Vacuum positive displacement

Vacuum systems (Fig. 21-12h) are characterized by material moving in an air stream of pressure less than ambient. The advantages of this type are that all the pumping energy is used to move the product and that material can be sucked into the conveyor line without the need of a rotaiy feeder or similar seal between the storage vessel and the conveyor. Material remains suspended in the air stream until it reaches a receiver. Here, a cyclone separator or filter (Fig. 21-12c) separates the material from the air, the air passing through the separator and into the suction side of the positive-displacement blower or some other power source. 

A useful summary of the typical equipment used for developing and maintaining process system vacuum is presented in Table 6-1. Also see Birgenheier [33]. The positive displacement type vacuum pumps can handle an overload in capacity and still maintain essentially the same pressure (vacuum), while the ejectors are much more limited in this performance and cannot maintain the vacuum. The liquid ring unit is more like the positive displacement pump, but it does develop increased suction pressure (higher vacuum) when the inlet load is increased at tlie lower end of the pressure performance curve. The shapes of these performance curves is important in evaluating the system flexibility. See later discussion. 

These units are useful in both vacuum and pressure ranges and can usually develop compression ratios of 3 1 to 10 1, depending on the inlet absolute pressure. The units are positive displacement in performance. 

Positive displacement pumps are self-priming by their normal operating action and are designed to cope with mnning dry. Centrifugal pumps are not inherently selfpriming, and need to be provided with assistance in the form of ejectors, as in Figure 32.56 or dry vacuum pump systems. In these systems, a dry vacuum pump allows air to be drawn out and where an automatic valve prevents... 

Reciprocating and rotary positive displacement pumps are commonly used where moderately low vacuum is required, about 10 mmHg (0.013 bar), at moderate to high flow rates such as in vacuum filtration. 

A slurry is being filtered at a net rate of 10,000 gal/day by a plate and frame filter with 15 frames, with an active filtering area of 1.5 ft2 per frame, fed by a positive displacement pump. The pressure drop varies from 2 psi at start-up to 25 psi after 10 min, at which time it is shut down for cleanup. It takes 10 min to disassemble, clean out, and reassemble the filter. Your boss decides that it would be more economical to replace this filter with a rotary drum filter using the same filter medium. The rotary filter operates at a vacuum of 200 mmHg with 30% of its surface submerged and rotates at a rate of 5 min/rev. If the drum length is equal to its diameter, how big should it be ... 

The lowest pressure which can be achieved in a vacuum vessel. The so-called ultimate pressure Pgi j depends not only on the pump s suction speed but also upon the vapor pressure p j for the lubricants, sealants and propellants used in the pump. If a container is evacuated simply with an oil-sealed rotary (positive displacement) vacuum pump, then the ultimate pressure which can be attained will be determined primarily by the vapor pressure of the pump oil being used and, depending on the cleanliness of the vessel, also on the vapors released from the vessel walls and, of course, on the leak tightness of the vacuum vessel itself. 

A Roots vacuum pump (see Fig. 2.17) Is a rotary positive-displacement type of pump where two symmetrically-shaped Impellers rotate Inside the pump casing past each other In close proximity. The two rotors have a aoss... 

Positive displacement vacuum pumps. Measurement of performance characteristics. Part 1 - Measurement of volume rate of flow (pumping speed) 12/93... 

Procedures Polyamic Acid films were cast by delivering 1.5 to 2.0 ml of resin to the center of a non-spinning j6 mm dia. wafer with a positive displacement bellows-type pump Spinning then progressed via IK rpm/sec ramp to the final spin speed for 20 seconds. Films were dried 30 minutes at 90° 10°C while held horizontally in a 25" mercury vacuum. B-stage cures were performed at 205°C for one to two hours in a forced air oven. Unless noted otherwise, all annealing was done at 350°C in Op for 30 minutes. 

Figure 5.6. Components of pneumatic conveying systems, (a) Rotary positive displacement blower for pressure or vacuum, (b) A rotary airlock feeder for fine materials (Detroit Stoker Co.), (c) A four-compartment receiver-filter (Fuller Co., Bethlehem, PA), (d) A two-stage cyclone receiver, (e) The Fuller-Kinyon pump for cement and other fine powders. Powder is fed into the aeration chamber with a screw and is fluidized with compressed air (Fuller Co., Bethlehem, PA).

The most commonly used vacuum pumps are steam-jet ejectors and several positive-displacement pumps, which are shown in Figures 5.1 and 5.2. Some of the characteristics of vacuum pumps are given in Table 5.1. A prime consideration when selecting a vacuum pump is the compatihility of a gas with a seal fluid. To avoid these problems, there is a trend toward using dry pumps where a seal fluid or lubricant is not used [60]. 

Figure 5.2 Positive-displacement vacuum pumps. References with permission.

Figure 5.7 shows that positive-displacement compressors, like vacuum pumps, are divided into two main classes reciprocating and rotary. Table 5.4 lists characteristics of these compressors. Ludwig [14] discusses compression equipment and calculation methods in detail. 

The simplest feeding system would involve a manually prepared fluoride solution and a flow-paced feed pump that introduces the fluoride solution into a water supply at a fixed rate. The requirements for auxiliary equipment are minimal. This system would include a platform scale, dissolving tank with a manual paddle or electric mixer for stirring, and a solution tank with a solution feed pump, this is all the equipment needed. Depending on the type of pumps (non-positive displacement) used to feed the fluoride solution, a vacuum breaker can be incorporated into the design of the pump feeding system to prevent pulling unmetered quantities of fluoride solution into the water system in the event of a low-pressure situation. 

A rotary-piston pump is an oil-sealed, positive-displacement vacuum pump. The oil both lubricates the pump and seals the discharge from the suction side of the pump. As the piston rotates, gas enters a chamber, as shown in Figure 5.2. Then, the inlet port closes, and the gas is compressed in the chamber until the discharge valve opens, exhausting the gas to the atmosphere. Possible contamination of the oil with condensable vapors, usually water, is a problem. One way condensation can be avoided is by reducing the partial pressure of the condensable gases by allowing air to leak into the cylinder, which is called a gas ballast. 

Vacuum blowers (vacuum pumps) are used to draw waste nitrogen, carbon dioxide, and water vapor from the adsorber vessels during the evacuation step. There are two rotary-lobe blowers operating in series. They are positive displacement machines. A timing gear arrangement maintains the close tolerances that are essential for ef cient operation. 

Photo 77 This pump uses both vacuum and a positive displacement unit to move oil. 

For vacuum production reciprocating, rotary positive displacement and liquid ring pumps can be used to pressures from 200 to 10 mmHg. Alternatively steam ejectors can be used when water self is a product, as in evaporation of aqueous solutions. The residual pressures can be lowered under 1 mmHg by using several ejectors in series. 

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