Ballast pump

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. 

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. 

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...

Therefore, the condenser must be backed by a gas ballast pump (see Fig. 2.41) and hence aiways works - iike the Roots pump - in a combination. 

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. 

In sizing the combination of condenser and gas ballast pump, the following points must be considered ... 

For a mathematical evaluation of the combination of condenser and gas ballast pump, it can be assumed that no loss of pressure occurs in the condenser, that the total pressure at the condenser entrance p,Q,., is equal to the total pressure at the condenser exit, p,g, 2... 

The higher air partial pressure Pp2 at the condenser exit is produced by an accumulation of air, vi/hich, as long as it is present at the exit, results in a stationary flow equilibrium. From this accumulation of air, the (eventually throttled) gas ballast pump in equilibrium removes just so much as streams from the entrance (1) through the condenser. 

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... 

Water vapor is frequently removed by pumps that operate with water or steam as a pump fluid, for example, water ring pumps or steam ejector pumps. This depends considerably on circumstances, however, because the economy of steam ejector pumps at tow pressures is generally far inferior to that of rotary pumps. For pumping a vapor - gas mixture in which the vapor portion is large but the air portion is small, the vapor can be pumped by condensers and the permanent gases, by relatively small gas ballast pumps (see Section 2.1.5). 

Fig. 2.73 Areas of application for gas ballast pumps and condensers pumping water vapor (o. G. = without gas ballast)...

For the pumping of water vapor, gas ballast pumps and combinations of gas ballast pumps, Roots pumps, and condensers are especially suitable. 

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... 

It must again be noted that the water vapor tolerance of two-stage gas ballast pumps is frequently lower than that of corresponding single-stage pumps. 

Normally, Roots pumps are not as economical as gas ballast pumps for continuous operation at pressures above 40 mbar. With very large pump sets, which work with very specialized gear ratios and are provided with bypass lines, however, the specific energy consumption is indeed more favorable. If Roots pumps are installed to pump vapors, as in the case of gas ballast pumps, a chart can be given that includes all possible cases (see Fig. 2.74). 

Region B A main condenser, a Roots pump with a bypass line, an intermediate condenser, and a gas ballast pump. 

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. 

Roots pumps and gas ballast pumps alone can now continue pumping. 

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. 

Using vacuum condensers (see Section 2.15). These act as selective pumps and should be sized so that the downstream gas ballast pump will not receive more vapor than the amount corresponding to the appropriate vapor tolerance. 

Forevacuum pressure is too high Check the forevacuum allow the gas ballast pump to run for a longer period of time w/ith the gas ballast valve open. It may be necessary to change the oil in the forepump. 

At 40 °C 1% RM is possible with a more standard pumpset a large two-stage gas ballast pump or for production plants better 1 roots pump plus a small gas ballast pump. 

The vacuum necessary for efficient performance and the methods of achieving and maintaining it are important considerations for any insulation. The pumpdown characteristics of the NRC material are quite favorable. A pumpdown from atmospheric pressure to 10 mm Hg is achieved in routine practice of calorimetric runs within 3 to 4 hr, using a 6-cfm rotary gas ballast pump and a 2-in. diffusion pump. After the vessel is filled with liquid nitrogen, the pressure generally drops by a factor of 10 to 10" mm Hg. In a completely seal-welded vessel there is no measurable pressure rise. 

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