METHOD FOR DETERMINING THE ABSOLUTE HUMIDITY AND APPARATUS FOR CARRYING OUT THE METHOD

Abstract

A method and a device for determining the absolute humidity of a gas sample of a process gas are disclosed. The process gas is filtered for purification, before the gas sample obtained in this way is directed via a chilled mirror hygrometer. The process gas is withdrawn via an inner tube, which is located in a filter cartridge. The filter cartridge projects into a gas flow of the process gas. The inner tube has openings via which a gas sample that is representative over the length of the filter cartridge can be obtained and which allows for an even backwashing of the filter cartridge.

Claims

1-15. (canceled)

16. A method, comprising: conducting a process gas through a filter with a filter cartridge for purification to obtain a purified gas sample; withdrawing the process gas via an inner tube located in the filter cartridge, with the filter cartridge projecting into a gas flow of the process gas; conducting the purified gas sample via a chilled mirror hygrometer for determining an absolute humidity of the gas sample of the process gas; connecting a backwash device to the inner tube; introducing a purge gas into the inner tube for backwashing in order to remove particle deposits from an outer side of the filter cartridge; and conducting the purge gas via a plurality of openings, arranged on a circumference of the inner tube, into an annular gap between an outer side of the inner tube and an inner side of the filter cartridge, with the openings being arranged at an opening spacing from an adjacent opening in a longitudinal direction, wherein the opening spacing is smaller at a first end of the inner tube than at a second end of the inner tube, so that more purge gas is conducted out of the inner tube in a region of the first end than in a region of the second end of the inner tube, with the first end projecting deeper into the gas flow of the process gas than the second end.

17. The method of claim 16, wherein the plurality of openings are arranged over a length of the inner tube, via which openings the gas sample filtered by the filter cartridge enters into a longitudinal channel of the inner tube and from there is fed to the chilled mirror hygrometer.

18. The method of claim 16, wherein the purge gas is conducted from the inner tube against an end-side closure body of the filter cartridge, with end-side openings of the plurality of openings in the inner tube adjoining the closure body, so that part of the purge gas is deflected from the closure body into the end-side openings.

19. The method of claim 16, wherein, when the process gas is withdrawn for humidity determination, the purified gas sample enters the inner tube via same ones of the plurality of openings as the purge gas from the inner tube, so that the gas sample is withdrawn via a representative cross-section of the gas flow in which the filter cartridge is located.

20. The method of claim 18, wherein the inner tube is located in a slot filter as a filter jacket, the method further comprising attaching the inner tube and the filter jacket to the end-side closure body and to an end cap, with the end cap including an opening and an adapter tube connected to the opening of the end cap, with the purge gas being introduced via the adapter tube and the opening of the end cap into the opposite inner tube.

21. Apparatus for determining the absolute humidity, the apparatus comprising: a filter cartridge designed to filter a process gas to obtain a gas sample; a chilled mirror hygrometer connected to a withdrawal point for the gas sample from a gas flow of the process gas; and an inner tube located in the filter cartridge and designed to receive the gas sample via an annular gap between a filter jacket of the filter cartridge and the inner tube and to conduct the gas sample to the chilled mirror hygrometer, said inner tube including a plurality of openings distributed over a length of the inner tube for introducing the gas sample into a longitudinal channel of the inner tube, wherein the openings are each arranged at an opening spacing from an adjacent one of the openings in a longitudinal direction of the inner tube, with the opening spacing at a first end of the inner tube being smaller than at a second end of the inner tube.

22. The apparatus of claim 21, further comprising a backwash device connected to the inner tube and designed to conduct a purge gas into the inner tube in order to remove particle deposits from an outer side of the filter cartridge.

23. The apparatus of claim 21, wherein openings of the plurality of openings of the inner tube in adjacent relationship to the first end adjoin an end-side closure body of the filter cartridge.

24. The apparatus of claim 22, further comprising: a backwash line connected to the inner tube; and a shut-off valve arranged in a gas sample line and being closed when the backwash line is open in order to protect the dew point hygrometer.

25. The apparatus of claim 24, further comprising a control unit operably connected to the backwash device and designed to start the backwash device in dependence on a trigger signal and to control the shut-off valve in such a way that only the filter cartridge is purged.

Description

[0027] It is shown in:

[0028] FIG. 1 a side view of a filter cartridge;

[0029] FIG. 2 a longitudinal section of the filter cartridge of FIG. 1;

[0030] FIG. 3a perspective view partially in section of a further embodiment of a filter cartridge;

[0031] FIG. 4 a schematic illustration of a depth filter;

[0032] FIG. 5 a schematic illustration of a surface filter;

[0033] FIG. 6 a schematic illustration of an apparatus for determining the absolute humidity.

[0034] The invention shows a filter cartridge 1 with a cylindrical cross-section with the diameter A of, e.g., 50 to 70 mm, in particular 60 mm. The lower end in the image plane is the first end 2 of the filter cartridge 1. It projects into a gas flow which is not shown in greater detail. The upper end in the image plane is the second end 3. It is attached to a wall of a flow channel. The filter cartridge 1 is connected to a chilled mirror hygrometer (not shown in greater detail) via an adapter tube 4 in fluid-conducting manner for a gas.

[0035] The adapter tube 4 has a smaller diameter than the filter cartridge 1. The adapter tube 4 is shorter than the filter cartridge 1. It is attached to a circular disk-shaped end cap 5. The other end 2, which projects freely into the gas flow, also has a disk-shaped closure body 6, so that the filter cartridge 1 has an overall cylindrical cross-section. The internal structure is explained with reference to FIG. 2.

[0036] Located in the filter cartridge 1 is an inner tube 7 with several openings 8 which are spaced apart from one another in longitudinal direction and distributed about the circumference and which are arranged at an opening spacing E relative to one another. In this exemplary embodiment, the cylindrical filter cartridge 1 has a length of 240 to 260 mm, with 10 openings being distributed over the length. The diameter of the openings in this exemplary embodiment example is 8 mm. There are four straight rows of openings 8, which are offset by 90 relative to each other in circumferential direction of the filter cartridge 1. From the illustration in FIG. 2, it can be seen that the opening spacing E between the two openings 8 shown at the second end 3 is greater than at the first end 2 of the filter cartridge 1.

[0037] The openings 8 at the first end 2 are directly adjacent to the closure body 6, so that a gas that is introduced into the inner tube 7 can escape directly adjacent to the closure body 6. The outgoing gas, the so-called purge gas, can enter an annular gap 9. The annular gap has a cylindrical cross-section, as the filter jacket 10, which surrounds the cylindrical filter cartridge 7, is also cylindrical. The filter jacket is connected to the closure body 6 and the end cap 5. The filter jacket 10 involves a slot filter that functions like a depth filter. The upper end cap 5 is also connected to the closure body 6 via the inner tube 7. The inner tube 7 has an outer diameter F that is greater than the diameter of the adapter tube 4. A width of the annular gap is approximately 10% of the diameter of the filter jacket 10. In a preferred exemplary embodiment, the filter jacket has a length of 245 mm and a diameter of 60 mm. The inner tube 7 is slightly shorter than the filter jacket 10 because the closure body 6 and the end cap 5 are slightly stepped radially inwards and therefore assume a shorter distance between them. Located in the end cap 5 at the second end is an opening 11 into which the adapter tube 4 is inserted.

[0038] The perspective view according to FIG. 3 clearly shows that the openings 8 are respectively arranged offset by 90 about the circumference and are arranged in several longitudinal sections spaced apart in longitudinal direction, with four openings of the same diameter D being arranged evenly about the circumference in each longitudinal section. In the exemplary embodiment of FIG. 3, the opening spacing E of all openings is identical in longitudinal direction. However, the nearest openings 8 adjacent to the second end 3 are arranged at a greater distance from the end cap 5 than the openings 8 in the region of the closure body 6.

[0039] The inner tube 7 fulfills two functions. On the one hand, it is used to receive filtered process gas, which is fed to the chilled mirror hygrometer as purified gas sample. The purified process gas first passes through the filter jacket 10 into the annular gap 9 and from there via the openings 8 into a central longitudinal channel 12 of the inner tube 7. The gas sample is conducted via the opening 11 and the adapter tube 4 via a gas sample line to the chilled mirror hygrometer. The arrangement of the openings and the length of the filter cartridge 1 make it possible to take a representative gas sample across the cross-section of the flow of the process gas, of which the absolute humidity is to be determined. The process gas is usually loaded with particles, for example in the form of a powder. The filter jacket 10 is in particular . . .

[0040] a gap filter and as such a so-called depth filter (FIG. 4). In a depth filter, the process gas 13 loaded with powder reaches the filter jacket 10 in the form of a gap filter. On the opposite side, the process gas emerges cleaned of particles. This involves the purified gas sample 14, which is fed to the longitudinal channel 12 of the inner tube 7 for further examination.

[0041] The separation effect of the depth filter is based, i.a., on the blocking effect. The blocking effect is based on the fact that movements of the center of mass of a particle to be separated pass a filter surface, with the particle still impacting the filter surface as a result of its geometric expansion and adheres through adhesion. As the particle diameter increases, the probability of contact between the particle and the filter surface increases. Ultrafine particles are mainly separated by diffusion, while particles with a diameter greater than 0.5 m are dominated by the blocking effect and inertia. In contrast to surface filtration (FIG. 5), the particles are separated solely by the filter medium. By narrowing the flow channels as a result of particle separation, the separation effect is improved with a simultaneous increase in pressure loss. The deposition of particles on the surface of the filter medium in the form of a filter cake is undesirable in depth filtration, as this makes it more difficult for subsequent particles to enter the filter medium and can lead to locally increased pressure losses. When using slot filters, it is therefore particularly important to clean the filters regularly or to backwash them in accordance with the invention. By designing the inner tube or the size of the annular space and the arrangement of the openings to suit the requirements, defined pressure ratios can be adjusted in the annular gap 9 in order to effectively remove particle deposits using one or more pressure surges with purge gas.

[0042] FIG. 6 shows a possible application for such a filter cartridge. A spray drying facility is shown purely schematically. A liquid to be dried, for example, milk powder, is introduced into a drying tower 15 and dried by a process gas 13 in the form of hot air. The milk powder is subsequently dried further in a fluidized bed 16.

[0043] Hot air 13 is also supplied as process gas for this purpose. Provision is made in the Invention for a measuring point M in immediate vicinity of the drying tower 15. The filter cartridge is arranged here, as shown in FIGS. 1 to 3. Via a heated gas sample line 17, the gas sample is fed to a chilled mirror hygrometer 18, shown purely schematically, by which the absolute humidity of the gas sample 14 is determined. The measuring process is controlled by a control unit 19. This control unit also serves as a backwash of the filter cartridge 1 by guiding air as purge gas through the adapter tube 4 into the inner tube 7 and via the annular gap 9 to the inside of the filter jacket 10. Particle deposits on the outside are thereby loosened from the filter jacket 10 and the filter jacket is cleaned. Backwashing takes place in dependence on a trigger signal, for example time-controlled.

Reference Sign

[0044] 1filter cartridge [0045] 2first end of 1 [0046] 3second end of 1 [0047] 4adapter tube [0048] 5end cap [0049] 6closure body [0050] 7inner tube [0051] 8opening in 7 [0052] 9annular gap [0053] 10filter jacket [0054] 11opening in 5 [0055] 12longitudinal channel from 7 [0056] 13process gas [0057] 14gas sample [0058] 15drying tower [0059] 16fluidized bed [0060] 17gas sample line [0061] 18chilled mirror hygrometer [0062] 19control unit [0063] Adiameter of 1 [0064] Blength of 10 [0065] Eopening spacing [0066] Fdiameter from 7