10.3.3 Air ejector pump systems
Figure 4.41 shows a typical arrangement, using air as the motive to the ejector. The system comprises an ejector stage and a liquid-ring pump stage, which combines to give two stages of compression of the condenser air/vapour mixture.
Figure 4.41 (a) illustrates the system when operating to maintain vacuum in the condenser. The air/vapour mixture from the condenser is first drawn through the vapour cooler. This is a direct condenser which receives recirculated seal water, of towns water quality. This cooler reduces the vapour quantity of the mixture before compression by the ejector stage. The ejector uses atmospheric air as the motive fluid and compresses the air/vapour mixture to a pressure of about 135-170 mbar.
The condensate from the vapour cooler and the compressed air/vapour mixture pass to the single-stage liquid-ring pump, where the air is compressed to atmospheric pressure and discharged to a separate tank, along with the seal water. The tank is vented to atmosphere, and there is also an air connection back to the air ejector. Seal water in the tank is pumped back to the vapour cooler and liquid-ring pump via a CW-cooled heat exchanger.
For vacuum raising duties, the air ejector stage is automatically isolated until a condenser pressure of 85 mbar is attained. Figure 4.41 (b) illustrates the flow paths necessary to achieve this duty.
If vacuum maintaining equipment is inadequate for fast vacuum raising, booster pump arrangements (similar to those described for hydraulic air pump systems) are incorporated, these booster pumps are arranged to shut down automatically at about 50 mbar.