When pumping water vapor in a large industrial plant, a certain quantity air is always involved, which is either contained in the vapor or originates from leaks in the plant (the following considerations for air and water vapor obviously apply also in general for vapors other than water vapor). Therefore, the condenser must be backed by a gas ballast pump (see Fig. 2.41) and hence always works - like 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.
For pumping larger quantities of water vapor, the condenser is the most economical pump. As a rule, the condenser is cooled with water of such temperature that the condenser temperature lies sufficiently below the dew point of the water vapor and an economical condensation or pumping action is guaranteed. For cooling, however, media such as brine and refrigerants (NH3, Freon) can also be used.
In sizing the combination of condenser and gas ballast pump, the following points must be considered:
a) the fraction of permanent gases (air) pumped simultaneously with the water vapor should not be too great. At partial pressures of air that are more than about 5% of the total pressure at the exit of the condenser, a marked accumulation of air is produced in front of the condenser surfaces. The condenser then cannot reach its full capacity (See also the account on the page Pumping gases (wet process) on the simultaneous pumping of gases and vapors).
b) the water vapor pressure at the condenser exit – that is, at the inlet side of the gas ballast pump – should not (when the quantity of permanent gas, described in more detail on the Pumping gases (wet process) page, is not pumped simultaneously) be greater than the water vapor tolerance for the gas ballast pump involved. If – as cannot always be avoided in practice – a higher water vapor partial pressure is to be expected at the condenser exit, it is convenient to insert a throttle between the condenser exit and the inlet port of the gas ballast pump. The conductance of this throttle should be variable and regulated (see page: Calculating conductance) so that, with full throttling, the pressure at the inlet port of the gas ballast pump cannot become higher than the water vapor tolerance. Also, the use of other refrigerants or a decrease of the cooling water temperature may often permit the water vapor pressure to fall below the required value.
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 tot 1, is equal to the total pressure at the condenser exit, p tot 2 ( 2.23)
Ptot1 = ptot2
The total pressure consists of the sum of the partial pressure portions of the air pp and the water vapor pv: ( 2.23a)
pp1 + pv1 = pp2 + pv2
As a consequence of the action of the condenser, the water vapor pressure pD2 at the exit of the condenser is always lower than that at the entrance; for (2.23) to be fulfilled, the partial pressure of air pp2 at the exit must be higher than at the entrance pp1, (see Fig. 2.43), even when no throttle is present.
The dashed lines are those for an ideal condenser (ptot 2 ≈ ptot 1). pD: Partial pressure of the water vapor, pL: Partial pressure of the air.
The higher air partial pressure pp2 at the condenser exit is produced by an accumulation of air, which, 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.
All calculations are based on (2.23a) for which, however, information on the quantity of pumped vapors and permanent gases, the composition, and the pressure should be available. The size of the condenser and gas ballast pump can be calculated, where these two quantities are, indeed, not mutually independent. Fig. 2.42 represents the result of such a calculation as an example of a condenser having a condensation surface of 1 m2, and at an inlet pressure pv1, of 40 mbar, a condensation capacity that amounts to 33lbs (15kg) / h of pure water vapor if the fraction of the permanent gases is very small. 1 m3 of cooling water is used per hour, at a line overpressure of 3 bar and a temperature of 53.6°F (12°C). The necessary pumping speed of the gas ballast pump depends on the existing operating conditions, particularly the size of the condenser. Depending on the efficiency of the condenser, the water vapor partial pressure pv2 lies more or less above the saturation pressure pS which corresponds to the temperature of the refrigerant. (By cooling with water at 53.6°F (12°C), pS, would be 15 mbar (see Table XIII in Section 9)). Correspondingly, the partial air pressure pp2 that prevails at the condenser exit also varies. With a large condenser, pv2 ≈ pS, the air partial pressure pp,2 is thus large, and because pp · V = const, the volume of air involved is small. Therefore, only a relatively small gas ballast pump is necessary. However, if the condenser is small, the opposite case arises: pv2 > pS · pp2, is small. Here a 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.
In practice, the following measures are usual:
a) A throttle section is placed between the gas ballast pump and the condenser, which can be short-circuited during rough pumping. The flow resistance of the throttle section must be adjustable so that the effective speed of the pump can be reduced to the required value. This value can be calculated using the equations given on the pumping gases (wet process) page.
b) Next to the large pump for rough pumping a holding pump with low speed is installed, which is of a size corresponding to the minimum prevailing gas quantity. The objective of this holding pump is merely to maintain optimum operating pressure during the process.
c) The necessary quantity of air is admitted into the inlet line of the pump through a variable-leak valve. This additional quantity of air acts like an enlarged gas ballast, increasing the water vapor tolerance of the pump. However, this measure usually results in reduced condenser capacity. Moreover, the additional admitted quantity of air means additional power consumption and increased oil consumption. As the efficiency of the condenser deteriorates with too great a partial pressure of air in the condenser, the admission of air should not be in front, but generally only behind the condenser.
If the starting time of a process is shorter than the total running time, technically the simplest method - the roughing and the holding pump - is used. Processes with strongly varying conditions require an adjustable throttle section and, if needed, an adjustable air admittance.
On the inlet side of the gas ballast pump a water vapor partial pressure pv2 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).
With a view to practice and from the stated fundamental rules, consider the two following cases: