The third major method of feeding evaporators is forced liquid recirculation, as shown in Fig. 6.12, where liquid from either a mechanical pump or a gas-pressure pumping arrangement supplied the evaporators.
A greater liquid flow rate enters the evaporator than is evaporated, so a mixture of liquid and vapor leaves the evaporator and passes back to a vessel that separates vapor from liquid. The separated liquid, along with the makeup
liquid admitted through the level-control valve, drops to the pump for recirculation to the evaporators. Both forced-recirculation systems and flooded coils boast the advantages of good heat transfer and saturated vapor passing to the compressor. The assurance of overfeeding under varying loads is probably stronger with forced recirculation, however, and all oil accumulates at one location—the separating vessel—rather than in the individual evaporators.
Properly designed flooded evaporators and evaporators operating with liquid recirculation operate with equal effectiveness. What, then, is the basis for choosing between flooded coils and a liquid recirculation system serving multiple coils? With a liquid recirculation system the pump and separating vessel add to the first cost, which is not true of flooded coils. On the other hand the maintenance cost of the system using flooded coils is usually high because of the multiple locations where oil must be removed and because of the maintenance cost on the controls of a large number of evaporators. These considerations lead to favoring flooded coils for small systems (perhaps three or four evaporators) and pumped liquid recirculation for larger installations.