Gas ballast

External cryogenic requirements None required 1. Helium hquefier 2. Helium compressor 3. Liquid helium storage tank 4. liquid nitrogen storage tank 5. Helium gas ballast tank Not apphcahle... 

The four stage exhaust box Includes the oil box separator, the demister pad, the oil mist eliminator, and the synthetic oil baffle. Additional features include ein automotive type spin-on oil filter, a built-in inlet anti-suckback valve that prevents oil from being drawn into the system when the pump is stopped, eind a built-in gas ballast, available on the RA version, which permits pumping with high water vapor loads. 

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]. 

If not connected to an inert gas line, the gas ballast valves on all vacuum pumps should be plugged to prevent accidental introduction of air into the pump while hydrogen is being used. 

In the case of dust filters which are used to protect gas ballast pumps and roots pumps, the percentage restriction value for the various pressure levels are listed in the catalog. Other components, namely the condensate separators and condensers, are designed so that they will not reduce pumping speed to any appreciable extent. 

Shortly before beginning of compression period, the front surface of the rotating plunger frees gas ballast opening -commencement of gas ballast inlet... 

The gas ballast facility as used in the rotary vane, rotary plunger and trochoid pumps permits not only pumping of permanent gases but also even larger quantities of condensable gases. 

The gas ballast facility (see Fig. 2.13) prevents condensation of vapors in the pump chamber of the pump. When pumping vapors these may only be compressed up to their saturation vapor pressure at the temperature of the pump. If pumping water vapor, for example, at a pump temperature of 70 °C, the vapor may only be compressed to 312 mbar (saturation vapor pressure of water at 70 °C (see Table XIII In Section 9)). When compressing further, the water vapor condenses without Increasing the... 

Shown schematically in Fig. 2.14 is the pumping process with and without gas ballast as it takes place in a rotary vane pump when pumping condensable vapors. 

With the gas ballast valve open the temperature of the pump inaeases by about 10 °C. Before pumping vapors the pump should be operated for half an hour with the gas ballast valve open). 

Pump chamber is separated from the vessel - now the gas ballast valve, through which the pump chamber is filled with additional air from outside, opens - this additional air is called gas ballast... 

Dixharge valve is pressed open, and particles of vapor and gas are pushed out -the overpressure required for this to occur is reached very early because of the supplementary gas ballast air, as at the beginning the entire pumping process condensation cannot occur... 

Eq. (2.3) shows that when using gas ballast (B 0) vapors can also be pumped without condensation if no gas is present at the intake of the pump. The gas ballast may also be a mixture of non-condensable gas and condensable vapor as long as the partial pressure of this vapor... 

Fig. 2.16 Partial pressure p of water vapor that can be pumped with the gas ballast valve open without condensation in the pump, as a function of the pump temperature for various partial pressures Pi of air. The lowest curve corresponds to the water vapor...

According to equation 2.4 an Increase In the gas ballast B would result In an Increased water vapor tolerance In practice, an Increase In B, especially In the case of single-stage gas ballast pumps Is restricted by the fact that the attainable ultimate vacuum for a gas ballast pump operated with the gas ballast valve open becomes worse as the gas ballast B Increases. Similar considerations also apply to the general equation 2.3 for the vapor tolerance p p. ... 

At the beginning of a pump down process, the gas ballast pump should always be operated with the gas ballast valve open. In almost all cases a thin layer of water will be present on the wall of a vessel, which only evaporates gradually. In order to attain low ultimate pressures the gas ballast valve should only be closed after the vapor has been pumped out. LEYBOLD pumps generally offer a water vapor tolerance of between 33 and 66 mbar. Two-stage pumps may offer other levels of water vapor tolerance corresponding to the compression ratio between their stages -provided they have pumping chamber of different sizes. 

Generally atmospheric air Is used as the gas ballast medium. In special cases, when pumping explosive or toxic gases, for example, other permanent gases like noble gases or nitrogen, may be used (see Section 8.3.1.3). 

E 250 (single-stage rotary plunger pump, operated without gas ballast). In... 

Fig. 2.41 Condenser (I) with downstream gas ballast pump (II) for pumping of large quantities of water vapor in the rough vacuum mnge (III) - adjustable throUle...

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