Mechanical pumps condensable vapors

All exhaust from a mechanical pump should be vented to a fume hood regardless of the room s ventilation quality or the type of pumped gases. Each time you bring new samples into vacuum conditions, your system is pumping at atmospheric pressured Because pump oils have low vapor pressures, and pump oils themselves are considered nontoxic, there is little concern for breathing pump oil mist. However, there may be dangers from trapped vapors within the pump oils. Regardless, there is little reason to breathe the pump oil mist if it can be avoided. Check with the manufacturer or distributor of your pump for an oil mist filter for your pump. If you use a condensate trap, be sure you position your exhaust line so that material does not drain back into the pump (see Fig. 7.14). 

There is no one good way to prevent condensable vapors from affecting a mechanical pump. There are, however, two directions that one can take in dealing with the problem One is to limit them from getting to the pump, the other is to prevent them from affecting the pump once they are present. Neither is the best approach, and usually it takes combinations of the two to deal effectively with the problem. An alternative approach is to constantly change the pump oil. This solution however, is neither cost- nor time-effective. 

To prevent (or limit) condensable vapors from getting to a pump, traps [either of chilled or chemical design (see Sec. 7.4 on traps and foreline traps)], are used. Depending on the type of trap used, there are opportunities for vapors to pass on to the mechanical pump. Thus, one cannot depend fully on traps of any kind, and one must also deal with vapors at the pump itself. 

To prevent (or limit) condensable vapors that reach a pump from affecting the mechanical pump oil, a gas ballast (also called a vented exhaust) is used. The gas ballast allows a small bit of atmosphere (up to 10%) into the pump during the compression stage so that the gas from the system is only part of the gas in the pump at the time of greatest compression. Thus, at the time of compression, the total percentage of condensable vapor within the pump is much less than there would be otherwise. Because the gas prior to being expelled is at a lower pressure, less of the vapor can be compressed into a liquid. Then, as the veins sweep into the vacuum side of the pump, no condensed vapor can expand back into a vapor. 

In a mercury diffusion pump, the mercury is heated to the point of vaporization. This vapor travels up into the condenser area where it is ejected at supersonic speeds from little holes. The vapor knocks any wandering gas molecules down toward the mechanical pump outlet which can then expel them from the system. The vapor later condenses and collects in the heating pot for reuse. 

The movement of condensable vapors from the mechanical pump can potentially decrease diffusion pump performance. If your system has diffusion and mechanical pumps, there should be a trap between the two pumps in addition to the cold trap between the system and the diffusion pump (see Fig. 7.30). The use of properly designed and placed cold traps can allow diffusion-pumped vacuum systems to achieve vacuums in the region of 10 9 torr35 and greater ... 

Liquid-nitrogen cold traps stop condensable vapors from traveling between mechanical pumps, diffusion pumps, and the rest of the system. They also protect the helium leak detector from possibly contaminating materials (such as silicon-based diffusion pump oil). 

This procedure should always be done when running any vacuum system without the use of traps because of the damage some condensable vapors can do to vacuum gauges. f On a regular vacuum system, this can be done by either (a) separating the diffusion pump from the rest of the system by stopcocks or valves or (b) the use of a nitrogen cold-trap between the mechanical and diffusion pump. 

Attachment of the distillation apparatus through a joint to a glass high-vacuum line with forepump and mercury diffusion pump is recommended. Distillation may be carried out in ordinary apparatus with a mechanical pump only, but even minute traces of oxygen leaking into the apparatus will react at once with the hot diphenylphosphine vapors, causing smoke to appear in the still head and condenser. 

Never use a mechanical vacuum pump before placing a mixture of dry ice and isopropyl alcohol in a Dewar flask [Fig. 7(c)] around the trap and never pump corrosive vapors (e.g., HCl gas) into the pump. Should this happen change the pump oil immediately. With care, it will give many years of good service. The dry ice trap condenses organic vapors and water vapor, both of which would otherwise contaminate the vacuum pump oil and exert enough vapor pressure to destroy a good vacuum. 

The mechanical pump used this way is called a fore pump. The mercury or oil is condensed by a water condenser at E to be recycled. The vapor pressure of the diffusion pump liquid sets the lower pressure limit that the pump can reach. 

Diffusion Pump. A diffusion pump, shown in Fig. 7.13a, works quite differently from a rotary pump. As noted in Fig. 7.13b, a liquid at low vapor pressure represented by B is first vaporized and then ejected at high velocity in a downward direction by means of a jet and condensed on the cold wall of the pump. The gas being pumped, represented by A, enters the vapor stream and also is propelled downward. Gas A is then removed through the discharge line by a mechanical pump such as a rotary pump. 

Ballast orifice (vacuum technology) An orifice upstream of the mechanical pump that can be used to allow dilution of the pumped gas with dry gas to ensure that vapors in the pumped gas do not condense during compression in the mechanical pump. The ballast orifice also allows the foreline portion of the vacuum pumping manifold to return to ambient pressure in case the mechanical pump stops because of a power failure or a broken belt. This avoids Suck-back. 

Demister (vacuum technology) A baffle on the exhaust of an oil-sealed mechanical pump used to condense oil vapors to reduce the loss of oil from the pump. 

Diluent gas (vacuum technology) Dry gas used to dilute a vapor-containing gas to the point that the vapor will not condense during compression in a mechanical pump. See also Ballast valve. 

Orifice, ballast (vacuum technology) An opening that conhnuously allows gas from the outside to bleed into the forehne of a pumping system. This prevents suck back in the case of a power failure. By using dry air into the orifice, moist air is diluted to the point that water vapor is not condensed by compression in the mechanical pump. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

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