In a horizontal feedwater heater with a partial length subcooling zone the end plate located at the entrance to the subcooling zone is the only non-welded barrier that prevents the condensing zone steam from entering the subcooling zone. The end plate is usually two or three inches thick and the tube holes in the end plate are drilled to tolerances similar to that of the tubesheet. As the steam from condensing zone tries to enter the subcooling zone it condenses in the tight spaces between the active tube and the tube hole in the end plate forming a liquid barrier that prevents further ingress of steam into the subcooling zone. With usage and wear the gap between the active tube and the tube hole in the end plate increases thereby weakening the liquid barrier. The liquid barrier is completely lost when the tubes are plugged. Steam from condensing zone enters the subcooling zone and disrupts the performance of the subcooling zone, the performance of the feedwater heater and the efficiency of the power plant. This problem is faced by all horizontal feedwater heaters with subcooling zones that are in operation in power plants worldwide.
This loss of performance can be eliminated by employing the patent pending Maarky concept of “dual end plate subcooling zone with a water seal”. The “dual end plate subcooling zone with water seal” concept comprises of two end plates separated by a short distance. The gap between the two end plates is filled with condensate thereby forming a triple barrier to ingress of condensing zone steam into the subcooling zone. The performance of the subcooling zone and the longevity of the heater are preserved.
This paper discusses the design of subcooling zone in present day feedwater heaters, degradation of performance of subcooling zone and the improvements brought about by the patent pending Maarky concept of “dual end plate subcooling zone with water seal”.