The design of a conventional gravity-flow steam heating system is such that the heat-emitting units cannot be placed at a level lower than the water level in the boiler. The movement of the condensation depends on gravity and, therefore, must move from a higher to a lower level until it reaches the boiler (at the lowest level). This is illustrated by the one-pipe steam heating system in Figure 10-1. Each heating unit has a single pipe through which it receives the hot steam and returns the condensate in the opposite direction. The dependence on gravity to return the condensate to the boiler places certain limitations on the design of the heating system unless mechanical means are used to compensate for the lack of gravity. A condensate pump serves this purpose.
Many different types of condensate pumps have been developed for use in steam heating systems. Screw, rotary, turbine, reciprocating, and centrifugal pumps are some of the types used for this purpose. One of the most common uses of condensate pumps in low-pressure steam heating systems is the motor-driven centrifugal pump equipped with receiver (tank) and float-control automatic switch.
In operation, condensation enters the receiver and fills the tank. A float connected to an automatic switch rises with the water until the tank is almost full. At that point, the float closes the switch and starts the pump motor. The water is pumped from the receiver and the float drops, causing the switch to open and shut off the pump motor.
The centrifugal pump shown in Figure 10-2 is used to pump condensation from a lower level return line to one at a higher level, or against a higher pressure. These units are also used to pump condensation from a flash tank to a boiler (see Figure 10-3) and for other special applications.
As shown in Figure 10-4, the basic components of this pump consist of an impeller (A), with an inlet at its center rotating on a shaft (D). The condensation enters the inlet orifice and flows radially through vanes to the outer periphery (F) of the impeller; it has approximately the same velocity as the periphery. The head of pressure developed by the pump is the result of the velocity imparted to the condensation by the rotating impeller.
When the condensation leaves the outer periphery of the impeller, it flows around the volute casing (B) and through the discharge orifice (E) of the pump. A wear ring (C) is provided to prevent bypassing of the condensation.
Condensate pumps are available in either single or duplex units. The latter are used in installations where it is necessary to have a pump available for use at all times. A duplex unit is actually two condensation pumps fitted with a mechanical alternator. Both pumps feed into the same receiver. If one pump malfunctions, the other starts automatically and continues to provide uninterrupted pumping service for the system.
Vertical condensation pumps are available for use in installations where the space for the pump is limited, where the returns run below the floor, or where it is undesirable to place a horizontal pump in a sunken area. Vertical condensation pumps are also available in both single and duplex units.
The use of a condensate pump in a two-pipe steam heating system is shown in Figure 10-5. Note the arrangement of gate and check valves on the discharge side of the pump. This type of system is commonly referred to as a condensate-return steam heating system. Using a condensate pump to return the condensation provides greater design flexibility for the system. The major disadvantage is that larger steam traps and piping must be used than in vacuum heating systems.
Condensate pumps can also be used as mechanical lifts in vacuum steam heating systems (see Figure 10-6). By connecting the vent outlet of the condensation pump to a return line above the level of the vacuum heating pump, the same vacuum return condition is maintained in the piping below the water level as in the rest of the system. In this arrangement, the only purpose of the condensate pump is to lift the condensation from the lower level to the higher one without reducing the capacity of the vacuum heating pump.