Hogger Ejector Air Cooled Condenser

They create a vacuum or suction to remove air, non-condensables and water vapor from the condensers.

http://www.uniquesystems.com/pvs/brochures/Bulletin_PVS-80025201-ARS_-_Ejector_Air_Removal_Systems.pdf

http://www.brighthubengineering.com/marine-engines-machinery/74427-how-does-an-air-ejector-work/

http://www.croll.com/products/powersystems.php

http://www.globalspec.com/FeaturedProducts/Detail/UniqueSystems/Ejector_Air_Removal_Systems/99833/0

Actually I asked the very same question when I was a co-op student crawling around my first generating station. As JP stated above, the purpose of an ejector is to remove condensables (water vapor) and noncondensables (gases) from the condenser space.

The hogger is a high volume device, moderate vacuum device. It's specific job is to quickly evacuate the condenser and the back-end of the LP (Low Pressure) turbine before the plant starts up after any prolonged shutdown.

You will also notice that there is a vacuum pump on the condenser as well. This is a low volume, high vacuum pump that provides the very high vacuum that is required for proper operation of the turbine. It operates whenever the plant is in operation, removing any noncondensables that have found their way into the feedwater.