Positive-displacement pumps are usually of the gear type where the meshing of gears conveys the liquid around the periphery of the gears, as in Figure 8.11a or in a combination of internal and external flow9 as in Figure 8.11b. When the pump speed is constant, the flow rate delivered by the pump is ideally constant, but because of leakage, which increases with an increase in pressure difference, the performance characteristics are typically as shown in Figure 8.12.
The positive-displacement pump delivers approximately a constant flow rate, so this type of pump is best suited to those liquid recirculation systems where the flow rate is essentially constant, and not, for example, to systems where thermostatically controlled solendoid valve open and close the liquid lines serving coils. If the liquid supply lines were restricted, the positive-displacement pump reacts by building up more pressure, so a pressure relief valve opens when the pressure rises above the pressure setting. The relief line discharges back to the low-pressure receiver. Because a shut-off valve in this line is necessary for servicing the pump, there is a possibility of accidentally trapping liquid between the shut-off valve and the relief valve. To avoid trapping cold liquid which could expand upon warm-up and possibly rupture the pipe or valve, some manufacturers provide a two-way relief valve which can relieve in the opposite direction at a very high pressure. Another approach to limiting the discharge pressure is to operate the pump with a variable-speed drive in which the speed is regulated by controlling to the desired discharge pressure.
The early liquid recirculation systems used positive-displacement pumps, but in recent years the centrifugal type has almost universally replaced them. This is not to suggest that the positive-displacement pump has no role in industrial refrigeration systems. It is ideally suited for liquid-transfer assignments, such as pumping liquid from a low-pressure vessel to one at high pressure. The positive-displacement pump can easily develop the high differential pressures that might be needed for this assignment, and normally this application does not require throttling to low flow rates which could overload the pump motor.