Vacuum heating pumps are used to maintain the vacuum in mechanical vacuum heating systems by removing air, vapor, and condensation from the lines. Many vacuum pumps are available in both single and duplex units, and are designed to automatically adjust themselves to the varying conditions of the system. The duplex units have an advantage over the single pumps, because they provide automatic standby service. If one of the pumps in a duplex unit should happen to malfunction, the other one cuts in and picks up the load.
A vacuum pump is operated either by steam or electricity. Steamdriven vacuum pumps are sometimes used in high-pressure steam systems, but these pumps have been generally replaced by automatic motor-driven return-line pumps. Examples of vacuum heating pumps are shown in Figures 10-7 and 10-8.
In operation, the pump is started before the steam enters the system. When the pump removes the air from the lines, steam quickly fills the radiators of the system. The radiators remain full of steam, because the air is automatically removed as fast as it accumulates. By quickly exhausting the air and condensation from the system, the vacuum pump causes the steam to circulate more rapidly, resulting in faster warm-up time and quieter operation.
The condensation of steam in the lines creates the vacuum, and the pump maintains it by continuing to pump air from the system. The vacuum maintained by the pump is only a partial one, because it is not possible with this device to extract all the air. Each stroke of the pump piston or plunger removes only a fraction of the air, depending on the percentage of clearance in the pump cylinder, the resistance of valves, and other factors; hence, an infinite number of strokes would theoretically be necessary to obtain a perfect vacuum, not considering line and pump resistance.
Vacuum pumps designed to remove only air from a system are referred to as dry pumps. Those that remove both air and condensation are called wet pumps. When a wet pump is used, the condensation is pumped back to the boiler. In operation, the air, being heavier than steam, passes off through thermostatic retainer valves to the pump. When the steam reaches the retainer valves, they close automatically to prevent the steam passing into the dry return line to the pump and breaking the vacuum. The air from the pump is passed into a receiver, where it is discharged through an air vent. The condensation is pumped back to the boiler generally by means of a centrifugal pump.
In most vacuum systems, the pump is controlled by a vacuum regulator and a float control. The vacuum regulator cuts in when the vacuum drops to a preset level and cuts out when the vacuum reaches its highest point. The float control operates independently from the vacuum regulator, starting the pump when condensation reaches a certain level in the receiver.
Two typical installations in which vacuum heating pumps are used are illustrated in Figures 10-9 and 10-10. In the vacuum air-line heating system, shown in Figure 10-10, thermostatic-type air-line valves are used instead of radiator air vents. The primary purpose of the vacuum heating pump is to expel air from the system.
The vacuum return-line system is very similar to a condensation return steam heating system, except that a vacuum pump is used to provide a low vacuum in the pipes and to return the condensation to the boiler. Because of the vacuum condition, smaller steam traps and piping can be used.
An accumulator tank must be installed in a vacuum pump steam heating system if the returns are below the inlet connection of the vacuum pump receiver. As shown in Figure 10-10, the condensate flows by gravity from the baseboard heating units to the accumulator tank, where it is lifted to the vacuum pump receiver.