What is a gas ballast and how does it work?

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The gas ballast facility developed in 1935 by Wolfgang Gaede inhibits the occurrence of condensation of the vapor in the pump. 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.

Benefits of a gas ballast

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 158°F (70°C), the vapor may only be compressed to 312 mbar (saturation vapor pressure of water at 158°F (70°C) (see Table XIII)). When compressing further, the water vapor condenses without increasing the pressure. No overpressure is created in the pump and the exhaust valve is not opened. Instead the water vapor remains as water in the pump and emulsifies with the pump’s oil. This very rapidly impairs the lubricating properties of the oil and the pump may even seize when it has taken up too much water.

Operating principle

Before the actual compression process begins (see Fig. 2.13), a precisely defined quantity of air (“the gas ballast”) is admitted into the pumping chamber of the pump. The quantity is such that the compression ratio of the pump is reduced to 10:1 max. Now vapors which have been taken in by the pump may be compressed together with the gas ballast, before reaching their condensation point and ejected from the pump. The partial pressure of the vapors which are taken in may however not exceed a certain value. It must be so low that in the case of a compression by a factor of 10, the vapors cannot condense at the operating temperature of the pump. When pumping water vapor this critical value is termed the “water vapor tolerance”.

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.

Two requirements must be met when pumping vapors:
1) the pump must be at operating temperature.
2) the gas ballast valve must be open.
(With the gas ballast valve open the temperature of the pump increases by about 50°F (10°C). Before pumping vapors, the pump should be operated for half an hour with the gas ballast valve open).

Table XIII Saturation pressure p5 and vapor density eD of water in a temperature range from -148°F (-100°C) to +284°F (+140°C)

Fig. 2.13 Working process within a rotary vane pump with gas ballast

1–2 Suction
2–5 Compression
3–4 Gas ballast inlet
5–6 Discharge

Fig. 2.14 Diagram of pumping process in a rotary vane pump without and with gas ballast device when pumping condensable substances.

a) Without gas ballast

1) Pump is connected to the vessel, which is already almost empty of air (70 mbar) – it must thus transport mostly vapor particles
2) Pump chamber is separated from the vessel – compression begins
3) Content of pump chamber is already so far compressed that the vapor condenses to form droplets – overpressure is not yet reached
4) Residual air only now produces the required overpressure and opens the discharge valve, but the vapor has already condensed and the droplets are precipitated in the pump.

b) With gas ballast
1) Pump is connected to the vessel, which is already almost empty of air (70 mbar) – it must thus transport mostly vapor particles
2) 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
3) Discharge 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
4) The pump discharges further air and vapor

Simultaneous pumping of gases and 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:

(2.1)

Where: pvapor = is the partial pressure of vapor at the intake of the pump.
p_perm = is the total pressure of all pumped permanent gases at the intake of the pump.
p_vapor,sat = is the saturation pressure of the pumped vapor, depending on temperature (see Fig. 2.15).
p_sum = p_exhaust + Δp_valve + Δp_exhaust filter
Δp_valve = is the pressure difference across the exhaust valve which amounts depending on type of pump and operating conditions to 0.2 ... 0.4 bar.
Δp_exhaust filter = is the pressure difference across the exhaust filter amounting to 0 ... 0.5 bar.

Fig. 2.15 Saturation vapor pressures: Table with temperatures

Example

With a rotary vane pump with an external oil mist filter in series, a mixture of water vapor and air is being pumped. The following values are used for applying eq. (2.1):

The pressure of the water vapor in the air/water vapor mixture must not exceed 23 % of the total pressure of the mixture.

Water vapor tolerance

An important special case in the general considerations made above relating to the topic of vapor tolerance is that of pumping water vapor. According to PNEUROP water vapor tolerance is defined as follows:

“Water vapor tolerance is the highest pressure at which a vacuum pump, under normal ambient temperatures and pressure conditions (68°F/20°C, 1013 mbar), can continuously take in and transport pure water vapor. It is quoted in mbar”. It is designated as P_W,O.

Applying equation (2.3) to this special case means:

(2.4)

If for the gas ballast gas atmospheric air of 50 % humidity is used, then p_vapor, g.b. = 13 mbar; with B/S = 0.10 – a usual figure in practice – and p_sum (total exhaust pressure) = 1330 mbar, the water vapor tolerance p_W,0 as function of the temperature of the pump is represented by the lowest curve in diagram Fig. 2.16. The other curves correspond to the pumping of water vapor-air mixtures, hence p_perm = p_air O), indicated by the symbol p_L in millibar. In these cases a higher amount of water vapor partial pressure p_w can be pumped as shown in the diagram. The figures for p_W,0 given in the catalogue therefore refer to the lower limit and are on the safe side.

Fig. 2.16 Partial pressure pw 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 pL of air. The lowest curve corresponds to the water vapor tolerance pw,o of the pump.

According to equation 2.4 an increase in the gas ballast B would result in an increased water vapor tolerance p_W,0. 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_vapor.

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.

Other gases as ballast

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.

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