Method Of And Apparatus For Pumping Gas Under Cryogenic Conditions

Abstract

In pumping a gas under cryogenic conditions, condensation surfaces disposed within a receiver containing the gas to be pumped are cooled to a predetermined temperature while the surfaces are thermally insulated from any heat within the receiver. During initial cooling the gas is not permitted to flow over the condensation surfaces. When the predetermined temperature is reached, the gas within the receiver, which is under vacuum conditions, is circulated over the condensation surfaces. The amount of gas flowing over the condensation surfaces is regulated in accordance with the temperature, vacuum and other conditions within the receiver.

Description

SUMMARY OF THE INVENTION

The present invention is directed cryogenic a method of and apparatus for pumping gases under cryogenic conditions wherein the gas being pumped flows over deep-cooled condensation surfaces, and, more particularly, it is concerned with the enclosure for the condensation surfaces within the receiver and with the means for admitting gas for passage over the condensation surfaces.

In the past, it has been known to use a refrigerator to establish the cryogenic pumping conditions for the starting operation and then to maintain these conditions at the operating temperature during pumping. The output of the refrigerator is established so that the required low temperature cooling is possible in a sufficiently short time. Experience has shown that the refrigeration output required to establish the low temperature conditions is much greater then is required for the continuous pumping operation at pressures below 10.sup.-.sup.4 mm. Hg for maintaining the operating temperature. During normal operating conditions, the heat directed to the condensation surfaces is predominantly by heat radiation whereas the heat of condensation liberated by the condensation of the gases being pumped is not of any decisive importance. In bringing the operating temperature to the desired level under cryogenic operating conditions, the refrigeration output has to be rated in accordance with the desired short cooling period, as a result, if the working speed demanded for a modern industrial pumping station is to be attained, the initial refrigeration output is required to be several times higher than that necessary for actual pumping operation. In cryogenic pumping operations conducted by the supply of liquefied gases from a storage tank, there is no problem in obtaining the greater refrigeration output required for the initial cooling phase since it can be supplied by a correspondingly larger refrigerant supply from the storage tank.

Therefore, it is the primary object of the present invention to improve the economic efficiency of a cryogenic pumping operation which employs refrigeration apparatus.

In the prior art where the given output of the refrigerator was employed for cooling to the desired operating temperature, a radiation protection was employed around the condensation surfaces. Such radiation protection usually consisted of an arrangement of sheetlike members shielding against heat radiation. However, while these sheets permitted the gas being pumped to pass over the condensation surface, they constituted a flow obstacle which caused a disadvantageous reduction in the pumping rate. Accordingly, the present invention is directed to overcoming this disadvantageous construction of the prior art.

In accordance with the present invention, in the method of pumping a gas under cryogenic conditions and employing a refrigerator to obtain the desired low temperature conditions the novel characteristic involves the thermal insulation of the condensation surfaces from the heat within the receiver while the surfaces are being cooled to a predetermined temperature.

In carrying out the cryogenic pumping operation under high vacuum conditions the surfaces for reducing the gas to the necessary low temperatures are disposed within a space containing the gases to be pumped and an adjustably movable device, operable from the exterior of the enclosed space, is arranged to admit the flow of gas over the cryogenically cooled surfaces, and the movable device is arranged to provide a thermal shield for the surfaces while the operating temperature is being established. In a known high vacuum cryogenic pumping system, in which the cryogenically cooled surfaces over which the gases pass are arranged in a space communicating with the space to be evacuated, an enclosure device has been used for temporarily separating the cooled surfaces from the space to be evacuated. This prior device overcame the problem of previous cryogenic pumping systems wherein the water vapor which precipitated on the condensation surfaces had to be retained until the process being carried out under a vacuum was completed. This system was not only time consuming and required a considerable amount of refrigerant, but it also posed the danger that a sudden rise in the temperature within the receiver, which could easily occur in the course of such a process, could lead to the release of the water vapor from the condensation surfaces and result in an undesired increase in pressure. By employing the above mentioned enclosure for the condensation surfaces, it was possible to seal the surfaces from the space containing the gas to be evacuated and to thaw the water condensed thereon discharging it through a separate pumping line without interfering with the vacuum within the space containing the gas and otherwise interrupting the process. Such operation required that the closure device was designed to afford a high vacuum seal between the space containing the condensation surfaces and the space containing the gases to be pumped. The arrangement which served only to thaw the condensate during operation is not an element of the present invention.

To attain the objectives of the present invention, as set forth above, the closure device for the cryogenically cooled condensation surfaces of the prior art is not sufficient. Under the present invention, it is necessary during the initial cooling phase to provide a temporary thermal separation between the condensation surfaces and the gas to be pumped. The thermal separation can be effected by providing a thermal shield to enclose the condensation surfaces. Such a thermal shield can be provided by cooling the surfaces of the shield or by the application of a thermal insulating material. A simple covering about the condensation surfaces, as disclosed in the prior art, is not sufficient because the surface of the covering facing toward the condensation surfaces acts as a heat radiation source of the same intensity as the receiver, which forms the space containing the gas without the covering, and the condensation surfaces would be exposed to the same temperatures. On the other hand, as compared to the prior art arrangement, it is not necessary, in accordance with the present invention, to provide a gas-proof seal between the space containing the gas and the space containing the condensation surfaces.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention .

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a somewhat schematic vertical sectional view of an apparatus constructed in accordance with the present invention;

FIGS. 2a and 2b show another arrangement of apparatus in accordance with the present invention; and

FIGS. 3a and 3b show still another embodiment of apparatus in accordance with the present invention for pumping gas under cryogenic conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a receiver 2 encloses a chamber 1 which is to be evacuated in effecting the cryogenic pumping operation. Condensation surfaces which are deep-cooled during the cryogenic pumping operations are formed with the chamber 1 by means of a cooling coil 4 through which a refrigerant is passed. The refrigerant is circulated in a closed cycle flowing into the cooling coil 4 through and inlet line 5 and passing out through an outlet line 6 for flow through a refrigerator 7. In accordance with the invention, the condensation surfaces provided by the coil 4 are enclosed during the period in which the surfaces are brought to a predetermined low temperature. The closure for the condensation surfaces is provided by a vertically movable hood member 8 which covers the sides and upper surface of the coil protecting it from any heat supply within the chamber 1 of the receiver 2. A column 9 extends into the receiver 2 through its base and is secured to the hood member. A vacuum-proof seal is provided by a packing member 10 for the opening through which the column 9 enters the receiver. Exteriorly of the receiver, the column 9 is connected to a threaded spindle 11 and by means of gear wheels 12 mounted on the spindle and a servomotor 13, the column 9 and the hood 8 can be lifted or lowered within the receiver. As indicated in dot-dash lines in FIG. 1, the hood 8 can be lifted from the position enclosing the condensation surfaces to a position designated by the reference character 8' fully exposing the condensation surfaces to the gas within the chamber 1, or the hood can be positioned in any location intermediate the fully closed and fully opened positions in accordance with the operating conditions of the pumping system.

The exterior surface of the hood 8 is covered with a thermal insulating material 14 which prevents the condensation surfaces from being exposed to any heat within the receiver during the time when the surfaces are being cooled to the operating temperature.

The servomotor for positioning the hood is controlled by a control device which is connected to a temperature sensor 15 located adjacent the coil 4 for providing a continuous measurement of the temperature of the condensation surfaces and thereby regulating the servomotor.

Before the pumping operation is commenced in the apparatus disclosed in FIG. 1, the protective hood is closed separating the cooling coil 4 from the remainder of the chamber 1. The condensation surfaces provided by the coil 4 are reduced to a predetermined temperature and the chamber 1 is evacuated by a forepump 16 through a line 17 containing a valve 18 which is in the opened position. When starting vacuum for the pumping operation, for example 10.sup.-.sup.4 mm. Hg and the predetermined temperature of the condensation surfaces, for example, 20.degree. Kelvin, have been reached, the protective hood 8 is opened and the pumping operation commences under full suction capacity. The establishment of the vacuum conditions within the receiver can be observed on the gauge 19 mounted on the receiver.

FIGS. 2a and 2b, a receiver 21 provides a complete enclosure for a chamber 22 arranged to contain a gas to be pumped under cryogenic conditions. In the lower portion of the chamber 22 a cooling coil 24 forms the condensation surfaces for reducing the gas to the desired low temperature. An inlet line 25 and and outlet line 26 circulate a refrigerant through the cooling coil 24. A heat radiation shield 23 surrounds the coil 24 and is formed along the sides and lower surfaces of the coiling coil 24 by a housing member 27 and the lower surface of this housing member is cooled by another cooling coil 28 soldered to it and through which another refrigerant flows at a higher temperature than that in the cooling coil 24 forming the condensation surfaces. The heat shield is completed by a radiation shield 29 which covers the upper surface of the cooling coil. The radiation shield 29 is constructed of a number of chevron-shaped sheets 30 arranged to prevent heat radiation within the receiver from being directed against the cooling coil 24. Encircling the angle sheets 30 is another cooling pipe 31 having an inlet line 32 and an outlet line 33 for flowing refrigerant through the pipe. The sheets 30 are supported by a frame 34 and, in turn, the frame is supported on a shaft 36 which extends through the wall of the receiver at 35 through a device effecting a vacuum seal. In addition, the inlet and outlet lines 32 and 33 for the cooling pipe 31 enter and leave the chamber 22 through the shaft 36. The upper part 29 of the heat shield 23 enclosing the condensation surfaces can be pivoted by means of a shaft 36 between a position, as shown in FIG. 2b, where it covers the condensation surfaces to a position, shown in FIG. 2a, where the gases within the receiver can flow over the condensation surfaces. By selectively positioning the upper cover of the condensation surfaces the flow of gas over the cooling coil 24 can be regulated. The shaft supporting the upper cover can be pivoted by hand or by a motor drive, such as shown in FIG. 2b comprising a gear transmission means 37 and a servomotor 38. A control device 39 is connected to the servomotor for positioning the upper cover of the heat shield. The servomotor is connected to a pressure gauge 40 in such a manner, that after the predetermined temperature, for example 20.degree. Kelvin, has been reached the condensation surfaces remain available for pumping as long as the pressure in the receiver does not exceed a predetermined value. In addition, a forepump 41 is connected to the receiver through a preevacuation line 42 which contains a valve 43 for establishing the desired vacuum conditions.

In FIGS. 3a and 3b, another embodiment of the present invention is shown providing individual adjustment of the various elements forming the heat shield enclosure for the condensation surfaces which afford a particularly advantageous gas throttling arrangement. A receiver 45 encloses the space containing the gas to be pumped and, as with the other embodiments disclosed, the condensation surfaces are provided in the lower region of the receiver by means of a cooling coil 46. A plurality of vertically disposed heat protection sheets are provided which are rotatable about their vertical axes as indicated by FIG. 3b. The individual sheets 47 extend between an upper plate 48 and a lower plate 49 and can be rotated jointly by means of a transmission device composed of a central gear 50 and individual gears 51 each secured to the upper end of one of the heat protection sheets 47. By means of the transmission device the plates can be rotated selectively to any position between the fully opened and the fully closed positions. For actuation of the transmission device, a vertically arranged shaft 53 is secured at its upper end to the driving gear 50 and extends downwardly through the cooling coil 46 and out of the receiver 45 through a packing device 52. Exteriorly of the receiver the shaft may be driven by hand or by means of a motor and gear transmission means as is shown in FIG. 3a.

Connected to the motor 54 is a control device 55 which is connected to a pressure gauge 56 for measuring the pressure within the receiver and to a temperature sensor 57 positioned adjacent the cooling coil 46 for determining the temperature of the condensation surfaces. During operation the condensation surfaces are completely enclosed by the heat protection sheets 47 until the predetermined temperature, that is about 20.degree. Kelvin, is reached and then the gas inflow is regulated as a function of the pressure within the receiver to maintain a certain pressure, that is, about 10.sup.-.sup.4 mm. Hg and the flow is throttled to a minimum. However, above this pressure the throttling is increased in an amount based on the quantity of the condensing gas in accordance with the constant refrigeration output. The constant refrigeration output depends on the condensation temperature and the heat of condensation of the gas being pumped. Accordingly, the gas inflow over the condensation surfaces must be adjusted so that at a given refrigeration output the corresponding quantity of gas is admitted to flow over the surfaces. The devices disclosed in FIGS. 2b and 3a employed, in accordance with the present invention, to insulate the condensation surfaces from the heat within the receiver at least during that part of the cooling of the surfaces when a predetermined temperature is being established, which temperature corresponds essentially to the operating temperature during pumping. Subsequently, by movably displacing the enclosure about the cooling coil, the condensation surfaces are fully exposed to the heat within the receiver. In accordance with this arrangement, it has been found that a better utilization of the refrigerating apparatus is achieved in that the operating temperature and hence the full suction capacity of the pump, is reached in a much shorter time after commencing the cooling operation than would be possible with the gas supply unthrottled and the condensation surfaces unprotected from heat during the initial cooling period despite the unhindered gas inflow and therefore stronger pumping during that time.

In accordance with the general concept of the invention, the thermally insulating enclosure about the cooling coil can include any measures which effectively prevent the heat within the receiver from impinging on the condensation surfaces. It can be appreciated, of course, that a complete thermal insulation of the condensation surfaces is not possible. However, to attain the object of the present invention, a thermal insulation easily obtained by known measures, such as in tanks for liquefied gas, is sufficient.

Claims

What we claim is:

1. A method of pumping a gas under cryogenic conditions comprising the steps of providing an enclosed space for the gas to be pumped, positioning deep cooled condensation surfaces within the enclosed space, thermally insulating the condensation surfaces from the heat supply within the enclosed space while cooling the condensation surfaces to a predetermined low temperature and while colling the condensation surfaces preventing the flow of the gas to be pumped over the surfaces, and regulating the flow of gas to the condensation surfaces after the predetermined temperature is reached.

2. A method, as set forth in claim 1, wherein establishing vacuum conditions within the enclosed space for pumping the gas under cryogenic conditions.

3. A method, as set forth in claim 2, wherein cooling the condensation surfaces to a temperature of about 20.degree. Kelvin and regulating the gas flow over the condensation surfaces for maintaining the vacuum conditions at about 10.sup.-.sup.4 mm. Hg.

4. Apparatus for pumping gas under cryogenic conditions comprising walls forming a closed chamber containing the gas to be pumped, means for forming condensation surfaces within the chamber for cooling the gas to the requisite low temperature, means for enclosing said condensation surfaces within said chamber for preventing the gas within said chamber from flowing over the condensation surfaces, means for thermally insulating said enclosing means for preventing heat within said chamber from affecting said condensation surfaces, and means for throttling the flow of gas within said chamber over said condensation surfaces.

5. Apparatus, as set forth in claim 4, wherein said enclosing means for said condensation surfaces comprising wall means surrounding said condensation surfaces and including a hood member, and said thermal insulating means comprising thermal insulation material covering said wall means including said hood for preventing heat from within said chamber from contacting said condensation surfaces.

6. Apparatus, as set forth in claim 5, wherein said means for throttling the flow of gas comprising a shaft extending into said chamber and being secured to said hood, and means for engaging said shaft exteriorly of said chamber for displacing said hood member from its position enclosing said condensation surfaces for variably admitting flow of gas to said condensation surfaces.

7. Apparatus, as set forth in claim 4, wherein said enclosing means comprising a housing enclosing a portion of said condensation surfaces, a cover for cooperation with said housing for completely enclosing said condensation surfaces, a cooling coil mounted on said housing for circulating a cooling medium therethrough for removing heat from said housing and preventing the heat from reaching said condensation surfaces, and said cover comprising a frame, a plurality of metallic plates mounted within said frame and arranged to prevent heat from within said chamber from contacting said condensation surfaces, and a second coil encircling said frame for removing heat therefrom.

8. Apparatus, as set forth in claim 7, wherein a rotatable shaft extending through said wall means into the interior of said chamber and being secured at its inner end to said cover, and means located exteriorly of said chamber for rotating said shaft and displacing said cover from said housing and thereby admitting gas within said chamber to flow over said condensation surfaces within said housing.

9. Apparatus, as set forth in claim 4, wherein said enclosing means comprising a pair of spaced plates located on opposite sides of said condensation surfaces, a plurality of rotatable plate sections extending between said plates and being arranged in combination with said plates to completely enclose said condensation surfaces.

10. Apparatus, as set forth in claim 4, wherein said means for throttling the flow of gas comprising a shaft member centrally located within said plate sections and extending into said chamber from the exterior of said wall means, a drive gear mounted on said shaft, a plurality of individual gears each mounted on one of said plate sections and in meshed engagement with said drive gear, and means for rotating said shaft whereby said drive gear is rotated and in turn individually rotates said plate sections for selectively admitting gas for flow over said condensation surfaces.

11. Apparatus, as set forth in claim 4, wherein said condensation surfaces comprising a cooling coil located within said chamber, and conduit means connected to said cooling coil for circulating a refrigerant therethrough.

12. Apparatus, as set forth in claim 4, wherein means being connected to said chamber for establishing a vacuum therein.

13. Apparatus, as set forth in claim 4, wherein a temperature sensor positioned within said chamber adjacent said condensation surfaces, a control device operatively connected to said temperature sensor, and drive means for opening and closing said throttle means being in communication with said control device which regulates the extent of the opening of said throttle means by said drive means.