Cryogenic Gas Trap

Abstract

The invention is concerned with cryogenic traps having a large trapping range. They comprise a reservoir (8) for a cryogenic liquid, and connected at the upper part thereof, a tube (11). The tube (11) is wound from the bottom to the top at 11a about a metallic hood (12) coaxial to the reservoir (8), then at 11b above said hood (12). These members are placed altogether in a housing (30) comprising at level 11b, an input pipe (4) for a gaseous flow, and at the upper level of the reservoir (8), an evacuation opening (5), the edges of which are in contact with a tube (13) linking the space comprised between the hood (12) and the reservoir (8) with the exterior of the housing (30).

Description

The present invention relates to cryogenic gas traps on the surface of which the gases to be eliminated from a chamber or from a gaseous mixture in circulation are condensed, the said surface being cooled by a boiling cryogenic liquid.

A cryogenic gas trap was described in French Pat. No. 1,547,694,wherein is provided, in the direction of gaseous flow from which it is desired to entrap certain constituents, an upstream exchanger comprising one metal tube and a hood or metal skirt in thermal contact with the said tube, a downstream exchanger comprising a reservoir intended to receive a cryogenic liquid, arranged within the hood, the downstream end of the said tube in the direction of gaseous flow communicating with the upper part of the said reservoir.

The traps described in French Pat. No. 1,547,694 are particularly suitable for lyophilization of aqueous and non-aqueous solutions. For these applications, the trap actually acts as a pump and, after use, it is, in practice, plugged by the ice formed or any other solid substance.

On the other hand, in conventional vacuum applications such as industrial metallurgy, metallization and vacuum distillation plants, it is necessary for the trap to be traversed permanently by the gaseous flow by causing the gases which cross it to lap the cold surfaces in a manner to deposit thereon the substances to be entrapped. It is necessary, moreover, for the trap to preserve good conductance compatible with the desired vacuum. For all these applications, the traps described in the aforementioned patent required certain adaptations.

The traps used in the field of industrial vacuum are usually formed by coils or plates positioned directly in the pumping channel or in the working chamber. They are cooled by a mechanical refrigerator or by the circulation of liquid nitrogen. For installations of small size, these traps may have the shape of a cylindrical tank filled with a coolant about which circulates the gas to be pumped.

All these traps are limited in efficiency, have a small capacity and have a very restricted trapping range. Their cooling performance is usually defective. When they use a refrigerating unit, they require a certain maintenance and are of limited life span.

One object of the invention is to improve the performances of cryogenic traps in order to render them particularly usable and advantageous in the field of industrial vacuum technology.

Another object of the invention is to improve the circulation of gases in said traps so as to obtain a large capacity, progressive trapping and a good cooling performance.

The cryogenic traps in accordance with the present invention are characterized in that the metal wall of the hood is impervious to the gaseous flow and in that the upstream and downstream exchangers are contained entirely within a housing comprising a first opening enabling the introduction of the gaseous flow, situated at a level comprised between the upper level of the hood and the top of said housing and a second opening enabling the evacuation of the gaseous flow, communicating solely with the space between the reservoir and the hood at a location situated at the upper level of the reservoir.

Preferably, a third exchanger precedes the upstream exchanger, in the direction of gaseous flow, and is situated above the metal hood.

The cryogenic traps according to the invention have a large trapping range due to the use of a plurality of exchangers, the temperature zones of which are between the boiling point of the cryogenic liquid and the ambient temperature.

With these traps, moreover, any entraining of the condensates by the gaseous flow evacuated from the trap is avoided.

The cryogenic traps of the invention are capable of operating automatically continuously for long periods of time. They are reliable for they use no moving member such as the compressor of a mechanical refrigerator. They are easy to use, do not require any particular maintenance and have a good life span. They offer, moreover, a good cooling performance.

FIG. 1 shows a view in section along A--A of FIG. 2 of a trap according to the invention provided with various accessories.

FIG. 2 shows a view in section along B--B of FIG. 1 of this same trap.

The description which follows with reference to the drawing does not restrict the scope of the present invention.

A cryogenic trap according to the invention is located in a housing 30, within which circulates the gaseous mixture to be treated. This housing is a cylindrical receptacle provided with a convex lower end. This housing is formed primarily by a metal enclosure 1. It is coated externally with a heat insulant 2. The housing 30 is closed at its top by a circular plate 3 by means of bolts (not shown).

It is provided with an opening enabling the gaseous mixture to be introduced, formed by an input pipe 4 provided with a flange. It is also provided with another opening 5 enabling the gaseous mixture to be evacuated. The pipe 4 is arranged at a tangent with respect to the side wall of the housing. This enables a whirling movement to be imparted to the gaseous mixture introduced.

The housing 30 is also provided at its lower end with an orifice 6 for evacuating the condensate accumulated during the trapping action. This orifice is closed during operation of the trap by a disc 7.

The actual cryogenic trap is formed by three exchangers which trap the gases to be eliminated from the gaseous mixtures to be treated at lower and lower temperature levels.

The coldest exchanger is formed by a reservoir or vessel 8 intended to receive a cryogenic liquid by means of a pipe 9. The pipe 9 passes through the circular plate 3 in sealed manner and is closed by a joint 10.

The intermediate temperature exchanger 11a is formed by the tube 11 connected with the vessel 8 at its upper end and wound from the bottom to the top in a coil about a metal skirt 12, coaxial to the vessel 8, to which skirt the tube 11 is welded to improve the thermal contact. This skirt 12 has substantially the shape of a cylinder surmounted by a cone through the apex of which passes the pipe 9 in sealed fashion.

The metal skirt 12 is impervious to the gaseous flow and is thus similar to a hood. The metal skirt 12 is, moreover, provided with a pipe 13 placing the space between the vessel 8 and the skirt 12 in direct communication with the exterior of the housing 30. This pipe 13, provided with a flange, thus passes through the opening 5 of the housing which it closes in sealed fashion. The axis of this pipe is radial with respect to the metal enclosure 1.

The hottest exchanger 11b is formed by an extension of the tube 11 wound from the bottom to the top in a coil coaxially to the exchanger 11a, above the metal skirt 12. The tube 11 emerges from the housing at 14 after having passed through the circular plate 3.

The cryogenic trap according to the drawing also comprises means for introducing a warm fluid, enabling the condensates to be liquified or vaporized, so that they may be evacuated from the trap. It is usually a sprinkler 15 arranged above the exchanger 11b and about the pipe 9. It is supplied with warm water or steam by means of a valve on a tube 16 passing through the circular plate 3 in sealed fashion.

The cryogenic liquid is contained in a supply reservoir 17. A transfer pipe 18 enables the liquid to be introduced into the vessel 8 after passing through the joint 10 in sealed fashion.

A control device shown diagrammatically at 19, enables the level of cryogenic liquid to be regulated between two points in the vessel 8. To this end, the signal supplied by a level probe 20, formed by two heat sensitive members 21 and 22 operates an electro-valve 23 on the transfer pipe 18.

The metal skirt 12 and the tube 11 are made from a metal which is a good conductor of heat, such as copper. In the case of a corrosive gaseous flow, the entrapping surfaces can be coated with an adequate coating which is electrolytic or plastic.

The trap operates like a counter-current heat exchanger. During operation thereof the vessel 8 is supplied with cryogenic liquid from the reservoir 17 through the transfer pipe 18. The level of cryogenic liquid is regulated between two points by the assembly 19, 20, 21, 22, 23.

The cryogenic liquid vaporizes in the vessel and thus maintains its walls at the lowest temperature level. The vapors produced cool the part 11a of the tube 11 to an intermediate temperature level. Then the vapors progress into the part 11b of the tube 11. They cool this part of the tube to the highest temperature level. The vapors then leave the tube 11 at 14.

The gaseous mixture from which it is desired to entrap certain constituents, enters the housing through the tube 4. The gaseous flow is thus given a whirling movement which prolongs its travel within the trap. It effects pre-cooling on the third exchanger 11b . Certain gases can already be entrapped at the highest temperature level. During its descent between the enclosure 1 and the skirt 12, the gaseous flow leaves on the upstream exchanger 11a the substances which can be entrapped at the intermediate temperature level. Then the gaseous flow encounters the base of the metal enclosure 1, makes a half turn and penetrates between the skirt 12 and the vessel 8. The gases which can condense at the lowest temperature level are entrapped on the downstream exchanger 8. The gaseous flow emerges from the trap by means of the pipe 13.

The trap thus constitutes a multi-stage condenser and the nature of the condensate varies according to the location in question and the time. This arrangement enables optimum use of the coldness of the cryogenic liquid, thus a good cooling performance of the trap.

The thickness of the layer of the condensed substances increases progressively with the operation of this trap, together with the residual pressure of these substances at the outlet from the trap. The increase in thickness of the condensate causes a decrease in conductance of the trap. The limit values allowed for conductances and residual pressure of the entrapped substances determine the saturation time of the trap. At the end of this time the trap is isolated, the evacuation orifice 6 opened and warm water or vapor is injected through the sprinkler 15. The condensate melts or vaporizes and escapes through the orifice 6. The trap can then be put back into operation.

By way of example, with a trap according to FIG. 1 supplied with liquid nitrogen, the following temperature levels were obtained for the different exchangers :

Exchanger 11b : average temperature of - 30.degree.C ;

Exchanger 11a : temperature between - 50.degree.C and - 70.degree.C, enabling, for example, water vapor to be entrapped.

Exchanger 8 : temperature of - 196.degree.C, enabling the substances having a higher vapor pressure to be entrapped.

The dimensions of such a trap are as follows :

overall length : 1m. 20

overall width : 0m. 50

diameter of the housing : 0m. 30

internal diameter of the tubes 4 and 13 : 10 cm.

Such a trap has a water capacity of 10 kg when stoppered. It enables all the substances having a vapor pressure greater than or equal to that of hydrochloric acid gas, and more particularly ammonia, water vapor, chlorine, etc., to be entrapped for a residual tolerated output pressure of 10.sup.-.sup.4 mm of mercury.

The amount of liquid nitrogen used in this trap depends on the condensation and its inherent losses. The losses due to condensation are equivalent to 12.5 liters of liquid nitrogen per kg condensed water. The consumption due to inherent losses of the trap depends upon the pressure in the trap and, the trap itself being insulated, reach a value :

0.75 l/h for a pressure greater than 0.3 mm of mercury

0.5 l/h for a pressure equal to 0.01 mm of mercury

0.25 l/h for a pressure equal to 0.003 mm of mercury.

To these amounts must be added those due to cooling of the gaseous flow when the trap is operating at pressures greater than 0.3 mm of mercury and the losses in the transfer pipe.

The trap, the capabilities of which have just been explained, has been used for example in an iron works where zirconium and titanium are worked by melting under vacuum in an arc furnace. By placing the trap in the pumping circuit just before the primary pumping unit, the second opening of the trap communicating with the input of a primary pumping apparatus, rapid deterioration of the primary pump and degradation of the properties of the lubricating oil due to the presence in the gaseous flow of chloride and hydrochloric acid vapors are avoided.

The cryogenic traps described may be used to advantage in the field of industrial vacuum technology, vacuum metallurgy, metallization vacuum distillation and evaporation.

Claims

What I claim is:

1. A cryogenic gas trap comprising, in the direction of gaseous flow from which it is desired to entrap certain constituents, an upstream exchanger comprising one metal tube and a metal hood in thermal contact with the said tube, a downstream exchanger comprising a reservoir intended to receive a, cryogenic liquid, arranged within the hood, the downstream end of the said tube in the direction of gaseous flow outside the tube communicating with the upper part of the said reservoir, the wall of the metal hood being impervious to the gaseous flow and the upstream and downstream exchangers being contained entirely within a housing comprising a first opening enabling the introduction of the gaseous flow, situated at a level comprised between the upper level of the hood and the top of said housing, a second opening enabling the evacuation of the gaseous flow, communicating solely with the space between the reservoir and the hood at a location situated at the upper level of the reservoir, the path of said gaseous flow being from said first opening past the outside of said metal hood and then into said hood past the outside of said reservoir and then out said second opening.

2. A trap according to claim 1, wherein a third exchanger precedes the upstream exchanger in the direction of gaseous flow and is situated above the metal hood.

3. A trap according to claim 2, wherein the third exchanger comprises at least one other tube wound in a coil, one end of which communicates with the upstream end of the said tube in the direction of the gaseous flow.

4. A trap according to claim 1, wherein the first opening comprises a tube, the axis of which is at a tangent with respect to the wall of the housing.

5. A trap according to claim 1, wherein the edges of the second opening are in contact with a pipe linking the said space and the exterior of the housing.

6. A trap according to claim 1, wherein is provided an orifice for evacuating the condensates and means for injecting a warm fluid within the housing.