Why used hogger and ejector in steam turbine?

Answer 1

A steam turbine condenser equipped with ejectors is usually fitted with two ejectors. The larger of the two is called the hogger and the smaller the huffer.

Steam turbines are designed for efficiency and thus attempt to extract as much energy from the steam passing thru as is physically possible. Energy is extracted as both temperature and pressure decrease. The lower the temperature and pressure at the steam turbine's exhaust the more energy can be extracted from the steam.

For this reason it is desirable for the steam turbine condenser (the exhaust) to be at the lowest temperature and pressure possible. The hogger, or hogging jet, is a large steam powered ejector that causes the steam condenser to operate under a near perfect vacuum whilst the steam turbine is in the start-up phase. In steady state operation the cooling effect of the condenser cooling water of the steam causes the condenser to operate under a nearly self sustained vacuum. The hogger is necessary in the start-up phase to prevent steam from condensing to water in the final stages of the turbine before the steam flow and cooling effect of the condenser can establish vacuum.

The huffer or huffing jet runs continuously whilst the turbine is operating to remove any non-condensable gasses (i.e. air) that might leak into the condenser. It is nearly impossible to perfectly seal a condenser and turbine from air-in leakage.

It should be noted that modern, large steam turbines do not employ steam ejectors to establish and maintain condenser vacuum. Modern designs utilize motor driven mechanical vacuum pumps.