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Both gravity and pumped condensate returns, along with makeup water are returned and blended together in the surge tank. All the flows drop directly to the surface water.
The surge tank serves as the holding tank for makeup water going to the deaerator. Normally, all returning condensate are blended with the colder makeup water. This blending helps avoid sudden temperature changes in the deaerator.
The blended condensate is transferred from the surge tank to the deaerator by the continuously running transfer pumps. A modulating level controller ensures a constant water level in the deaerator by modulating the flow of blended condensate through the modulating transfer valve.
Next the condensate flows into the top of the deaerator and through a spring-loaded, stainless steel spray nozzle that breaks down the water into a fine mist.
The droplets of mist then cascade down through a series of baffles in the spray tray. The cascading breaks down the droplets into a thin film. The thin film allow rapid heating of the water and release of non-condensable gases.
Steam constantly flows directly into the deaerator through the submerged preheat tube and the preheat tube above the water line (below the tray). The injected steam:
Keeping the water temperature at the saturation point (i.e., boiling point) is critical for effective removal of oxygen and non-condensable gases. Therefore, preventing wide temperature fluctuations within the deaerator is critical to achieving and maintaining its rated performance level (e.g., .005 cubic centimeters of oxygen remaining per liter of water).
As the condensate droplets fall from the spray tray, the rising steam scrubs (i.e., releases oxygen and other non-condensable gases) the falling droplets. As the droplets falls into the surface water of the deaerator, the steam bubbling up from narrow slits in the preheat tube purges the last of the oxygen in the chamber water.
The released oxygen and other non-condensable gases rise up through the spray tray baffles and escape through the orifice vent valve at the top of the deaerator. (Note: the vent should be piped to an area suitable for a small steam plume).
The size of the vent hole drilled in the orifice vent valve is based on the lbs/hr of the unit. The orifice vent must be kept closed during operation of the unit.
The transfer pumps of the surge tank run continuously so that the modulating transfer valve always has a flow available to it.
As the water level in deaerator falls, the modulating level controller detects the decrease and sends a signal to open the modulating transfer valve wider allowing an increased flow of blended condensate from the surge tank into the deaerator.
When the volume of blended water transferred to the deaerator exceeds the volume of condensate returning to the surge tank, the water level in the surge tank begins to fall.
A solenoid valve controls the flow of makeup water into the surge tank. The solenoid valve, in turn, is controlled by a float switch in the surge tank. In the real world, the surge tank may have multiple float switches (e.g., low water alarm, high water alarm, low water cutoff of transfer pumps). This animation uses a single float switch for illustrating the float control principle. In addition on the deaerator until have HWA, LWA, LWC and also a float switch to turn on second transfer pump in case of first transfer pumps failure.
At the low water level, a float switch sends a signal to open the solenoid valve letting makeup water flow into the surge tank.
As the makeup water flows into the surge tank, the water level begins to rise.
If for some reason the water level continues to drop due to failure of the float switch, solenoid, etc... the float for a low water alarm (if present) would send a signal to turn on alarm. If the water level would continue to drop after the alarm, a low water cutoff (if present) wold turn the transfer pumps to prevent them from running dry and destroying the pumps.
At high water level, a float switch sends signal to solenoid valve shutting off flow of makeup water in the surge tank.
Because the temperature of blended condensate is still cooler than the water stored in the deaerator, causing the pressure in the deaerator to fall slightly.
The pressure sensors immediately detect the pressure decrease and sends signal to the steam regulator to increase the flow of steam into both the submerged preheat tube and the preheat tube above the water line (below the tray).
The increased steam flow raises tank pressure back to normal.
The modulating level controller senses water level restored and sends signal to modulating transfer valve to reduce flow of blended condensate into the deaerator. Cycle repeats.