10.4 Quick-start plant requirements
10.4.1 Type of plant
More recently, a new design of liquid-ring pump has led to improvements in the flexibility of air extraction equipment by replacing the single-stage quick-start unit with a two-stage vacuum raising and- maintaining unit.
This new unit (Fig 4.43) consists of two stages with both rotors being mounted on a common shaft, a seal water cooler, and an air/water separator. In the hogging mode, only the first-stage vacuum pump operates and it handles the entire system load, as condenser pressure is pulled down from atmospheric to about 235 mbar(a). During hogging, the volume of air handled by the first stage is greater than the inlet capacity of the second stage, and is discharged through a check valve into a separator and to atmosphere.
At approximately 235 mbar(a), the second-stage vacuum pump handles the entire volume of gas discharging from the first stage. Figure 4.43 illustrates how the second-stage pump suction closes the check valve, and the two-stage combination pulls the condenser down to operating pressure. The second stage discharges into the separator and then to atmosphere and, because the interstage pressure is sub-atmospheric, the suction at the first stage of the pump can be lower than 34 mbar(a).
Other design differences incorporated in this system include stationary inlet and outlet ported cones which fit inside a rotor, instead of suction and discharge end plates, and the provision of a plate-type titanium seal water cooler instead of a tubular cooler.