US3436965A

US3436965A - Air-purge units for radiation pyrometers

Info

Publication number: US3436965A
Application number: US579078A
Authority: US
Inventors: Thomas Land
Original assignee: Land Pyrometers Ltd
Current assignee: Land Infrared Ltd
Priority date: 1966-08-04
Filing date: 1966-09-13
Publication date: 1969-04-08
Anticipated expiration: 1986-04-08
Also published as: GB1092590A

Description

April 8, 1969 T. LAND 3,436,965

AIRPURGE UNITS FOR RADIATION PYROMETERS Filed sept. 15, 196e sheet of 2 By g waz-LW, C-Jf, M14.

f Attorney5 prl 8, 1969 T, LAND 3,436,965

AIR-PURGE UNITS FOR RADIATION PYROMETERS Filed sept. 15, 196e sheet Z of 2 2; f/ E Z/A F 2 Inventor Attorney U.S. Cl. 73-355 4 Claims ABSTRACT OF THE DISCLOSURE An air-purging unit for a radiation pyrometer which includes a tube disposed about the window end of the pyrometer to :form an extension and an outer wall Iwhich together with the tube forms an annular purging air chamber about the window end of the tube. An outlet for the air in the chamber is provided by an annular diffusing plate and a converging tubular extension which is attached to the outer wall and terminates in a viewing opening for the pyrometer and an outlet for the purging air. The purging air may be admitted directly to the annular chamber, or it may rst be admitted between the tube and the outer wall.

This invention relates to air-purge units for radiation pyrometers of the type having a casing with a transparent window (which may be a lens) near one end, through which radiation from a source of high temperature to be measured is directed to a detector disposed towards the other end.

Since such a pyrometer must have a sight-path free from smoke, dust, and fumes, it has been known to apply a current of air in the neighborhood of the window to discharge the air so as to purge the space in front of the window and thus provide a column of clear air moving axially away from the window. It is however found that the production of the current of air can result in a recirculation from the unclean atmosphere into which the current discharges, so that the window itself can become soiled by deposit of dust and dirt entrained in air moving in the opposite direction to the main discharge of air. Moreover, the purging air may itself contain water droplets or dust, which again may be deposited on the window from such air taking part in the recirculation.

The object of the invention is to provide an air-purging unit for a radiation pyrometer for the use of purging air onto the pyrometer in such a manner as to discourage counterilow of either the atmosphere into which the air is discharged, or the purging air itself, towards the window of the pyrometer.

According to the present invention, an air-purging unit for a radiation pyrometer of the type referred to comprises a tube, an annular chamber surrounding the tube with an inlet for the admission of purging air to the chamber, an annular diffusing plate surrounding one end of the tube and forming a diffusing outlet from the chamber, a tubular extension from the periphery of the diffusing plate and converging to an outlet, coaxial with the tube, which outlet has a cross-sectional area less than the annular area of the diffusing plate, and support means to position a pyrometer to the unit coaxially with the tube and at the end remote from the diffusing plate, so that the tube forms a continuation of the window end of the pyrometer.

Because of the diffusing plate, the purging air emerges coaxially into the tubular extension of the unit from the annular chamber, with a velocity that is uniform across the cross section of the diffusing plate. The air then passes along the tubular extension with a slowly increasing States atent O 3,436,965 Patented Apr. 8, i969 ice velocity, because of the convergence to the smaller area of the tube outlet, as compared lwith the area of the plate. Because of the coaxial entry into the extension, there is no tangential component likely to cause air to swirl in the extension. Accordingly, any tendency to the creation of a low-pressure region at the axis of the mounting, such as would result from the increase of any initial tangential velocity in the convergent flow of the air towards the outlet of the extension, is eliminated. The increase in air velocity along the extension is accompanied by a consequent fall in pressure, so that a positive pressure is maintained by the purging air in the region immediately in front of the pyrometer window within the continuation of the window end formed by the tube. This pressure reinforces the converging liow of purging air in preventing movement towards the window of the atmosphere into Which the purging air discharges from the outlet of the converging extension of the unit into the sight-path of the pyrometer, as lwell as avoiding flow of the purging air towards the window.

The annular chamber may be extended rearwardly by a spaced outer casing of the unit which surrounds the support means for the pyrometer to .form a jacket. An inlet for the air to the unit may therefore be provided at the rear end of the jacket so that purging air may be irst admitted into the space formed by the jacket and the unit before entering the annular chamber. This form of unit is advantageous if the pyrometer itself is located in hot surroundings, because the flow of purging air along the jacket serves to keep it cool.

Various forms `of diffusing plates may be used to give a distributed flow of air from the annular chamber into the tubular extension, each plate producing substantial uniformity of velocity because of the significant pressure drop across the thickness of the plate arising yfrom the air pressure existing in the annular chamber being subdivided by the diffusing passages in the plate. A preferred diffusion plate is made from sintered metal, eg., bronze. Therefore, the closely packed substantially spherical particles of the sintered metal, of about 0.5 mm. in diameter, form multitudinous air passages from which the air emerges, so that the flow at only a few millimetres from the emergent surface is perpendicular to the surface of the plate.

Other features of the invention will appear from the following description of two forms of air-purging unit shown in the accompanying drawings, in which:

FIGURE 1 is a longitudinal axial -section of an airpurging unit provided with a cooling jacket for a pyrometer with a generally tubular casing shown in partial section, supported in it; and

FIGURE 2 is a corresponding section of another form of air-purging unit, and the window end of a pyrometer supported in it, the left-hand end of the unit and pyrometer being omitted.

In FIGURE 1, the pyrometer 1 consists of a cylindrical casing 2 having at its window end a lens 3 and towards the other end a detector 4 in a mounting forming a field stop, a calibrating aperture 5 being disposed between the lens and the detector. A slightly protruding ring 6 at the Window end enables the window end of the pyrometer 1 to be supported in a tube 7, which also supports the rear end 8, where the pyrometer carries an end fitting 9 bearing a connector 10 for the leads (not shown) from the detector 4.

The tube 7 is part of an air-purging chamber formed to enclose the pyrometer 1 and to provide for purging air to be effectively used beyond the window end of the pyrometer. The front end of the tube 7 carries a flange 11 of a tube 12 having a slightly divergent bore and forming a coaxial continuation of the window end of the pyrometer.

The tube '12 is surrounded by an annular chamber 113, at the front of which is an annular diffusing; plate 14. Purging air, to be hereinafter explained, ows from chamber 13, through the plate 14, and into a tubular extension 15, converging from the periphery of the plate 14 to a coaxial outlet 16 of cross-sectional area less than the total annular area of the plate 14. The elements 12, '14, and 15 and the chamber 13 are the essential parts of the air-plugging unit.

The extension has a ange 17 Secured to a ange l of an outer casing 19 of the unit, spaced from the support tube 7 to form a jacket space 20 round the pyrometer 1. At the rear end of the casing 19, an inlet 21 provides for the admission of purging air, at say 25()y mm. water gauge, to the jacket space 20, so that the air cools the pyrometer as it ows towards the chamber 13. Although shown conventionally as entering at right-angles, the inlet 21 may be tangential to the space 20, so that the air swirls around the space, to improve the cooling effect. Interior of the ange 18, the chamber 13 increases to the outer diameter ofthe plate 14, from the lesser outer diameter of the space 20, part of the chamber 13 also being interior of the ilange 17.

A further ange 22 on the extension 15 enables the whole unit to be secured to a plate 23 having a sighting hole 24- through which the purging air issues into the sight path ofthe pyrometer. A lug 25 on the casing `19 provides for alternative or additional supporting of the unit, as by a ball fitting 26.

The purging air entering the chamber 13 from inlet 21 and through the jacket space can only escape through the diiusing plate 14 and into the converging extension 15 as a multitude of streams parallel to the axis of the unit and the pyrometer. The streams are thereafter caused to converge towards the circular outlet 16 of the extension 15. The cross-sectional area of the outlet 16 (say 2000 mm?) is less than the area of the plate 14 (say 300() mm2), so that the converging streams acquire a steady increase of velocity towards the outlet. In this Way, there is a pressure gradient from inside the tube 12 towards the outlet 16. This, together with the absence of tangential swirl on the emergence of the streams of purging air from the plate 14, ensures that there is neither any flow back towards the lens 3 of soiled atmosphere into which the air emerges beyond the hole 24 in the plate 23 nor any flow of the purging air towards the lens. Consequently, the sight-path of the lens is kept clear both inside the extension and in the column blown clear in the atmosphere beyond the hole 24 into which the pyrometer 1 is directed. A slight positive pressure is maintained inside the tube 12, i.e., in the region in front of the lens 3, and this further prevents flow of atmosphere containing water droplets and/or dust towards the lens.

With the dimensions above indicated, which correspond to diameters of about 75 mm. for the plate 14 and 55 mm. for the outlet 16, and with the convergence of the extension 15 to a cone of a 30 included angle, the air velocity across the outlet 16 has been found to be constant within plus and minus 15%, to produce a clear column extending at least 11/2 metres beyond the outlet.

In FIGURE 2, the annular chamber 13 of the air-purging unit is formed in an intermediate section 27 of the unit, with

flanges

28, 29 secured respectively to the flange 17 of the extension and the flange 18 of the casing 19. The section 27 forms an enclosing rear end 30 for the chamber 13, with purging air being admitted directly to the chamber by an inlet 21A. The section 27 also includes an integral tube 12A as an extension of the window end of the pyrometer 1. i

What I claim is:

1. An air-purging unit for a radiation pyrometer of the type having a casing with a window which may be a lens, at one end and a detector toward the other end, the unit comprising:

a tube disposed about the window end of the pyrometer as a coaxial continuation thereof;

an outer wall means surrounding said tube and forming therewith, an annular purging air chamber;

an inlet pipe on said outer wall means for admission of purging air into said chamber;

an annular diffusing plate between said tube and said outer wall means located at the end of said tube thereby forming a diffusing outlet for purging air from said chamber; and

a convergent open-ended tubular extension on said outer wall means with its larger end aiiixed to the periphery of said diffusing plate whereby purging air entering the chamber from said inlet pipe may emerge through said diusing plate and into said convergent tubular extension as a multitude of streams perpendicular to the face of said diliusing plate, and thereafter outlet through the smaller end of said tubular extension thereby insuring that no air flow of soiled atmosphere from outside said tubular extension toward the lens, nor any ow of purging air toward the lens, will interfere with the sight path of the lens.

2. An air-purging unit according to claim 1 wherein said diffusing plate comprises a sntered metal.

3. An air-purging unit according to claim 1 wherein said outer wall means comprises a jacket surrounding the pyrometer casing in spaced relation thereto, and wherein said inlet pipe is located on said jacket so that purging air may be iirst admitted into the space formed by said jacket and the casing before entering said annular chamber.

4. An air-purging unit according to claim 1 wherein said outer wall means comprises a spaced outer wall integral with said tube, and wherein said inlet pipe is located on said spaced outer Wall for admission of purging air directly to said annular chamber.

References Cited UNITED STATES PATENTS 2,074,641 3/1937 Clark 73--355 XR 2,369,624 2/ 1945 Vollrath 73--355 XR 2,497,783 2/1950 Markson 73-355 2,909,908 10/ 1954 Pastahov. 3,345,873 10/'1967 Lellep 73-351 FOREIGN PATENTS 620,766 3/ 1949 Great Britain.

LOUIS R. PRINCE, Primary Examiner.

F. SHOON, Assistant Examiner.