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When vapors must be pumped, in addition to the factors working pressure and pumping speed, a third determining factor is added namely the vapor partial pressure – which may vary considerably in the course of a process. This factor is decisive in determining the pumping arrangement to be installed. In this regard, the condensers are very important adjuncts to rotary displacement pumps. They have a particularly high pumping speed when pumping vapors. This page covers pumping of water vapor (the most frequent case). The considerations apply similarly to other non-aggressive vapors.
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 low pressures is generally far inferior to that of mechanical 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 mechanical pumps operating with gas ballast.
Comparatively, then, a pump set consisting of a Roots pump, condenser, and backing pump, which can transport 220lbs (100kg)/h of vapor and 39lbs (18kg)/h of air at an inlet pressure of 50 mbar, has a power requirement of 4 – 10 kW (depending on the quantity of air involved). A steam ejector pump of the same performance requires about 60 kW without altering the quantity of air involved. For the pumping of water vapor, gas ballast pumps and combinations of gas ballast pumps, Roots pumps, and condensers are especially suitable.
The ratio of vapor partial pressure pv to air partial pressure pp is decisive in the evaluation of the correct arrangement of gas ballast pumps, as shown previously by equations 2.2 and 2.3. Therefore, if the water vapor tolerance of the gas ballast pump is known, graphs may be obtained that clearly give the correct use of gas ballast pumps for pumping water vapor (see Fig. 2.73). Large single-stage rotary vane pumps have, in general, an operating temperature of about (60 to 80 °C) and hence a water vapor tolerance of about 40-60 mbar. This value is used to determine the different operating regions in Fig. 2.73. In addition, it is assumed that the pressure at the discharge outlet port of the gas ballast pump can increase to a maximum of 1330 mbar until the discharge outlet valve opens.
At a saturation vapor pressure pS of 419 mbar at 170°F (77 °C), according to equation 2.2, the requirement is given that pv < 0.46 pp, where
pv is the water vapor partial pressure
pp is the partial pressure of air
pv + pp = ptot total pressure
This requirement is valid in the whole working region of the single- stage rotary vane pump – hence, at total pressures between 10-1 and 1013 mbar
In this region the water 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 vane pump does not exceed the admissible value. The correct dimensions of the condenser are selected depending on the quantity of water vapor involved. At a water vapor tolerance of 60 mbar, the lower limit of this region is
pv > 6O + 0.46 pp mbar
The lower limit of region C is characterized by the lower limit of the working region of this pump. It lies, therefore, at about ptot = 1 mbar. If large quantities of vapor arise in this region, it is often more economical to insert a condenser: 44lbs (20kg) of vapor at 28 mbar results in a volume of about 1000 m3. It is not sensible to pump this volume with a backing pump. As a rule of thumb:
A condenser should always be inserted at the pump’s inlet if saturated water vapor arises for a considerable time.
As a precaution, therefore, a Roots pump should always be inserted in front of the condenser at low inlet pressures so that the condensation capacity is essentially enhanced. The condensation capacity does not depend only on the vapor pressure, but also on the refrigerant temperature. At low vapor pressures, therefore, effective condensation can be obtained only if the refrigerant temperature is correspondingly low. At vapor pressures below 6.5 mbar, for example, the insertion of a condenser is sensible only if the refrigerant temperature is less than 32°F (0°C). Often at low pressures a gas - vapor mixture with unsaturated water vapor is pumped (for further details, see the page on condensers. In general, then, one can dispense with the condenser.
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. Operating the Roots pump with a frequency converter, thus limited the speed of the pump at rougher pressure, 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).
As there is merely a compression between the Roots pump and the rotary vane pump, the following applies here too:
pv < 0.46 pp
The requirement is valid over the entire working region of the pump combination and, therefore, for total pressures between 10-2 and 40 mbar (or 1013 mbar for Roots pumps with a bypass line or frequency converter drive).
This combination is economical only if large water vapor quantities are to be pumped continuously at inlet pressures above about 40 mbar. The size of the main condenser depends on the quantity of vapor involved. The intermediate condenser must decrease the vapor partial pressure below 60 mbar. Hence, the gas ballast pump should be large enough only to prevent the air partial pressure behind the intermediate condenser from exceeding a certain value; for example, if the total pressure behind the Roots pump (which is always equal to the total pressure behind the intermediate condenser) is 133 mbar, the gas ballast pump must pump at least at a partial air pressure of 73 mbar, the quantity of air transported to it by the Roots pump. Otherwise, it must take in more water vapor than it can tolerate. This is a basic requirement: the use of gas ballast pumps is wise only if air is also pumped!
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. The page on condensers explains the best possible combination of pumps and condensers
The lower limit of the water vapor partial pressure is determined through the compression ratio of the Roots pump at the backing pressure, which is determined by the saturation vapor pressure of the condensed water. Also, in this region the intermediate condenser must be able to reduce the vapor partial pressure to at least 60 mbar. The stated arrangement is suitable – when cooling the condenser with water at 59°F (15°C) – for water vapor pressures between about 4 and 40 mbar.
In this region D the limits also depend essentially on the stages and ratios of sizes of the pumps. In general, however, this combination can always be used between the previously discussed limits – therefore, between 10-2 and 4 mbar.