The terms liquid recirculation and liquid overfeed are sometimes used interchangeably and refer to the practice of delivering a greater rate of liquid refrigerant to the evaporator than evaporates. So at the exit of the coil a mixture of liquid and vapor flow out of the evaporator, as shown in Fig. 8.1. If there is desire to differentiate between liquid overfeed and liquid recirculation, it might be that liquid overfeed applies to the coil, while a liquid recirculation system incorporates the additional equipment to accommodate the overfeeding of the coil.
The overfeed operation contrasts to regulating with a controlled expansion valve the liquid flow rate to the evaporator so that it precisely matches the rate of evaporation.
The flooded evaporator, discussed in Chapter 6 and arbitrarily not included in the category of liquid recirculation, also operates in a liquid-overfeed manner, resulting in no superheat at the outlet of the evaporator. Flooded evaporators are supplied with high-pressure liquid like DX coils. Those configurations classified in this chapter as liquid recirculation have a central liquid-vapor separator capable of serving multiple evaporators. Furthermore, the liquid recirculation is forced by either mechanical pumps or by gas-pressure pumping.