Tray Type Deaerators: Parallel Flow vs. CounterflowDeirdre Patterson
There are two different types of tray units. The first type of tray unit is a parallel downflow unit where steam travels in the same direction as the water. As the water enters through the water inlet on top of the tank, steam enters through the side of the deaerator and moves down through the trays with the water (hence, the name parallel downflow). The major disadvantage of a parallel downflow tray unit is that it does not comply with HEI standards because the steam is not contained within the stainless-steel box. The shell of the pressure vessel is exposed to the high temperatures of the steam that has been used to remove the oxygen. Not being HEI compliant means that many customers, specifically those in the United States, cannot use a parallel downflow tray unit.
The other type of tray unit is a counterflow unit. Whereas, the parallel flow unit had steam running the same direction as the water flow, counterflow units do the opposite. As water enters through the spray valves on the top side of the deaerator, the steam enters from below the stainless-steel box and is always contained within said box. This eliminates the opportunity for steam to encounter the shell of the chamber. This method is approved by HEI and is the most commonly used.
Counterflow Tray-Type Deaerator (per HEI):
KCD specializes in the design of counterflow tray-type deaerators which prevent any carbon steel pressure retaining components from encountering corrosive undeaerated water or vent gases. The counterflow design is the only design that meets the requirements of HEI and protects the vessel from all corrosive gases, not just concentrated gases.
Stages of Counterflow Deaerators:
Stage 1: Incoming water flows through KCD’s variable orifice spray valves and the steam filled vent condensing chamber as a thin-walled, hollow cone spray pattern. Latent heat transfer is instantaneous because of the intimate water to steam exposure. As the water reaches the tray stack, at which point stage one is complete, its temperature is within 2ºF of the counter flowing saturated steam temperature, and most dissolved oxygen and free carbon dioxide has been removed. Nearly all of the steam has now condensed, permitting the non-condensable gases to be carried through by the remaining steam.
Stage 2: During stage two, the preheated water flows over the tray stack and is vigorously scrubbed by the counter flowing steam. The water zigzags its way though a counter-current of pure steam and leaves the tray stack virtually free of oxygen and carbon dioxide.