Multifunctional molten salt corrosion testing system

Abstract

The invention belongs to the technical field of corrosion testing, and particularly relates to a multifunctional fused salt corrosion testing system which mainly comprises a corrosion reaction kettle and a fused salt buffer tank, a plurality of fixing clamping grooves are formed in the corrosion reaction kettle and mounted on a lower flow guide plate, an operation hole is formed in the top of the corrosion reaction kettle, and the fused salt buffer tank is mounted in the operation hole. The two ends of the corrosion reaction kettle are connected with circulating pipelines through flanges; a high-temperature molten salt medium in the molten salt buffer tank is conveyed into the corrosion reaction kettle through the micro industrial circulating pump, and a test piece fixed on a fixed clamping groove in the corrosion reaction kettle is scoured. The corrosion reaction kettle is a main reaction device of the testing system, is connected with an external circulating pipeline through a flange, and is convenient to disassemble.

Description

Technical Field

[0001] The invention belongs to the technical field of corrosion testing, in particular to a multifunctional molten salt corrosion testing system. Background Art

[0002] The existing molten salt corrosion test system mainly uses the static immersion corrosion test method, that is, the test piece is immersed in the molten salt; some also use the slow strain rate method, that is, a constant tensile stress is applied to the test piece and immersed in the molten salt medium. The molten salt medium of these two methods is static.

[0003] In actual applications, molten salt is mostly in a flowing state in systems and equipment. In addition to being affected by static corrosion from molten salt, equipment or materials are also affected by medium erosion and other stress interactions, and the two have completely different characteristics. The existing molten salt corrosion test has a single test condition and can only test the molten salt corrosion under static conditions, which is far from the actual engineering application conditions and cannot be tested under multiple conditions.

[0004] To this end, the present invention provides a multifunctional molten salt corrosion testing system, which can superimpose multiple conditions such as different medium flow rates, different temperatures, different stress sizes, etc. to realize various corrosion type tests. Summary of the invention

[0005] In order to make up for the deficiencies of the prior art, at least one technical problem raised in the background technology is solved.

[0006] The technical solution adopted by the present invention to solve its technical problems is: a multifunctional molten salt corrosion testing system described in the present invention includes a corrosion reactor and a molten salt buffer tank, a plurality of fixed slots are arranged in the corrosion reactor, and a plurality of the fixed slots are installed on the lower guide plate. An operation hole is opened on the top of the corrosion reactor, and a circulation pipe is connected with both ends of the corrosion reactor through a flange. The circulation pipe is connected with the molten salt buffer tank, an outlet control valve and an inlet control valve are arranged on the circulation pipe, a micro-industrial circulation pump is arranged on the circulation pipe, and a salt discharge valve connected with the interior is opened at the bottom of the corrosion reactor.

[0007] Furthermore, an electric heater is arranged in the molten salt buffer tank, a temperature controller for controlling the electric heater is arranged in the molten salt buffer tank, and a temperature transmitter is installed on the circulation pipeline.

[0008] Furthermore, the upper guide plate and the lower guide plate are each provided with a plurality of through holes penetrating through the upper guide plate and the lower guide plate, so as to increase the disturbance degree of the flowing molten salt.

[0009] Furthermore, the lower guide plate is provided with a fixing slot for fixing the test piece.

[0010] Furthermore, a visual window is fixedly installed on the top of the corrosion reactor, and the visual window is made of high-temperature resistant quartz material.

[0011] Furthermore, a through groove connected to the interior is opened on the top of the corrosion reactor, a "U"-shaped fixed frame is fixedly installed in the corrosion reactor, a high-temperature resistant cleaning cloth is arranged in the fixed frame, a sliding rod is slidably connected in the through groove, one end of the sliding rod is in the fixed frame, and the high-temperature resistant cleaning cloth is sleeved on the surface of the sliding rod.

[0012] Furthermore, two telescopic components symmetrically distributed with sliding rods are arranged in the through slot, and the telescopic components are composed of a plurality of hollow rectangular plates slidably connected, and one end of the telescopic component away from the through slot is fixedly connected to the sliding rod.

[0013] Furthermore, the open end of the fixed frame is rotatably connected to two symmetrically arranged baffles via a torsion spring.

[0014] Furthermore, a plurality of hemispherical bosses are fixedly installed in the fixed frame, a plurality of hollow rectangular plates of the telescopic assembly are sealed and slidably connected, the bosses and the telescopic assembly are connected through a connecting pipe, and the end of the connecting pipe away from the telescopic assembly is close to the high temperature resistant cleaning cloth.

[0015] Furthermore, a filter screen is fixedly installed inside the connecting pipe, and two symmetrically arranged arc-shaped guide plates are fixedly installed at the bottom of the filter screen.

[0016] The beneficial effects of the present invention are as follows:

[0017] 1. A multifunctional molten salt corrosion testing system described in the present invention transports the high-temperature molten salt medium in the molten salt buffer tank to the corrosion reactor through a micro-industrial circulation pump, thereby flushing the test piece fixed on the fixed slot in the corrosion reactor. The corrosion reactor is the main reaction device of this test system. The corrosion reactor is connected to the external circulation pipeline through a flange for easy disassembly. There are an upper guide plate and a lower guide plate inside the corrosion reactor. There is an operating hole above the corrosion reactor, through which the test piece can be placed or removed. There is also a visual window on the operating hole, through which the condition of the test piece in the corrosion reactor can be observed. The bottom of the corrosion reactor is equipped with a salt discharge valve, which is used to discharge the salt in the corrosion reactor after the experiment. The system designed by the present invention can realize molten salt corrosion testing under conditions of different medium flow rates, different temperatures, and different stress sizes; realize experimental testing of different corrosion types such as uniform corrosion, stress corrosion, and crevice corrosion; realize corrosion experimental testing in flowing molten salt; and simulate actual application conditions to test materials. The present application integrates multiple functions in one, greatly reducing the cost of material selection for use in molten salt media, and has important promotion and application value in the fields of solar thermal power stations, molten salt thermal energy storage technology, and other industrial fields.

[0018] 2. The multifunctional molten salt corrosion testing system described in the present invention slides the sliding rod in the sliding groove, so that the sliding rod slides with the telescopic component and the high temperature resistant cleaning cloth in the fixed frame, so that the top of the high temperature resistant cleaning cloth abuts against the visual window, and the visual window is efficiently wiped and cleaned as the high temperature resistant cleaning cloth moves. The above-mentioned reciprocating operation can quickly clean the visual window without opening the corrosion reactor, thereby ensuring the uninterrupted experiment, and allowing the staff to observe the situation in the corrosion reactor in real time. The high temperature resistant cleaning cloth will continuously squeeze and collide with several bosses when entering the fixed frame, thereby shaking off the molten salt remaining during the top cleaning, and reflowing into the corrosion reactor through the gap between the baffle and the fixed frame. As the sliding rod moves with the telescopic assembly, the telescopic assembly not only blocks impurities and seals the corrosion reactor, but also continuously squeezes the internal gas as it returns to its original position and contracts following the sliding rod, so that the internal gas is blown into the high-temperature resistant cleaning cloth and the fixed frame through the connecting pipe, thereby assisting in cleaning the high-temperature resistant cleaning cloth and the fixed frame and reducing the molten salt on the surface of the high-temperature resistant cleaning cloth and the molten salt in the fixed frame. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below in conjunction with the accompanying drawings.

[0020] Figure 1 It is a schematic diagram of the workflow of the present invention;

[0021] Figure 2 It is a structural schematic diagram of the corrosion reactor in the present invention;

[0022] Figure 3 It is a partial cross-sectional structural schematic diagram of the corrosion reactor in the present invention;

[0023] Figure 4 It is a structural schematic diagram of the fixed frame in the present invention;

[0024] Figure 5 The present invention Figure 3 A schematic diagram of the structure at A;

[0025] Figure 6 The present invention Figure 4 Schematic diagram of the structure at point B.

[0026] In the figure: 1. Corrosion reactor; 2. Operation hole; 3. Upper guide plate; 4. Lower guide plate; 5. Visual window; 6. Fixed slot; 7. Flange; 8. Outlet control valve; 9. Molten salt buffer tank; 10. Electric heater; 11. Micro industrial circulation pump; 12. Inlet control valve; 13. Circulation pipeline; 14. Temperature transmitter; 15. Salt discharge valve; 16. Temperature controller; 17. Through slot; 18. Fixed frame; 19. Sliding rod; 20. High temperature resistant cleaning cloth; 21. Telescopic assembly; 22. Baffle; 23. Boss; 24. Connecting pipe; 25. Filter; 26. Guide plate. DETAILED DESCRIPTION

[0027] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0028] like Figures 1 to 6 As shown, a multifunctional molten salt corrosion testing system described in an embodiment of the present invention includes a corrosion reactor 1 and a molten salt buffer tank 9. A plurality of fixed slots 6 are arranged in the corrosion reactor 1, and the plurality of fixed slots 6 are installed on the lower guide plate 4. An operation hole 2 is opened on the top of the corrosion reactor 1. A circulation pipe 13 is connected to both ends of the corrosion reactor 1 through a flange 7. The circulation pipe 13 is connected to the molten salt buffer tank 9. An outlet control valve 8 and an inlet control valve 12 are arranged on the circulation pipe 13. A micro-industrial circulation pump 11 is arranged on the circulation pipe 13. A salt discharge valve 15 connected to the inside is opened at the bottom of the corrosion reactor 1.

[0029] Specifically, an electric heater 10 is provided in the molten salt buffer tank 9, a temperature controller 16 for controlling the electric heater 10 is provided in the molten salt buffer tank 9, and a temperature transmitter 14 is installed on the circulation pipe 13. The upper guide plate 3 and the lower guide plate 4 are each provided with a plurality of through holes penetrating through them. The fixed card slot 6 is fixed to the lower guide plate 4 by bolts, and its function is to fix the test piece, which can be a thin sheet for testing uniform corrosion, or a C-shaped ring or U-shaped ring for testing stress corrosion. A visual window 5 is fixedly installed on the top of the corrosion reactor 1, and the visual window 5 is made of high-temperature resistant quartz material.

[0030] During the test, the micro industrial circulation pump 11 transports the high-temperature molten salt medium in the molten salt buffer tank 9 to the corrosion reactor 1, flushing the test piece fixed on the fixed slot 6 in the corrosion reactor 1. The corrosion reactor 1 is the main reaction device of this test system. The corrosion reactor 1 is connected to the external circulation pipeline 13 by a flange 7 for easy disassembly (welding can also be used, depending on the specific situation). There are upper guide plates 3 and lower guide plates 4 inside the corrosion reactor 1. There is an operating hole 2 above the corrosion reactor 1, through which the test piece and the upper guide plate 3 and the lower guide plate 4 can be placed or removed. There is also a visual window 5 on the operating hole 2, which can observe the test piece in the corrosion reactor 1. The bottom of the corrosion reactor 1 is equipped with a salt discharge valve 15, which is used to discharge the salt in the corrosion reactor 1 after the experiment.

[0031] It should be noted that the upper guide plate 3 and the lower guide plate 4 are covered with through holes to increase the disturbance degree of the flowing molten salt. On the one hand, the molten salt above and below the upper guide plate 3 and the lower guide plate 4 can pass freely, and on the other hand, the flowing molten salt from the circulation pipe 13 can be restricted to a certain extent between the upper guide plate 3 and the lower guide plate 4, thereby increasing the disturbance degree of the molten salt.

[0032] The micro industrial circulation pump 11 has a speed regulation function, which can realize the circulation of molten salt in the system, so that the test piece in the corrosion reactor 1 is in a flowing molten salt environment. The speed regulation function of the micro industrial circulation pump 11 can also realize molten salt corrosion test under different flow rate conditions. The temperature controller 16 accurately controls the molten salt temperature of the system according to the temperature measured by the temperature transmitter 14 on the circulation pipeline 13 and the set temperature value, and realizes molten salt corrosion test under different temperature conditions. The inlet control valve 12 and the outlet control valve 8 mainly play the role of controlling the on and off of the circulation system, and the role of isolating and repairing the corrosion reactor 1.

[0033] The system designed in this application can realize molten salt corrosion testing under different medium flow rates, different temperatures, and different stress conditions; realize experimental testing of different corrosion types such as uniform corrosion, stress corrosion, and crevice corrosion; realize corrosion experimental testing in flowing molten salt; and simulate actual application conditions to test materials. This application integrates multiple functions in one, greatly reducing the cost of material selection for use in molten salt media, and has important promotion and application value in the fields of solar thermal power stations, molten salt thermal energy storage technology, and other industrial fields.

[0034] A through slot 17 communicating with the interior is provided on the top of the corrosion reactor 1. A "U"-shaped fixed frame 18 is fixedly installed in the corrosion reactor 1. A high temperature resistant cleaning cloth 20 is arranged in the fixed frame 18. A sliding rod 19 is slidably connected in the through slot 17. One end of the sliding rod 19 is in the fixed frame 18. The high temperature resistant cleaning cloth 20 is sleeved on the surface of the sliding rod 19. Two telescopic components 21 symmetrically distributed with the sliding rod 19 are arranged in the through slot 17. The telescopic components 21 are composed of a plurality of hollow rectangular plates slidably connected. One end of the telescopic component 21 away from the through slot 17 is fixedly connected to the sliding rod 19.

[0035] Specifically, the open end of the fixed frame 18 is rotatably connected to two symmetrically arranged baffles 22 through a torsion spring. Several hemispherical bosses 23 are fixedly installed in the fixed frame 18, and several hollow rectangular plates of the telescopic assembly 21 are sealed and slidably connected. The bosses 23 and the telescopic assembly 21 are connected through a connecting pipe 24, and one end of the connecting pipe 24 away from the telescopic assembly 21 is close to the high temperature resistant cleaning cloth 20. A filter screen 25 is fixedly installed inside the connecting pipe 24, and two symmetrically arranged arc guide plates 26 are fixedly installed at the bottom of the filter screen 25.

[0036] During operation, when the molten salt flows in the corrosion reactor 1 and splashes onto the visual window 5, after a long period of multiple sputtering, there will be molten salt on the visual window 5, and it is difficult for the staff to observe the situation in the corrosion reactor 1 through the visual window 5. By sliding the sliding rod 19 in the through slot 17, the sliding rod 19 slides with the telescopic component 21 and the high temperature resistant cleaning cloth 20 in the fixed frame 18. At this time, the high temperature resistant cleaning cloth 20 leaves the fixed frame 18 and rotates to open the baffle 22. The high temperature resistant cleaning cloth 20 is no longer bound after being separated from the fixed frame 18, so that it returns to its original size, so that the top of the high temperature resistant cleaning cloth 20 abuts against the visual window 5, and the visual window 5 is efficiently wiped and cleaned as the high temperature resistant cleaning cloth 20 moves. The above-mentioned reciprocating operation can quickly clean the visual window 5 without opening the corrosion reactor 1, thereby ensuring the uninterrupted experiment, and at the same time allowing the staff to observe the situation in the corrosion reactor 1 in real time. After the cleaning is completed, the high temperature cleaning cloth 20 is brought into the fixed frame 18 by sliding the sliding rod 19. Before entering the fixed frame 18, the high temperature cleaning cloth 20 will rotate again to open the baffle 22 and then enter the fixed frame 18. The high temperature cleaning cloth 20 will continuously squeeze and collide with several bosses 23 when entering the fixed frame 18, so that the molten salt remaining during the top cleaning will be shaken off and flow back into the corrosion reactor 1 through the gap between the baffle 22 and the fixed frame 18. And as the sliding rod 19 moves with the telescopic component 21, the telescopic component 21 will continuously squeeze the internal gas while shielding impurities and sealing the corrosion reactor 1, and when following the sliding rod 19 to reset and shrink, it will continuously squeeze the internal gas, so that the internal gas is blown into the high temperature cleaning cloth 20 and the fixed frame 18 through the connecting pipe 24, thereby playing an auxiliary cleaning role for the high temperature cleaning cloth 20 and the fixed frame 18, and reducing the molten salt on the surface of the high temperature cleaning cloth 20 and the molten salt in the fixed frame 18. At the same time, the arc-shaped guide plate 26 guides the gas to the surroundings, thereby increasing the cleaning area. At the same time, when the connecting pipe 24 is used to suck air into the telescopic component 21, the filter screen 25 can play a role in blocking the entry of molten salt.

[0037] Working principle: During the test, the micro industrial circulation pump 11 transports the high temperature molten salt medium in the molten salt buffer tank 9 to the corrosion reactor 1, flushing the test piece in the fixed slot 6 fixed on the lower guide plate 4 in the corrosion reactor 1. The corrosion reactor 1 is the main reaction device of this test system, and the corrosion reactor 1 is connected to the external circulation pipeline 13 through the flange 7. There are upper guide plates 3 and lower guide plates 4 inside the corrosion reactor 1, and there is an operation hole 2 above the corrosion reactor 1. The test piece and the upper guide plate 3 and the lower guide plate 4 can be placed or removed through the operation hole 2. There is also a visual window 5 on the operation hole 2, which can observe the test piece in the corrosion reactor 1. The bottom of the corrosion reactor 1 is equipped with a salt discharge valve 15, which is used for the salt discharge work in the corrosion reactor 1 after the experiment. The micro industrial circulation pump 11 has a speed regulation function, which can realize the circulation of molten salt in the system, so that the test piece in the corrosion reactor 1 is in a flowing molten salt environment. The speed regulation function of the micro industrial circulation pump 11 can also realize molten salt corrosion test under different flow rate conditions. The temperature controller 16 accurately controls the molten salt temperature of the system according to the temperature measured by the temperature transmitter 14 on the circulation pipe 13 and the set temperature value, so as to realize the molten salt corrosion test under different temperature conditions. The inlet control valve 12 and the outlet control valve 8 mainly play the role of controlling the on-off of the circulation system and isolating the corrosion reactor 1 for maintenance.

[0038] When the molten salt flows in the corrosion reactor 1 and splashes onto the visual window 5, after a long period of multiple sputtering, there will be molten salt on the visual window 5, and it is difficult for the staff to observe the situation in the corrosion reactor 1 through the visual window 5. By sliding the sliding rod 19 in the through slot 17, the sliding rod 19 slides with the telescopic component 21 and the high temperature resistant cleaning cloth 20 in the fixed frame 18. At this time, the high temperature resistant cleaning cloth 20 leaves the fixed frame 18 and rotates to open the baffle 22. The high temperature resistant cleaning cloth 20 is no longer bound after leaving the fixed frame 18, so it returns to its original size, so that the top of the high temperature resistant cleaning cloth 20 abuts against the visual window 5, and the visual window 5 is efficiently wiped and cleaned as the high temperature resistant cleaning cloth 20 moves. After the cleaning is completed, the high temperature resistant cleaning cloth 20 is brought into the fixed frame 18 by the sliding rod 19. Before entering the fixed frame 18, the high temperature resistant cleaning cloth 20 will rotate to open the baffle 22 again, and then enter the fixed frame 18. When the high temperature cleaning cloth 20 enters the fixed frame 18, it will continuously squeeze and collide with several bosses 23, so that the molten salt remaining during the top cleaning will be shaken off and flow back into the corrosion reactor 1 through the gap between the baffle 22 and the fixed frame 18. And as the sliding rod 19 moves with the telescopic component 21, the telescopic component 21 will not only block impurities and seal the corrosion reactor 1, but also continuously squeeze the internal gas when following the sliding rod 19 to reset and shrink, so that the internal gas is blown into the high temperature cleaning cloth 20 and the fixed frame 18 through the connecting pipe 24, thereby playing an auxiliary cleaning role for the high temperature cleaning cloth 20 and the fixed frame 18, and reducing the molten salt on the surface of the high temperature cleaning cloth 20 and the molten salt in the fixed frame 18. At the same time, the arc-shaped guide plate 26 guides the gas to the surroundings, thereby increasing the cleaning area. At the same time, when the connecting pipe 24 is used to inhale air into the telescopic component 21, the filter 25 can play a role in blocking the entry of molten salt.

[0039] The above-mentioned front, back, left, right, top and bottom are all based on the figures in the specification. Figure 1 As a benchmark, according to the person's observation perspective, the side of the device facing the observer is defined as the front, the left side of the observer is defined as the left, and so on.

[0040] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the scope of protection of the present invention.

[0041] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A multifunctional molten salt corrosion test system, characterized in that: The invention comprises a corrosion reactor (1) and a molten salt buffer tank (9), wherein an upper guide plate (3) and a lower guide plate (4) are installed in the corrosion reactor (1), a plurality of fixed slots (6) are arranged in the corrosion reactor (1), and the plurality of fixed slots (6) are installed on the lower guide plate (4), an operation hole (2) is provided on the top of the corrosion reactor (1), and a circulation pipeline (13) is connected to both ends of the corrosion reactor (1) through flanges (7), the circulation pipeline (13) and the molten salt buffer tank (9) are connected, an outlet control valve (8) and an inlet control valve (12) are arranged on the circulation pipeline (13), a micro industrial circulation pump (11) is arranged on the circulation pipeline (13), and a salt discharge valve (15) connected to the inside is provided at the bottom of the corrosion reactor (1).

2. A multifunctional molten salt corrosion testing system according to claim 1, characterized in that: An electric heater (10) is arranged in the molten salt buffer tank (9), a temperature controller (16) for controlling the electric heater (10) is arranged in the molten salt buffer tank (9), and a temperature transmitter (14) is installed on the circulation pipeline (13).

3. A multifunctional molten salt corrosion testing system according to claim 1, characterized in that: The upper guide plate (3) and the lower guide plate (4) are both provided with a plurality of through holes penetrating through the upper guide plate (3) and the lower guide plate (4).

4. A multifunctional molten salt corrosion testing system according to claim 3, characterized in that: The fixing slot (6) is fixed to the lower guide plate (4) by means of bolts. The fixing slot (6) can fix the test piece by means of bolts and a clamp. The test piece is a thin sheet with a regular shape, or a C-shaped ring or a U-shaped one.

5. A multifunctional molten salt corrosion testing system according to claim 1, characterized in that: A visual window (5) is fixedly mounted on the top of the corrosion reactor (1), and the visual window (5) is made of high-temperature resistant quartz material.

6. A multifunctional molten salt corrosion testing system according to claim 1, characterized in that: The top of the corrosion reactor (1) is provided with a through groove (17) which is connected to the interior. A "U"-shaped fixed frame (18) is fixedly installed in the corrosion reactor (1). A high-temperature resistant cleaning cloth (20) is arranged in the fixed frame (18). A sliding rod (19) is slidably connected in the through groove (17). One end of the sliding rod (19) is located in the fixed frame (18). The high-temperature resistant cleaning cloth (20) is sleeved on the surface of the sliding rod (19).

7. A multifunctional molten salt corrosion testing system according to claim 6, characterized in that: Two telescopic components (21) symmetrically distributed with a sliding rod (19) are arranged in the through slot (17); the telescopic components (21) are composed of a plurality of hollow rectangular plates connected in a sliding manner; and one end of the telescopic components (21) away from the through slot (17) is fixedly connected to the sliding rod (19).

8. A multifunctional molten salt corrosion testing system according to claim 7, characterized in that: The open end of the fixed frame (18) is rotatably connected to two symmetrically arranged baffles (22) via a torsion spring.

9. A multifunctional molten salt corrosion testing system according to claim 8, characterized in that: A plurality of hemispherical bosses (23) are fixedly mounted in the fixed frame (18); a plurality of hollow rectangular plates of the telescopic assembly (21) are sealed and slidably connected; the bosses (23) and the telescopic assembly (21) are connected via a connecting pipe (24); an end of the connecting pipe (24) away from the telescopic assembly (21) is close to the high-temperature resistant cleaning cloth (20).

10. A multifunctional molten salt corrosion testing system according to claim 9, characterized in that: A filter screen (25) is fixedly mounted inside the connecting pipe (24), and two symmetrically arranged arc-shaped guide plates (26) are fixedly mounted at the bottom of the filter screen (25).

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