US2944405A - Conservation arrangement - Google Patents

Description

' July 12,1960 c, BASORE ETAL 2,944,405

CONSERVATION ARRANGEMENT Filed 001;.

H i "E .Y

INVENTORS. Q

g za kgMaz/a/ United States Patent "i CONSERVATION Clebume A. Basore, Auburn, Al a.,, and William L. Ban

ham, Baton Rouge, La., assignors, by mesne assignments, to Union Tank Car Company, Chicago, 11]., a

corporation of New Jersey Filed Oct. 27, 1955, Ser. No. 543,060

18 Claims. (Cl. 62-54) This invention relates'to means and method of prevent ing vapor losses in storage tanks and more particularly to a. means and method of reducing dififerential pressure between the interior and exterior of a storage tank.-

Storage tanks today are usually of the type. WhlCh are mounted above ground and exposed to the elements of nature. These tanks, generally speaking, have a relatively thin wall which defines a space containing liquid in the bottom portion and fumes, gases and vapors in the top orchamber portion In the 24 hour cycleof ,a 'day the storage tank is usually subjected in the day time to the rays of the sun and general heating, and then at'nig'ht to ape-nod of cooling when the temperature of the ambient air is,

below-{the temperature to which the tank was previously heated; This cycle of heating and cooling ,is welljknown in the art, hence is not discussed in more detail.

During the heating phase, it is apparent that the rays of the sun are directedfrom various angles and at vary.

ing. ntensity on the tank, with substantially no rays. be-

ing directly against the bottom. Hence, an uneven hea,t-.

ing wi'thin thetank takes place whereby the liquid surface. of the liquid within the tank is heated to a higher temperature than the remaining liquid. This uneven heating results in a relatively cool liquid. core. within the Forthis andother reasons the prior art tanksuhaveusual 1y been provided with a pressure-vacuumrelief: valve; which opens at a predetermined"difierential pressure ,to.

allow either escape of. the. vapor .to, the atmos here when a pressure is built, up in the tank or the takinginof air when a partial vacuum isinducedin the tank.

2,944,405 Patented July 12, 1960 tank. In the latter instance, the fluid constituting, the

vapors and gases in the chamber portion of the storage tank must, of necessity be condensable in order for the apparatus to function properly.

In the present invention, contrary to the prior art, the vapors and gases in the chamber portion ofthe storeage tank need not. be condensable, .even though some condensation thereof'maytake place as hreinafter be described. I a V s Briefly, the apparatus of the present invention includes a circulation conduit or passage meanswhich commnni cates with the vapor phase of the contentsof a storage tank at a plurality of points. Circulation of the vapor through the conduit is induced by a temperaturedifierential created by a heat exchanger in the path of the conduit. Many means may be employed as acoolantin the heat exchanger; however, the preferred embodiment of our invention, as hereinafter disclosed, utilizes the: liquid phase of thecontents of the tank .as this coolant.

The method of our invention, wherein this apparatus is employed, may broadly be described asa process ofinduc- -ing conditions favorable to equilibrium distillation; or,

depending on the chemical nature. .of the liquid, .condi tions iavorable to maximum absorbing,- or dissolving of the vapor phase into the liquid phase. ,As pointed out; n above, the contents of'a storage tank. are heatcdaunevenl y during the day portion of a 24hour time cycle.

these climatic conditions, the temperature. ofthe surface of the liquid Phase Within the .tank is much highenthan. theren ainingliquid. In thevapor phase there isalso;

uneven heating, with the vapors adjacent the walls and vapors adj c nt the top being heated t a hi e t mperav ture than the vapor in the central portion of the tank...

.B-Y p ng he appa atu of this inventi ntt e vapo s wh c r n h u t are caus.ed, y P s ns i u d@ from-the liquid phase adiac 'nt these. vapors to coalesce .Itlis, apparent. that, many disadvantages accompany operation of the pressure-vacuum relief valve. Upper--- I most amongthese disadvantages'is that .an'appreciable amount. of valuable product islostwhen the vaporspass:

into the a'ir. Another disadvantage is thatatheai hi h.

is taken intfo'the, tank during partial vacuum is available:-

for expansion and Pressure build-upon a following day and'becomes a reactant to accelerate the corroding .of the inside of the tank.

In the pastmany devices have been. used in an attempt to reduce the pressure build-up in storage tanks and to prevent-vapor losses therefrom; Generally theseprior' devices, may be classified as expansion roofs, floating roofs, vapor bags and refrigeration mechanisms, The devices which incorporate the expansion roots or vapor flags-operate on a principal-of providing. additional volume into=whichthej vapors may expand, while therefrigeration mechanism attempt toconvert. the vapors back into liquid'to bereturned to the liquid phase in the storage blanket over the liquid surface. Of course, during .t is processsome of-the condensed vapors return .tothel-iquidr;

and become heavier. Theheavier 'vapors,..some .ofwhich may belcond'ensed ,but. do not necessarily have to becondensed; are directed by the conduit, down along the liquid surface to form what may best be described as;

a heavy vapor blanket, Concurrently-the hot vapors xfrom' the sides and'top ofthe tank are urged up and through the conduit where they, in turn, are cooled" As the operation continues, movement of theheavy vapors inzthe heat exchanger tendsto aspirate additionallhot vapors whichare cooled and distributed as additionalivaporo phase,

w nds-railwa cool vapor over the. liquid surfaceequilibrium drstillation'conditions are set up whereby-the rate of distillation or evaporation is reduced. tFurthe s,

C001" 3199 t n t coo th l qui surfa thus;

farther reducing the tendency of the liquid to. evapor e The immediate effect'fof' setting up. equilibrium. distilla-.

tion conditions is to causev substantial absorption Ofzillfi vapors of the blankctrinto the liquid phase, to. appreciably reduce the positive pressure within the. storage Under these conditions the above operation reducesth POSITIVE Pressure in th tank Iosuc'h an extent that suhstantially no vapors are lost through operationoftheconventionalpressure-vacuum relief valve'. l

On the ,other hand, ,at night the system is ,not operated hence there isa pressure build upto overcome the partiah vacuum normally encountered, While this .operation isnot completely understood at this time, it, in all probability-,

maybe described as'a gradual release of vapor from the supersaturated liquid surface and a gradual: returnIbf thesystem to one where the. vapors andthe'liquid. are n equilibrium at the existing, temperature. The: pressure build up to overcome .the naturally encountered vaeu maybe at end d t t e; st. that he reduction. ole-4 1 Under pressure and volume of the vapors over the liquid caused by natural cooling, promotes the evaporation of part of the more volatile liquids into the vapor space. The resulting increase in the volume of the vapors nullifies the reduction in volume mentioned above. This is an application of Henrys law.

Accordingly, it is an object of the present invention to provide .a means and method of maintaining the pressure, within a partially filled storage tank which is exposed to the atmosphere, within predetermined limits during the normal heating and cooling cycle of a day.

Another object of our invention is to provide a means and method of creating and maintaining a relatively high concentration of vapors or a heavy vapor blanket over the liquid surface of a-liquid within a storage tank.

Another object of our invention is to provide a means and method of cooling the liquid surface of the liquid within a storage tank.

Another object of our invention is to provide a means and method for reducing vapor losses in a storage tank partially filled with liquid.

Another object of our invention is to provide a means for reducing vapor losses in storage tanks which means is economical to construct and install on conventional storage tanks, efiicient in operation and durable in structure.

Other and further objects and advantages of our invention will become apparent from the following description when taken in conjunction with the accompanying drawings wherein like characters of reference designate corresponding parts throughout the several views, and wherem:

Figure l is a schematic, side elevational view of a conventional storage tank with the apparatus constructed in accordance with this invention being installed thereon.

Figure 2 is an enlarged fragmentary view, partly in section, of the heat exchanger of the apparatus shown in v Figure 1.

Figure 3 is an enlarged fragmentary view of a portion of the apparatus herein disclosed and illustrating a modified embodiment of the invention. a

Referring now in detail to the embodiment chosen for purpose of illustration, numeral denotes the base of a conventional storage tank having the usual cylindrical body or side 11, and an upwardly concave formed top 12 While we have illustrated but a single embodiment of a storage tank, it will be apparent to those skilled in the art that our invention, as hereinafter disclosed, is

applicable to many types of storage tanks and that the spe- I cific configuration of storage tank shown is but one of these types.

The conventional storage tank is also shown as having the customary pressure-vacuum relief valve 13 in top.

12, which valve communicates with the interior of the tank to allow escape of vapors when a pressure is built up within the tank, and also to permit the taking in of ambient air when a vacuum is induced in the tank. According to the prior art, this pressure-vacuum relief valve 13 may be set to open when there is A to 4 /2 ounces per square inch or more of pressure exerted by the fluid within the tank or when there is a vacuum of approximately ounce per square inch or more within the tank. Of course, the pressure-vacuum relief valve may be set to function at other ditferential pressure limits; however, it is within the above stated limits of pressure and vacuum that the present invention is particularly adapted to operate.

For purpose of illustration, the storage tank is shown in Figure 1 as being partially full of liquid 14. The type and chemical composition of the liquid is not of great importance in the operation of this invention; however, as pointed out above, the present invention is adapted to volatile liquids of the type usually found in storage tanks and particularly petroleum and its products. The surface of liquid 14 may be higher or lower in the tank, depending upon the quantity of liquid stored. As seen in Figure 1, the liquid surface 15 dividm the tank into 4 a chamber portion 16, and a liquid portion, containing liquid 14.

According to our invention, a pipe 17 is mounted on the roof 12 with the inlet port 18 o f the pipe projecting through roof 12 to communicate with chamber portion 16. Pipe 17 extends upwardly' and externally of the storage tank and then projects horizontally and downwardly to terminate above roof 12. In some instances it may be found desirable to provide a valve, such as valve 19 in the passage of pipe 17. This valve 19 is used to open and close the passage way of pipe 17 to thereby permit or prevent travel of fluid through pipe 17.

The free end of pipe 17 communicates with a heat exchanger denoted generally by numeral 20. In the present embodiment this heat exchanger 20 is mounted above roof 12 and is connected thereto by a pipe 21 which provides a channel between heat exchanger 20 and chamber portion 16 of the storage tank. Pipe 21 is so connected to roof 12 that its discharge port 22 is spaced from the inlet port18. The exact location of the respective ports is not critical and it is apparent to those skilled in the art that a plurality of ports and passage ways may be provided for the passage of fluid to and from the heat exchanger. discharge port 22 direct the fluid toward liquid surface 15.

As best seen in Figure 2, heat exchanger 20 includes a hollow casing 23 and plates 24, 24. Plates 24, 24 are normally disposed horizontally in casing 23 to dividethe same into an upper cavity 25 in communication with pipe 17, a lower cavity 26 in communication with pipe 21' and a central cavity or intermediate chamber 27. Disposedvertically within casing 23 are a plurality of spaced heat exchanger pipes 28 which are retained in position by their respective ends being seated in appropriate apertures in plates 24, 24 thereby providing a plurality of spaced passageways between cavities 25 and 26.

It will now be seen that we have provided a completely may'travel from chamber portion 16, up through pipe 17 past open valve 19 and into upper cavity 25. Thence, down through the channels of pipe 28 through lower cavity 26, through pipe 21 to return to chamber portion 16.

It should be noted that any number of pipes 26 within heat exchanger 20 may be provided, that is, there may be one pipe or a plurality of pipes depending upon the? size. and other easily determinable technical characteristics of the storage tank to which the invention is ap- L plied.

To provide a means of circulating a coolant to and f from the heat exchanger 20, a suitable pump 29 is posi tioned adjacent the liquid containing portion of the storage tank. A liquid supply tube 30 is connected between the suction end of pump 29 and extends through side 11 of the storage tank to provide communication between the central portion of liquid 14 and the pump 29 so that.

pump 29 is provided with a source of relatively cool liquid. The discharge of pump 29 is connected to dischargetube 31 which extends upwardly to terminate at heat exchanger 20 with the mouth of tube 31 in com-.

munication with central cavity 27.

To return the liquid in-central cavity 27 to the liquid container portion of the storage tank a return tube 32 nected to sidell since, in this way, circulation of liquid- 14 is accomplished when pump 29 is operating. In some.

instances it may be found desirable to extend liquid supply tube 30 to approximately the axial center of the It is desirable, however, that pipe 21 and 1 solution.

5- storage or to other locations to provid-e'access to the relatively cool portion of liquid 14.

It is readily apparent to those skilled in the art that other sources of coolant are available and that in locationsrwhere an abundant watersupply is available, it may be found advantageous to connect liquid supply tube 30 to the water supply and discharge this circulated water externally of the storage tank, by relocating the discharge end of return tube 32. For this purpose gate valves 31a and 32a are provided in the tubes 31 and 32, respectively.

It will be noted that pump 29 may be any type of conventional pump which will supply sufficient head and capacityto circulate the coolant in sufficient volume for the purposes hereinafter described. It will also be understood that pump 29 may be operated in any conventional manner but since it ispreferably to coordinate the action of pump 29 with the build up of pressure within the storage tank, a pressure static switch 33 is connected in. communication with chamber portion 16 to actuate pump 29 through appropriate electrical connections (not shown) whereby pump 29 is started and stopped in response to the action of switch 33- Valves such as valves 34 and 35 may be connected to supply tube 30 and return tube 32, respectively, to seal the coolant in the tubes when the pump is not in operation. 7 V

In the operation of the invention, it will be understood that the relief valve v1.3 is setto open at a predetermined pressure'above atmospheric. It will also be understood that switch 33 may be arranged to operate at a pressure lessthan the opening pressure 'for the operation of valve 13 whereby upon pressurebuild up within chamber portion 16, switch 33 will be actuated to startpump 29 beforevalve lit-opens. As the'su'n comesup in-th'e morning,- its rays strike the skin or surface of the storage. tank, and warms it. Thus, heat is transferred to the gases, vapors, fluids, and liquids which are internally adjacent the surface of the tank. As the sun continues its travel, it is apparent that its rays are directed more and more against roof 12 and therefore, that portion of the fluid within the storage tank which is immediately adjacent roof 12 is usually heated to a greater extent than the remaining vapors and gases. .Further, heat is transmitted, probably to a large extent by radiation from the inner surface of roof 12 and also to some extent by convection, to liquid surface 15. It is thus seen that a condition is set up within the tank where the mean temperatures of the gases and vapors in chamber portion 16 of the tank are increasedand the temperature of liquid surface 15 is increased leaving a relatively cool volume of liquid 14 in that area of the tank below liquid surface 15. -This condition resultsin a build-up of pressure in chamber'po'rtion lfi-because of expansion of the gases and vapors within chamber portion 16 andbecause, of evaporation of liquid from the liquid surface.

Now, according to Henrys law, the quantity of gas dissolved in a given quantity of solvent or solution is directly proportional to the gases partiallpressure overthe Henrys law applies for the individual components of a gaseous mixture as well as for a single pure gas, the concentration of each component dissolved being, according to the law, proportional toits own partial pressure and not the total gas pressure. Theoretically Henrys law holds true over the range of pressures and temperatures contemplated by this invention. Deviations from this law will occur if liquid 14 and the gases in ,storagetanks, those deviations which do occur will usually tend to decrease rather than increase the total pressure exerted in chamber portion 16. Further, Raoults law states that in vapor-liquid equilibrium the partial pressure of any component: in a liquid isequal. to the product of its mole fraction inthe liquid and the-vapor pressure of the pure component at the same temperature. From the above laws it will be seen that the area in chamber 16 directly over the liquid surface 15 has a pro.- found efiect on the rate of distillation or dissociation. In general it can be said those liquids which have a high vapor pressure have a. correspondingly low boiling point since boilingoccurs when the vapor pressure of the con.- stituent under standard conditions reaches atmospheric pressure. Thus, thelow boiling point constituents of liquid 16 exerts relatively higher partial pressures. f

Assume now that liquid 14 is a petroleum product containing for example pentane and hexane. As-the storage tank is heated by the sun, the lighter fractions, pentane, will distill or dissociate first and then with further heating the heavier fractiomhexane, in larger quantities will distill or dissociate from the liquid. With this distillation or dissociation, the pressure within chamber 16 increases sufliciently to actuate pressure static switch 33 to start pump 29. Thus, relative cool liquid from liquid 14 is circulated through central cavity 27. This circulation tends to cool those vapors and gases inthe heat exchanger pipes 28 to cause the vapors to coalesce, become heavier and pass by gravity and by heat difierential circulation down through lower cavity 26 through pipe 21 into chamber 16. Because these coalesced vapors are generally heavier than the surrounding vapors they continue to travel downadjacent liquid surfacedS where'some of the vapors'reassociat'e with the liquid' 14 while. a substantial portion of the remaining coalesced vapors distribute themselves irmnediately above l quid surface 15. v

The coalescing of the'vapor as described'above hasan' 1 asperating effect within heat: exchanger pipes 28 to thus draw additional vapors and gases through pipe 17 and into the .heatexchanger pipes 28. Thus it is seen that a circulation of vapors and gases is set up wherein a portion of the vapors and gases in chamber portion 16 is cooled, coalesced'and returned to chamber portion,- as. a vapor blanket along'liquid surface 15. 1

Attentionis now directed to Figure 3 which showsfthe other conditions such as severe climatic conditions 1'8? quire a vapor flow greater than that induced by the gravity flow described above. 1 V r a It will'be understood that the immediate effect, of the circulation of liquid 14 in'heat exchanger 20 is an appre-v ciable'lowering in the positive pressure within the storage tank. This lowering of pressure cannot be entirely explained by a siinplecooling of'the vapors. Instead, the concentration of vapors immediately above the liquid surface 15, cools the liquid surface and set up. equilibrium distillation conditions. Thevapors immediately above the liquid surface 15, that is, the vapor blanket issuch that any additional pressure build-up will cause a forcing of a this vapor into the liquid. Further, the mole fraction concentration of each vapor is such as to discourageits complementary fractionin the liquid phase from vaporizing. a

At night the vapors of the vapor blanket tends to dissi-l pate itself into the other gases and vapors, thereby =tending to oppose the creation of a vacuum in the tank.

It will be obvious to those skilled in the art that many variations may be madein the embodiment chosen for purpose of illustration without departing from the scope of my invention as defined by the appended claims.-

We claim:

' 1. In a conservation apparatus for use with a volatile liquid storage tank, said tank defining a liquid containing portion and a chamber portion wherein Yaporscollect;

7 the combination of a plurality of openings communicating with the chamber portion, passage means interconnecting said openings and establishing closed constant volume communication therebetween, said passage means being disposed externally of said chamber portion and temperature changing means operatively associated with said passage means to cool said vapors and induce circulation thereof through said passage means, said temperature changing means comprising a chamber surrounding a segment of said passage means and additional means associated with said chamber for circulating a substance therethrough to efiect a temperature change of the segment of said passage means, said circulating means comprising passageways for establishing a closed communicating circuit between the chamber and a source of said substance external of the chamber, and a pump mechanism in at least one of said pasageways to eflect said circulation.

2. A conservation apparatus according to claim 1, wherein one of said openings communicates with said chamber portion at a point vertically above the other of said openings.

3. The conservation apparatus of claim 1, wherein there is additionally provided means to selectively control the operation of said mechanism to provide intermittent circulation of said substance.

4. A conservation apparatus according to claim 3, wherein said last mentioned means is responsive to the pressure'in said chamber portion.

5. A conservation apparatus according to claim 3, wherein said last mentioned means comprises a pressure switch arranged to operate said mechanism when the pressure in said chamber portion reaches predetermined limits above or below atmospheric pressure.

6. A conservation apparatus according to claim 1, wherein said source is said liquid containing portion, and the substance comprises the liquid in said last mentioned portion.

7. A conservation apparatus according to claim 5, wherein said source is said liquid containing portion, and said substance is the liquid in said last mentioned portion.

8. In a method of conserving vapors in atank used for storing volatile liquids, said tank having a liquid containing portion and a vapor containing portion; the steps of accommodating emission of part of the vapor in said vapor portion from one point in said vapor portion, circulating said vapor externally of said vapor portion, changing the temperature of said circulating vapor during said circulation by moving liquid from said liquid portioninto such physical relation with the circulating vapor to effect the heat transfer therebetween, and depositing said circulated vapor into said vapor portion. 7

9. The method of claim 8, wherein there is additionally provided a step of controlling the circulation of the liquid insuch a manner that saidliquid circulation is responsive to the pressure in said vapor portion.

10. Apparatus for retarding pressure build up in a storage tank mounted at least partially above ground and incidentally subjected to the rays of the sun, said storage tank also having a conical roof and a chamber portion below said roof and a liquid containing portion below said vapor containing portion, said liquid containing portion normally having liquid therein and said chamber portion normally containing vapors from said liquid therein, means to release said vapors from said chamber portion when said pressure in said storage tank reaches a predetermined pressure above the pressure of said ambient air and to permit the influx of ambient air into said storage tank when said storage tank reaches a predetermined vacuum below the pressure of said ambient air, comprising a first pipe mounted externally of said storage tank at the apex of said roof, the entrance to said pipe being in communication with said chamber portion, said first pipe extending upwardly and then horizontally and then downwardly, a vertically disposed second pipe mounted on said root and having an exit in communication with said chamber portion at a position spaced from said entrance of said first pipe, said second pipe being aligned with the downwardly extending portion of said'first pipe, a heat exchanger externally of said storage tank and connected between the free end of said first pipe and on the top of said second pipe, and means for circulating said liquid from said liquid containing portion through said heat exchanger and back to said liquid containing portion.

11. Apparatus for retarding pressure build up in a storage tank mounted at least partially above ground and incidentally subjected to the rays of the sun, said storage tank having a roof and a chamber portion below said roof and a liquid containing portion below said vapor containing portion, said liquid containing portion normally having liquid therein and said chamber portion normally containing vapors from said liquid therein, comprising a first pipe mounted externally of said storage tank on said roof, the entrance to said pipe being in communication with said chamber portion, a second pipe mounted on said roof and having an exit in communication with said chamber portion at a position spaced from said entrance of said first pipe, a heat exchanger mounted above said storage tank and connected between the free end of said first pipe and on the free end of said second pipe, said heat exchanger being provided with an upper cavity and a central cavity and a lower cavity in vertical relationship to each other, said upper cavity communicating with said'first pipe, said lower cavity in communication with said second pipe, said heat exchanger being further characterized by a plurality of vertically disposed'heat exchanger pipes extending through said centralcauity and forming a path between said upper cavity'and said lower cavity, conduit means for the circulation of saidliq'uid from said liquid containing portion through said central cavity and back to said liquid containing portion, pump means connected to said conduit means for circulating said liquor, and control means responsive to the pressure in said tank for actuating said pump means.

12. Apparatus for retarding pressure build up in a storage tank mounted at least partially above ground and incidentally subjected to the rays of the sun, said storage tank being provided with a pressure-vacuum relief valve which opens when the pressure differential between the ambient air and the interior of the storage reaches predetermined limits, said storage tank also having a conical roof and a chamber portion below said roof and a liquid containing portion below said vapor containing portion, said liquid containing portion normally having liquid therein and said chamber portion normally containing vapors from said liquid therein, said pressure-vacuum relief valve being mounted in said roof in communication with the upper portion of said chamber portion to release said vapors from said chamber portion when saidpressure in said storage tank reaches a predetermined pressure above the pressure of said ambient air and to permit the influx of ambient air into said storage tank when said storage tank reaches a predetermined vacuum below the pressure of said ambient air, comprising a first pipe mounted externally of said storage tank on said roof, the entrance to said pipe being in communication with said chamber portion at the apex of said roof, a vertically disposed second pipe mounted on said roof and having an exit in communication with said chamber portion at a position below and spaced from said entrance of said first pipe, a heat exchanger externally of said storage tank and connected between the free end of said 'first' pipe and on the top of said second pipe, said heat exchanger being provided with an upper cavity and a cen tral cavity and a lower cavity in vertical relationship to each other, said upper cavity communicating with said first pipe, said lower cavity communicating with said second pipe, said heat exchanger being further characterized by a plurality of vertically disposed heat exchanger pipes extending through said central cavity and forming a path between said upper cavity and said lower cavity, conduit means for circulation of said liquid from said liquid containing portion through said central cavity and back to said liquid containing portion, pump means connected to said conduit means for circulating said liquid in said conduit means, and means responsive to pressure with said tank for actuating said pump means.

13. Apparatus for retarding pressure build up in a storage tank mounted at least partially above ground and incidentally subjected to the rays of the sun, said storage tank being provided with a pressure-vacuum relief valve which opens when the pressure differential between the ambient air and the interior of the storage reaches predetermined limits, said storage tank also having a roof and a chamber portion below said roof and a liquid containing portion below said vapor containing portion, said liquid containing portion normally having liquid therein and said chamber portion normally containing vapors from said liquid therein, said pressure-vacuum relief valve being mounted in said roof in communication with the upper portion of said chamber portion to release said vapors from said chamber portion when said pressure in said storage tank reaches a predetermined pressure above the pressure of said ambient air and to permit the influx of ambient air into said storage tank when said storage tank reaches a predetermined vacuum below the pressure of said ambient air, comprising a first pipe mounted externally of said storage tank on said roof, the entrance to said pipe being in communication with said chamber portion, a vertically disposed second pipe mounted on said roof and having an exit in communication with said chamber portion at a position spaced from said entrance of said first pipe, a heat exchanger externally of said storage tank and connected between the free end of said first pipe and on the top of said second pipe, said heat exchanger being provided with an upper cavity and a central cavity and a lower cavity in vertical relationship to each other, said upper cavity communicating with said first pipe, said lower cavity communicating with said second pipe, said first and second heat exchanger pipes extending through said central cavity and forming a path between said upper cavity and said lower cavity, and means for circulating said liquid from said liquid containing portion through said central cavity and back to said liquid containing portion.

References Cited in the file of this patent UNITED STATES PATENTS 977,281 Ely July 11, 1911 1,490,782 Milligan Apr. 15, 1924 1,619,661 Field Mar. 1, 1927 2,059,942 Gibson Nov. 3, 1936 2,083,611 Marshall June 15, 1937 2,313,087 Parr et al. Mar. 9, 1943 2,682,154 Wilkinson June 29, 1954 FOREIGN PATENTS 685,425 Germany .Q Dec. 18, 1939 954,926 France June 20, 1949

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