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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.
An option exists to make the baffles in the dome removable. If the baffles are removable, the dome will include an oval, steel manway to provide access to the baffles.
Pumped condensate returns into the dome. Boiler feed pump sends water to boiler as needed. Water level controller on boiler controls boiler feed pumps.
A small amount of makeup water flows continuously:
The fine spray, created by the spray nozzle, 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 the baffles tray, they form a thin film that enhances the steam scrubbing.
Steam constantly flows into both the submerged preheat tube in the receiver and the preheat tube in the dome. The purpose of the preheat is 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.
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.
Even though the makeup water is preheated, its temperature is still cooler than the tank water. Therefore, its presence causes the water temperature in the tank to fall slightly.
The temperature sensor immediately detects the temperature decrease in the receiver and sends a signal to the steam regulator to increase the flow of steam into both the submerged preheat tube in the receiver and the dome.
The steam regulator includes a pressure reducing valve that ensures the steam pressure released into either the submerged preheat tube or the preheat tube in the dome does not exceed 5-7 psig.
The increased steam flow raises water temperature 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.