CA2203587C

CA2203587C - A heat exchanger tube supporting plate and a process for manufacturing it

Info

Publication number: CA2203587C
Application number: CA 2203587
Authority: CA
Inventors: Hideyuki Ohta; Kazutoyo Murata; Setsuo Tokunaga; Takahisa Hattori; Hiroshi Hirano; Mikio Nagatsune; Ikuo Ohtake
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1996-04-25
Filing date: 1997-04-24
Publication date: 2007-01-02
Anticipated expiration: 2017-04-24
Also published as: JPH09292198A; CA2203587A1; FR2748099B1; JP3342289B2; FR2748099A1

Abstract

A heat exchanger tube supporting plate which reduces or prevents scaling of magnetite, etc., and a process for manufacturing it. This may be accomplished by chrome coating the plate.

Description

A BEAT EXCHANGER TUBE SUPPORTING PLATE
AND A PROCESS FOR MANUFACTURING IT
BACKGROUND OF THE INVENTION
The. present invention relates to a heat exchanger which may be used in thermoelectric, nuclear power plants and chemical plants. More particularly, the present invention relates to a heat exchanger tube supporting plate and a process of manufacture for such plate wherein the tube supporting plate has a coating or a method of reducing the impact of depositing iron oxide such as magnetite from water.
In heat exchangers used in thermoelectric, nuclear plants and chemical plants, etc., the heat exchanger tube supporting plate arranged therein is usually positioned horizontally to support the vertically disposed heat exchanger tubes. The materials generally used to construct the heat exchanger tube supporting plate are carbon steel and SUS stainless steel. Figure 6 illustrates an example showing the inside of a conventional vertical shell-and-tube heat exchanger 10 having an outer shell 11. The inside of the exchanger is constructed with a bundle of heat exchanger tubes 13. The heat exchanger tubes 13 are so arranged that the bundle of heat exchanger tubes is supported by a plurality of tube supporting plates 18.
The heat exchanger tubes 13 are U-shaped and both ends of each tube are inserted into a tube plate 12. The tube plate 12 is formed integral with the outer surface, that is, the shell 11. A heated intake fluid 14 at 150°C to 350°C flows into a water tank 15 and passes through the heat exchanger tubes 13, giving off heat, as described hereafter. The fluid thereby cools down to a lower temperature and flows out of the water tank 16 as cooler fluid I4'.
Many heat exchanger tubes 13 having a variety of curving radii form the heat exchanger tube bundle which is enclosed within a wrapper trim 17.
At the same time, this bundle is supported by a plurality of substantially horizontal tube supporting plates 18. Feedwater 20 flows in through a feedwater nozzle 19 and flows between the wrapper trim 17 and the shell 11. The feedwater then turns at the upper surface of the tube plate 12 and rises along the heat exchanger tubes 13. At this time the feedwater takes heat away from the heated fluid 14 in the tubes 13 by thermal exchange, rises in temperature, boils, and turns into steam. The steam 21 flows out of a steam nozzle 25 and heads, for example, to a steam turbine.
The heat exchanger tube supporting plates 18 must have rigidity and mechanical strength which allow the plates to support the heat exchanger tubes 13. Nonetheless, the feedwater 20 meets with little resistance as it is pumped up from the plate 12 toward the steam nozzle 25. Conventionally, the feedwater contains some iron oxide (scale) such as rust generated in the system and the scale gradually deposits on the tube supporting plates 18. This reduces the cross-sectional areas of the feedwater path holes 36.
Conventional heat exchanger tube supporting plates are con-structed of carbon steel, stainless steel, etc. The film fabricated on the surface of carbon steel or stainless steel is made of cubic crystals having a spinet structure. Iron oxide (scale) such as Fe,304 contained in the feedwater of the heat exchanger is also made of cubic crystals having a spinal structure.
Accordingly, these two tend to bond to each other. Experience has shown that iron contained in the feedwater deposits on the surface of the supporting plates as scale.
The amount deposited will increase as the heat exchanger is operated for long time periods.
In the tube supporting plate of the heat exchanger, there are many holes having a diameter of, for example, about 15 mm. With the passage of time, the aforementioned scale deposits in the holes and substantially reduces the cross-sectional areas of the path holes. This results in decreased water flow rate, unstable feedwater level due to the difference in the cross-sectional areas of the feedwater path, and the like.
SUMMARY OF THE INVENTION
An object of the present invention is to resolve these problems.
According to the principles of the present invention, a heat exchanger tube supporting plate is provided having means or a method of reducing the impact of depositing iron oxide which is extremely effective.
In accordance with a first embodiment of the present invention, a heat exchanger is provided having heat exchanger tubes and at least one tube supporting plate for supporting such tubes, which tube supporting plate is chrome-coated.
In another embodiment, the chrome-coated heat exchanger tube supporting plate has a hexagonal crystalline chrome oxide coating thereover.

Still a further embodiment of the present invention includes a heat exchanger having a body with an inlet nozzle for feedwater and an outlet nozzle for steam. The heat exchanger body also has an inlet fluid tank and an outlet fluid tank. A plurality of heat exchanger tubes are provided, mounted in the body, each of the tubes respectively having a first end communicating with the inlet tank, and a second end communicating with the outlet tank. Means for introducing heated fluid into the inlet tank are also included so that the heated fluid flows into the respective first ends of the heat exchanger tubes and out of the second ends into the outlet tank, whereby the heat of the heated fluid heats the feedwater via the tubes and turns the feedwater into steam emitted from the outlet nozzle. Also included is at least one tube supporting plate for supporting the plurality of tubes in the body, the tube supporting plate being chrome-coated.
In an embodiment constructed in accordance with the principles of the present invention, a heat exchanger is provided having a plurality of heat exchanger tubes arranged substantially vertically and a tube supporting plate arranged substantially horizontally. The tube supporting plate is chrome-coated.
In a further embodiment of the present invention, a heat exchanger has a tube supporting plate with at least one hole therein through which the feedwater passes as it moves between the inlet and the outlet nozzles.
It is also an object of the present invention to teach a process for providing a protective coating for a heat exchanger tube supporting plate.
Such process includes the steps of chrome plating the plate in a plating bath having an iron concentration of 0.5 grams per liter or less.

Another object of the present invention is to provide a process which includes the additional step of oxidizing the chrome-plated tube supporting plate to obtain a hexagonal crystalline chrome oxide layer.
Still, a further object includes carrying out this additional step at a 5 temperature of 200°C for two hours.
In a broad aspect, then, the present invention relates to a tube support plate for a heat exchanger in a nuclear power plant, said heat exchanger having a body with an inlet nozzle and an outlet nozzle forfeedwaterto pass through said inlet nozzle or steam to pass through said outlet nozzle, and a plurality of heat exchanger tubes mounted in the body through the tube support plate, with the tube support plate having holes for the feedwater to pass through and for supporting the heat exchanger tubes in said body characterized in that the tube support plate has a chrome coating that is iron free.
In another broad aspect, then, the present invention relates to a heat exchanger for a nuclear power plant having a body with an inlet nozzle, an exit nozzle, at least one heat exchanger tube extending through a tube support plate for circulating said water and steam in the said body with said tube support plate having holes for the feedwater to pass through and for supporting said heat exchanger tube mounted therein, characterized in that the tube support plate has a chrome coating that is iron free.
In a further broad aspect, then, the present invention relates to a process of manufacture of a tube support plate with feedwater holes for a heat exchanger having a body with inlet and exit nozzles for the passage of feedwater and steam respectively and heat exchanger tubes supported by the tube support plate characterized by chrome plating the tube support plate in a bath with a density of iron at a concentration of no greater than 0.5 g/litre or less and oxidizing the chrome coating to form a chrome oxide layer such that there is no iron content in the surface of the said support plate with feedwater holes.

5a BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:
Figure 1 (a) is a top view of a heat exchanger tube supporting plate in accordance with a first embodiment of the present invention; Figure 1 (b) is a cross-sectional view of Figure 1 (a) taken along the line B-B; and Figure 1 (c) is a magnified cross-section of a tube supporting plate coated with chrome;
Figure 2 is a graphical waveform showing the comparative test results for magnetic scaling on the chrome-coated tube supporting plate of Figure 1 (c) and of a conventional plate;
Figure 3 is a magnified cross-section of a chrome oxide-coated tube supporting plate in accordance with a second embodiment of the present invention;
Figure 4 is a graphical waveform showing comparative iron concentrations of a chrome plating bath versus iron contents of a chrome layer obtained by a third embodiment of the present invention as compared to conventional methods;

Figure 5 is a graphical waveform comparing the amount of scale deposited on a chrome-plated heat exchanger tube supporting plate in accordance with the third embodiment of the present invention and a conventional plate;
and Figure 6 is a sectional view conceptually illustrating the internal structure of a vertical shell-and-tube heat exchanger.
DESCRIPTION OF THE PREFERRED EMBODIIVVIENTS
Referring to the drawings and more particularly to Figures 1 (a) to 1 (c), Figure 1 (a) is a top schematic view of the heat exchanger tube supporting plate 35 which may be one of a plurality of tube supporting plates such as plates 18 shown in Figure 6. In Figures 1(a) and 1(b), the heat exchanger tube supporting plate 35 is formed with holes 36 through which passes the feedwater such as feedwater 20 of Figure 6. Heat exchanger tubes 37 pass through and are supported by the plate 35. The heat exchanger is filled with the system water, such as shown in Figure 6, within a range of 250°C to 350°C
inside for . exchanging thermal energy.
EXAMPLE NO. 1 In Example No. 1, incorporating the principles of the present invention, the entire surface of the heat exchanger tube supporting plate 35 of the heat exchanger which the liquid contacts is coated with a chrome layer 40, as illustrated in Figure 1(c) (thickness: 25 ~ S~,m). In Example No. 1, the chrome layer 40 is fabricated using a hard chrome-plating solution ("sergeant"
solution) for a plating solution, at a current density of 0.2 A/cmz, a temperature of 50°C, and a plating time of 1.5 hours. After fabricating the chrome layer 40 in this manner, a magnetite scaling test was provided on the chrome layer 40 on the tube supporting plate 35 under the power plant feedwater simulated conditions (285°C; 60 atm; NH3: 0.5 ppm; NZH4: 0.5 ppm). The chrome layer 40 showed nearly no magnetite scaling while an AISI type 304 stainless steel plate showed a scaling which was. ten (10) times greater than the scaling formed on the supporting plate 35 coated in the manner described in Example No. 1.
This is best seen in Figure 2, which is a plot of the relative scaling amount versus the dme for the scaling test. In Figure 2, it can be seen that the plot representing the chrome-plated tube supporting plate showed substantially zero amount of scaling while the plot representing the conventional AISI type 304 stainless steel plate showed a relative scaling amount ten (10) times greater.
In Example No. 1, the chrome thickness for coating the surface of a carbon steel or stainless steel tube supporting plate is preferably S~cm to 100~.m, and one can apply general chrome plating for the coating.
EXAMPLE NO. 2 The second example incorporating the principles of the present invention is described referring to Figure 3. In Figure 3 is shown a heat exchanger tube supporting plate 45; 46 is a chrome-coating layer; and layer 47 is a hexagonal crystalline chrome oxide layer. This hexagonal crystalline chrome oxide layer 47 is obtained by oxidizing the aforementioned chrome layer 40 illustrated in Figure 1(c) at 200°C for 2 hours. Its thickness was about O.l~cm.

A magnetite scaling test was provided on the hexagonal crystalline chrome oxide layer 47 under the same conditions described in Example No. 1.
The chrome layer 47 showed nearly no magnetite scaling even after 9,000 hours, which was one-one hundred fiftieth (1/150) of that shown on an AISI type 304 stainless steel plate.
In other words, in Example No. 1, the amount of magnetite scaling on the chrome layer 40 after 2,000 hours was 0.05 mg/cm2 or less, which was about one-tenth (1/10) of the amount of scaling shown on an AISI type 304 stainless steel plate (0.52 mg/cm2). In Example No. 2, the amount of magnetite scaling on the hexagonal crystalline chrome oxide layer 47 was 10 ~cg/cm2 even after 9,000 hours which was about one-one hundred fiftieth 1/150 of that shown on an AISI type 304 stainless steel plate (1.5 mg/cm2).
The chrome thickness for coating the tube supporting plate surface is within the same range as the above. The chrome film thickness of the hexagonal crystalline chrome oxide coated on the aforementioned chrome layer is preferably about O.Ol~cm to l.O~cm. This hexagonal crystalline chrome oxide can be fabricated by appropriately oxidizing the pre-coated chrome layer.
The thin chrome surface layer coated on the heat exchanger tube supporting plate naturally turns into hexagonal crystalline chrome oxide. In addition, by oxidizing the chrome coated on the tube supporting plate, a thicker chrome oxide layer than the naturally fabricated chrome oxide layer may be fabricated thereon. These chrome oxide layers in existence can provide a countermeasure or a method of reducing the impact of depositing cubic crystalline iron oxide crystals (scales). As a result, variations in areas, such as reduced or varied cross-sectional areas in the feedwater path of a tube supporting plate caused by scaling on the tube supporting plate, can be reduced or prevented, thus providing a stable thermal exchange.
'EXAMPLE NO. 3 In general, chrome plating uses conditions such as an iron concentration of 6 g/liter or less; current density of 0.1 A/cmZ to 0.4 A/cm2;
a temperature of 30°C to 60°C; a chromic acid anhydride concentration of 200 g/liter to 250 g/liter; and a sulfate concentration of 2.2 g/liter to 2.5 g/liter.
However, in accordance with the principles of the present invention, an iron concentration of 0.01 g/liter; a chromic acid anhydride concentration of 240 g/liter; a current density of 0.2 A/cm2; and a temperature of 50°C was used. A
process time of one (1) hour was used for plating a heat exchanger tube supporting plate. As a result, no iron was measured in the obtained chrome-plated layer.
As a control, a heat exchanger tube supporting plate was chrome plated using the same chrome-plating bath as aforementioned, except that the iron concentration was 6 g/liter while maintaining the same conditions. Figure 4 illustrates the measurements resulting from conditions used for Example No. 3 and for the comparative control example. It is clear from Figure 4 that no iron was measured in the chrome layer obtained from the chrome-plating bath whose iron concentration was 0.5 g/liter or less, while some iron was measured in the chrome layer obtained from the chrome-plating bath whose iron concentration was more than 0.5 g/liter.
Furthermore, Figure 5 illustrates a relative comparison of scaling on the tube supporting plate over a long dme period under the conditions of 5 Example No. 3 and the control. It is apparent from Figure S that the method used s in Example No. 3, detailed above, significantly reduced the amount of scale deposited on the tube supporting plate. Thus, from Figure 5, it is seen that the relative scaling with an iron concentration of 0.01 g/liter in a plating bath was substantially zero even after 750 hours. On the other hand, with an iron 10 concentration of 6 g/liter in a plating bath, the resulting relative scaling was fifteen (15) times greater. In addition, the relative scaling of the control continued to increase with time above 750 hours, while the relative scaling using the method incorporating the principles of the present invention continued at substantially the zero level.
In summary, the heat exchanger tube supporting plate, which tube supporting plate is processed in accordance with the principles of the present invention, substantially prevents scaling of iron oxides such as magnetite, thus always providing a stable exchange of thermal energy in a heat exchanger having such a tube supporting plate. In addition, the chrome-plating process conditions according to the principles of the present invention eliminate iron from a chrome-plated layer, thus providing a process for manufacturing a heat exchanger tube supporting plate which can reduce or substantially prevent scaling.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims

1. ~Tube support plate for a heat exchanger in a nuclear power plant, said heat exchanger having a body with an inlet nozzle and an outlet nozzle for feedwater to pass through said inlet nozzle or steam to pass through said outlet nozzle, and a plurality of heat exchanger tubes mounted in the body through the tube support plate, with the tube support plate having holes for the feedwater to pass through and for supporting the heat exchanger tubes in said body characterized in that the tube support plate has a chrome coating that is iron free.

2. ~The tube support plate as defined in claim 1, further comprising a coating of hexagonal chrome oxide over the chrome-coat.

3. ~A heat exchanger for a nuclear power plant having a body with an inlet nozzle, an exit nozzle, at least one heat exchanger tube extending through a tube support plate for circulating said water and steam in the said body with said tube support plate having holes for the feedwater to pass through and for supporting said heat exchanger tube mounted therein, characterized in that the tube support plate has a chrome coating that is iron free.

4. ~Process of manufacture of a tube support plate with feedwater holes for a heat exchanger having a body with inlet and exit nozzles for the passage of feedwater and steam respectively and heat exchanger tubes supported by the tube support plate characterized by chrome plating the tube support plate in a bath with a density of iron at a concentration of no greater than 0.5 g/litre or less and oxidizing the chrome coating to form a chrome oxide layer such that there is no iron content in the surface of the said support plate with feedwater holes.

5. ~The heat exchanger of claim 3, wherein said tube support plate further~
comprises a coating of hexagonal chrome oxide over the chrome coat.