Some of the advantages of liquid recirculation systems over those operating with evaporators fed directly with high-pressure liquid are:
1. More effective use of evaporator heat-transfer surface. In nonliquid recirculation systems, some of the surface is used to superheat the leaving refrigerant with resultant low heat-transfer coefficients and reduced temperature differences between the refrigerant and the fluid being cooled. The heat transfer capability of the liquid recirculation system is often compared to the direct-expansion evaporator with liquid feed controlled by a superheat-control valve which is also called a thermostatic expansion valve and is described in more detail in Chapter 11 on refrigerant control valves. This expansion valve, as Fig. 8.4 shows, requires superheat in order to open, so the refrigerant near the exit of the evaporator is at a higher temperature than the saturation temperature prevailing in the upstream portion of the evaporator. In the superheating section of the evaporator the temperature difference between the fluid being cooled and the refrigerant is diminished, and furthermore, the heat-transfer coefficient between the evaporator surface and the superheated refrigerant is lower than the boiling coefficient elsewhere in the evaporator. In liquid recirculating systems the evaporating temperature prevails throughout the evaporator and the heat-transfer coefficient remains high.
2. The refrigerant entering the compressor is close to saturated vapor conditions. The separating vessel(s) in Figs. 8.2 and 8.3 protect the compressor from receiving liquid in the vapor, and the vapor possesses little or no superheat. With low superheat entering the compressor, the discharge temperature from the compressor is also moderated. Indeed, the two-stage system of Fig. 3.15 provides close to saturated vapor for the high-stage compressor, but the liquid recirculation system does the same for the low-stage compressor.
3. Flash gas is removed in the machine room rather than having it appear after the expansion valve at each evaporator. The pressure drop of the refrigerant in the evaporator tubes can thereby be reduced. The same advantage applies to the suction line from the evaporator (also refer to Disadvantage la.)
4. Valves controlling the liquid feed to the evaporator receive liquid at a uniform pressure, regardless of the condensing pressure. An expansion valve fed with liquid directly from the condenser experiences a high feed pressure in summer and a low one in winter. In a liquid recirculation system, the pump provides the same pressure year-round.
5. In the case of ammonia systems, oil which escapes the compressors carries to the low-pressure side of the system where it separates from the ammonia. This oil must be periodically drained and with individual feeding of evaporators with high-pressure liquid, oil must be drained periodically from each evaporator. With liquid recirculation, however, particularly if the coil undergoes periodic defrosts, the oil from the evaporator is brought back to the liquid/vapor separator so that oil draining is necessary at only one location.