Can someone please explain to me how a hogging ejector works? i believe it has something to do with pulling vacuum. And the purpose of the holding ejector, how does it work and is it simply for maintaining the vacuum?

Many thanks.

 

A hogger is a term someone somewhere thought would be a good description for a large air ejector that can be used to "bring up" (establish) vacuum on a condenser when little or no steam is being condensed, such as during start-up of a steam turbine.

Once steam actually starts condensing the rapid decrease in volume that results from the steam condensing back into water actually is the "source" of the majority of the vacuum in the condenser. So, a large method of establishing and maintaining vacuum is no longer required.

A holding air ejector is not a term I'm familiar with, but it might refer to an air ejector that is put into service after steam has started flowing through the turbine and is actually condensing in the condenser. It's probably put into service and then the hogger is taken out of service.

The smaller holding air ejector is then doing exactly what it says: removing air and other non-condensable gases from the condenser, and assisting only marginally with maintaining the vacuum, since the condensing steam is the main source of the vacuum. If any air and non-condensable gases are not removed from the condenser they will begin to occupy space in the condenser and reduce vacuum, so that's the main purpose of any "holding" air ejector.

I've seen hoggers that were AC motor-driven, steam-driven, and air-driven (instrument air, though these are huge consumers of instrument air flow so I haven't seen them in quite a while and only on smaller steam turbine units).

Most "holding" air ejectors I've seen were single stage steam-driven air ejectors (essentially a venturi) to reduce electrical load on the plant. Older plants seemed to have two- or three-stage steam-driven air ejectors.

By the way, I did an Internet search using the term 'hogger air ejector' (without the quotes) and a lot of really sear results with some really good information came up.

 

The basic fundamental principle of ejector is nozzle effect.Steam at high pressure of around 10bar is expanded in the nozzle and creates a pressure which is lower than the pressure available in the condenser at nozzle outlet. So all non condensables and air travels towards the nozzle outlet and mixes with steam.

The heat available in the steam air mixture is removed with the help of low pressure feed water.

Condensate formed will be drained to recovery vessel and no condensables will be vented to atmosphere.

 

thanks a lot to both of u