Machinery's Handbook, 31st Edition
VACUUM
2823
Vacuum Vacuum is any pressure lower than atmospheric. It can be measured in psi or Pa as a neg ative gauge pressure, or as a positive absolute pressure.Other common units for vacuum are inches of mercury (inHg), millimeters of mercury (mmHg), millibar, and torr. A per fect vacuum has zero absolute pressure, or - 14.7 psi ( - 29.92 inHg). Flow in low to me- dium vacuum (down to 10 –4 torr) is characterized as viscous flow, while in high (below 10 –4 torr) and ultra-high vacuum (below 10 –8 torr) it is characterized as molecular flow. Tubing and Fittings for Vacuum Applications.— Tubing and fittings used for vacuum ap - plications must be rated for vacuum use to prevent collapse. When working with vacuum, the resistance to flow created by tubing and fittings is discussed in terms of conductance. Conductance can be calculated as follows: C Q ∆ p = ---- where C is conductance in l/sec, p is pressure in torr, and Q is flow rate in torr–l/sec. For dry air at 20 ° C in the molecular flow region of vacuum, estimate conductance of long tubes as follows: C 75 D 3 L = --- where L is tube length in inches, D is tube inner diameter in inches, and C is in l/sec. Effective pumping speed at the end of a tube with conductance C can be calculated using the following equation: 1 where S is the pumping speed of the pump by itself, and C is conductance of the tube. The conductance of tubes in series and parallel can be combined according to the following equations. Conductance changes with pressure, so effective pumping speed is best calculated piecemeal at incremental pressures. C p = C 1 + C 2 where C p is the conductance of tubes 1 and 2 in parallel. 1 C s --- = 1 C 1 --- 1 C 2 + --- where C s is the conductance of tubes 1 and 2 in series. Vacuum Pumps and Generators.— Vacuum pumps are essentially compressors in re- verse. Where deep or continuous vacuum is required on a large evacuation volume, it is common to use a fast evacuation pump to remove the gas, and then a deep vacuum pump is used to reach the desired vacuum level. Vacuum deeper than - 14.7psi ( - 29.92 inHg) is not possible. Vacuum generators usually use a venturi to generate vacuum when compressed air is supplied. Single and multi- stage units are widely available to generate vacuum as deep as - 25 inHg ( - 85 kPa). Suction flow for vacuum generators is moderate, with values of 7 SCFM (200 l/min) commercially available. Vacuum generators consume compressed air, so these tend to be more costly to run than vacuum pumps. Vacuum generators are sometimes combined into devices called “vacuum ejectors” which provide a puff of posi - tive air pressure when the vacuum is halted. This provides a positive release function when picking and placing components using vacuum. S eff ---- 1 C -- 1 S = + -- Types of Vacuum Pumps: Vacuum pumps can employ pistons, vanes, and lobes like com- pressors or pumps. There are also turbomolecular pumps, diffuser pumps, chemical reac- tion pumps, gettering pumps, ion pumps, and cryogenic pumps. Vacuum pumps tend to fall into two main categories: transfer pumps and capture pumps.
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