A high specific surface area tube air preheater that can avoid low-temperature corrosion of flue gas

Abstract

The present invention discloses a high specific surface area tube air preheater capable of avoiding low-temperature corrosion of flue gas, which has one tube pass and two shell passes, and includes an intermediate partition, heat exchange tubes and a shell; the intermediate partition is vertically arranged in the middle of the shell, dividing the shell into an air preheating section I and an air preheating section II with a communicating bottom, an air inlet section is arranged on the top side of the air preheating section I, and an air outlet section is arranged on the top side of the air preheating section II; there are multiple heat exchange tubes, which are vertically and spacedly laid inside the shell, and the heat exchange tubes located in the air preheating section I are arranged with a progressive tube pitch that becomes denser from the air inflow side. The air preheater of the present invention adopts a shell-side U-shaped two-pass co-current flow and a single-pass arrangement of heat exchange tubes, with a compact structure and small floor area. The heat exchange tubes at the inlet section are arranged with a gradually denser pitch, which can effectively reduce the risk of low-temperature corrosion at the inlet section of the air preheater.

Description

Technical Field

[0001] The present invention relates to the technology of waste heat recovery and utilization, and particularly to a high specific surface area tube air preheater that can avoid low-temperature corrosion of flue gas. Background Art

[0002] An air preheater is a common auxiliary equipment for furnaces that recovers the waste heat of flue gas, raises the temperature of the air entering the furnace, reduces the fuel consumption of the furnace, and improves the combustion condition. Compared with a rotary air preheater, a traditional tubular air preheater has the characteristics of being not easy to leak and being easy to replace and overhaul. However, due to the low air inlet temperature, the traditional tubular air preheater often encounters the problem of low-temperature corrosion during operation, which seriously affects the service life and safe operation of the equipment. On the other hand, the traditional tubular air preheater generally uses straight round tubes, with low heat exchange efficiency and large volume. And because the heat exchangers are mostly arranged in a cross-flow pattern, in order to reduce the flow resistance on the flue gas side outside the tubes, a relatively low flue gas flow rate is generally adopted. On the one hand, this reduces the overall heat transfer coefficient of the heat exchanger. On the other hand, limited by the heat transfer temperature difference in the cross-flow arrangement, the outlet flue gas temperature of the air preheater is generally relatively high, and the waste heat of the flue gas cannot be recovered to the maximum extent. Summary of the Invention

[0003] In order to overcome the deficiencies of the traditional tubular air preheater, the present invention provides a high specific surface area tube air preheater that can avoid low-temperature corrosion of flue gas. It adopts a shell-side U-shaped two-pass co-current flow and a single-pass arrangement of heat exchange tubes, with a compact structure and small floor area. The inlet section adopts a gradually denser pitch arrangement, which can effectively reduce the risk of low-temperature corrosion in the inlet section of the air preheater.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] A high specific surface area tube air preheater that can avoid low-temperature corrosion of flue gas has one tube pass and two shell passes, and includes an intermediate partition, heat exchange tubes, and a shell;

[0006] The intermediate partition is vertically arranged in the middle of the shell, dividing the shell into an air preheating section I and an air preheating section II that communicate at the bottom. The top side of the air preheating section I is provided with an air inlet section, and the top side of the air preheating section II is provided with an air outlet section;

[0007] There are multiple heat exchange tubes, which are vertically and spacedly laid inside the shell, and the heat exchange tubes in the air preheating section I are arranged with a gradually denser tube pitch starting from the air inflow side;

[0008] The gaps between the heat exchange tubes form an air flow channel, and the inside of the heat exchange tubes forms a flue gas flow channel.

[0009] In this way, the air flow is in a U shape, enabling counter-flow and co-flow heat exchange with the flue gas inside the heat exchange tubes. Meanwhile, for the heat exchange tubes in the first air preheating section, the tube pitch is adjusted to vary the space outside the tubes, with the tube pitch changing from sparse to dense. This can reduce the air flow velocity at the inlet section, thereby decreasing the heat transfer coefficient on the air side at the inlet, ensuring that the tube wall temperature at the inlet section is higher than the acid dew point temperature, and thus avoiding low-temperature corrosion of the heat exchange tubes at the inlet section.

[0010] Furthermore, the flue gas enters the first air preheating section and the second air preheating section from the top of the heat exchange tubes. In this way, the first air preheating section is for co-flow heat exchange. The low-temperature air at the inlet first exchanges heat with the high-temperature flue gas, enabling a relatively high tube wall temperature and further avoiding low-temperature corrosion. The second air preheating section is for counter-flow heat exchange, expanding the heat transfer temperature difference of the heat exchange tube wall and helping to reduce the heat exchange area of the second air preheating section (the main heat exchange section).

[0011] Furthermore, the heat exchange tubes are twisted elliptical high-specific-surface heat exchange tubes. In this way, the elliptical tubes are formed by spiral twisting, which can effectively enhance the external flow disturbance and internal swirl flow of the tubes, and improve the convective heat transfer coefficients inside and outside the tubes.

[0012] Furthermore, straight circular tubes are provided at both ends of the twisted elliptical high-specific-surface heat exchange tubes for airtight connection with the top and bottom of the shell.

[0013] Furthermore, adjacent twisted elliptical high-specific-surface heat exchange tubes contact each other at the maximum diameter-changing convex points to form a self-supporting structure. In this way, it can make the characteristics of variable space and variable flow field be formed among the tube bundles.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] 1. It adopts a single-tube-pass and double-shell-pass structure form, and the tube row pitch of the air in the first shell-pass stage uses a tube pitch that is sparse at the inlet side and becomes dense towards the outlet. This not only greatly improves the heat exchange performance of the air preheater but also overcomes the risk of low-temperature corrosion at the inlet section of the traditional tubular air preheater. Thus, ordinary carbon steel tubes can be used as the heat exchange tube material, reducing the material cost of the air preheater. Moreover, it can achieve the maximum temperature drop of the flue gas, thereby obtaining the maximum energy-saving benefit.

[0016] 2. It uses twisted elliptical high-specific-surface tubes as heat exchange elements, increasing the air-side flow velocity and strengthening the heat exchange inside and outside the tubes, thereby improving the overall heat exchange effect and heat recovery level, reducing the comprehensive cost of waste heat recovery, and achieving better energy conservation and emission reduction effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a front view plane schematic diagram of the air preheater of the present invention;

[0018] Figure 2Schematic diagram of the tube layout for the first stage of air preheating. The upper part is the air inlet side, and the lower part is the intermediate baffle side;

[0019] Figure 3 Wall temperature distribution diagram of the heat exchange tubes for the first stage of air preheating;

[0020] Explanation of reference numerals: 1 - air inlet section; 2 - first stage of air preheating; 3 - second stage of air preheating; 4 - air outlet section; 5 - intermediate baffle; 6 - heat exchange tube; 7 - housing. Detailed implementation manners

[0021] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners.

[0022] As Figure 1 shown, a high specific surface area tube air preheater capable of avoiding low-temperature corrosion of flue gas includes an air inlet section 1, a first stage of air preheating 2, a second stage of air preheating 3, an air outlet section 4, an intermediate baffle 5, heat exchange tubes 6, and a housing 7.

[0023] The housing 7 is the main body of the air preheater. The intermediate baffle 5 is vertically arranged in the middle of the housing 7, dividing the interior of the housing 7 into a first stage of air preheating 2 and a second stage of air preheating 3 that communicate at the bottom. The air inlet section 1 is arranged at the upper left of the housing 7 and communicates with the first stage of air preheating 2. The air outlet section 4 is arranged at the upper right of the housing 7 and communicates with the second stage of air preheating 3, forming a U-shaped flow space with a double shell pass.

[0024] The heat exchange tubes 6 are vertically and spacedly laid inside the housing 7. The gaps between the heat exchange tubes 6 form an air flow channel, and the inside of the heat exchange tubes 6 forms a flue gas flow channel. To improve the comprehensive heat exchange performance of the air preheater, the heat exchange tubes 6 are high specific surface area tubes formed by spirally twisting elliptical tubes, and straight pipe sections are provided at both ends thereof for easy welding to the upper and lower end faces of the housing 7.

[0025] During operation, the flue gas is divided into two streams from the top of the air preheater and enters the heat exchange tubes 6 in the first stage of air preheating 2 and the second stage of air preheating 3 respectively. Flowing from top to bottom, it passes through the air preheater and then merges into the subsequent flue gas purification system. The air flowing in from the air inlet section 1 flows downward along the first stage of air preheating 2 in a co-current manner, enters the second stage of air preheating 3 at the bottom and then turns upward, at this time, it exchanges heat with the flue gas in the tubes in a counter-current manner, and finally flows out through the air outlet section 4.

[0026] To control the wall temperature of the heat exchange tubes in the inlet section to be higher than the acid dew point temperature so as to avoid low-temperature corrosion, as Figure 2As shown, in the first stage of air preheating section 2, extending from the air inlet side towards the middle partition, the heat exchange tubes 6 are arranged from sparse to dense. By adjusting the area of the flow cross-section outside the tubes, the convective heat transfer coefficient outside the tubes is controlled, and then the tube wall temperature is controlled to be higher than the acid dew point temperature.

[0027] To better illustrate the control of the tube wall temperature at the inlet section of this patent, taking the acid dew point of the flue gas as 74 °C and the ambient air temperature as 21 °C as an example, refer to Figure 3 , starting from the air inlet side, the transverse tube pitch of the 1st - 7th rows is 126 mm, the transverse tube pitch of the 8th - 11th rows is 79 mm, and the transverse pitch from the 12th row onwards is 63 mm. The longitudinal pitch is 63 mm and is equal to the major axis of the twisted elliptical tube, so that the longitudinal tube bundle can form self-support. Then the tube wall temperature of the first row of tubes is 74.1 °C, the second row is 75.2 °C and the air temperature rises to 26.5 °C. The tube wall temperature of each subsequent row of tubes is higher than 74 °C. At the same time, the air temperature gradually increases, and for the tube rows with the same transverse tube pitch, the tube wall temperature will also increase as the number of rows increases. Generally, the tube wall temperature of each row of tubes can be maintained above the acid dew point, thus avoiding low-temperature corrosion.

[0028] In summary, for the high specific surface area tube air preheater that can avoid low-temperature corrosion of flue gas in this application, the structural design adopts a shell-side U-shaped two-pass co-current flow type. Among them, the first stage of air preheating section 2 adopts a co-current flow type anti-low-temperature corrosion layout, and the heat exchange tubes are arranged in a gradually denser tube row layout from the air inlet side. The second stage of air preheating section 3 adopts a counter-current flow layout. The heat exchange tubes 6 for flue gas waste heat recovery adopt twisted elliptical heat exchange tubes, which have good heat transfer enhancement effects, reduce the pressure drop while ensuring the gas flow rate, are not prone to vibration, and have strong anti-fouling ability. Compared with the traditional air preheating device, the volume of the air preheater in this application is reduced by 30 - 50%, and the weight is reduced by 35 - 55%, realizing high-efficiency energy-saving operation of flue gas waste heat recovery.

[0029] The above embodiments are only for illustrating the technical concept and features of the present invention. The purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and it should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A high specific surface area tube air preheater capable of avoiding low-temperature corrosion of flue gas, having one tube pass and two shell passes, characterized in that: It includes an intermediate partition plate (5), heat exchange tubes (6) and a housing (7); The intermediate partition plate (5) is vertically arranged in the middle of the housing (7), dividing the housing (7) into an air preheating section I (2) and an air preheating section II (3) with a common bottom. An air inlet section (1) is provided on the side of the top of the air preheating section I (2), and an air outlet section (4) is provided on the side of the top of the air preheating section II (3), forming a U-shaped flow space with a double shell pass; There are multiple heat exchange tubes (6), which are vertically and spacedly laid inside the housing (7), and the heat exchange tubes (6) located in the air preheating section I (2) are arranged with a progressive tube pitch that becomes denser from the air inlet side; The gaps between the heat exchange tubes (6) form an air flow channel, and the inside of the heat exchange tubes (6) forms a flue gas flow channel; The flue gas enters the air preheating section I (2) and the air preheating section II (3) from the top of the heat exchange tubes (6); The heat exchange tubes (6) are twisted elliptical high specific surface heat exchange tubes; Both ends of the twisted elliptical high specific surface heat exchange tubes are provided with straight circular tubes for airtight connection with the top and bottom of the housing (7); Adjacent twisted elliptical high specific surface heat exchange tubes are in contact with each other at the maximum diameter-changing convex points to form a self-supporting structure; During operation, the flue gas is divided into two streams at the top of the air preheater and enters the heat exchange tubes (6) in the air preheating section I (2) and the air preheating section II (3) respectively. Flowing from top to bottom, it passes through the air preheater and then merges into the subsequent flue gas purification system. The air flowing in from the air inlet section (1) flows downward along the air preheating section I (2) in a co-current manner, enters the air preheating section II (3) at the bottom and then turns upward. At this time, it exchanges heat with the flue gas in the tubes in a counter-current manner and finally flows out through the air outlet section (4).

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