The gas ballast

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. 

When simultaneously pumping permanent gases and condensable vapors from a vacuum system, the quantity of permanent gas will often suffice to prevent any condensation of the vapors inside the pump. The quantity of vapor which may be pumped without condensation in the pump can be calculated as follows  

1) Pump is connected to the vessel, which is already almost empty of air (70 mbar) - it must thus transport mostly vapor particles 

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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. 

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... 

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). 

The gas ballast pump has the function of pumping the fraction of air, which is often only a small part of the water-vapor mixture concerned, without simultaneously pumping much water vapor. It is, therefore, understandable that, within the combination of condenser and gas ballast pump in the stationary condition, the ratios of flow, which occur in the region of rough vacuum, are not easily assessed without further consideration. The simple application of the continuity equation is not adequate because one is no longer concerned with a source or sink-free field of flow (the condenser is, on the basis of condensation processes, a sink). This is emphasized especially at this point. In a practical case of non-functioning of the condenser - gas ballast pump combination, it might be unjustifiable to blame the condenser for the failure. 

Pv2 Ps Pp2 relatively large gas ballast pump is required. Since the quantity of air involved during a pumping process that uses condensers is not necessarily constant but alternates within more or less wide limits, the considerations to be made are more difficult. Therefore, it is necessary that the pumping speed of the gas ballast pump effective at the condenser can be regulated within certain limits. 

On the inlet side of the gas ballast pump a water vapor partial pressure p 2 is always present, which is at least as large as the saturation vapor pressure of water at the coolant temperature. This ideal case is realizable in practice only with a very large condenser (see above). 

Pumping of permanent gases with small amounts of water vapor. Here the size of the condenser - gas ballast pump combination is decided on the basis of the pumped-off permanent gas quantity. The condenser function is merely to reduce the water vapor pressure at the inlet port of the gas ballast pump to a value below the water vapor tolerance. 

Important note During the process, if the pressure in the condenser drops below the saturation vapor pressure of the condensate (dependent on the cooling water temperature), the condenser must be blocked out or at least the collected condensate Isolated. If this is not done, the gas ballast pump again will pump out the vapor previously condensed in the condenser... 

In this region the vrater vapor pressure exceeds the admissible partial pressure at the inlet. The gas ballast pump must, therefore, have a condenser inserted at the inlet, which is so rated that the water vapor partial pressure at the inlet port of the rotary pump does not exceed the admissible value. The correct dimensions of the condenser are selected depending on the quantity of water vapor involved. For further details, see... 

With an ideally leak-free vessel, the gas ballast pump should be isolated after the required operating pressure is reached and pumping continued with the condenser only. Section 2.1.5 explains the best possible combination of pumps and condensers. 

With short-term drying, the separation of the condenser filled with condensed water is particularly important, because the gas ballast pump would continue to pump from the condenser the previously condensed... 

With longer-term drying processes, it suffices to shut off the condensate collector from the condenser. Then only the remaining condensate film on the cooling tubes can reevaporate. Depending on the size of the gas ballast pump, this reevaporation ensues in 30 - 60 min. 

An increased test gas (helium) background level can be lowered by opening the gas ballast valve and introducing gas which is free of the test gas (helium-free gas, fresh air). The dissolved helium will be flushed out, so to speak. Since the effect always affects only the part of the oil present in the pump body at the particular moment, the flushing procedure will have to... 

If an oil-filled rotary pump is operated without an oil separation and return device, then it will be necessary to expect a certain amount of oil consumption, the extent of which will depend on the size of the pump and the nature of the operations. In the worst case this can amount to about 2 cm3 for every cubic meter of air pumped (at STP and including the gas ballast also drawn in). Figure 8.1 makes it possible to predict the amount of oil loss to be expected in practical situations. The example demonstrates that greater oil losses must be expected when operating the pump with gas ballast. This situation, vi/hich is generally valid, is alvi/ays to be taken into account in practice. 

The gas ballast principle (See Fig. 2.14). This increases considerably the amount of vapor which the pump can tolerate. 

Normal N 62 pump oil Is to be used to pump basic (alkaline) solutions. Sodium hydroxide and caustic potash solutions should not be pumped In their concentrated form. Ammonia is highly amenable to pumping with the gas ballast valve closed. Alkaline organic media such as methylamlne and dimethylamlne can also be pumped satisfactorily, but with the gas ballast valve open. 

When handling hydrogen It Is necessary to make note of the hazard of creating an explosive mixture. The gas ballast valve may In no case be opened when dealing with hydrogen. The motors driving the pumps must be of explosion-proof design. 

The low molecular weight alkanes such as methane and butane can be pumped with the gas ballast valve closed or using Inert gas as the gas ballast and/or at inaeased temperature of the pump. But important -Increased explosion hazard ... 

Once operating temperature has been reached, methanol and ethanol can be extracted without using gas ballast (N 62 pump oil). To pump higher molecular weight alcohols (e.g. butanol) the gas ballast valve will have to be opened or other protective measures will have to be implemented to prevent condensation. 

Keep the gas ballast open when pumping carbon tet and other nonflammable solvents. Use N 62 pump oil. 

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