The liquid recirculation subsystem in which the liquid is motivated by a mechanical pump includes the following main components: pump, liquid distribution piping, refrigerant controls at the coil, the coil with its defrost controls, the liquid/vapor return line, and the low-pressure receiver. Several types of pumps serve liquid recirculation systems, including positive displacement pumps(Section 8.7), open-type centrifugal pumps (Section 8.8), and hermetic centrifugal pumps (Section 8.10). The pump must be selected in conjunction with the design of the liquid- distribution system (Section 8.12) to provide the required flow rate of liquid refrigerant. Special attention must be dedicated to the inlet piping to the pump (Section 8.11) so that no vaporization of liquid occurs at the pump inlet (cavitation). A specific requirement to avoid cavitation is to provide adequate net-positive-suction head (Section 8.9).