JPH0285697A - Plate-type heat exchanger - Google Patents

Abstract

PURPOSE:To prevent crevice corrosion from occurring at a gap construction part and enhance durability of an apparatus by depositing a film of Zn or a Zn alloy on a surface for contact with a liquid medium, of stainless steel heat transfer surfaces at least one of which is used as the surface for contact with the liquid medium. CONSTITUTION:A seawater passage 2 and a passage 3 for water to be cooled are provided in gaps between a multiplicity of plates 1 disposed in parallel, and the seawater and the water to be cooled are brought into heat exchange with each other through the plates 1 as heat transfer surfaces. Gaskets 4 are placed between the plates 1, along the peripheral edges of the plates. A multiplicity of gaps are inevitably generated in the vicinity of the gaskets, in the vicinity of liquid inlet and output ports, etc. Stainless steel surfaces 12a, 12b for contact with a liquid, facing the gap 13, are coated with plates layers 14a, 14b of Zn or a Zn alloy. With Zn thus present, corrosion of stainless steel is prevented even when Zn is converted into zinc hydroxide, so that it is possible to prevent crevice corrosion and to enhance reliability of an apparatus.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a plate heat exchanger made of stainless steel that can use a liquid containing a large amount of 02-, such as from drought, as a liquid medium.

[Background of the invention]

Stainless steel WJ heat exchangers, in which the heat transfer surface of the heat exchanger is made of stainless steel, are popular, but they have not been widely used for seawater. This is because even stainless steel suffers from crevice corrosion when seawater passes through it.

Stainless steel is an extremely corrosion resistant material.

Due to its corrosion resistance, it is widely used in various types of heat exchange equipment used in water environments, but when assembled as a single device, it is inevitable that there will be gaps in parts due to welding, caulking, gasket parts, etc. . Stainless steel has excellent corrosion resistance in general flat parts, but if it has a structure with gaps, it may corrode, so-called crevice corrosion, depending on the usage environment. Crevice corrosion of stainless steel is induced by chlorine ions contained in water, and the higher the C2 concentration and the higher the temperature, the stronger the crevice corrosion becomes.

Therefore, when used for purposes such as cooling high-temperature water with seawater, the gaps often corrode in a short period of time.

To prevent crevice corrosion in stainless steel, measures such as cathodic protection using an external power source or sacrificial anode method are widely applied in water heaters, etc. If the system is complex, it may be difficult to install it in all the liquid passages, or even if it is installed, it may not be sufficiently effective, and it will also be costly.

For this reason, expensive materials with Mo added or with significantly increased Cr content are used to prevent crevice corrosion, but this is not perfect, and as of now seawater heat exchangers are free from crevice corrosion. It can be said that no general-purpose equipment using ordinary stainless steel has appeared that can function semi-permanently.

[Purpose of the invention]

The purpose of the present invention is to obtain a heat exchanger that is resistant to crevice corrosion using general-purpose stainless steel without using special stainless steel containing a large amount of expensive corrosion-resistant elements, and is particularly economical. The aim is to provide a seawater heat exchanger.

[Structure of the invention]

One aspect of the present invention relates to a heat exchanger in which a heat transfer surface is formed of stainless steel, and at least one surface of the heat transfer surface is a surface in contact with a liquid medium.

(1) A stainless steel plate heat exchanger in which a Zn or Zn alloy coating is adhered to the liquid contact side surface of the stainless steel heat transfer surface.

(2) A plate heat exchanger made of stainless steel, in which a Zn or Zn alloy coating is adhered to the surface of the stainless steel heat transfer surface in contact with liquid, and a chromate coating is further applied to the surface of this coating.

(3) A stainless steel plate heat exchanger made of a Zn or Zn alloy coating adhered to the liquid contact side surface of the stainless steel heat transfer surface, and a zinc hydroxide coating further applied to the surface of this coating. exchanger, and.

(4) A plate heat exchanger made of stainless steel is provided, in which a zinc hydroxide coating is deposited on the liquid-contacting surface of the heat transfer surface made of stainless steel.

The plate heat exchanger of the present invention has extremely good crevice corrosion resistance, and can be satisfactorily used as a seawater heat exchanger even when general-purpose stainless steel is used. The heat exchanger of the present invention is suitable for use as a liquid-to-liquid heat exchanger or a gas-to-liquid heat exchanger, in which the heat transfer surface is made of stainless steel and liquid flows on one side. The effect is great when the heat exchanger is configured as a large plate heat exchanger.

(Detailed Description of the Invention) Figure 1 shows an example of the structure of a plate heat exchanger that cools high-temperature water (water to be cooled) by exchanging heat with seawater. A seawater passage 2 and a cooled water passage 3 are formed in the gap between the plates l, and each plate l
Heat is exchanged between the two using the surface as the heat transfer surface. A gasket 4 is interposed between each plate 1 so as to go around the periphery of the plate, and includes a seawater supply passage 5 for introducing seawater into each passage 2, a seawater outlet passage 6 for taking out waste water from each passage 2, and A hot water supply passage 7 that introduces water to be cooled into each passage 3 and a hot water outlet passage 7 that takes out water to be cooled from each passage 4 are connected, and each seawater passage 2 and water passage 3 to be cooled flow in opposite directions. A water passageway has been formed. The structure of such a counterflow type plate heat exchanger itself is well known, and stainless steel heat exchangers are also known in which the plates serving as the heat transfer surface are made of stainless steel, but the surroundings of the gasket.

Many gaps inevitably occur around the liquid inlet and outlet.
In addition, the plates that serve as heat transfer surfaces are made of corrugated plates, embossed plates, etc. to increase the heat transfer area and to make the flow of liquid turbulent to increase heat exchange efficiency. The amount of gap increases.

For example, in Fig. 2, corrugated plates la and lb are stacked alternately so that their wave directions cross and are in contact at the ridge lines of each wave, forming a zigzag-shaped seawater passage 2 and a cooled water passage. 3 is formed between both plates, as shown in the -m1 plane. When such corrugated plates are crossed, countless gaps are created at the crossing points. In other words, in order to increase heat exchange efficiency,
The more uneven the heat transfer surface is, or the corrugated plate is used to expand the heat transfer area and create turbulence, the more gaps there will be. I can say that there will be more. Conversely, this indicates that even a heat exchanger with good performance suffers from serious crevice corrosion. The present invention has successfully prevented crevice corrosion in stainless steel plate heat exchangers that have a large number of crevices.

3 to 6 schematically show the gap corrosion prevention structure of the present invention for the gap portion of the heat exchanger. In these figures, 11a and llb are stainless steel heat transfer surfaces,
12a, ], 2b are the surfaces of the heat transfer surfaces on the liquid contact side.

13 indicates a gap on the liquid side.

FIG. 3 shows the basic configuration of the present invention corresponding to the above (1).
4a, 14b (preferably film thickness of 0.5 g/m" or more)
It is covered with When the other surfaces 15a and 15b of the heat transfer surfaces 11a and llb are also surfaces in contact with liquid (in the case of a liquid-liquid heat exchanger), it is preferable to form a similar coating.

It is omitted from the figure (the same applies to Figures 4 to 6 below).

That is, the outermost surface (the surface in contact with the liquid) forming the gap 13 consists of plating layers 14a and 14b that are in close contact with the stainless steel heat transfer surfaces 11a and llb. It is necessary that the plating layers 14a and 14b are in close contact with the entire surface of the heat transfer surface forming the gap 13. Heat transfer surfaces that do not form gaps 13, and therefore stainless steel heat transfer surfaces where crevice corrosion is not a problem, are outside the scope of the present invention, but in cases where it is advantageous to cover them with a plating layer for convenience of device manufacturing. Just cover it.

Furthermore, if a Zn or Zn alloy plating layer exists on a surface other than the surface of this gap 13, this portion acts as a sacrificial anode, and the zinc hydroxide film formed here acts as a barrier for oxygen reduction. It suppresses the rise in natural potential and has an advantageous effect on preventing crevice corrosion. Therefore, forming a similar plating layer on the stainless steel heat transfer surface forming the gap and on other surfaces is more advantageous in preventing gap corrosion. However, it is necessary to keep the plating layer in close contact with at least the liquid-contacting surface of the stainless steel heat transfer surface forming the gap.

FIG. 4 shows the coating structure of the heat transfer surface of the present invention corresponding to (2) above, in which the stainless steel heat transfer surfaces 12a, 12b facing the gap 13 are coated with a Zn or Zn alloy plating layer 14a, 14b, and chromate films 16a and 16b formed on the surface thereof. That is, the stainless steel surfaces 12a, 12b facing the gap 13
are adhered with plating layers 14a, 14b of Zn or Zn alloy, and chromate treatment films 16a, 16b are formed on the surfaces of the plating layers 14a, 14b on the liquid contact side.

FIG. 5 shows the coating structure of the heat transfer surface of the present invention corresponding to (3) above, in which the stainless steel heat transfer surfaces 12a and 12b facing the gap 13 are coated with Zn or Zn alloy plating N 14a, 14b and IQs 17a and 17b formed on the surface of the same lead hydroxide coating. That is, the stainless steel surfaces 12a, 12 facing the gap 13
b is closely adhered with plating layers 14a, 14b of Zn or Zn alloy, and zinc hydroxide coatings 17a, 17t+ are formed on the liquid-contacting surfaces of these plating layers 14a, 14b. After the plating layers 14a, 14b are formed on the lined lead hydroxide coatings 11i17a and 17b, the surface portion of the plated zinc is preferably converted into zinc hydroxide by a chemical reaction to form a zinc hydroxide coating.

FIG. 6 shows the coating structure of the heat transfer surface of the present invention corresponding to (4) above, in which the stainless steel heat transfer surfaces 12a, 12b facing the gap 13 are covered with zinc hydroxide coatings 17a, 17b. It is something. These zinc hydroxide coatings 17a, 17b
is preferably formed by chemically changing the entire thickness of the plating layer in the structure of (3) above to zinc hydroxide,
As a result, zinc hydroxide coatings 17a, 17b can be formed on the stainless steel surfaces 12a, 12b.

In any of these structures (1) to (4), the gap on the liquid contact side of the heat transfer surface is formed between two stainless steel surfaces, or when only one surface forming the gap is made of stainless steel. However, by forming a coating having the above-described structure on the surface of the stainless steel in contact with liquid, it is possible to prevent crevice corrosion on the heat transfer surface. Further, the shape of the gap is not limited to the illustrated example.

The structure of the present invention can be applied to any gap that causes crevice corrosion. In this case, the coating must be formed in close contact with the liquid-wetted surface of the stainless steel, and there is no particular problem if the coating is formed on the liquid-wetted surface of the stainless steel other than the gap, and it is preferable. There are many cases.

Traditionally, Zn coatings have generally been used to prevent corrosion of common steel. In ordinary steel, Zn sacrificially melts and corrodes immediately when metal Zn is consumed, so the period of time that metal Zn remains is an indicator of durability. The phenomenon in which Zn disappears through sacrificial melting can be assumed to occur in Zn-coated stainless steel as well as in Zn-coated materials for ordinary steel. That is, it is common to think that once Zn on stainless steel dissolves and disappears, the anticorrosive effect of Zn disappears, just like ordinary steel.

However, the present inventors investigated the anticorrosion effect of coating stainless steel with Zn from various aspects in more detail, including after dissolving Zn. It was discovered that it has the effect of preventing corrosion of stainless steel even when it corrodes and becomes zinc hydroxide. It was also found that the amount of Zn consumed was smaller in stainless steel than in ordinary steel. Therefore, the plate heat exchanger of the present invention has a remarkable effect in that the zinc hydroxide exerts its anticorrosion effect even after the metallic Zn is used up, and can prevent crevice corrosion over a long period of time. . The zinc hydroxide coating exhibiting this anticorrosion effect may be formed in advance, or the Zn coating may be modified into a zinc hydroxide coating during use of the heat exchanger. Furthermore, if the outermost layer is chromate treated, the anticorrosion effect will be even better. As for the anticorrosion mechanism in the crevice, firstly, Zn in the crevice is difficult to elute, and while Zn is present, the anticorrosion effect by sacrificial dissolution works, and furthermore, zinc hydroxide dissolves in the crevice and becomes a corrosion product. It is thought that this works to suppress the decrease in pH within the gap and prevent corrosion of the substrate. Second, Zn outside the gap naturally acts as sacrificial corrosion protection.

Even if Zn melts and becomes zinc hydroxide, it is thought that this zinc hydroxide acts as a barrier to oxygen reduction, suppresses the rise in self-potential, and prevents corrosion.

This can be demonstrated from the test results conducted by the present inventors shown below.

Test example Figure 7 shows the material of 5US316 stainless steel and plating 1.
The results of examining the crevice corrosion of 5US316 stainless steel plated with Zn in four different sizes are shown. The gap structure was formed as shown in FIG. That is, plate-shaped test piece 1
A hole is made in the center of 8, and a Ti bolt 19 is passed through the hole and fixed with a Ti nut 22 via a Teflon gasket 20 and a Ti washer 21. As a result, a gap is formed between the Teflon gasket 20 and the test piece 18.

The test pieces were plated after bolt holes were drilled. Therefore, a plating layer is also present on the surface of the hole. The test is 3.5χNaCl
The test structure was immersed in an aqueous solution at 80°C.
Changes in self-potential and corrosion conditions within the gaps between the test pieces were observed.

From the results shown in FIG. 7, it can be seen that although the potential of the stainless steel material is high, it corrodes in a short period of time and decreases to the corrosion potential. On the other hand, stainless steel coated with +Zn has a low potential while Zn is present, and an anticorrosion effect due to sacrificial dissolution of Zn is observed. As Zn dissolves and is consumed, the potential increases. Zn in the gap has a narrow contact area with the liquid surface, suppressing the diffusion of Zn into the liquid, and the rate of consumption of Zn is slow (however, it gradually disappears over 1 hour and the potential increases over 2 hours).

However, those with a coating weight of 0.5 g/m'' or more do not corrode, and this is thought to be because the pH buffering effect of zinc hydroxide has worked effectively.

Table 1 shows the state of corrosion after conducting the test shown in FIG. 7 on various surface-treated materials for 6 months.

From the results in Table 1, it can be seen that even if zinc hydroxide is used, crevice corrosion will not occur if the coating weight of Zn is 0.5 g/m" or more. If the coating weight is less than this, the effect of Zn is insufficient. Therefore, the time required to prevent crevice corrosion is shortened.Also, it is thought that the greater the amount of plating, the more effective it is.

Table 2 shows the results when the same experiment was carried out for 10 days with different steel types; the effect of Zn is fully recognized even when the steel types are different.

Table 3 shows the crevice corrosion properties of stainless steel coated with Zn alloy, which were investigated under the same conditions as in FIG.

The effect of Zn coating is observed not only in Zn alone coating but also in Zn alloy coating. Even in these alloy coatings, crevice corrosion will not occur if the coating weight is 0.5 g/a'' or more.

1st table 2nd table! Tanomei 1 Table 3 tI1 ku(! [: ISS Il Story] In this way, zinc hydroxide is effective in preventing crevice corrosion in stainless steel.

Figure 9 shows a case in which a 23g7m'' (DZn plating is applied to a test piece and the surface of the Zn plating layer is treated with zinc hydroxide to form a Zn+zinc hydroxide coating, and a case in which a Zn+zinc hydroxide coating is applied to the surface of the Zn plating layer. For the case shown in Fig. 7, the results of crevice corrosion tests conducted under the same conditions as in
From the results shown in the figure, the change in potential is different for the Zn plating layer, the multilayer structure of Zn and zinc hydroxide, and the zinc hydroxide coating; In the multilayer structure treated with acid arsenic, the potential rises quickly; however, the subsequent potential change remains the same as that of Zn plating.
Almost the same. Although no corrosion protection due to sacrificial dissolution of tZn is observed in the coating structure consisting only of zinc hydroxide, the change in electric
Table 4 shows corrosion (a) after this test, which shows the same behavior as the change in potential after dissolving Zn in the plating material, but no crevice corrosion occurred in any material other than the material.

The test results in Table 4 and above show that the effect of the present invention is f! even in actual heat exchangers. It has been certified. In other words, the stainless steel plate type heat exchanger has countless gaps formed by overlapping the heat transfer surfaces having a large number of irregularities to form the seawater passage 2 and the cooled water passage 3, as described above with reference to FIG. For the exchanger, 5US316 is used as the heat transfer face plate material, and this 5US31
Approximately 40 g/m" of Zn coating on the heat exchanger plate of No. 5, 1
A heat exchanger with 80g/m" treatment, hydroxide treatment and chromate treatment was fabricated, and a durability test as a heat exchanger was conducted by circulating 60°C seawater for 30 days. After the test, the plate was dismantled. The inner surfaces of the heat exchangers were investigated, but no corrosion was observed on the contact surfaces between the plates or the contact surfaces of the gaskets.The same experiment was carried out using 5US31
This was done using the same material from No. 6 as the heat transfer surface.

Severe crevice corrosion was occurring on the contact surfaces between the plates on the sea channel side and on the contact surfaces of the gaskets.

As explained above, 5 The plate heat exchanger of the present invention does not cause crevice corrosion even in seawater. 8 The coating structure of the heat transfer surface according to the present invention includes electrolytic Zn plating, hot-dip Zn plating, vapor-deposited Zn plating, etc. Zn or Zn alloy is coated with Zn or Zn alloy. Zn or Zn alloy and its hydroxide are deposited by coating or thermal spraying, and Zn or Zn is further deposited in the gap.
Insert alloy foil or plate. The coating structure can be formed by such a treatment, and either method can achieve the prevention of crevice corrosion on the stainless steel heat transfer surface.

According to the present invention, crevice corrosion in the crevice structure is prevented not only in neutral water environments but also in salt solutions with high chlorine ions, and as a result, the durability of the equipment is significantly improved. In addition, according to the present invention, there is no need to use expensive materials containing a large amount of corrosion resistance improving elements such as Cr and Mo, resulting in cost reduction! It is also very beneficial from the point of view of W source. In addition, compared to constructing a heat exchanger using stainless steel as a heat transfer surface, it is not subject to design constraints to avoid crevice corrosion, so it is possible to create a heat exchanger with a complex structure and high heat exchange efficiency. Since it can be configured freely, it can make a significant contribution to the technical field of heat exchangers, and its effects are particularly great not only in general heat exchangers but also in the field of industrial heat exchange using seawater as a liquid medium.

[Brief explanation of drawings]

FIG. 1 is a partial schematic cross-sectional view showing an example of the blended heat exchanger of the present invention, FIG. 2 is a schematic cross-sectional view of a liquid passage for explaining the gap in the plate heat exchanger, and FIGS. Each figure is an enlarged cross-sectional view showing an example of a coating structure in a gap portion of a heat exchanger according to the present invention, and FIG. 7 is a diagram showing a change in self-potential over time in a gap corrosion test. Figure 8 is a side view showing the state of gap formation in the gap test;
FIG. 9 is a diagram showing the change in natural potential over time in another gap test. l...Plate that forms a heat transfer surface. 2. Seawater passage, 3. Cooled water passage. 4. Punkin member (gasket). 11a, llb...Stainless steel heat transfer surface. 12a, 12b: Surface of the heat transfer surface on the liquid contact side. 13...Liquid side gap on heat transfer surface. 14a, 14b... Zn or Zn alloy plating layer formed on a stainless steel heat transfer surface. 16a, 16b - Chromate coating. 17a, 17b...Zinc hydroxide coating.

Claims (5)

[Claims] (1) In a plate heat exchanger in which the heat transfer surface is formed of stainless steel and at least one surface of the heat transfer surface is a surface in contact with a liquid medium, the liquid contact side surface of the stainless steel heat transfer surface. A stainless steel plate heat exchanger made of a Zn or Zn alloy coating. (2) In a plate heat exchanger in which the heat transfer surface is formed of stainless steel and at least one surface of the heat transfer surface is a surface in contact with the liquid medium, the liquid contact side surface of the stainless steel heat transfer surface. A plate heat exchanger made of stainless steel, in which a Zn or Zn alloy coating is adhered to the base plate, and a chromate coating is further applied to the surface of this coating. (3) In a plate heat exchanger in which the heat transfer surface is formed of stainless steel and at least one surface of the heat transfer surface is a surface in contact with the liquid medium, the liquid contact side surface of the stainless steel heat transfer surface. A plate heat exchanger made of stainless steel, in which a Zn or Zn alloy coating is adhered to the surface of the stainless steel plate, and a zinc hydroxide coating is further applied to the surface of this coating. (4) In a plate heat exchanger in which the heat transfer surface is formed of stainless steel and at least one surface of the heat transfer surface is a surface in contact with the liquid medium, the liquid contact side surface of the stainless steel heat transfer surface. A stainless steel plate heat exchanger made of a zinc hydroxide coating. (5) The stainless steel plate heat exchanger for seawater according to claim 1, 2, 3, or 4, wherein the liquid medium flowing in contact with at least one of the heat transfer surfaces is seawater.