US3603704A

US3603704A - Radiant heat reflection in devices such as getter pumps

Info

Publication number: US3603704A
Application number: US866335A
Authority: US
Inventors: Mario Zucchinelli; Bruno Ferrario
Original assignee: SAES Getters SpA
Current assignee: SAES Getters SpA
Priority date: 1968-10-28
Filing date: 1969-10-14
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07

Abstract

A heat reflector having a number of discrete spaced surfaces which present a substantially continual heat-reflecting surface wherein adjacent surfaces are offset with respect to each other sufficiently to provide for the passage of gas molecules at pressures within the molecular flow region as well as at higher pressures. These reflectors which have a high conductance to gases are especially useful when employed with getter pumps wherein they minimize radiant heat losses and reduce the amount of current necessary to heat the getter material employed in these pumps.

Description

United States Patent Inventors Mario Zucchinelli;

Bruno Ferrurio, both of Milan, Italy Appl. No. 866,335 Filed Oct. I4, I969 Patented Sept. 7, I971 Assignee S. A. E. S. Getters S.p.A. Milan, Italy Priority Oct. 28, 1968 Italy 23038 A/68 RADIANT HEAT REFLECTION IN DEVICES SUCH AS GETTER PUMPS I3 Claims, 6 Drawing Figs.

US. Cl 417/51, 3 1 3/7 Int. Cl F04f 11/00, H01 j 7/16 Field of Search 417/48, 49,

[56] References Cited UNITED STATES PATENTS 2,482,043 9/1949 Veehemans et al 41 7/48 3,214,245 l0/l965 Peters, Jr 4l7/49 X Primary Examiner-Robert M. Walker Attorney-David R. M urphy PATENTEDSEP Tran 3503704 RADIANT HEAT REFLECTION IN DEVICES SUCK-ll AS GET'IER PUMPS Getter pumps employing a substrate coated with a head activable nonevaporable getter material are widely employed to produce and maintain vacuum in dynamic and static systems having pressures down to and including the molecular flow range of pressure. In some systems the getter pump can be placed in a separate envelope which is in fluid communication with the vacuum chamber of the system. In such cases essentially only the envelope is heated while supplying current to the pump. However in other systems the getter pump is generally mounted in the system itself causing two problems. First additional electric power must be supplied to the pump to maintain the getter material at the desired temperature while compensating for heat loss by radiation. Second the radiated heat increases the temperature of parts of the vacuum system causing undesirable gas evolution which reduces the degree of vacuum and can cause other problems. These getter pumps generally comprise a central resistance heater coaxially surrounded by the coated substrate which is frequently pleated. In operation current is passed through the central resistance heater radiating heat to the getter material heating and activating it. While such getter pumps have found wide acceptance they generally require a large amount of power principally due to the loss of heat from the getter pump by radiation especially when the pump is mounted in its nude version. Providing these pumps i.e. the nude ones, with commonly employed thermal insulation is impractical for a number of reasons. First many commonly employed thermal insulators are formed of fibrous particulate materials and are not compatible with advanced vacuum techniques. These insulators are prone to flake off causing problems within the chamber to be evacuated. Second most common insulation materials cannot withstand the high temperatures employed to degas these materials. Other insulation materials which have a low conductance to gas cannot be employed because they substantially reduce the pumping speed of the getter pump. Providing gas passages through such insulation materials generally also provides a path for the escape of radiant heat. The usual thermal reflectors employed in the vacuum art are formed by metal sheets composed of a number of strata where conductance to gas and volume occupied do not present problems. Radiant heat reflectors commonly employed for other purposes have been found to be unsuitable for use with getter pumps for the above and other reasons.

It is therefore an object of the present invention to provide a radiant heat reflector which is substantially free of one or more of the disadvantages of prior reflectors. Another object is to provide a novel combination of a getter pump and a radiant heat reflector.

A further object is to provide a heat reflector having a high conductance to gas at pressures within the molecular flow re gion as well as at atmospheric pressures.

A still further object is to provide a heat reflector constructed entirely of materials resistant to the temperatures commonly employed for degassing.

Additional objects and advantages of the present invention will be apparent by reference to the following detailed description and drawings wherein:

FIG. II is an exploded view of the getter pump of the present invention with a radiant heat reflector wherein the reflector is shown in FIG. la and the remainder of the getter pump is shown in FIG. lb;

FIG. 2 is a top view of the reflector of FIG. la;

FIG. 3 is a sectional view taken along line 3-3 of FIG. la;

FIG. 4 is a sectional view taken along line ll-43 of FIG. 2;

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3. According to the present invention the above and other objects are accomplished by providing a novel heat reflector comprising a plurality of discrete spaced surfaces, the surfaces presenting a substantially continual heat-reflecting surface, adjacent surfaces being offset with respect to each other sufficiently to provide for the passage of gas molecules at pressures within the molecular flow region as well as at higher pressures. Referring now to the drawings and in particular to FIG. la

there is shown in the form of a reflector 10 a nonlimiting embodiment of the present invention. The reflector l0 comprises an upper support Ill and a lower support 12. A plurality of straight strips 13 and offset strips 14 are attached at their ends to the upper support ill and the lower support l2.

As shown in FIGS. 2 and 4, the upper support ll is provided with a plurality of elongated passages l5, large holes l6, and small holes l7 whose function is to further increase the gas conductance of the reflector 10. The upper support ll also has an axially positioned hole 118 by which the reflector R0 is attached to the remainder of the getter pump as described below. As shown in FIG. 4 the upper support Ill is provided with an annular groove 19 adapted to receive the straight strips 113 and the offset strips 114. The upper support ll is also provided with an annular ring 20 adapted to position the upper end of the reflector on the remainder of the getter pump.

As shown in FIG. 5, the lower support 12 is likewise provided with an annular groove 21 adapted to contain the straight strips l3 and the offset strips 14. The lower support 12 has an inner surface 22 adapted to position the lower end of the reflector 10 on the remainder of the getter pump. The lower support 12 can be described as torroidal. As shown in FIGS. 4 and 5, the straight strips 13 are fixedly attached at one end in the annular groove 19 of the upper support 11 and at the other end in the annular groove 21 of the lower support H2. The offset strips 14 have a relatively long straight segment 23 attached to an upper inwardly angled segment 24 and a lower inwardly angled segment 25 which in turn are attached to an upper terminal segment 26 and a lower terminal segment 27. The

terminal segments

26 and 27 as well as the ends of the straight strips 13 are fixedly held in the grooves 19 and M by any convenient means such as spot welding diagrammatically shown in FIG. la as small indentions 28.

Referring now to FIG. llb there is shown a getter cartridge 30 comprising an upper retainer 31 and a lower retainer 32, both of generally cylindrical shape, having a screen 33 also of cylindrical shape fixedly therebetween. Axially disposed within the cartridge 30 is a rod 34 the upper end of which is threaded. Surrounding the rod 34 and within the cartridge 30 is an insulator 35 having wound thereon a wire 36 of high electrical resistance which can be connected to a source of power not shown in order to provide heat for the cartridge 30. Within the cartridge 30 and coaxially disposed around the rod 34 are a series of pleated strips 37 in stacked array coaxially held by means of the screen 33. Attached to the central portion of the strips 37 is a particulate nonevaporable getter material 38 which is heat activatable. The strips 37 are folded back and forth in a pleated manner in order to provide a very high total surface area of getter material 38 for gas sorption.

The getter pump of the present invention comprising the reflector l0 and the cartridge 30 is placed in operation by moving the reflector 10 downwardly until the rod 34 of the cartridge 30 passes through the hole 118 of the upper support lll whereupon the reflector E0 is fixedly held to the cartridge 30 by means of a nut not shown. The annular ring 20 fits snugly against the outside of the upper retainer 31 and the sur' face 22 of the lower support 12 fits snugly against the outside of the lower retainer 32. The wire 36 is heated by connecting it to any source of electrical power. The heat is radiated from the wire 36 to the strips 37 heating and activating the getter material 38, rendering it gas sorptive and creating a very low pressure within the cartridge 30. By virtue of this low pressure gases pass between the strips l3 and M and are sorbed by the getter material 38. When the getter pump is operating in the molecular flow region wherein the path of gas molecules is straight, the gas molecules can reach the getter material 38 by a path between the strips l3 and 14 such as shown by arrow 39.

As shown in FIG. 3 the length, w,, of each straight strip 13 is substantially equal to the width, w, of each offset strip 14 whereas the sum of the widths of all the straight strips 13 and all the offset strips 14 is substantially equal to the circumference of each annular groove 19 or 21. By virtue of this particular geometric arrangement and since the offset strips 14 have inwardly extending

segments

24 and 25, the central segment 23 of the offset strip 14 is a chord of a circle having a radius smaller than the radius of the annular grooves 19 and 21. Since heat is normally radially radiated from the cartridge 30 as shown by

arrows

40 and 41 that radiated heat which just misses the inner surface of the offset strip 14 must impinge and be reflected by the inner surface of the straight strip 13. Since these inner surfaces are rendered highly heat reflective by any convenient means such as polishing, the amount of heat produced by the wire 36 which escapes from the getter pump is greatly reduced. Since the annular ring 20 of the upper support 11 and the inner surface 22 of the lower support 12 hold the reflector l axially on the cartridge 30, points of contact between the reflector and the cartridge 30 are limited to a small portion of the upper reflector support 11 and the upper cartridge retainer 30 as well as the lower reflector support 12 and the lower cartridge retainer 32. Since the portions of the offset strip 14 are maintained out of contact with the cartridge further minimizing the heat loss.

All parts of the reflector 10 including the

supports

11 and 12 and the

strips

13 and 14 as well as the parts of the cartridge 30 are preferably constructed of materials which are both temperature and vacuum compatible and preferably are constructed of metals which can withstand the elevated temperatures such as 350 C. to 800 C. commonly employed in degassing. Examples of suitable metals include iron and stainless steel.

As used herein the molecular flow region refers to that pressure range wherein gas flows under conditions such that the largest internal dimension of a transverse section of the vessel is smaller than the mean free path of the molecules of the gas. In the molecular flow region the rate of flow of gas is limited not by collisions between molecules but by collisions of molecules with the walls of the vessel. For vessels commonly employed as vacuum tubes having a largest internal dimension of about 1 to 50 cm. the molecular flow region is generally from just above zero up to about 10 torr. As used herein the conductance (F) of the reflector 10 in the molecular flow region for a given gas is the ratio of throughput of gas (Q) to the partial pressure difference across the reflector 10 (P P,) in the steady state. it is measured in liters per second, and given by F=Q/(P P,); where P is the upstream pressure, and P is the downstream pressure. 7

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described above and as defined in the appended claims.

What is claimed is:

1. A heat reflector having a high conductance to gases comprising a plurality of discrete spaced surfaces, the surfaces gases comprising a plurality of strips having a heat reflective inner surface said strips being generally circularly disposed on at least two circles of different radii; adjacent strips being on different circles; the inner surfaces of all strips being substantially lperpendicular to the radii.

4. he reflector of claim 3 wherein the ends of all strips are aligned on the circle oflargest radius.

5. The reflector of claim 4 wherein the sum of the widths of the strips is equal to the circumference of the circle of the largest radius.

6. A cylindrical, radiant-heat reflector having a high conductance to gases comprising a plurality of planar strips having a heat reflective inner surface, said strips being circularly disposed as chords on two circles of different radii; alternate strips being on alternate circles; the heat-reflective surfaces of each strip being perpendicular to the radii.

7. A radiant heat reflector having a high conductance to gases comprising:

A. a first support;

B. a second support;

C. a series of straight strips fixedly attached at one end to the first support and at the other end to the second sup- P D. a series of offset strips fixedly attached at one end to the first support and at the other end to the second support; the offset strips being disposed between adjacent straight strips, whereby the straight strips and the offset strips present a continual heat-reflecting surface while permitting the passage ofgas therebetween. 8. A cylindrical, radiant heat reflector ofclaim 7 wherein adjacent straight strips are spaced from one another and have an inner heat reflective surface;

wherein the sides of the ends of the offset strips contact adjacent straight strips; each strip comprising two terminal segments each attached to two inwardly angled segments attached to a straight central segment leaving an inner heat reflective surface.

9. A getter pump comprising a substrate coated with a nonevaporable getter material, means for heating the nonevaporable getter material and means for reflecting radiant heat.

10. A getter pump of claim 9 wherein the means for reflecting radiant heat comprises a heat reflector having a high conductance to gases comprising a plurality of discrete spaced presenting a substantially continual heat-reflecting surface,

surfaces, the surfaces presenting a substantially continual heat reflecting surface, adjacent edges of adjacent surfaces being offset with respect to one another sufficiently to allow the passage of gases in a straight path between adjacent surfaces.

11. A getter pump comprising a circular pleated substrate coated with a nonevaporable getter material, means for heating the nonevaporable getter material and means for reflecting radiated heat said heat reflecting means substantially surrounding the circular pleated substrate.

12 A getter pump comprising a substrate coated with a nonevaporable getter material, means for heating the nonevaporable getter material and a heat reflector having a high conductance to gases comprising a plurality of discrete spaced surfaces, the surfaces presenting a substantially continual heat-reflecting surface, adjacent surfaces being offset with respect to each other sufficiently to allow the passage of gas.

13. A getter pump comprising a substrate coated with a nonevaporable getter material, means for heating the nonevaporable getter material and a radiant heat reflector having a high conductance to gases comprising a plurality of strips having a heat reflective inner surface said strips being generally circularly disposed on at least two circles of different radii; adjacent strips being on different circles, the inner surfaces of all strips being substantially perpendicular to the radii.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 603 704 D8td Sgpt 7 I 1 971 Inventor(s) Mario Zucchinelli. Bruno Eerrar'io It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line l delete "head", insert heat Col. 3, 'line 46 delete "10 insert "10' Signed and sealed this I 8th day of January 1 972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents DRM USCOMM-DC some-Pe a U 5 GOVERNMENT PRINTING CFFICE 969 0-366-134

Claims

1. A heat reflector having a high conductance to gases comprising a plurality of discrete spaced surfaces, the surfaces presenting a substantially continual heat-reflecting surface, adjacent surfaces being offset with respect to each other sufficiently to allow the passage of gas.

2. A heat reflector having a high conductance to gases comprising a plurality of discrete spaced surfaces, the surfaces presenting a substantially continual heat-reflecting surface, adjacent edges of adjacent surfaces being offset with respect to one another sufficiently to allow the passage of gas in a straight path between adjacent surfaces.

3. A radiant heat reflector having a high conductance to gases comprising a plurality of strips having a heat reflective inner surface said strips being generally circularly disposed on at least two circles of different radii; adjacent strips being on different circles; the inner surfaces of all strips being substantially perpendicular to the radii.

4. The reflector of claim 3 wherein the ends of all strips are aligned on the circle of largest radius.

7. A radiant heat reflector having a high conductance to gases comprising: A. a first support; B. a second support; C. a series of straight strips fixedly attached at one end to the first support and at the other end to the second support; D. a series of offset strips fixedly attached at one end to the first support and at the other end to the second support; the offset strips being disposed between adjacent straight strips, whereby the straight strips and the offset strips present a continual heat-reflecting surface while permitting the passage of gas therebetween.

8. A cylindrical, radiant heat reflector of claim 7 wherein adjacent straight strips are spaced from one another and have an inner heat reflective surface; wherein the sides of the ends of the offset strips contact adjacent straight strips; each strip comprising two terminal segments each attached to two inwardly angled segments attached to a straight central segment leaving an inner heat reflective surface.

10. A getter pump of claim 9 wherein the means for reflecting radiant heat comprises a heat reflector having a high conductance to gases comprising a plurality of discretE spaced surfaces, the surfaces presenting a substantially continual heat reflecting surface, adjacent edges of adjacent surfaces being offset with respect to one another sufficiently to allow the passage of gases in a straight path between adjacent surfaces.

12. A getter pump comprising a substrate coated with a nonevaporable getter material, means for heating the nonevaporable getter material and a heat reflector having a high conductance to gases comprising a plurality of discrete spaced surfaces, the surfaces presenting a substantially continual heat-reflecting surface, adjacent surfaces being offset with respect to each other sufficiently to allow the passage of gas.