Vacuum systems pumps

Chapter 2 acknowledges the fact that in the design of vacuum systems, pump sets and pipework of an appropriate size must be used and that it is vital that the flow of gases into and out of the system be quantified. Terms widely used in vacuum technology are defined and the calculation of flow and related quantities under the three major types of gas flow is discussed. 

VACUUM-SYSTEM PUMPING SPEED AND PIPE SIZE 6.72... 

Vacuum system. Components associated with lowering the pressure within a mass spectrometer. A vacuum system includes not only the various pumping components but also valves, gauges, and associated electronic or other control devices the chamber in which ions are formed and detected and the vacuum envelope. 

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

Optimum Design of Pumping, Compression, and Vacuum Systems... 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

A vacuum system can be constmcted that includes a solar panel, ie, a leak-tight, instmmented vessel having a hole through which a gas vacuum pump operates. An approximate steady-state base pressure is estabUshed without test parts. It is assumed that the vessel with the test parts can be pumped down to the base pressure. The chamber is said to have an altitude potential corresponding to the height from the surface of the earth where the gas concentration is estimated to have the same approximate value as the base pressure of the clean, dry, and empty vacuum vessel. 

Vacuum systems, largely for the semiconductor industry, are the main source of sales (see Semiconductors). The sales of all vacuum equipment, pumps (qv), valves, sensors (qv), etc, in the United States, including apphcations not in vacuum systems, generally exceed 500 X 10 /yr. A reasonably comprehensive hst of high vacuum manufacturers is supphed by the American Vacuum Society s exhibitor s hst. In Europe, a special issue of the journal A acuum serves similady. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

For materials of moderate to low porosity, a good starting vacuum level is 0.6 to 0.7 bar (18 to 21 in Hg), as the capacity of most vacuum pumps starts to fall off rapidly at vacuum levels higher than 0.67 bar (20 in Hg). Unless there is a critical moisture content which requires the use of higher vacuums, or unless the deposited cake is so impervious that the air rate is extremely low, process economics will favor operation at vacuums below this level. When test work is carried out at an elevation above sea level different than that of the plant, the elevation at the plant should be taken into account when determining the vacuum system capacity for high vacuum levels (>0.5 bar). 

The cost of the filter station includes not only the installed cost of the filter itself but also that of all the accessories dedicated to the filtration operation. Examples are feed pumps and storage facihties, precoat tanks, vacuum systems (often a major cost factor for a vacuum filter station), and compressed-air systems. The dehvered cost of the accessories plus the cost of installation of filter and accessories generally is of the same order of magnitude as the dehvered filter cost and commonly is several times as large. Installation costs, of course, must be estimated with reference to local labor costs and site-specific considerations. 

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. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

A vacuum system typically consists of one or more pumps which are connected to a chamber. The former produces the vacuum, the latter contains whatever apparatus requires the use of the vacuum. In between the two may be various combinations of tubing, fittings and valves. These are required for the system to operate but each introduces other complications such as leaks, additional surface area for outgassing and added resistance to the flow of gas from the chamber to the pumps. Additionally, one or more vacuum gauges are usually connected to the system to monitor pressure. 

A vacuum pump seal drum design which provides a liquid seal (hydraulic flame arrester) to mitigate flame propagation backward into the vacuum system. The seal liquid is an organic stream (mostly Cg aromatics) that comes from the vacuum pump discharge drum overflow. 

Figure 2-47. Acceptable pressure losses between the vacuum vessel and the vacuum pump. Note reference sections on figure to system diagram to illustrate the sectional type hook-ups for connecting lines. Use 60% of the pressure loss read as acceptable loss for the system from process to vacuum pump, for initial estimate. P = pressure drop (torr) of line in question Po = operating pressure of vacuum process equipment, absolute, torr. By permission, Ryans, J. L. and Roper, D. L., Process Vacuum System Design Operation, McGraw-Hill Book Co., Inc., 1986 [18].

In order to estimate an acceptable air inleakage rate for sizing a vacuum pump for use in the medium to high vacuum system, consider ... 

Figure 6-48A. Typical performance of high vacuum booster lobe-type high volume draw-down for evacuating vacuum systems before use of higher vacuum (lower absolute pressure) pump. By permission, Roots Division, Dresser Industries, Inc.

Booster vacuum pumps are used to shorten the pump-down on evacuation (time) of a vacuum system before switching to the smaller vacuum pump to maintain the system opening vacuum and to handle the air inleakage to the system. 

To prevent/reduce the undesirable condensation in the pump, a small hole is drilled in the pump head to admit air or other process non-condensable gas (gas ballast) into the latter portion of the compression stroke. This occurs while the vapor being compressed is sealed off from the intake port by the piston. By reducing the partial pressure of the vapor s condensables, the condensation is avoided. Obviously, this can reduce the capacity of the pump, as the leakage past the seals allows the gas ballast to dilute the intake volume of ba,se suction gas. For most process applications, the effect of this leakage is negligible, unless the vacuum system suction is below 1 torr [22]. 

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