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On applications with a large volume (i.e., 20 percent or more of system load) of condensate returns, a two-tank design (i.e., surge tank and deaerator) is typically used. On a two-tank design, the modulating valve is a "transfer connection". On single-tank design (i.e., deaerator only), the modulating connection is a "make-up connection".
On the two-tank design, both the pumped and gravity returns are pumped first into a surge tank rather than directly into the DA tank as illustrated on this animation which is based on a single-tank design. The returns are then pumped from the surge tank to the DA through a modulating "transfer" valve.
The overflow drainer keeps the pressurized steam within the tank but on overflow conditions, it allows water to flow to drain.
The orifice vent valve is required to draw steam from tank up through baffles and release the oxygen and other non-condensable cases, along with a small amount of steam, to atmosphere.
The pressure relief valve is required to prevent over-pressurizing tank. The relief valve is normally set at 15 PSIG.
Pumped condensate returns into spray tray.
Boiler feed pump sends water to boiler as needed. Water level control on boiler controls operation of boiler feed pumps.
A small amount of makeup water flows continuously:
The spray tray provides increased surface area for heat absorption and release of non-condensable gases (i.e., steam scrubbing).
As the droplets of makeup water cascade down spray tray, they form a thin film that enhances the steam scrubbing.
Tank typically pressured at 5 PSIG.
Steam constantly flows into the submerged preheat tube and into the preheat tube above the water line (below the tray) to:
The non-condensable, escaping gases (such as carbon dioxide and oxygen) heat the incoming makeup water as the gases rise through both the dome and vent condensor.
The non-condensable gases escape through the orifice vent valve. The size of the vent hole drilled is based on the lbs/hr of the unit. The orifice vent must be kept closed during the operation of the unit.
When the volume of water pumped to boiler exceeds the volume of condensate returns, the water of level of the tank begins to fall.
The modulating level controller immediately detects any water level drop and sends signal to modulating valve to increase flow of makeup water.
Because the make-up water temperature is cooler than the tank water, its presence causes the water pressure in the tank to fall slightly.
The pressure sensors immediately detect the pressure decrease and sends signal to the steam regulators to increase the flow of steam into the submerged preheat tube and the preheat tube above the water line (below the tray).
The steam regulator includes a pressure reducing valve that ensures the steam pressure released into the submerged preheat tube does not exceed 5-7 psig.
The increased steam flow raises water pressure back to normal.
The modulating level controller senses water level restored and sends signal to modulating valve to reduce flow of makeup water into tank.