injector or vacuum pump?

It should be ejector of course.

Both are commonly used.
Steam jet air ejector systems have no moving parts, but they use steam and have heat exchangers that need to be kept in order.
There are a few different vacuum pumps used, liquid ring are common because they seal using water, but they need clean cool water to work.

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As an end user, ejectors are great. If you can keep crap in the cooling water from fouling the condenser they are very reliable. LRVPs are usable but some - ours were from Nash - were maintenance-heavy and didn't have an exactly stellar reliability record. They didn't achieve as deep a vacuum as the ejectors either.

If you install ejectors insist that the mechanical maintenance team clean the condenser during a unit outage. It's not a big job and I have been on some awful return-to-service duties where the ejector simply wouldn't work properly, and the problem was the condenser being clogged with a couple of hundred kilos of silt. When mechs say the condenser is clean, the only acceptable answer is 'prove it'.

Assuming that feed water is being used as the coolant, a multi-stage ejector will provide the best reliability with a tolerable thermodynamic penalty. Liquid ring pumps can serve well, but their true efficiency is easily over-estimated. Liquid ring pumps can serve well for start-up duty with the ejectors being used for continuing operation. The cautions regarding maintenance requirements for liquid ring pumps are well deserved. Due to the lack of moving parts, ejectors systems are some of the most reliable devices in existence.

The details of the specific application will control the ultimate choice of equipment. Take care in the system energy analysis to account for the effects of the steam source and the feed water heating effects of the ejector system vs. the true cost of electric power and water consumption of liquid ring pumps. The electric power for the liquid ring pump comes from the end-product of the system rather than some mid-cycle extraction point for the steam.

Liquid ring pumps cannot be expected to pull as deep a vacuum as ejectors, but ejectors can work nicely in series with a liquid ring pump serving as the last stage to get the necessary deep vacuum while consuming less steam if that is actually beneficial for the particular system.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

The ultimate vacuum of wither system is limited by the temperature of the cooling water.
One issue with ejectors is that they do not get inspected enough.
The capacity and blank off pressure should be checked regularly and the orifice inspected, as well as maintenance of the condensers. They are not trouble free.

At the same time many power plants have run LR pumps for decades with no issues. In a tight system they have the advantage of being able to turn off excess pumps and reducing house load.

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