US3635039A

US3635039A - Vapor traps

Info

Publication number: US3635039A
Application number: US32452A
Authority: US
Inventors: Basil D Power; Roger D Oswald
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd
Priority date: 1969-04-28
Filing date: 1970-04-27
Publication date: 1972-01-18
Anticipated expiration: 1989-01-18
Also published as: DE2020755A1; FR2040345A1; GB1313135A

Abstract

In a refrigerated vapor trap for a vapor vacuum pump, the refrigerated trapping surfaces are arranged to be screened from external heat sources by additional water-cooled surfaces so that should the supply fail of the refrigerating liquid (such as liquid nitrogen) then the trapping surfaces reach a temperature which is still below ambient.

Description

o mted States Patent [15] 3,o35,o39 Power et al. Jan. 18, 1972 [54] VAPOR TRAPS 3,296,810 1/1967 l-lablanian ..62/55.5 [72] Inventors: Bull D. Power, Horsham; Roger D. 2'934257 4/1960 Power 7 3,019,809 2/1962 lpsen f gng'and 3,081,068 3/1963 lnil1eron.. [73] Assignee: The British Oxygen Company Limited, 3,137,551 6/ 6 Mark C 'a Iey England 1 Boyer [22] Filed: 1970 Primary Examiner-William J. Wye 21 Appl 32,452 AttorneyTownshend&Meserole [57] ABSTRACT [30] Foreign Application Priority Data In a refrigerated vapor trap for a vapor vacuum pump, the Apr. 28, 1969 Great Britain ..21602 f i t d trapping Surfaces are arranged to be screened from external heat sources by additional water-cooled sur- [52] US. Cl. ..62/55.5 faces so that should the supply faii of the refrigerating liquid [51] Cl Bold 5/00 (such as liquid nitrogen) then the trapping surfaces reach a [5 8] Fleld oi Search ..62/55.5 temperature which is Stiii below ambient [56] References Cited 11 Clairns Eigures UNITED STATES PATENTS 3 ,321,927 5/1967 Hood 6 2 5. 5

PATENTEUJAIIBIIR 3.635.039

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INVENTOR B19: 1' D.Po s1? AT TORNEYS VAPOR This invention relates to vapor traps, particularly for high vacuum systems employing vapor vacuum pumps.

In order to stop vapor from the vapor vacuum pump passing to the equipment being evacuated, it is known to interpose a refrigerated trap between the pump and the equipment. The refrigerant for high-vacuum systems is usually liquid nitrogen, which is arranged to abstract heat from an array of surfaces in the vapor path between the pump and the equipment. These trapping surfaces reach such a low temperature that any incident vapor particles freeze on to them.

The trap is usually supplied intennittently with liquid nitrogen, as be being topped-up manually. This has the disadvantage that when replenishing of the refrigerant (or, more accurately, cryogenic) liquid is discontinued, the trap tends to warm up towards the local ambient temperature, thus both releasing frozen vapor and failing to trap fresh vapor emitted by the pump.

It is known to attempt to overcome this problem by placing water-cooled baffles in the vapor path, so that the vapor condenses on them when there is no cryogenic liquid in the trap. This has the disadvantage that the trapping surfaces when combined with the baffles impede to an undesirable extent the flow of the fluid being evacuated.

The present invention aims at providing a refrigerated vapor trap in which the trapping surfaces are continuously maintained at a temperature generally below ambient, even in the absence from the trap of a crogenic liquid.

Accordingly the present invention provides a refrigerated vapor trap for a vapor vacuum pump, which is as claimed in the appended claims.

The present invention will now be described by way of example with reference' to the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional view of one form of trap according to the invention;

FIG. 2 is a diagrammatic view of a modified housing, and

FIG. 3 is a diagrammatic view of a further modified housing.

In a known form of refrigerated trap a set of trapping surfaces are in normal contact with a support member having in its two coolant passages. One passage is for liquid nitrogen or other cryogenic (or refrigerant) liquid, and the other is for tap water, or other fluid coolant, of which the temperature is normally below ambient. Before this known trap is operated, the water in the respective coolant passage has to be frozen so that there is no flow of water through the passage, while the cryogenic liquid flows through the other passage to keep the various trapping surfaces associated with it at a very low temperature. This state can be reached only by stopping the flow of water initially and using the liquid nitrogen to freeze the static water. Thereafter a valve or tap in the water passage has to be opened, but there is of course no ensuing flow because of the ice blockage. Thus when the apparatus is working normally, the trapping surfaces act to capture by freezing any vapor particles incident upon them.

When the flow or supply of liquid nitrogen is discontinued and the remaining nitrogen vaporized by head inleak, the temperature of the trapping surfaces starts to rise. This can continue until it reaches such a value that the frozen water in the support member is thawed. The trapping surfaces thereafter remain at about the temperature of the cooling water, which is now free to flow through the support member. Under these conditions the trapping surfaces are acting as water-cooled surfaces on which the vapor can condense and flow back into the vapor vacuum pump.

When fresh nitrogen or other cryogenic liquid is supplied, it is necessary for the flow of water to the support member to be stopped for a time, as the nitrogen cannot abstract enough heat to freeze the water when flowing.

This known arrangement has the disadvantages that the operator has to remember both to close and to open the water valve at the appropriate times; that the cyclic freezing and thawing of the support member, with the concomitant contraction and expansion thereof, require a trap which is more robust and massive than is necessary functionally, and that the trap has to be in a cooling water circuit different from that by which the vapor vacuum pump is cooled.

In the trap of the present invention, the flow of cooling water is continuous, and can be in series with the flow of cooling water to the vacuum pump. There is thus no need for the operator to concern himself with opening and closing the water valve at daily or other intervals, and because the cooling water is never frozen the trap can be of lightweight construction.

The refrigerated trap shown in the accompanying drawing includes an annular reservoir 2 for liquid nitrogen or other cryogenic liquid. The reservoir has a central cylindrical passage d in which is positioned an array of members 6 providing the trapping surfaces. These members 6 are supported at the lower end of the passage 4 by supports 8 of which only two are shown. This support is in good mechanical and thermal contact with the reservoir and members 6, and serve as a heat transfer path between the trapping surfaces and the reservoir 2. The reservoir, trapping surfaces, and supports are of stainless steel of which the outer surfaces are preferably treated, as by being polished, so that they are of low absorptivity so as to reduce heat transference by radiation from the surrounding housing and adjacent heat sources to the reservoir and the cryogenic liquid. In order to reduce undesired heat inleak to the reservoir 2 by conduction the reservoir inlet pipe 12 (which is also made of stainless steel) is relatively long and is thin-walled. If it is not sufficiently strong to support by itself the reservoir when full with liquid nitrogen against the stresses imposed on it in use, then reinforcing ties or struts of low thermal conductivity may extend between the reservoir and the adjacent housing, but these are omitted from the drawing for clarity. Encasing the reservoir 2 is a housing 114 of hollow cross section so that it is close to, but spaced from, the cylindrical outer surface and axially directed end surfaces of the reservoir. The housing 14 has wrapped around it a tubular coil 16, of which the inlet 18 is intended to be connected to a tap, and of which the outlet 20 is intended to be connected to the cooling coil of an associated vapor vacuum pump (not shown) In use, the refrigerated trap (including essentially the reservoir 2 and the housing M) is sealed to the upper end of the pump. The equipment (also not shown) to be evacuated is in fluidtight engagement with the upper surface of the housing 14.. With this arrangement the fluid to be evacuated from the equipment passes downwardly through an opening 22 in the housing 14 (which opening is aligned with the passage 4 in the reservoir 2), passes over the trapping surfaces 6, over a set of inclined louvres 24 in thermal contact with the housing M, and through a lower opening 26 in the housing.

The axes of the louvres 2d are parallel to each other, and the louvres extend across the mouth of the opening 26 and are inclined and positioned so that they prevent the trapping surfaces 6 from receiving heat by radiation except along specified directions. These louvres 24 are in thermal contact with the housing 14, so that they are maintained substantially at the temperature of the cooling water. They thus act as watercooled surfaces on which vapor from the vacuum pump can condense, but their primary function is to act as radiation shields. Thus they impede the flow of fluid to the pump from the equipment to a lesser extent than they would if designed as true baffles. With this arrangement, the trapping surfaces 6 can see only the cooled surfaces of the associated vacuum pump, and are not able to receive heat by radiation directly from the hot jet assembly of the vacuum pump.

In spite of the good thermal insulation of the reservoir and trapping surfaces from the housing, the almost complete encasement of the reservoir and surfaces by the outer housing and screen (kept at the temperature of the cooling water), plus the fact that all interconnecting ducts and supports are kept at cooling water temperature, ensure that, in the absence of cryogenic liquid, the temperature of the reservoir and trapping surfaces tends towards that of the cooling water and not to that of any adjacent heat sources, which might be at significantly higher temperature.

When the apparatus is working, the space between the reservoir 2 and the housing 14 is automatically evacuated by the vapor vacuum pump so that the reservoir is able to take up heat from the housing 14 substantially only by radiation through the space, and by conduction along the inlet pipe 12. The radiation heat inleak is kept to a low figure by causing the respective surfaces of the housing to have low emissivity: this can be done by polishing the surfaces which are preferably of stainless steel.

When the apparatus is working, the continuous flow of tap water through the coil 16 keeps the housing 14 at a temperature which is generally below the local ambient temperature.

Owing to the very small heat transfer from the housing to the reservoir, there is no danger of the water flowing through the coil 16 being frozen. The liquid nitrogen in the reservoir 2 slowly absorbs heat and is vaporized, but as long as there is any liquid nitrogen in the reservoir then the reservoir is substantially at the temperature of liquid nitrogen, i.e., l96 C. When stable operating conditions are reached, the trapping surfaces 6 assume this temperature (or come very close to it), whereas the louvres 26 are substantially at the temperature of the cooling water.

in operation of the vacuum vapor pump, vapor from the operating fluid, e.g., oil or mercury, tends to back-stream towards the equipment being evacuated. In doing so it passes through the louvres 26, which are sufficiently cold for a proportion of the vapor to condense on them and to drop back into the pump. However the remaining portion of the vapor tends to impinge on the trapping surfaces 6 to which the vapor particles immediately freeze, so that the space above and trapping surfaces, and particularly including the evacuated equipment, is virtually free of contamination by the vapor.

When all the liquid nitrogen in the reservoir 2 has vaporized (which can happen overnight or over a weekend when the equipment is not being used) the inleak of heat to the reservoir gradually raises its temperature. The present invention aims at keeping this undesired heat inleak to a very low value. By far the major portion of the surface area of the reservoir and its associated trapping surfaces is able to receive heat only from the housing 14. The remaining surfaces of the reservoir, which in practice means substantially only the inner cylindrical surface forming the passage 4, and the trapping surfaces 6, can receive a certain amount of heat through the

openings

22 and 26, but the solid angles through which they can receive heat from such external heat sources are relatively small. This has the effect that the temperature of the trapping surface 6 in particular can rise only as high as (or very little higher than the temperature of the cooling water, and therefore stays below ambient temperature. If this warming-up of the reservoir and trapping surfaces is allowed to continue, the temperature eventually stabilizes at about that of the cooling water, so that the trapping surfaces 6 come to function more as water-cooled surfaces than as refrigerated surfaces. This resultant decreased efficiency of trapping of the vapor particles is acceptable over the period when the apparatus is not used, and is still sufiicient to prevent gross contamination of the interior of the equipment by condensed vapor.

For some applications of the present invention it might be sufficient for the user not to suppiy cryogenic liquid, but to rely on the trapping surfaces 6 and the louvres 24 to keep the amount of vapor reaching the interior of the equipment at an acceptable value. However for greater efficiency of trapping, it would be necessary to use liquid nitrogen or other cryogenic liquid.

As shown in FIGS. 2 and 3 of the drawing, the solid angle through which heat can be radiated to the reservoir and trapping surfaces can be reduced still further, such as by extending the housing 14 appropriately. In FIG. 3 the housing has a portion 28 extending vertically upwardly to define an elongated opening 22, As shown in FIG. 2, the housing can be provided with a reentrant skirt 30 effectively screening the inner cylindrical surface of the reservoir. Both or similar modifications can be made if desired.

In FIGS. 2 and 3 the trapping surfaces and radiation shields have been removed for clarity.

Other modifications lying within the scope of the present invention can be used. Thus although the water-cooled louvres 24 have been stated as having their axes parallel with each other, they could take the form of a series of concentric frustoconical surfaces. Conversely, although the trapping surfaces 6 are shown as being in the form of such frustoconical members, they could take the form of parallel inclined slats arranged so that there is no linear path through the passage 4 which does not intersect the trapping surfaces, reservoir, or extensions thereof.

It will thus be seen that the trap of the present invention requires no sequential operation of water taps when it is to be used with cryogenic liquid, and that the trap can be of a relatively lightweight construction dictated more by its function than by the necessity to be mechanically robust.

We claim:

1. A refrigerated vapor trap for a vapor vacuum pump, including a reservoir or duct for liquid refrigerant, an internal passage in which is an array of trapping surfaces in thermal contact with the reservoir or duct, a housing encircling and spaced from the reservoir or duct and adapted to be cooled with the fluid coolant to a temperature nearer but below ambient, and additional fluid-cooled surfaces, not encircling the reservoir or duct, positioned on the same side of the trapping surfaces as that on which the vacuum pump is or is to be, positioned whereby, in the absence of the refrigerant liquid, the fluid-cooled housing and additional surfaces screen the trapping surfaces from external heat sources so that the trapping surfaces tend to assume a temperature which is nearer to that of the fluid coolant than to that of the ambient atmosphere.

2. The vapor trap as claimed in claim 1', in which the said additional surfaces consist of, or include, a louvered screen which extends across at least that end of the said passage which is nearer the vacuum pump with which the trap is to be used, so that the screen acts as a radiation shield, the screen being in thermal contact with the fluid coolant.

3. The vapor trap as claimed in claim 2, in which the louvres of the screen have their longitudinal axes extending in parallel with each other.

4. The vapor trap as claimed in claim 1, in which at least some of the trapping surfaces are provided by an array of frustoconical members spaced apart along the axis of the said passage.

5. The vapor trap as claimed in claim 1, in which the housing directly supports the said reservoir.

6. The vapor trap as claimed in claim 1, in which the said reservoir is in the fonn of an annular container having a right cylindrical central passage.

7. The vapor trap as claimed in claim 6, in which the reservoir, trapping surfaces and supports therefore extending between the reservoir and the trapping surfaces, are of stainless steel.

8. The vapor trap as claimed in claim 7, in which the external stainless steel surfaces are polished.

9. The vapor trap as claimed in claim 1, in which the additional surfaces are provided by extensions from the housing which reduce the solid angle through which radiated heat can be received by the reservoir or trapping surfaces from externally of the trap.

10. The vapor trap as claimed in claim 9, in which one additional surface is provided by a reentrant skirt which is spaced inwardly from the surfaces of the passage in the reservoir.

11. The vapor trap as claimed in claim 9, in which one or another additional surface is provided by a cylindrical exten-' sion thereof coaxial with the passage in the reservoir and forming an extension thereof.

Claims

1. A refrigerated vapor trap for a vapor vacuum pump, including a reservoir or duct for liquid refrigerant, an internal passage in which is an array of trapping surfaces in thermal contact with the reservoir or duct, a housing encircling and spaced from the reservoir or duct and adapted to be cooled with the fluid coolant to a temperature nearer but below ambient, and additional fluidcooled surfaces, not encircling the reservoir or duct, positioned on the same side of the trapping surfaces as that on which the vacuum pump is or is to be, positioned whereby, in the absence of the refrigerant liquid, the fluid-cooled housing and additional surfaces screen the trapping surfaces from external heat sources so that the trapping surfaces tend to assume a temperature which is nearer to that of the fluid coolant than to that of the ambient atmosphere.

2. The vapor trap as claimed in claim 1, in which the said additional surfaces consist of, or include, a louvered screen which extends across at least that end of the said passage which is nearer the vacuum pump with which the trap is to be used, so that the screen acts as a radiation shield, the screen being in thermal contact with the fluid coolant.

6. The vapor trap as claimed in claim 1, in which the said reservoir is in the form of an annular container having a right cylindrical central passage.

11. The vapor trap as claimed in claim 9, in which one or another additional surface is provided by a cylindrical extension thereof coaxial with the passage in the reservoir and forming an extension thereof.