What are called hermetic refrigerant pumps are actually semihermetic, because many elements can be dismantled for servicing. What is distinctive about the pump is that there are no moving metal parts, such as the shaft, that must be sealed to prevent refrigerant leaks to the outside. In contrast to hermetically sealed compressors where refrigerant vapor is in direct contact with insulated electrical conductors, hermetic pumps prevent refrigerant liquid from contacting the conductors. As the diagram of Figure 8.19 shows, the rotor with its lamination is encapsulated in a stainless steel envelope. The stator which contains the windings is separated from the refrigerant pump by another pressure-tight housing and also enclosed within a steel housing. For brevity, the semi-hermetic pump will hereafter be called hermetic.
Hermetic pumps are usually higher in first cost than open-type pumps, even those with the oil seal. To compensate for the additional first cost, purchasers of hermetic pumps expect lower maintenance costs. Both hermetic and open type pumps are widely used in liquid recirculation systems, especially those using ammonia as the refrigerant.
The basic diagram of the installation of the hermetic pump assembly is shown in Figure 8.20. This arrangement shares most features of the layout of the open-type pump including the provisions for adequate NPSH and a bypass line back to the vessel to assure that there is always some liquid flow through the pump even if valves in the discharge pipe network are closed. Assuring this bypass flow is even more critical in the hermetic pump than it is in the open type. The reason is that a small flow rate of liquid passes outside the rotor envelope to cool the motor. Failure of this liquid flow could result in overheating of the motor and shutdown of the pump.
A hand valve could be used to regulate the bypass flow, but because of the critical nature of this stream and the possibility of improper adjustment, an orifice (the minimum-flow orifice) of the size specified by the pump supplier is preferred. This bypass flow is sometimes sent to the discharge line of the lowstage compressor before the intercooler/desuperheater to desuperheat the discharge gas. Any liquid that fails to vaporize simply drains into the vessel which is designed for liquid/vapor separation. To enhance the desuperheating of the discharge gas, the bypass liquid can be atomized with a spray nozzle. This spray nozzle, which behaves as an orifice, can be chosen to duplicate the pressure-drop characteristics of the specified minimum-flow orifice.
Here is how the two orifices affect the performance of the pump assembly. The size of the minimum-flow orifice must be equal or larger than the size specified by the manufacturer, while the maximum-flow orifice must be of equal or smaller size. As Figure 8.20 illustrates, the flow from the pump first reaches the minimum-flow orifice and if the pump flow is indicated by Point A (Figure 8.21), all of that flow rate bypasses to the low-pressure receiver. The flow rate delivered to the system is zero and indicated by Point A´ in Figure 8.21. Throughout the entire range of flow of the pump some liquid passes through the orifice, and the flow rate delivered to the system is shown by the dashed line. The fact that the pressure drops as the flow increases will result in reduced bypass flow, but that is no problem because the flow through the system is adequate to assure that the pump work does not vaporize the liquid.
The maximum-flow orifice limits the flow rate so that the NPSH provided by the installation is not exceeded. As Figure 8.18 showed, it is at high flow rates that large NPSHs are needed. Point B in Figure 8.21 is the maximum system flow which overcomes the resistance of the maximum-flow orifice and the piping network with all control valves open. This realization means that the pump manufacturer in specifying the diameter of the maximum-flow orifice should
know the characteristics of the wide-open system, and combine that resistance with that of the orifice. This information is usually not available to the manufacturer, so the specification of the orifice normally results in a conservative choice. As control valves begin to close off, the performance rides up and to the left on the dashed performance curve and moves into the region of low values of required NPSH.