On the Inability of Multiple Exhausters to Eliminate Condenser Excess Back Pressure at High Air In-Leakage

A comprehensive theoretical development, confirmed by inplant measurements, will be summarized showing the dependence of condenser air in-leakage and exhauster capacity on build up of excess back pressure caused by the establishment of a zone of stagnation near the condenser’s air removal section. It has been generally believed that there will not be a zone of stagnation and an associated excess back pressure, above the design pressure, provided the exhauster capacity for air removal remains higher than the air in-leakage regardless of the leakage size. This paper will show, however, that excess back pressure can occur at high air in-leakage, even though exhauster capacity is adequate. A gradual increase in the concentration of non-condensables as steam scavenges these gases toward the air removal section is presented. This results in a reduction of individual tube heat transfer coefficient in the high air concentration region which is accompanied by an increase in condenser pressure. It will be shown that this may be the condition of heat transfer reduction that many authors have used throughout the 20^th century to describe the effect of air on all tubes in the condenser. An associated condition will also be discussed related to subcooling of the gas and vapor space between tubes within the region of reduced heat transfer. With high air in-leakage and high exhauster capacity, it will be shown that near the air removal section the large amount of air flow will scavenge steam through the subcooled region, directed toward the vent line, causing generation of a condensate mist that becomes suspended in the moving air. The reported theoretical explanation is derived using a condenser model developed earlier by the author. The results are confirmed from measurement data taken on an operating plant using a RheoVac^® condenser performance and diagnostic instrument, the description of which may be found in the open literature.