Types Of Vacuum Devices

Almost every commercially made vacuum-producing device can be used with systems in which pumps are primed by evacuation of air. Formerly water-jet, steam-jet, or air-jet primers had wide application, but with the increase in the use of electricity as a power source, motor-driven vacuum pumps have become more popular.

Ejectors Priming ejectors work on the jet principle, typically using steam, compressed air, or water as the operating medium. A typical installation for priming with an ejector is shown in Figure 4. Valve V1 is opened to start the ejector, and then valve V2 is opened. When all the air has been exhausted from the pump, liquid will be drawn into and discharged from the ejector. When this occurs, the pump is primed and valves V2, and V1 are closed, in that order.

An ejector can be used to prime a number of pumps if it is connected to a header through which the individual pumps are vented through isolating valves.

Dry Vacuum Pumps Dry vacuum pumps, which may be of either the reciprocating or the rotary oil-seal type, cannot accommodate mixtures of air and liquid. When they are used in priming systems, some protective device must be interposed between the centrifugal pump and the dry vacuum pump to prevent liquid from entering the vacuum pump. The dry vacuum pump is used extensively for central priming systems.

Wet Vacuum Pumps Any rotary, rotative, or reciprocating pump that can handle air or a mixture of air and liquid is classified as a wet vacuum pump. The most common type

FIGURE 5 Operating principle of the Nash Hytor pump (Nash Engineering)

used in priming systems is shown in Figure 5. This is a centrifugal displacement type of pump consisting of a round, multiblade rotor revolving freely in an elliptical casing partially filled with liquid. The curved rotor blades project radially from the hub and, with the side shrouds, form a series of pockets and buckets around the periphery.

The rotor revolves at a speed high enough to throw the liquid out from the center by centrifugal force. This forms a solid ring of liquid revolving in the casing at the same speed as the rotor, but following the elliptical shape of the casing. It will be readily seen that this forces the liquid to alternately enter and recede from the buckets as the rotor at high velocity.

Referring to Figure 5 and following through a complete cycle of operation in a given chamber, we start at point A with the chamber (1) full of liquid. Because of the effect of the centrifugal force, the liquid follows the casing, withdraws from the rotor, and pulls air through the inlet port, which is connected to the pump inlet. At (2) the liquid has been thrown outwardly from the chamber in the rotor and has been replaced with air. As rotation continues, the converging wall of the casing at (3) forces the liquid back into the rotor chamber, compressing the air trapped in the chamber and forcing it out through the discharge port, which is connected with the pump discharge. The rotor chamber is now full of liquid and ready to repeat the cycle. This cycle takes place twice in each revolution.

If a solid stream of liquid circulates in this pump in place of air or of an air and liquid mixture, the pump will not be damaged but it will require more power. For this reason, in automatic priming systems using this type of vacuum pump, a separating chamber or trap is provided so liquid will not reach the pump. Liquid needed for sealing a wet vacuum pump can be supplied from a source under pressure, with the shutoff valve operated manually or through a solenoid connected with the motor control. It is, however, preferable to provide an independent sealing liquid supply by mounting the vacuum pump on a base containing a reservoir. This is particularly desirable in locations where freezing may occur, as a solution of antifreeze can be used in the reservoir.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook

Post a comment