WO1990006482A1

WO1990006482A1 - Heat exchanger

Info

Publication number: WO1990006482A1
Application number: PCT/SE1989/000718
Authority: WO
Inventors: Erik Andersson; Endre Balint
Original assignee: Gadelius Sunrod Ab
Priority date: 1988-12-07
Filing date: 1989-12-07
Publication date: 1990-06-14
Also published as: SE8804412L; AU4745090A

Abstract

A heat exchanger designed to recover heat from a substantial gas flow is designed as a self-supporting, gastight unit, in which upper and lower transverse end boxes (10a, b; 11a, b), are connected by means of horizontal, longitudinal tubes (12a, b), said tubes forming the frame of the top and bottom walls. A plurality of vertical pipes (13a, b) are welded between the tubes (12a, b). The outermost pipes (13a) and the tubes (12a, b) are connected by means of flanges, to produce gastight membranes. The remaining pipes (13b) around which gas is flushed are provided completely or partially with external surface-enlarging elements (17).

Description

Heat exchanger

The present invention relates to heat exchangers, particularly heat exchangers included in a gastight waste heat boiler, and is aimed at offering a simpli¬ fied design in which supporting steel constructions have been eliminated and replaced with components in the circulation system. Water pipes in boiling and economizer surfaces are located vertically, thus en¬ abling gravity circulation in the boiling surface so that circulation pumps can be avoided. A steam dome may be supported by downcomers for the boiling surface.

The use of primarily external surface enlarging elements on the water pipes results in efficient thermal absorption and thus a compact construction.

Heat exchangers of this kind often find use in large plants such as coal-fired power plants and the like.

Heat exchangers of this kind can have a weight of serveral hundred tons, for example 1800 tons, and the gas flow through the gas channel wherein the heat exchanger is located can have a pressure of about 0.3 bar above the surrounding atmospheric pressure.

One inconvenience at heat exchangers of this general kind is that extensive and heavy support devices normally have to be used, partly in order to support the pipes, tubes and end boxes of the heat exchanger so that the heat .exchanger structure does not collapse under the influence of its own weight, partly in order to stiffen the plate walls of the channel which surrounds the heat exchanger unit.

At a known plant with the total weight 1800 tons, for ^* example, about 1200 tons of steel is used for heat transfer surfaces, and further about 600 tons of steel are used for supporting structures. At the known plant, the heat exchanger itself is made as an independent unit which is surrounded by portal frames which support it. A.gas channel with rectangular cross-section is built around the heat exchanger. In view of the fact that the channel cross-section has a height and width of several meters, and that the gas pressure in the channel is in the order of 0.3 bar, for example, it is realized that the plates which form the gas-tight walls of the channel require substantial structural support. At the known plant, such structural support is provided by longitudinal beams which in turn support against the portal frames.

Against this background, the object of the invention is to provide a heat exchanger device with simplified design in which the external supporting steel construction has been reduced, and in which the stability of the heat exchanger is improved.

The inventive heat exchanger is defined in the appended claim 1. Embodiments of the heat exchanger is defined in the enclosed subclaims.

The invention thus relates to a heat exchanger through which a heat-emitting gas shall flow substantially horizontally, and comprises upper and lower transverse end boxes, longitudinal tubes connecting the end boxes ^• horizontally, water pipes connecting the longitudinal tubes vertically, welded flanges connecting the longi¬ tudinal tubes and the outermost rows of the water pipes to form gastight defining walls for the heat exchanger, and also surface-enlarging elements on the water pipes between the vertical side walls thus produced. The vertical rows of water pipes will of course form reinforcements which prevent that the upper and lower gas channel walls are expanded. Likewise, the upper and lower sets of tubes, which are held together by the welded- in flanges, hold the upper and lower edge of the sides of the gas channels. As a complement, bracing rods can be arranged to connect between the central portions of the vertical s"ide walls of the gas channel.

By fitting the flanges between the tubes of the upper and ' lower set of tubes and between the pipes of the outermost rows of pipes, the heat exchanger will thus be provided with an external tubular shell which not only participates in the heat exchange but also defines a stiff gastight gas channel for the pressurized gas which is to give off its heat to the cooling fluid within the pipes and tubes of the heat exchanger. Finally the welded-in flanges also stiffen the heat exchanger to a self-supporting unit. It is even possible to provide such a stiffness and strength for the heat exchanger unit tlϊat it can be made at some remote location and transported to its place of use if so desired.

In order to control the flow of water in the pipes, there¬ by achieving more uniform heating, screens are suitably arranged in the longitudinal tubes between at least some of the connected water pipes, said screens in upper and lower tubes being displaced in relation to each other so •that the water in certain pipes is forced to move up¬ wardly whereas the water in adjacent pipes moves down¬ wardly. The screens are suitably in the form of plates assembled on a rod and having an inner diameter corres¬ ponding to that of the tubes.

To ensure an efficient flow of water, it is often sufficient for only one upper and one lower of the four end boxes to be connected to the tubes joined thereto, whereas the other two end boxes only constitute support elements.

A heat-exchanging unit such as a steam boiler, may- comprise a number of heat exchangers connected in series one after the other, separated by open spaces defined only by side walls, top or bottom walls constituting extensions of corresponding portions in the heat exchangers included therein.

The invention will be described in the following with reference to the accompanying drawings, in which

Figure 1 shows an end view of a heat exchanger according to the invention,

Figure 2 shows a longitudinal section through the heat exchanger shown in Fig 1, Figure 3 shows a horizontal projection, partially in section, of a part of the heat exchanger shown in Figure 1,

Figure 4 shows a horizontal section through two water pipes with external, surface-enlarging elements, Figure 5 shows a side view of parts of two pipes according to Fig 3, where the surface-enlarging elements have been inclined in order to increase the gas turbulence around the pipes,

Figure 6 shows a modified embodiment, in which portions of the surface-enlarging elements have been folded up with the object of further increasing the turbulence,

Figure 7 shows schematically a longitudinal section through a steam boiler comprising a number of heat exchangers as claimed, Figure 8 shows a part of the boiler shown in Figure 7, on a larger scale, and

Figure 9 shows a cross section through a part of the boiler shown in Fig 7.

Figures 1 - 3 show in principle the structure of a heat exchanger-according to the invention, suitable for connection after a source emitting hot gases flowing substantially horizontally. Together with other similar units, it may form a steam or hot-water boiler provided with hearth of arbitrary type, or be connected as waste gas boiler from a gas-turbine or process plant.

The heat exchanger comprises front upper and lower trans¬ verse end boxes 10a, 10b and rear upper and lower trans¬ verse end boxes 11a, lib. Between these extend upper and lower tubes 12a, 12b.

A plurality of vertical water pipes 13 are welded between these tubes, of which the pipes 13a connecting the outer tubes are suitably somewhat thicker than the pipes 13b in the central part of the heat exchanger.

This system of horizontal and vertical elements welded to each other provides sturdy unit in itself, thus eliminating the need for any supporting steel construction,

Connecting channels for the gas flowing through are inti¬ mated at 14 and these channels rest on feet 15.

To ensure a gastight construction, flanges 16 in the form of flats or rounds are welded at the top and bottom between the tubes 12a and 12b and between the outermost rows of water pipes 13a. The inner water pipes 13b are provided with external surface-enlarging elements 17, suitably of the type shown in Figures 3 and 4 so that a large contact surface to the gas is obtained within a limited space.

The dimensions of the surface-enlarging elements are adjusted to optimize utilization of each heat-exchanger unit in the boiler. Thus heating surfaces without sur¬ face enlargement or with little surface enlargement can be used in the superheater part, where the gas temperature is high and the transfer coefficient of the steam is poor.

Only two of the four end boxes 10a, b and 11a, b, respect¬ ively, i.e. 10a and lib, which are located diagonally opposite each other in the unit, are included in the water circulation system, whereas the other two boxes are only supporting elements.

Screens 18 are arranged in the tubes 12a and 12b to guide the flow of water from the inlet at box lib to the outlet at 10a. The screens 18 consist of plates corresponding to the inner diameter of the tube, spaced along and attached to a rod 19 running axially through the tube. The screens in an upper and a lower box, respectively, are displaced a half pitch so that the water is forced upwards in some pipes, whereas it will flow downwards in adjacent pipes.

Figure 2 shows screens placed so that the water flows upwardly and downwardly in alternate pipes. However, it is naturally possible to arrange for it to flow in the same direction in two or more adjacent pipes. Further- more, it is not absolutely necessary for the unit to be gastight and a small amount of leakage can be accepted within the fit tolerance.. Figure 4 shows a horizontal section through two pipes 13b, with external surface-enlarging elements 17. These are in the form of rectangular plates, and two such plates are intended to be welded to the pipe in the same plane. Each plate 17 has a substantially semi-circular recess for connection to the pipe and is provided at the other edge with three parallel notches 20 to reduce the risk of thermal deformation of the plate and to guide the heat in towards the pipe.

As can be seen in Figure 5, the plates 17 may be inclined somewhat in relation to the longitudinal axis of the pipes, thus increasing the turbulence in the gas, and thus the heat transfer.

In the embodiment shown in Figure 6 the plates are welded perpendicular to the longitudinal axis of the pipe, but the parts between the notches 20 have instead been bent up to increase the gas turbulence.

Figure 7 shows schematically a steam boiler designed for use as waste gas unit after a gas turbine (not shown) . The boiler itself comprises three heat-exchange units according to the invention, i.e. economizer/feed water heater 21, boiling surface 22, and superheater 23. As is evident from the enlarged details shown in Figures 8 and 9, the same components are included as are described in conjunction with Figures 1 - 2.

The boiler is provided with a steam dome 24, supported by downcomers 25 (see also Fig. 9) and connection tubes 26 from an upper collection box 10 for the boiling surface 22. Return feed water is introduced at 27 to a lower end box lie, and the heated water is removed from an upper end box 10c and carried through at least one pipe 28 to the steam dome 24, below the water level therein.

The downcomers 25, one on each side of the gastight side walls formed by the pipes 13a, communicate with a lower end box ll'd which distributes the water via longitudinal tubes 12c to a plurality of pipes 13b (not shown) , connected at the top to longitudinal tubes 12d which in turn are connected to an upper end box lOd. The latter is connected to the steam dome 24 via the previously mentioned tubes 26.

Steam is withdrawn from the dome through a pipe 29 connected to an upper end box lOe and, after superheating, is withdrawn through a pipe 30 connected to a lower end box lie.

Free spaces 31, 21, 33 are provided between the separate heat-exchanger units for inspection and maintenance, and are accessible through upper and lower manholes 34.

A heat exchanger 35 is provided to take advantage of the remaining heat in the waste gases. This heat exchanger is independent of the steam boiler, e.g. forms part of a district heating system, but is otherwise constructed in the manner described in conjunction with Figures 1 and 2.

It will be noted that each heat-exchanger unit has two

"active" end boxes and that in the case of the units 22, 21 and 35, an "active" end box from the preceding unit is utilized as supporting (blind) end box in the next unit.

Side walls, bottom and top walls in the free spaces 31, 32, 33 constitute extensions of corresponding parts of the heat-exchanger units and are thus satisfactorily water-cooled. ^'

Figures 8 and 9 show on a larger scale details of .parts of the boiler shown in Figure 7. No detailed description is necessary since the design of the components can be seen upon a comparison with Figures 1 and 2. Freshly added feed water is supplied via a pipe 36 to a small dome 37 on the steam dome 24. The water can thus be cascade-degasified and the gas diverted through pipe 38.

Within the scope of the appended claims, the components of the heat exchanger may vary in size and number depending on their application and the gas source to which they are connected. The pipes 13 provide bracing between the tubes in the top and bottom walls. Bracing between the vertical side walls (pipes 13a) is provided by trans¬ verse flats (not shown) , which extend between the walls and are welded to the surface-enlarging elements. The thermal expansion then agrees best with expansion in the other components. The tubes 13a forming the side walls may be provided with surface-enlarging elements corre¬ sponding to 17, but only on the inwardly facing sides of the pipes. This avoids an all too exposed gas path along the side walls.

Claims

C l a i m s

1. A heat exchanger through which a heat emitting gas shall flow substantially horizontally, comprising upper and lower sets of longitudinal tubes (12a, 12b) , water pipes (13a, 13b) connecting the longitudinal tubes vertically, w h e r e i n flanges (16) are welded in between adjacent tubes in each set (12a, 12b) and between adjacent pipes in each of the outermost rows of pipes (13a) , whereby the flanges (16a) , the upper and lower sets of tubes (12a, 12b) and the outermost rows of tubes (13a) form a gastight channel for the gas flow through the heat exchanger and also form a supporting and stiffening structure for the heat exchanger.

2. A heat exchanger as claimed in claim 1, w h e r e i n screens (18) are inserted in the longitudinal tubes (12a, 12b) between at least some of the connected water pipes (13a, 13b) , said screens in upper and lower tubes being displaced in relation to each other so that the water in certain pipes is forced to move upwardly whereas the water in adjacent pipes flows downwardly.

3. A heat exchanger as claimed in claim 2, w h e r e i n the screens (18) are in the form of plates assembled on a rod (19) and having an inner diameter corresponding to that of the tubes (12a, 12b) .

4. A heat exchanger as claimed in any of claims 1 - 3, w h e r e i n surface-enlarging elements (17) are fitted on the water pipe (13b) between the vertical side walls of the gas channel thus produced.

5. A heat exchanger as claimed in any of claims 1 - 4, w h e r e i n the channel is extended by a channel extension (31,21,33) at one end of the heat exchanger.