US 3893458 A
An improved heat-sterilizable patient ventilator characterized by a ported center-body, a shell formed of heat-sterilizable material mounted on the center-body and defining an hermetically sealed reservoir for confining under positive pressure a mixture of bacteria-free gas, and a pneumatic circuit including an oxygen delivery jet coupled with an absolute filtration system for delivering bacteria-free mixture of gases to the reservoir.
Description (OCR text may contain errors)
United States Patent Fletcher et al. July 8, 1975 1 HEAT STERILIZABLE PATIENT 3,068,858 12/1962 Bird et al. 128/1455 VENTILATOR 3,530,856 9/1970 Bird 128/1456 3,555,787 1/1971 Lustig 55/485  Inventors: James C. Fletcher, Admmistrator of 3,632,166 8/1972 Jacobs l28/145.8 the National Aeronautics and Space 3,717,147 2/1973 Flynn..... 128/1418 Administration, with respect to an 3,736,927 6/1973 Misaqi l28/l4$.6 invention of Alexander S. Irons, La Can Paul Muehter, Primary Examiner-Dalton L. Truluck Tu unga; Willie D. Kent, Los Assistant Examiner-Henry J. Recla I Angeles, all of Calif. Attorney, Agent, or Firm-Monte F. Mott; John R.  Find: Mar. 7, 1974 Manning; Wilfred Grifka A N 449,15  3 57 ABSTRACT An improved heat-sterilizable patient ventilator charj g f gg acterized by a ported center-body, a shell formed of 58] Field 58/142442 3 heat-sterilizable material mounted on the center-body 128/145 202 6 and defining an hermetically sealed reservoir for con- 146 5 08 6 :Iining undeg positive pressure a mixltue of bacteriaree gas, an a pneumatic circuit me u ing an oxygen SSIDIG 486 delivery jet coupled with an absolute filtration system References Cited for delivering bacteria-free mixture of gases to the reser UNITED STATES PATENTS v" 1,781,254 11 1930 Stelzner......................... 55/010. 33 Clams 7 Drawmg 138 g 176 160 84 l4 I2 32 ER R6 60 0 m so lax sees 34 a} R I 182 f we a 1| 4 SHEET I'M [NED JUL 8 m5 1 HEAT STERILIZABLE PATIENT VENTILATOR ORIGIN OF INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of I958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an intermittent positive pressure breathing apparatus, herein referred to as a patient ventilator, and more particularly to a heat-sterilizable patient ventilator having an improved pneumatic circuit.
2. Description of the Prior Art As a practical matter, non-heat sterilizable equipment often serves as a source of many types of hospitalassociated infections. Such equipment frequently is rendered non-heat sterilizable as a result of the materials employed in the construction of the equipment. Chemical agents, of course, cannot always be relied upon to sterilize equipment due to the inherent complexity of the equipment, the types and numbers of microorganisms present, and numerous other conditions indigenous to environments in which such equipment is employed.
Patient ventilators are found to be among the most culpable devices included in equipment having a propensity to foster propagation and spread of infectious microorganisms. It is postulated that these devices are particularly culpable because of their incompatibility with sterilization processes which can be relied upon to effect a complete removal or inactivation of associated infectious microorganisms. Moreover, infectious microorganisms tend to multiply quite rapidly within the humid environment normally found within such devices. It can, therefore, readily be appreciated that in the event a patient ventilator is incompletely sterilized, or has been sterilized and subsequently receives a charge of microorganisms, the device is capable of being contaminated rapidly due to the rapid growth rate of the microorganisms.
A typical patient ventilator is illustrated in US. Letters Pat. No. 3,068,856 which has been found adequate for many purposes. Ventilators of the type illustrated in the aforementioned patent can be adjusted to assist or control the rate and depth of pulmonary ventilation. Both the inspiratory and the expiratory phases of spontaneous respiration can be assisted by the ventilator to increase the gas volume during inspiration and enhance the outward flow of gases from the lungs of a patient during the expiratory phase of a breathing cycle.
As described in the aforementioned patent, oxygen from a pressurized tank enters the apparatus and is ducted to a pressurized reservoir via a venturi. Unfortunately, it is difficult to acquire bacteria-free oxygen from existing distribution systems. Therefore delivery of bacteria-free oxygen to and beyond the venturi of the patented ventilator cannot be guaranteed, simply because bacteria-ladened oxygen is mixed with filtered ambient, atmospheric gas from which particles, but not microorganisms, have been removed. As a result, mixtures of bacteria-ladened gases are delivered to the patients.
Sterilization of ventilators, of the type hereinbefore described, requires a complete disassembly of all components and a decontamination of each component with various chemical substances that often are hazardous to both materials and to personnel. Of course, the components must be reassembled in a bacteria-free environment in order to assure that the sterilized condition of the ventilator is maintained. Presently, facilities for reassembling the ventilators in a bacteria-free environment generally are not available. Moreover, once a ventilator of the type aforementioned has been sterilized, further precautionary measures must be taken to prevent microorganisms from again entering the ventilator. This frequently necessitates the use of sterilized, hermetically sealed storage containers.
Therefore, it can be appreciated that heretofore sterilization of patient ventilators has been deemed to be an expensive process in terms of the labor costs, the materials utilized, and the facilities required in the sterilization and storage thereof.
It is therefore the general purpose of the instant invention to provide a patient ventilator which is both heatsterilizable and substantially impervious to microorganisms found in ambient atmospheric air.
OBJECTS AND SUMMARY OF THE INVENTION It is an object of the instant invention to provide a heat-sterilizable patient ventilator.
It is another object to provide a heat-sterilizable patient ventilator which is impervious to microorganisms.
It is another object to provide a patient ventilator formed of heat resistant material which can readily be packaged and then sterilized in a fully assembled condition and readily stored in such condition.
These and other objects and advantages are achieved through the use of a heat-sterilizable patient ventilator which includes a ported center-body including an annular shoulder, a shell formed of a heat resistant material seated on the shoulder and defining in juxtaposition with the center-body an hermetically sealed reservoir for confining under positive pressure a bacteriafree mixture of ambient atmospheric gas and/or pure oxygen, a discharge conduit extended from the reservoir to a discharge fitting adapted to be received by a patient, and an absolute filter system for delivering bacteriafree gas to the reservoir, as will become more readily apparent by reference to the following description and claims in light of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross-sectional view of a heatsterilizable patient ventilator which embodies the principles of the instant invention.
FIG. 2 is a cross-sectional view taken line 2-2 of FIG. 1.
FIG. 3 is a sectional view taken generally along line 3-3 of FIG. 2, illustrating an arrangement of studs provided for securing a closure shell to the center-body of the patient ventilator.
FIG. 4 is a fragmented, partially sectioned view of an end portion of one of the studs shown in FIG. 3.
FIG. 5 is a cross-sectional view of a first absolute filter provided for filtering ambient atmospheric air as it is delivered to the patient ventilator.
FIG. 6 is an exploded view of a support ring employed in supporting bacteria retentive pads provided for the filter shown in FIG. 5.
generally along FIG. 7 is a cross-sectional view of a second absolute filter provided for filtering oxygen as it is delivered to the ventilator.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, with more specificity, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a heat-sterilizable ventilator, generally designated 10, which embodies the principles of the instant invention.
The ventilator 10 is provided with a center-body 12, preferably machined from a suitable aluminum casting. As a practical matter, the center-body 12 is of a type similar to the center-body shown and described in the aforementioned U.S. Letters Pat. No. 3,068,856. The center body 12 includes a vertically oriented oxygen conduit 14 through which pure oxygen is delivered to the patient ventilator. The conduit 14 is provided with a bi-stable valve 16 for controlling the flow of oxygen therethrough. Where desired, the valve includes a spool seated in a sleeve having appropriate, radially extended passages, not shown. An axially movable, valveactuating shaft 18 is integrally connected with the spool and terminates at its opposite ends in armatures 20. The shaft 18 serves to switch the valve 16 between its bi-stable conditions as appropriately directed, axial motion is imparted thereto. An axially adjustable magnet assembly 22 is supported in juxtaposition with each of the armatures and serves to assist in imparting axial motion to the shaft and to retain the shaft in a position to which it is advanced. It is to be understood that when the shaft 18 is moved in a first direction, the valve 16 is opened so that a flow of oxygen under pressure is established through the oxygen conduit 14; when the shaft 18 is moved in an opposite direction, the valve 16 is closed for thereby interrupting the flow of oxygen established through the conduit 14.
Also mounted on the shaft 18 there is a diaphragm 24 formed of silicone rubber having its peripheral surfaces hermetically sealed about a recess machined in one surface of the center-body for thus defining an expansible plenum chamber 26 whereby the shaft is advanced in opposite directions in response to changes in pressure within the plenum chamber.
The center-body 12 is provided with an annular shoulder 30 which serves as an annular seat for receiving a shell 32 in a mated relation for thus establishing an hermetically sealed reservoir 34 adjacent to the center-body 12. The shoulder 30 is provided with an annular groove within which there is seated an O-ring 36, also formed of silicone rubber, for thus establishing an hermetic seal between the adjacent surfaces of the shell 32 and the shoulder 30, without subjecting the shell to significant stress. It is important here to note that the shell 32 is formed of a material which will not degrade when exposed to heat-sterilization cycles, characterized by temperatures of 257 F. and a duration of approximately 6 hours. Since the material employed preferably is susceptible to molding, polysulfone serves quite satisfactorily for this purpose.
The shell 32 further serves as a mount for one of the magnet assemblies 22. Further, the shell is provided with a flow discharge port 38 connected with a conduit 40 through which a mixture of ambient atmospheric gas and oxygen is discharged from the reservoir 34 through a humidifier 42, to a fitting 44 adapted to be received within the mouth of a patient, not shown.
The humidifier 42, as a practical matter, is an in-line nebulizer, of a known configuration, which serves to deliver to the mixture of gases flowing through the conduit 40 an atomized liquid. Additionally, where so desired, a micronebulizer 46 is connected with the conduit 40 and serves to deliver medication to the mixture of gases passing to the mouthpiece or fitting 44. Of course, the conduit 40, the humidifier 42 and the nebulizer 46 also are formed from material which will not degrade during sterilization cycles. Where desired, the case for the humidifier 42, as well as the nebulizer 46, is formed of polysulfone, while the conduit is formed of Silastic tubing.
The fitting 44 includes an exhaust valve 48 normally maintained in a sealed condition through back pressure developed in a plenum chamber 50 connected with the oxygen conduit 14 through a pressure tube 52, also formed of Silastic rubber. The exhaust valve 48 includes a compression spring 54 acting on a closure plate 56 for closing an exhaust port, not designated. It is to be understood that the compression spring 54 permits the sealing plate 56 to be displaced as a patient exhales, or coughs, for discharging spent gas through a discharge orifice 58.
Moreover, the pressure tube 52 also serves to deliver oxygen under pressure to the in-line nebulizer 46 through a branch tube 60. This tube is connected between the nebulizer and the pressure tube 52, and serves to deliver a pressurized stream of oxygen to the nebulizer in a manner well understood by those familiar with such devices. Accordingly, a more detailed description of the nebulizer 46 is omitted in the interest of brevity.
Turning now to FIGS. 2, 3 and 4, it can be appreciated that the shell 32 is mounted by a plurality of studs 61 connected with the shell in a manner such that the shell remains in a substantially unstressed condition. The studs 61 are secured within the center-body 12, in any suitable manner and project in opposite directions therefrom. Each stud includes a distal portion 62 which includes an annular shoulder 64, FIG. 4. Seated on each of the shoulders 64 there is a stand-off washer 66 having a tubular body 68 and a radially extended flange 70. The tubular body 68 is received within a bore 72 fonned in the shell 32, so that the shell is permitted to seat on the flange 70. Preferably, an O-ring 74 also is mounted on the body 68 and seats against the external surface of the shell 32. A conventional washer 76 seats on the O-ring 74 while a nut 78 is threaded onto the distal end portion of the stud 61, whereupon the O-ring 74 is crushed for thus forming an hermetic seal between the bore 72 and the stand-off washer 66. Consequently, the nut 78 can be suitably torqued without introducing stress in the shell 32. This, of course, tends to obviate deformation and crazing when the ventilator is subjected to a heatsterilization cycle.
The reservoir 34 communicates with the plenum chamber 26 through a plurality of pressure equalizing bores 80 through which pressures established within the plenum chamber and the reservoir are substantially equalized. As a practical matter, it should be apparent that so long as gas is extracted from the reservoir 34, via the fitting 44, pressure within the plenum chamber 26 is reduced and the valve 16 remains open. However, in the event pressure within the reservoir 34 is elevated,
via the bores 80, sufficiently for expanding the plenum chamber 26, the diaphragm 24 is displaced for repositioning the shaft 18 whereupon valve 16 closes. The magnet assemblies 22 serve to insure a complete repositioning of the shafts 18 as the armatures 20 are alternately positioned within the flux fields of the magnet assemblies for thus assuring a seating of the shaft 18 in either of its alternate positions as it comes to rest following axial displacement.
As a practical matter, the center-body 12 is provided with an additional bore 82, downstream from the valve I6, through which the conduit 14 communicates directly with the reservoir 34 for metering pure oxygen to the reservoir, whereby an elevated pressure is developed within the reservoir at any time the valve I6 is opened. Hence, it should be appreciated that the valve I6 is a normally closed valve.
As a practical matter, a shell 84 is mounted on the center-body 12 in a manner quite similar to that in which the shell 32 is fixed to the center body. However, it is important to note that the shell 84 merely serves as supporting structure for a magnet assembly 22. Therefore, it is to be understood that the shell 84 need not be hermetically sealed relative to the center-body.
It is here important to note that ambient atmospheric gas is introduced into the reservoir 34 via a filtration system generally designated 90. The filtration system 90 includes a conduit 92 which extends through the center-body I2 and terminates in a flow control valve 94. The flow control valve 94 includes a ported housing 95 having formed therein a multiplicity of radially extended ports 96. A spring-loaded closure plate 98, biased by a spring 99, is seated in the housing and closes the valve by seating against the adjacent end surface of the conduit 92. It will therefore be appreciated that as pressure within the conduit 90 is caused to exceed the combined forces of the spring 99 and pressure established within the reservoir 34, the plate 98 lifts off its seat against the bias of its spring for opening the ports 96.
As should readily be understood by those familiar with the administration of oxygen to patients, it is necessary that oxygen rich gases delivered to a patient should be so mixed as to include approximately 85 percent ambient atmospheric air in order to avoid various undesirable side effects. Therefore, the opposite end of the conduit 92 is connected with an absolute filter 100, through which ambient atmospheric gas is introduced to the patient ventilator. Consequently, it is necessary that the filter 100 have a capacity which accommodates such requirements. Moreover, patients being treated frequently are in a debilitated condition which requires that the ambient atmospheric air be readily conducted through the filter in order to sustain respiration with minimal effort. Thus, the filter 100 must also possess a capacity for accommodating passage of atmospheric gas in the presence of a minimal pressure differential established thereacross.
The filter I00 is provided with a filter housing 102, also formed of a heat-sterilizable material such as polysulfone. Within the housing 102 there is seated, in series, a dust filter unit 104 upon which is seated a microbial filtration unit 106, and a media migration filter unit I08, seated, in turn, upon the microbial filter unit 106. The dust filtration unit I04 preferably is formed of a fine open-pore urethane foam, such as that developed by the Foam Division of the Scott Co., Chester, Pennsylvania, which removes particulate matter from ambient atmospheric gases as it enters the filter I00 via an entrance port 110. This unit is sustained in position through a support ring 112 seated on an annular lip I14 circumscribing the entrance port IIO. As a practical matter, the support ring 112 is of an annular configuration with the midportion thereof being closed by a screen 116 of any suitable mesh affixed thereto.
The microbial filter unit I06 includes a plurality of bacteria retentive pads 118, each of which is supported by a support ring I [2. As a practical matter, each of the support rings 112 is provided with a plurality of spacer pins 120 which prevent the pads 118 from being crushed between the support rings I12. The pads I18 are formed from filter material, preferably a very fine spun glass material such as that designated FM-O04 and manufactured by Owens-Corning Fiberglas Corp., Toledo, Ohio. This material is known to have a capability for removing all bacteria from gases passing therethrough.
The media migration filter unit I08 also is formed of open-pore urethane foam and serves to assure that no particles of the fiberglass material which becomes disassociated from the pads I I8 is permitted to pass from the filter I00 to the conduit 92. Once ambient atmospheric gas is drawn through the filter 100, it is delivered to the valve 94, via a venturi throat I22 having its downstream end communicating with the reservoir 34, via the ports 96. The purpose of the venturi 122 is to achieve a mixing of pure oxygen with ambient atmospheric gas prior to an introduction of resulting mixture into the reservoir 34.
In order to achieve a mixing of pure oxygen with the ambient atmospheric gas, within the venturi I22, there is provided an oxygen jet I24 mounted at the distal end of a tubular conduit I26 which, in turn, is connected with the oxygen conduit 14 and communicates therewih through a port 128. The jet 124 is located immediately upstream from the entrance to the venturi I22 and is disposed in coaxial alignment therewith. The jet 124 serves to direct a stream of pure oxygen, derived from the oxygen conduit 14, axially into the throat of the venturi 122 whereupon a mixing of the oxygen with ambient atmospheric gas delivered through the conduit 92 is effected prior to its being discharged into the reservoir 34.
In order to control the rate of flow of oxygen through the oxygen conduit 14, there is provided a flow-rate valve 130 having a tubular body 132 within which there is provided a plurality of ports I34. Thebody 132 is supported by a suitable boss I36 while the ports 134 are axially spaced at distances such that they become aligned, simultaneously, with the opening for the bore 82 and the conduit 126, as the body is advanced through the boss 136. Thus, the flow of oxygen through the flow-rate valve 130 can be controlled by axially displacing the body 132. Consequently, while it is preferable to administer to a patient gases consisting of percent ambient atmospheric air, the ratio of oxygen gas to ambient atmospheric gas can be varied where so desired.
Of course, as hereinbefore mentioned, it is difficult to acquire a practical source of bacteria-free oxygen, therefore, a filter I38 is provided between the oxygen conduit I4 and a selected source of oxygen, not shown; The purpose of the filter 138 is similar to that of the fil= ter in that this filter is an absolute filter providiirl for removing microorganisms inherently present in oxygen commonly found in distribution systems. The filter 138 includes a heat-sterilizable housing 160 capable of withstanding pressures of at least 60 p.s.i. Within the housing there is provided a filter unit formed of alternating filter pads 162 and 164. The pads 162 also are formed of fine open-pore urethane foam, similar to material employed in fabricating the dust filter unit 108, while the pads 162 are formed from spun glass material similar to the material employed in the pads 118. It should therefore be apparent that through the filter 138 it is assured that the oxygen introduced into the patient ventilator is introduced in a bacteria-free condition.
The housing 160 is closed at each of its opposite ends by ported end plates 170 and 172. The end plate 170 includes a fitting 174 for coupling the filter to a source of oxygen while the end plate 172 includes a fitting 176 for connecting the filter to the conduit 14.
OPERATION it is believed that in view of the foregoing description, the operation of the device will readily be understood and it will be briefly reviewed at this point.
Assuming that the pressure within the reservoir 34 is maximized, it can be appreciated that the pressure within the plenum chamber 26 also is maximized so that the valve 16 is in an "off" condition. Similarly, the plate 98 of the valve 94 is seated in sealing relation with the adjacent end of the conduit 92. A patient may now insert the fitting 44 into his mouth and begin inhalation. As the patient inhales, pressure is reduced within the reservoir 34 so that the plate 98 unseats with respect to the distal end of the conduit 92 and the diaphragm 24 begins to collapse, due to the reduction of pressure within the plenum chamber 26. Thus, movement of the shaft 18 is initiated for switching the valve 16 to its on condition, whereupon a stream of oxygen is established through the filter 138 and the oxygen conduit 14. The gas of this stream rendered bacteria-free by the filter 138 is discharged into the reservoir 34, via the bore 82, and into the conduit 92, via the jet 124. As the patient continues to inhale, ambient atmospheric air is drawn through the filter 100 of the dust filtration unit 104, whereupon foreign particles are removed there- 4 from. As the ambient atmospheric gas advances through the filter, it is passed through the microbial filtration unit whereupon all microorganisms are extracted therefrom. The ambient atmospheric air then continues through the final media migration filtration unit whereupon all foreign particles finally are removed from the gas, particularly particles of fiberglass wool which may have become suspended in the gas as it is passed through the microbial filtration unit.
The gas then passes from the filter to the venturi 122, through the conduit 92, and is there mixed with oxygen delivered from the jet 124, and thereafter passed to the reservoir 34. Gas thus passed to the reservoir 34 is delivered via the conduit 40 through the humidifier 42 and the nebulizer 46 to a patient, at the fitting 44. Of course, once inhalation is completed and exhalation is initiated a back pressure develops within the reservoir 34 causing the plate 98 to reseat at the distal end of the conduit 92 and the plenum chamber 26 again to be pressurized for switching the valve 16 to its off condition. As the patient exhales, the plate 56 of the fitting 44 is unseated, against the compression spring 54, so that the discharge orifice 58 is open. Thus, the patient normally discharges the air confined within his lungs to ambient atmosphere.
it should here be noted that the filtration system is completely sealed against an introduction of microorganismbearing gas. Thus, the only opening through which contamination of the patient ventilator is accommodated is the fitting 44. Since the fitting 44 can readily be sealed by a suitable bag or similar structure attached thereto, it is possible to store the ventilator 10 in a completely sterile condition.
A desired sterile condition is readily achieved simply by subjecting the assembled patient ventilator 10 to a temperature suitable for destroying microorganisms contained therewithin. Since the various components of the patient ventilator are formed of materials readily adapted to withstand the required temperatures, the patient ventilator can be sterilized and stored in only a fraction of time heretofore required, and stored in a practical and economic manner.
In view of the foregoing, it should readily be apparent that the ventilator of the instant invention provides a practical solution to the perplexing problem of avoiding an introduction of infectious organisms into patients as they are treated employing a patient ventilator.
Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the illustrative details disclosed.
What is claimed is:
1. in a heat-sterilizable patient ventilator of the type including a ported center-body having means defining a circumscribing annular shoulder, a shell mounted on said shoulder for defining in juxtaposition with the center-body an hermetically sealed reservoir for confining gas consisting essentially of a bacteria-free mixture of ambient atmospheric gas and pure oxygen, and means for discharging said mixture from said reservoir including a first tubular gas conduit extended from said reservoir to a discharge fitting, the improvement comprising:
means for delivering bacteria-free gas to said reservoir including,
A. a second tubular gas conduit of relatively constant diameter extending from said reservoir having means defining at one end thereof a gas discharge port communicating with said reservoir and means defining at the other end thereof an ambient gas intake port remotely related to said gas discharge port;
B. filter means connected to the second tubular gas conduit at the said other end thereof for substantially sealing said ambient gas intake port against entry of microorganisms including:
1. a filter housing,
2. a dust filtration unit seated in said housing formed of Scott filter foam,
3. a microbial filtration unit seated in said housing in juxtaposition with said dust filter unit including a plurality of screen-supported bacterial retentive pads formed of FM-004 fiberglass filter material, and
4. a media migration filtration unit seated in said housing in juxtaposition with said microbial filtration unit formed of Scott filter foam; and
C. means including a gas jet coaxially related to said second tubular gas conduit for discharging into the second conduit a stream of gas consisting essentially of pure oxygen.
2. The improvement of claim I wherein said patient ventilator is fabricated from materials adapted to withstand heat of predetermined temperatures for periods of predetermined durations without experiencing substantial degradation.
3. The improvement of claim 1 further comprising filter means for filtering bacteria from said stream of gas consisting essentially of pure oxygen.
4. The improvement of claim 3 further comprising means for mounting said shell in a substantially unstressed condition including means defining a studreceiving opening in said shell, a stud extended through said opening having an annular shoulder disposed in juxtaposition with said shell, a stand-off washer seated on said shoulder and extended through said bore, and an O-ring concentrically related to said stand-off washer for hermetically sealing said bore.
5. The improvement of claim 4 further comprising means defining an annular groove circumscribing the shoulder of said center-body, and an O-ring seated in said groove in contiguous engagement with the adjacent surface of said shell.
6. The improvement of claim I wherein said second tubular gas conduit includes a venturi, the downstream end of which is disposed in juxtaposition with said reservoir.