Method For Increasing Effective Scavenging Vent Steam Within Heat Exchangers Which Condense Vapor Inside Long Tubes

Abstract

A device for scavenging condensate from long heat exchange tubes by recirculating the steam which is passed through one portion of a U-shaped tube bundle through another portion of the tube bundle and thus reducing the build-up of slugs of liquid in the tubes by increasing the velocity therethrough without reducing the heat transfer surface and also protecting the welds which seal the tubes to the tube sheet from thermal shocks and stresses by incorporating a thermal sleeve in the device which cooperates with the scavenging of condensate from the tube to provide optimum protection.

Description

BACKGROUND OF THE INVENTION

This invention relates to heat exchangers and more particularly to a device for scavenging condensate from the tubes of a heat exchanger.

When there are a number of tubes condensing in parallel, variations in condensing produces different flow patterns in different tubes, which result in condensate build-up in certain tubes. After the condensate plugs the tubes, the flooded tubes purge themselves and the cycle begins again, resulting in thermal shocks and stresses, which in time cause failure adjacent the area where the tubes are welded to the tube sheet.

One solution to this problem is to vent a portion of the steam to a lower pressure receptacle, increasing the flow through the tubes and thereby scavenging the condensate, which collects therein; however, unless very large quantities of steam are vented, this method is ineffective in keeping all of the tubes free from plugs of condensate. From a thermodynamic standpoint, the greater the quantity of vented steam, the lower the thermal efficiency of the system and, therefore, the more costly it is to operate.

SUMMARY OF THE INVENTION

In general, a heat exchanger, having a shell, a head, a tube sheet having a plurality of holes, and a plurality of long tubes having at least one end thereof secured in the holes in the tube sheet, when made in accordance with this invention, has the head divided into an inlet portion and an outlet portion and the heat exchanger is adapted to heat a first fluid passing through the shell and over the outer surface of the tubes by removing heat from a second fluid which passes through the tubes. A portion of the second fluid changes state from a gas to a liquid in the tubes. The heat exchanger further comprises, in combination, a first manifold disposed within the outlet portion of the head and a second manifold disposed within the inlet portion of the head, the manifolds being in fluid communication with each other and each manifold having means for placing the manifold in communication with a plurality of tubes, which have their ends associated with the respective portions of the head and the first manifold has a port disposed therein for draining liquid therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings, in which:

FIG. 1 is a sectional view of a moisture separator reheater incorporating a liquid scavenging device made in accordance with this invention;

FIG. 2 is an enlarged partial sectional view showing the scavenger device; and

FIG. 3 is an enlarged partial sectional view showing how the scavenging device is connected to the tubes to form a thermal shield surrounding the area where the tube is welded to the tube sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings in detail, FIG. 1 shows a moisture separator reheater 1 incorporating a condensate scavenging device 3 made in accordance with this invention. The reheater 1 is a shell 5 and tube 7 heat exchanger in which a first fluid, such as steam, flows within the shell 5 and passes over the outer surface of the tubes 7 picking up heat from a second fluid, in this case also steam, which flows through the tubes 7. The second fluid in giving up heat has a portion thereof change state from a gas, steam, to a liquid, water or condensate.

The reheater 1 comprises the axially elongated shell 5, a flanged and dished head, are disposed on one end of the shell, the end on the right as shown in FIG. 1, and a hemispherical head 11 disposed on the other end of the shell, the end on the left as shown in FIG. 1. A bundle of U-shaped or U-tubes 7 extend lengthwise across the upper portion of the shell 5, a meshed pad or moisture separator 13 traverses the shell horizontally adjacent the axis thereof, and a distributor plate 15 is disposed horizontally below the meshed pad 13. A first fluid inlet nozzle 17 is disposed in the flanged and dished head 9 and a plurality of first fluid outlet or discharge nozzles 19 are disposed in the upper portion of the shell 5. A hot well 21 and drain nozzle 23 are disposed in the lower portion of the hemispherical head 11. A tube sheet 25 having a plurality of holes 27 disposed therein to receive the end of the U-tubes 7 which are seal welded thereto is disposed in the upper portion of the hemispherical head 11 so that the tubes 7 extend lengthwise across the upper portion of the shell 5. A hemispherical head 29 is made integral with the tube sheet 25, of course a head of some other design could also be utilized.

A dividing plate 30 divides the head 29 into two portions, an inlet portion 31 having an inlet nozzle 33 for the second fluid and disposed to be in fluid communication with the inlet end of all of the U-tubes forming the tube bundle and an outlet portion 35 having an outlet nozzle 37 for the second fluid and disposed to be in fluid communication with the outlet end of all of the U-tubes.

As shown best in FIG. 2, the scavenging device 3 comprises a first manifold 41 disposed in the outlet portion 35 of the head 29 and a second manifold 43 disposed in the inlet portion 31 of the head 29. The manifolds 41 and 43 are placed in fluid communication with each other by a conduit 45 extending therebetween. Each manifold comprises a tube sheet 47, peripheral walls 49, and a cover plate 51.

The tube sheets 47 are generally disposed parallel to the tube sheet 25 of the heat exchanger and have holes 53 which register with a plurality of the holes 27 in the tube sheet 25. Short tubes 55 are received by the holes 53 and extend into less than a majority of the ends of the U-tubes 7 in the associated portion of the head 29. Generally, the header 41 in the outlet portion 35 of the head 29 is in communication with the lower tubes disposed therein. However, some of the outside tubes may be included to form a generally crescent-shaped header, in other words, the tubes connected to the header 41 are the tubes initially contacted by the first fluid as it enters the tube bundle.

Generally, the header 43 in the inlet portion of the head 29 is in communication with the central tubes of the tube bundle and the shape of this header is generally rectangular. However, the headers 41 or 43 may take any shape, the general arrangement being that the headers are disposed so that U-tubes are only in communication with one of the headers.

As shown in FIG. 3, the short tubes 55 are generally expanded into the header tube sheet 47 by rolling or other means. The short tubes 55 may be sufficiently smaller in diameter than the U-tubes to provide an annular space therebetween. A seal 57 is formed between the short tubes 55 and the U-tubes 7 by expanding a short portion of the short tube 55 by rolling or other means. As shown in FIG. 3 the rolling may be such that only a small annular ring contacts the outer tube 7 forming a seal 57 therebetween and providing an annular space 59 which cooperates with the short tubes 55 to act as a thermal shield to protect the weld areas from thermal shocks produced by plugs of water which collect in the tubes 7 and cause thermal stresses and premature failure of the tubes adjacent the welds. It is, of course, understood that other rolling procedures and sealing arrangements could be utilized.

A drain nozzle 61 is disposed in the lower portion of the manifold 41 disposed in the outlet portion 35 of the head 29 to bleed off condensate flowing from the tubes 7. And a vent nozzle 63 is disposed in the outlet portion 35 of the head 29 to vent steam therefrom.

The operation of the reheater and scavenging device 3 is as follows:

The first fluid, wet steam, enters the inlet nozzle 17, passes through the distributor plate 15 and flows through the meshed pad 13, wherein large droplets of water are removed and then the first fluid flows over the outside of the U-tubes picking up heat and raising its temperature to become a superheated fluid and then flows out the discharge nozzles 19.

The second fluid, high temperature steam, enters the inlet portion of the head 29 and flows into the ends of the U-tubes not connected to the header 43. The second fluid then flows through the tubes 7 giving up its heat and condensing a portion thereof. Some of the tubes may discharge directly into the outlet portion 41 of the head 29, while other tubes discharge through the short tubes 55 into the header 41 disposed in the outlet portion 35 of the head 29. In the header 41 steam separates from the water and the water flows through the drain nozzle 61 while the steam flows up through the conduit 45 and into the header 43 in the inlet portion 31 of the head 29. The header 43 distributes the steam through the short tubes 55 to the U-tubes which discharge in the outlet portion of the head 29 causing a portion of the second fluid to make two passes through the tubes, which effectively increases the flow rate through all the tubes without increasing the mass flow rate or increasing the quantity of steam vented from the discharge portion of the head 29. Thus, by selecting those tubes which are particularly susceptable to periodic condensate plugging the scavenging device, hereinbefore described, effectively increases the scavenging in the tubes and thereby prevents condensate plugging and subcooling, and also provides a thermal shield to protect the weld areas against thermal shocks and stresses thereby eliminating premature tube failures of the tubes adjacent the welds.

Claims

What is claimed is:

1. A heat exchanger having a shell, a head, a tube sheet connected to said head and having a plurality of holes, and a plurality of long tubes having their ends secured in the holes disposed in the tube sheet, the head portion having means to divide it into an inlet portion and an outlet portion, the heat exchanger being adapted to heat a first fluid flowing through the shell and over the outer surface of the tube by removing heat from a second fluid which passes through the tubes, a portion of said second fluid changing state from a gas to a liquid, in combination with; a first manifold disposed within the outlet portion of the head, and a second manifold disposed in the inlet portion of the head, means providing fluid communication between said manifolds and each manifold having means for placing the manifold in sealed fluid communication with a plurality of tubes which have their ends associated with the respective portions of the head, and said first manifold having a drain port disposed therein for draining liquid therefrom.

2. A heat exchanger as set forth in claim 1, wherein the second manifold is in fluid communication with less than a majority of the tube ends associated with the inlet portion of the head.

3. A heat exchanger as set forth in claim 1, wherein the tubes are U-shaped and the first manifold is in fluid communication with the tubes contacted by the coolest first fluid and the second manifold is in fluid communication with tubes other than those that are in communication with the first manifold.

4. A heat exchanger as set forth in claim 1, wherein the means placing the manifold in fluid communication with the ends of the tubes are short tubes extending from the manifolds into the ends of the long tubes.

5. A heat exchanger as set forth in claim 4 and further comprising a seal between the long tubes and the short tubes extending therein.

6. A heat exchanger as set forth in claim 4, wherein the short tubes are expanded in the ends of the long tubes forming a seal therebetween.

7. A heat exchanger as set forth in claim 4, wherein the short tubes have an outer diameter sufficiently smaller than the inner diameter of the long tubes to provide an annular space therebetween and the short tubes are expanded to produce at least one small annular area of contact between the tubes thereby forming a seal therebetween.

8. A heat exchanger as set forth in claim 4, wherein the short tubes have an outer diameter sufficiently smaller than the inner diameter of the long tubes to produce an annular space therebetween and the short tubes being expanded to form two small annular areas of contact between the tubes thereby forming seals therebetween.

9. A heat exchanger as set forth in claim 4, wherein each header comprises a second tube sheet generally parallel to the tube sheet of the heat exchanger, the shape of the second tube sheet depending on the arrangement of U-tubed ends which are in fluid communication with the header.

10. A heat exchanger as set forth in claim 1, wherein the discharge portion of the head has venting means disposed therein.