This paper represents a case study of a replacement single-zone vertical feedwater heater with a pump forward drains system. When replacing a feedwater heater, it is important to review, evaluate and adjust the NLL of the new heater since it may not be the same as the NLL of the existing heater in order to achieve the same performance, particularly when changing the tube material. This paper strives to explain a method that was used in order to predict changes in both the feedwater and drains outlet temperatures based on varying liquid level. Single-zone feedwater heaters are typically sized and rated with a drains outlet temperature that is equal to the saturation temperature of the incoming bleed steam. Depending on the normal liquid level (NLL) setpoint, there is some subcooling that occurs due to the submerged tube surface that is typically not accounted for in the heater rating, this is especially true in a vertical channel down configuration. The outlet drains of a condensing zone only heater are still required to be safely transported in the liquid state to either another heater or to the condenser, therefore an adequate amount of subcooling is desired in order to preclude flashing and two-phase flow from occurring within the drains piping. The amount of subcooling that can be expected is related to the amount of surface that is submerged and therefore is directly tied to the liquid level inside the heater. Intuitively, more subcooling can be achieved by raising the liquid level; however, this has the effect of reducing the surface area available for condensation and ultimately the feedwater outlet temperature. In a pump forward drains system, this has an additional negative effect in that the drains are then combined with the outlet feedwater and reduces the feedwater temperature further prior to entering the next heater. It was desired to optimize the normal liquid level such that the drains outlet temperature would be subcooled enough to protect the downstream heater drain pump, but not to raise the level to a point that would significantly lower the inlet temperature to the next feedwater heater, resulting in inefficiencies in the overall heat cycle and overloading the downstream heater.
Performance Prediction of a Single-Zone Vertical Feedwater Heater
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Svensson, E, & Mirowski, W. "Performance Prediction of a Single-Zone Vertical Feedwater Heater." Proceedings of the ASME 2010 Power Conference. ASME 2010 Power Conference. Chicago, Illinois, USA. July 13–15, 2010. pp. 147-152. ASME. https://doi.org/10.1115/POWER2010-27105
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