five evaporators, liquid recirculation is chosen.