When To Choose Liquid Recirculation

The advantages and disadvantages of liquid recirculation were listed in Section 8.3. Liquid recirculation competes against flooded-coil evaporators and direct expansion. The advantages of liquid recirculation become dominant in lowtemperature installations and where there are multiple evaporators, particularly if they are located some distance from the machine room. In a two-stage system, the low-temperature evaporators might be served by a liquid recirculation system, and the intemediate-temperature evaporators may operate in direct expansion. It is also possible to install liquid recirculation on the intermediate temperature evaporators with the flash-tank/intercooler serving as the lowpressure receiver.

In addition to low-temperature applications where liquid recirculation now predominates, the concept may have advantages in higher-temperature systems used for air conditioning16. Improved heat-transfer coefficients in the evaporator and the positive feeding of multiple evaporators are advantages of liquid recirculation in these cases too. Furthermore, such air-conditioning systems are likely to use a halocarbon refrigerant, and a recirculation system will convey oil on the low-pressure side to the low-pressure receiver where a central oil return can be provided, rather than running the risk of oil accumulation in some evaporators.

Liquid recirculation is more advantageous when serving tube-type evaporators in contrast to evaporators where boiling takes place outside tubes in the shell of the evaporator. Properly applied flooded evaporator coils operate with the same effectiveness of coils served by liquid recirculation. Since oil return provisions must be made for each coil in systems with flooded evaporators, when the number of coils exceeds three to five, it is usually more effective to choose liquid recirculation which has only one point of oil removal—the lowpressure receiver.