10.3.4 Steam ejector pump systems
Figure 4.42 shows a typical arrangement, using steam as the motive fluid to the ejector. The system comprises an ejector stage and a two-stage liquid-ring pump which combine to give three stages of compression of the condenser air/vapour mixture for the vacuum maintaining duty.
The condenser air/vapour mixture is first drawn through a direct contact pre-condenser to reduce the quantity of water vapour before compression by the ejector. The pre-condenser uses main CW as the cooling medium, which is drawn from a break tank by the condenser vacuum, and sprayed into the pre-condenser. The condensate from this condenser is pumped to waste by a cooling water extractor pump.
The motivating steam for the ejector is vent steam from the de-aerator which would otherwise be dumped to the main condenser. Since de-aerator pressure varies with load, a control valve maintains the steam pressure to the ejector at a minimum pressure of 1.22 bar which corresponds to the minimum operating pressure of the de-aerator.
On leaving the ejector, the compressed condenser air and ejector steam discharge to an inter-condenser where the steam is condensed, and the remaining air/vapour mixture passes to the liquid-ring pump.
The inter-condenser also uses the main CW as its cooling medium which is drawn from a break tank using the condenser vacuum. Condensate from the inter-condenser is cascaded to the pre-condenser for removal by the cooling water extraction pump. The air/vapour mixture from the inter-condenser is passed to the two-stage liquid-ring pump where it is compressed to atmospheric pressure.
The liquid-ring pump operates with a recirculation service water supply which comes from the service tank. Water from the tank passes via a CW cooled heat exchanger to the liquid-ring pump. For vacuum raising duty, the ejector steam valve is isolated.
The advantages of this system are due to the compression across the steam ejector, combined with effective pre- and post-condensing which increases the liquid-ring pump suction pressure. The reduced mixture quantity handled by the pumps, therefore, makes it economically attractive for vacuum holding duties. However, the effect is to reduce the vacuum raising capability of the liquid-ring stage, and larger vacuum raising pumps are required with this system.