Abstract:
This device comprises a closed heating chamber ( 36 ) extending through the wall ( 3, 4, 5 ) of the tank and connected to this wall, a feed pipe ( 22 ) suitable for feeding the heating chamber ( 36 ) with a heating fluid having a temperature above the temperature of the cryogenic fluid, and an exhaust pipe ( 23 ) intended for discharging the heating fluid, each of the pipes intended for discharging the heating fluid, each of the pipes ( 22, 23 ) passing through an outer wall ( 20 ) of the heating chamber ( 36 ). The device is particularly useful in the delivery of ultrapure helium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pressure-regulating device for a tank of a cryogenic fluid, especially a helium tank, which comprises a closed heating chamber extending through the wall of the tank and connected to this wall. 
     It furthermore relates to a plant for delivering fluid from a cryogenic tank. 
     The invention applies, for example, to the delivery of ultrapure helium for the microelectronics industry. 
     BACKGROUND OF THE INVENTION 
     Cryogenic tanks have a very efficient thermal insulation. When gas is withdrawn from such a tank, the pressure, which is typically a few bar relative, drops because the heat influx is too low to compensate for the loss of fluid. Consequently, when gas is withdrawn, the pressure in the tank may drop excessively with respect to the requirements of the user network. 
     In order to keep the pressure in the tank constant, heat has to be supplied to the tank during withdrawal. 
     For this purpose, pressure-regulating devices for cryogenic tanks are known which use an electrical resistor as heating element, in combination with electrical safety means should there be a power failure. However, the known solutions are expensive if the emergency electrical supply has to operate for a long period. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an inexpensive pressure-regulating device which can provide a cryogenic tank with heat over a long period. The invention must furthermore guarantee that the contents of the container are not contaminated, even in the case of ultrapure fluids. 
     For this purpose, the subject of the invention is a pressure-regulating device characterized in that it includes a feed pipe suitable for feeding the heating chamber with a heating fluid having a temperature above the temperature of the said cryogenic fluid, and an exhaust pipe intended for discharging the heating fluid, each of the said pipes passing through an outer wall of the heating chamber. 
     The device according to the invention may include one or more of the following characteristics taken by themselves or according to any of their technically possible combinations: 
     the device includes a controlled valve inserted in the feed pipe and connected via its control part to a pipe for using the fluid in the tank so as to open the controlled valve when the pressure in the tank drops below a predetermined threshold; 
     the device includes second heating means, especially electrical resistors; 
     the second heating means are inserted into the heating chamber, preferably near the outlet of the feed pipe; 
     an insulating sleeve is provided on the inner wall of the tank, around a mid-section of the heating chamber, dividing the heating chamber into an insulated outer region and an uninsulated inner region; 
     the outlet of the feed pipe lies within the uninsulated region, near the inner end of the heating chamber; 
     the inlet of the exhaust pipe lies within the uninsulated region, near the insulated region; 
     the exhaust pipe is covered with thermal insulation means which extend from the outside of the heating chamber through its outer wall and approximately as far as the inlet of this pipe; 
     the heating gas has, under its conditions of use, a dew point below the temperature of the cryogenic fluid contained in the tank; 
     the cryogenic fluid and the heating gas consist of helium; and 
     the pipes are composed of a material which is a poor thermal conductor, especially an epoxy resin. 
     The subject of the invention is also a plant for delivering a fluid, comprising a tank for this fluid, which is in cryogenic form, equipped with a heating device as defined above, a use pipe, connecting the tank to a use station, and a heating gas source connected via a feed pipe to the heating device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood on reading the description which follows, given solely by way of example and with reference to the drawings in which: 
     FIG. 1 is schematic view of a helium delivery plant according to the invention; and 
     FIG. 2 is a longitudinal sectional view on a larger scale of the pressure-regulating device connected to the cryogenic tank. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cryogenic tank  1  contains helium  2  in the supercritical state, at a very low temperature, typically between 4 and 45 K. It is of a known type and is formed by an outer wall  3 , an inner wall  4  and a central wall  5  which are spaced apart, the spaces being filled with a material which is a good thermal insulator and a vacuum being created therein. The central wall  5  additionally includes means which allow it to be cooled by the fluid leaving the tank during withdrawal. 
     The tank includes a neck  6  for the heating device, a withdrawal pipe  7  and a safety valve  8 . The tank  1  is connected to a use station  9  via, in succession, the withdrawal pipe  7 , an intermediate pipe  10 , an atmospheric heater  11 , two valves  12 ,  13  between which a filter  14  is provided, and a use pipe  15 . The latter is equipped with a use valve  16  which controls the helium withdrawal. This valve has a construction such that, when there is a power failure, it is in the flow position. 
     A finger  17  extends through the neck  6  and the walls  3 ,  4 ,  5  of the tank  1 . It is provided at its inlet with a flange  18  fastened to the inlet of the neck  6 . Inserted into the finger  17  is a heating device  19  provided with a closure flange  20  which is removably fastened to the flange  18  by means of bolts  21 . A feed pipe  22  and an exhaust pipe  23  extend through the flange  20 , as does an electrical heating rod  24 . 
     A discharge valve  25  is connected via an outlet valve  26  and a heater coil  27  to the exhaust pipe  23 . 
     A stand  28  supports bottles  29  of heating helium at room temperature, the bottles being connected via a regulator  30  and a pipe  31  to a valve  32 . Inserted into the pipe  33  which connects the valve  32  to a feed valve  34  of the feed pipe  22  is a controlled dome valve  35 . Its dome is connected to the pipe  15  so that when the pressure in the pipe  15  falls below a certain threshold, the valve  35  opens, allowing heating gas to pass into the pipe  33 . 
     FIG. 2 shows in more detail one embodiment of the device used for regulating the pressure. 
     The heating chamber  36  is bounded by the finger  17 , the flange of the tank  18  and the closure flange  20  forming the outer wall. An insulating sleeve  37 , which is connected to the inner wall  4  of the tank  1 , surrounds part of the finger  17 . The feed pipe  22 , to which the feed valve  34  is connected, passes through the flange  20  and extends almost as far as the bottom of the heating chamber  36 . The said pipe is preferably made of an epoxy resin. The heating rod  24 , the electrical connection  38  of which is located outside the chamber  36 , is placed inside this chamber, reaching almost as far as the bottom of the finger  17 . Its resistor  39  is wound around the end part of the feed pipe  22 . 
     The exhaust pipe  23  is surrounded by an evacuated tube  40 , which tube extends from the outside of the heating chamber  36 , through the flange  20 , virtually as far as the end of the insulating sleeve  37 . Likewise, the opening of the exhaust pipe  23  is placed approximately level with the end of the insulating sleeve  37 . 
     Two regions in the heating chamber  36  may be distinguished: an insulated outer region  41  covered by the neck  6 , the walls  3 ,  4 ,  5  and the insulating sleeve  37 , and an uninsulated inner region  42 . 
     The plant operates in the following manner: 
     When the pressure of the helium  2  in the tank  1  is high enough, within the limit permitted by the safety valve  8 , the pressure in the pipe  15  is also high enough for the valve  35  to close the pipe  33 . Consequently, no heating gas is introduced into the heating chamber  36 . Heat influx is reduced by the low conduction of the materials, the thermal path extended by the insulation  37  and the helium-cooled central wall  5 . 
     If gas is consumed at the use station  9 , fluid is withdrawn from the tank  1 . The gas is taken via the pipes  7  and  10  to the heater  11 , where it is heated to room temperature, passes through the valves  12 ,  13  and the filter  14  and then enters the pipe  15 . 
     Because of this withdrawal, the pressure drops in the tank  1 . In normal operation, the electrical rod  24  is supplied by the electrical mains, under the control of pressure-controlled means (not shown). The inside of the heating chamber  36  is then heated by the resistor  39  of this rod when the pressure in the tank falls below a predetermined threshold. 
     If the resistor  39  does not operate, for example should there be a power failure, the pressure continues to drop so that the pressure also drops in the control dome of the valve  35 . When the pressure falls below a predetermined threshold, the dome opens the valve  35 , thereby allowing the heating gas to flow. Heating gas then escapes from the bottles  29  and, after expansion in the expander  30 , flows into the pipe  31 . 
     The gas flows through the valve  32  and the controlled valve  35  and flows through the pipe  33  and the feed valve  34  and then into the feed pipe  22 , from where it reaches the heating chamber  36 . 
     The heating gas then supplies heat in the section which is not covered by the insulating sleeve  37 , through the wall of the finger  17 , thereby heating the helium  2  contained in the cryogenic tank  1 . This has the result of raising the pressure in the tank  1 . 
     When, because of the continuous supply of the heating chamber  36  with heating gas, the pressure in the chamber rises above a certain threshold, the heating gas is discharged via the discharge pipe  23 , the heater coil  27 , the outlet valve  26  and the discharge valve  25 . 
     When the pressure in the tank  1 , and consequently in the pipe  15 , has risen sufficiently, the dome of the valve  35  stops the flow of the heating gas into the pipe  33 . 
     Thus, the heating is stopped and the pressure in the tank no longer rises, except because of the heat influx, which is very small. 
     Thus, should there be a power failure, the use of such a device heats the tank  1  in a simple, inexpensive and automatic manner. In order to maximize the heat delivered to the helium in the tank  1 , the outlet of the feed pipe  22  and the inlet of the exhaust pipe  23  are far apart. For the same purpose, the feed pipe  22  is not provided with a thermal insulation, unlike the exhaust pipe  23 .