Patent Publication Number: US-4224981-A

Title: Feed-water heater for steam power plants

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to feed-water heaters for steam power plants. 
     In steam power plants, a supply of steam at high pressure and high temperature flows through a steam turbine to drive the turbine as the steam expands. At an intermediate stage in the turbine, steam is bled from the turbine and conducted in- one or more feed-water heaters to heat the water that is finally supplied to the boiler, thereby increasing the efficiency of the system. A conventional feed-water heater includes a housing surrounding a heat exchanger. The feed-water for the boiler flows through a number of heat exchangers, and steam bled from the turbine flows into the housing to heat the water flowing through the heat exchangers. 
     If the feed-water heater housing is under vacuum during operation, a certain amount of atmospheric air will enter the pipeline system and as a result will enter the heater. In order to avoid having the noncondensable gases interfere with the transfer of heat between the heat exchanger surface and the steam, the noncondensable gases must be removed from the heater. The noncondensable gases remain stagnant around the heat exchanger surface, and reduce the thermodynamic efficiency of the feed-water heater. The accumulation of large quantities of noncondensable gases can also lead to severe corrosion of the heat exchanger tubing, especially if the chemical composition favors corrosion, such as ammonia contained in air. 
     Various proposals have been made for removing the noncondensable gases. It has been proposed, for example, to install baffles in the feed-water heaters to provide the proper flow of ths steam that is to be condensed. At the end of the condensation path, the noncondensable gases are then removed by suction devices. An example of such a system is disclosed in Mitteilungen der VGB, Number 102, June 1966, pages 184 to 190. These prior proposals have the main disadvantage that relatively high steam pressure losses, i.e. thermodynamic losses, will occur. This is especially true in the case of high output units because the distance between the individual baffles must be relatively close. 
     It is an object of this invention to design a feed-water heater which avoids the disadvantages of the prior and which will make it possible to remove the noncondensable gases efficiently from the feed-water heater. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, the heat exchanger tubes through which the feed-water flows are arranged in a U-shape, with a central partition which blocks the flow of steam through the bundle directly from the steam inlet to the low temperature water inlet side. The steam flows around the heat exchanger tubes to reach the low temperature water inlet side. In the center of the bundle of heat exchanger tubes, on both sides of the partition, a plurality of deaerating pipes are positioned. The deaerating pipes are provided with openings spaced lengthwise of the heat exchanger tubes. This arrangement minimizes the accumulation of noncondensable gas in the interior of the heat exchanger tubes, and yet does not result in high steam pressure losses. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention is illustrated in the accompanying drawings in which: 
     FIG. 1 is a longitudinal cross-sectional view of the feed-water heater; and 
     FIG. 2 of the feed-water heater along the line II--II in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawings, like reference numerals have been used for like parts. The feed-water heater of this invention includes a housing 1 which is provided with a steam inlet 2 and a condensate outlet 3. The housing 1 forms a steam chest 4. A pair of support members 5 are mounted in the housing to support a bundle of heat exchanger tubes 6 through which the feed-water flows. The heat exchanger tubes 6 are bent in the shape of a U, and are shown schematically in FIG. 1. A partition 7 is mounted between the supports 5 in the interior of the heat exchanger tubes 6 to divide the tubes into a low temperature section at the left side of the partition, as viewed in FIG. 1, and a high temperature section at the right side of the partition, as viewed in FIG. 1. Deaerating pipes 8 are mounted in the interior of the heat exchanger tubes 6 on opposite sides of the partition 7. The deaerating pipes 8 are provided with openings 9 at spaced intervals along the length of the pipes. The deaerating pipes are closed at the upper end, as viewed in FIG. 1. At the lower end, the pipes are connected with a suction fitting 10 for receiving the gases that flow into the deaerating pipes. Baffles 11 are arranged horizontally around the tubes 6 and spaced apart axially of those tubes 6. 
     A conventional manifold e.g. a water box is provided at the lower end of the housing 1 for conducting feed-water into the low temperature section of the heat exchanger tubes 6 and out of the high temperature section, from which it flows into the next heater, and finally into the boiler. 
     In operation, bleeder steam from the turbine flows through the steam inlet 2 into the heater. At the same time, low temperature feed-water is introduced to the low temperature section of the heat exchanger tubes 6. The steam flows around the heat exchanger tubes in the direction shown by the arrows in FIGS. 1 and 2, thereby heating the water contained in the tubes. As the steam condenses, the condensate is removed through the outlet 3. As the steam flows around the heat exchanger tubes, a low pressure region is formed in the center of the tubes on opposite sides of the partition 7. The noncondensable gases contained in the steam are drawn inwardly into this low pressure region and the gases pass into the deaerating pipes 8 through the openings 9. The gases are then drawn off from the deaerating pipes 8 through the suction fitting 10. 
     The feed-water heater, in accordance with this invention, has the advantage that due to the partition 7 between the low temperature section and the high temperature section of the heat exchanger tubes steam does not flow directly from the inlet 2 through the high temperature section and then into the low temperature section. Instead, the steam flows around the tubes of each section and enters the low temperature section from the steam chest 4. By placing the deaerating pipes 8 on both sides of the partition, the noncondensable gases are extracted from the heater at the end of the steam condensation path. Furthermore, the point of lowest pressure within each half of the heat exchanger is located near the partition, as are the deaerating pipes also. Since the steam is flowing at a minimum velocity across the entire surface of the heat exchanger tubes, the steam pressure loss within the nest of tubes will be held to a minimum and a proper extraction of noncondensable gases is achieved. 
     While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may be made therein without departing from the invention, as set forth in the claims.