US2439617A

US2439617A - Immersible electrical heater

Info

Publication number: US2439617A
Application number: US603341A
Authority: US
Inventors: Robert C Belgau
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-07-05
Filing date: 1945-07-05
Publication date: 1948-04-13
Anticipated expiration: 1965-04-13

Description

April 1948. k. c. BELGAU 2,439,617

IMMERSIBLE ELECTRICAL HEATER Filed July 5, 1945 I 4 Sheets-Sheet l INVENTOR.

April 13, 1948.

R. c. BELGAU 2,439,617

IMMERSIBLE ELECTRICAL HEATER Filed July 5, 1945 4 Sheets-Sheet 2 PEESSZ/IFES 75 7' TO HEB TE 12 1N VENTOR,

CU? flver/ eyau J fiewawq ATTORNEY.

4 Sheets-Sheet 3 R. C. BELGAU IMMERSIBLE ELECTRICAL HEATER Filed July 5, 1945 April 13, 1948;

INVENTOR. fiaer/Cfzema an? ATTORNEY.

April 13, 1948. BELG'AU 2,439,617

IMMERSIBLE ELECTRICAL HEATER Filed July 5, 1945 4 Sheets-Sheet 4 INVENTOR.

Patented Apr. 13, 1948 UNITED STATES PATENT OFFICE IMMERSIBLE- ELECTRICAL "HEATER Robert 'Ci Belgau, Detroit,"Mich.

Application July .5, 1945, SerialNo. 603,341

12 Claims. 1

This invention relates to heating -apparatus, and more particularly-to electrical apparatus for heating liquids and solid materials, the heat units being generated-at, or within, the. process vessel.

In one-form of the invention, the invention comprises a tubular heating device which is immersed in the product to be heated and includes means for vaporizing aliquid-the vaporsofwhich flow through a heat-radiating system, also: immersed in-the material, to transfer the-heat of the vapors thereto.

There are'many industrial processes wherein it is desired to heat" a-material at a rapid rate, while insuring that no part of the material is brought to a temperature-which'would produce burning or deterioration thereof. The requirement 'for accurately controlled heating means is especially stringent in the case'of thermo-plastic compositions and foodstuffs, both ofwhich deteriorate above acertain critical-temperature.

"Such materials haveinthe past been heated by various methods. One method comprises a central steam generating plant, utilizing coal or other fuel, to supply superheated steam-tothe various process tanks in which the products'are heated. The temperature of the superheated steam may in such installations be accurately controlled,-so as to limit the-maximum temperature to which the material-will be subjected; but where high temperatures are required as for many'modern processes, the steam plant must be operated at correspondingly high pressuresupwards of 250 to'300 pounds per square inch. The high-pressuremethod is very costly and unsatisfactory in many cases, because-in-order to heat a single containenit isvnecessary that-the entire steam generating plantbe put into operation; with consequent excessive heatingcosts. Heat losses in conveying the high-pressure steam to the point of use are considerable, and the system as a whole lacks flexibility; -the-pumps, valves and accessories demand costly maintenance, and boilers must beperiodically re-tubed.

and fire-boxes rebuilt at great cost. 'Further-= more,'where process vessels are ntted with steam coils therewithin; a-considerable amount of labor is required to disconnect and remove the coils when periodically cleaning the process vessel.

Combustion gases are being applied to the heating ofa liquid heating medium at the site of use, the heating medium transferring itsheat to the material being heated. This method is very satisfactory for many applications, but leecause of fire hazard it is not satisfactory or desirable for heating many inflammable products, or Where, for other reasons, electricity is preferred, nor is it satisfactory for installations where flexibility is 'of primary. consideration.

Electrical apparatus for heating the heattransfer medium-or for directly heating the material, .isalso being used, and generally consists ofan electrical heating element which is immersedin the heating medium or liquidmaterial in such manner that without the use of a circulating means, no effective circulatory actionis obtained, so that burning of the material or a prolonged heating cycle may result; or by the use of .stripheaters attached to the outside of the container, in such-manner that a large proportion of the heat generated is radiated or transmitted to the container wall as ambient heat instead of direct heat. In any case where the heat is applied externally to the container, the container walls and external heating mass must, of course, be brought to a temperature above that towhich the contents are being heated; and because the heat flows more readily Within the metallic wall, in a direction parallel to the surfaces thereof, than directly through the Wall at right angles to the surfaces, the diffusion prolongs the heating cycle with consequent increased heating costs.

In any ofthe' systems just referred to, there is' likely to be a considerable lag between the time when the heat supply'is first turned on and the time when the heating area reaches its'maxlmum safe potential, above that to which the product-'must'be heated. Then, when the maximumoperatingtemperature of the heating-mass is reached, and the current supply is turned off, heat units continue to flow from the higher potentialinto the product until the potential no longer exists, or until the current supply isagain turned on. 'Ihis is called overshooting. The greater the heating mass the longer the time lagsand thegreater the degree of overshooting, with the result-that it isdifiicult to maintain close -temperature control. This is a very objectionable condition in applications where fluctuating temperatures are harmful to the product.

Furthermore, in applicationswhere stripheaters are attached-directly to the exterior walls ofthe-productcontainer, -hot spots occur because of the difficulty in providing uniform heat distribution throughout the heatedwall area.

=In-the-present"invention, the heat units are generatedelectrically at the site of use, and within the-heating area which is submerged within, and indirect contact with, the liquid heating medium so that all heat units are directly transmitted thereto. As a portable immersion heating system the device is complete andindependent' of any other equipment, except for the vessel'intowhich it can be immersed, and the only attachments to this device are the electrical cables which can be connected to the current supply by means of conventional male and female receptacles, thereby eliminating the dismantling labor usually required where other heating systems are employed. The electrical resistance elements are so arranged that they are immersed in the liquid heating medium and are of relatively small cross-sectional area with the total mass of the heating area being relatively small, thereby providing a rapid rate of heating, and of cooling, so that the objectionable lag and overshooting is reduced to a negligible amount. All heat units generated by the resistance elements are transmitted directly into the heating medium. A system of stacks, or bafile plates, induces a thermo-syphon action during the heating cycle and the heated medium is thus partially vaporized; and, in turn, transmits heatunits to the material being heated.

An object of the present invention is to provide a heating device of simplified construction which will avoid the disadvantages of the prior art devices, in that it will be independent of other units and dependent only upon a supply of electricity at or within the process vessel.

A'further object of the invention is to provide a heating unit which may be readily inserted in and removed from the container of material to be heated, and which may be used to supplement the stationary heating means usually connected to the container, so as to heat the material to the desired temperature quickly and uniformly with a minimum of current consumption.

A further object of the invention is to provide a heating unit which will transfer substantially all of the heat units directly into the material to be heated, the container for such material becom heating unit which may be immersed in the material to be heated, and wherein no part of the unit will be heated above a temperature representing the maximum degree to which the mate rial may be safely subjected.

Further objects and advantages of the invention will be apparent from the following description, taken in connection with the appended drawings, in which:

Fig. 1 is a view in end elevation of a portable heating unit embodying the invention, the cover plate being removed to disclose the electrical connections;

Fig. 2 is a longitudinal section of the same, taken on the line 22 of Fig. 1;

Fig. 3 is a plan view of the same;

Fig, 4 is a sectional view of the vaporizing part of the apparatus, taken on the line 44 of Fig, 2 and with the heater elements removed.

Fig. 5 is a view in horizontal section of a heating cell which is a component part of the immersion heating system but modified and designed to be permanently installed in the vapor-jacket of the vessel which contains the product to be Fig. 8 is an end elevation of the apparatus shown in Fig. 7; and

Fig. 9 is a diagrammatic representation of the electrical control means of the heating unit.

Before explaining in detail the present invention it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, because the invention is capable of other embodiments according to the size, shape and other requirements to accommodate various applications. Also it is to be understood the phraseology or terminology employed herein is for the purpose of description and not of limitation.

The apparatus shown in Figs, 1-4 comprises a cylindrical tank In having an end wall l2 of dished shape, being preferably welded in place and designed to resist pressures, either superatmospheric or subatmospheric, existing within the tank. The other end of the tank is closed by means of an end wall l4 which has formed therein, preferably by welding, or casting, a chamber l 8 which serves to house the terminal ends of the electrical heating elements and their connections. The chamber I6 is formed of a cylindrical wall I! and an inner wall l9, and is sealed by means of a gasket and cover plate l8 securely held in position against end wall It by means of studs 20, secured in the wall l4 and provided with nuts 22.

A plurality of tubular sheaths 24 extend horizontally through the inner wall IQ of chamber l6, one end of each sheath being fixed in place in apertures in said wall by welding or other means. The forward, or open, end of each of the sheaths 24 extends through the wall l9 into the chamber IE, to permit insertion into the sheaths of electrical resistance elements 25, of a conventional type, each having both terminals at one end thereof, to accommodate electrical terminals 21. Leads 28 interconnect the terminals 21 and extend through a pair of cables 30 to connect to a source of current supply, not shown. The rear ends of sheaths 24 are closed by caps 32, so that liquid Within the tank Ill cannot enter the sheaths.

Supported centrally within the tank l0 and secured in anyconvenient manner, as by tack-welding to wall I9, are a plurality of vertical vanes or baflle-plates 34 which extend centrally and longitudinally between the sheaths 24 so as to direct the flow, b thermo-syphon action of the heating medium contained in the tank Iii, upwardly over the heated sheaths, thus tending to heat the liquid quickly and uniformly and at the same time maintaininga cooling effect on the heated sheaths.

The baffle plates 34 are joined together by upper bars 36 and lower bars 38 to form a rigid grid, the two lower sets of bars being spaced from each other by central bafiie plates 40 which extend somewhat beyond the other baille plates 34 toward the bottom of the tank l0.

The heating medium 42 contained in tank In is maintained at a normal liquid-level indicated at 44, above the upper-most of the sheaths 32. The heating medium may be water, Prestone, mercury,

' alcohol, non-volatile oil, diethylene glycol, Dowtherm or the like, having suitable characteristics to accommodate the desired operating temperature according to the requirements of the application; The space within tank Ill above the liquid level 44 is herein referred to as the vapor chamher 45.

The upper surface of tank It is provided with an aperture within which is welded a pipe nipple 46, internally threaded to receive a threaded pipe nid e re mi nd. hich fse irs as xit t ia isi tl his-i ei ;the r p ionr; eetin the.

hausted by a vacuum pum v he nomaloper he apso e; ,.;.e. snre-w hini hen nks-will ai ess h s. iteena undsinera uare r nchi. he w? has i heiv nor pres ure within the system also lowers the boiling point ofi e s atinaimed-ium ana-othwwim ssures .betrrer qrmanceh se eatmte m ttanee,-;-bec use many sta c cens d summaris resen at li'. Q5T:. -K g hei rationi te rih atin stem.=w erein.-th normal operating pressure ex ed fteen;;pounds; en quare nch he PI... oemp nhea neimed um. hieh-beil:at.35 2= to Q hat the one a-t na press re, i v n: tt inn ra ura f qs5Q 1- sare-belo iteen: .i ounds ;;sueh ns neer= .presence; s,- not required.

A vertical conduit 54 extends upwardly-from the chamber I t being s cu ed inwapertures in-.th e upper walls-pfi 1 2 chamberi l 6 and of the tank; I 0,11 as by? welding Th onduit accommodates hes le fi altahlesi .ch-pa. s upwardly in h sq iduitz nifl lh hroughzanvoutlet 56 xt nding horizontallyitherefrom ataa point; near the pperend-o ihe cond t,

At its upper end conduit 54 is provided with a o rl nting flange .-58,- secu red .thereto as by. welde. ing and hrgugh the flangeextends a pressure trans m tin tu-be 60, A. sum-p. 52:;is secured. to the under; surface of the mounting flange through which the tube. 66,-:extendsasshown in-.Fig. 2. 'Il 'e,;h ting medium enters the sumpby means of.a;t be; 6l atheilower end fiiuofwhichextends hrouehan ape tu e in-the aHH or the chamber I 6 in;$116111position;v as to open immediately 1 below thecnq fmal liquid level inthe. tank I 0. The upper en stube 6t;,ex-tends:into and terminates adjacent he iuppfir portion ;of.-th'e. sum-p well above-din A lower; end -of tube; .6 0;:thesarrangement forminga liq id trap to insure thatthe vaporized med'um; will;;not enter into ,the :pressurestat; whic would be; damagedsby the vapor 1 tempera! tur la -conventional. pressurestat is mounted upon fiange .58, with .a suitable: gasket :therebet-ween toassure weak-moot seal,'and is. anchoredin aeee ri eansnoi; ol s-56.1. Aitubular portion-v 68,, connects with pipe -Gil -and with U .:of the instrument Body 510,; contains aomechanism o convention ii n truct oniv.shownad a r mat c lis -in Fish 9 as sqmpris nga diaphra m I: which respons e to pressureswithin the vapor chamber- 45 Increase 0i such; pressures beyond 1 a pred terminediva uems hby adjustments: the knob. 14;oauses a switch-fl3 in a-relay circuit 15;: to be opened, :therehyadeenergizing therelay, and openin switch H in ,the. maini circuit; formed by,-.le adi 2.8;: om ingiswitchq'll deenergizes the n Q i E t: 1dt l d'sQfit h iupply'Ofclectricity to I t e re ance-e ements; i'iuntil :the' pre r h n 11. P I UISfidhaIallenpto a. .p ln wherein. i hfwrwi-ll aga n be:.,c1.osed- The.

h nsid rahle require theta ev-ic is designed imp y. 1S; :cutrent tqresi tance elements 6 :m y-

lsoibe. controfledmanually,by means f a man-. ual switQh1 The -.adjustable knobjM ,ispror.

vlded' -at the; top ofthe pressurestat, to enable h s eretorgtd adjust the instrument for. the

" desiredoperating temperature. The ,pressurestat.

may beiof oqliventional construction; and because suchmonstruotion is well known, it is. not .further describ edihel'einia The heating unit thus far: describedis designed to be placediinzaprqcess tanklfi to heat the conten hereon-which may be athermoplastic compos n, foodstuffs, Or' any. other. type. of product i ogbe p iq essedi; The location of the 58- ispreferablyso arranged that the heatmayJ-be suspendedavithin process tank ubmer ed ,withini theproduct therein, wit outlet 56.-posi tion8.d above thetanksides to perm th e leadsgjd tqpass from the outlet to a ome, o ur ntsupply, as to a switch upon an 'adjac nt wall The lower side of outlet 55.may be prpvided with; a bracket 69. i which rests upon thenpper; edge of the. container to support the heating system-i so, that it clears the bottom of tank; 1%. Aibracket 18 may likewise be attached to theradiatingH-tube ,82 ;to provide a means .for

supporting.oneendofithe heating unit, and rests upon the top sides, or ends, of .the process tank with the-heating unitsuspendedin the tank and with;all;heatediareas,well submerged within the "productbeingheatedn The levelv of material in tank l6 is indicatedsat 85. This. constructionpermits oi instant-removal :ofthe. heating unit from the; process tank without breaking any con- ...nections -and whether the heating. unit is hot or r COIQ A plurality of.radiatingtubes 80.; 8|, 82 of any suitable size 01115 13118 aretconnected to the int fiQli. ottank llliandextend upwardly therefrom 1 1,ch relation:as. to beentirely immersed in the materialbeing .heated. Each. of the tubes 80, 8l ,;.82 is,-,-provi dedwith a vapor inlet84-c0nnecting to, thesvapor space within tank It), adjacenttheuppermost portion thereof, and with a l q iidi-OutktBEiWhich extends downwardly into a nk i flrto apoint. well below the liquid level 44 therein The radiating tubes operate in a manneij similar to that of steam radiators, conducting the vaporized heating .mediumto the upper .;,portion.;of =the tube where the heat of vaporizat;than byiheapplication of heat from the outside surfaces; Thisform ofthe invention ma be used to advantage in installations wherein the process tank, --indicated at 1B',has no heating means permanently connected thereto, orin which the container;haspermanently connected heatin means which'gs not' a'dequate to bring the material up toitemperaturequickly enough, as in resuming operations. after. a shut-down. If for example, the tank 16 contains thermo-plastic material 1; whichpbecomes solid at room temperatures, the

portable apparatus disclosed in Figs. 1 to 4 may be.placed. inthetank before a shut-down and turnedroit-by means of the manual switch 19 Uponresuming operations after the shutdown, theecurrent. may-besturned onand the normal 7 heating means likewise turned onto bring the material up to the desired temperature within a minimum length of time. As a further means of utilizing the heating apparatus of this type, it may be placed in the process tank prior to a shut-down, and the regulator knob 74 set to maintain a relatively low temperature below the normal operating temperature, so that a correspon'dingly shorter period of time will be required to bring the material !up to operating temperature when operations are resumed, while not causing deterioration of the material such as occurs if it is maintained over too long a period of time at higher temperatures. Regardless of how the device is used, it will be noted that no the heating medium, the temperature of which is controlled by the pressurestat 10; or by radiation or conduction to the cover plate I8, which is so spaced from the ends of the resistance elements 26 that its temperature will not exceed the controlled temperature of the heating medium within the tank I 0.

For installations wherein it is desirable to mount the heating device fixedly in a container of liquid to be heated, the form of the invention 7 shown in Figs. 5 and 6 may be employed. In this form of the invention, the plate 88 corresponds to the end wall I4 in Figs. '1 to 4, and the chamber 90 is similar to chamber I6 in those figures. The plate 88 is secured as by welding to the vertical wall of the process tank, indicated at 92, adjacent the bottom thereof, and is provided with threaded studs 94 for securing on the outside of the tank an electrical outlet box 96, of

tion with Figs. 1 to 4. The electrical leads for e the resistance element are indicated only diagrammatically at I02 in Figs. 5 and 6, the electlical connections and pressure control being similar to those shown in Figs. 1, 2 and 9.

The resistance elements I00 are slidably encased in sheaths I04, which are closed at their inner ends by caps I06 in the same manner as described in connection with caps 32 in Figs 1 to 4, to seal the resistance elements against contact with the liquid being heated. In-order to direct the circulation of the liquid about the sheaths M4, a plurality of vertical ballle members I 08,-I 09 are interposed between the various sheaths I04, and held in assembled relation by upper and lower bars H0, III. The bars III are flanged and welded to the baiile members I09, which extend downwardly below the lower edges of bafile members I08, in order that the resulting grid of bafile members will conform generally to the cylindrical shape of the chamber 90. An end plate H4 is secured as by welding to the ends of the battle members, to provide better thermo-syphon action of the heating medium. I

The form of the invention shown in Figs. 7 and 8 is similar to that shown in Figs. 5 and 6, and the parts are designated by similar reference numerals, except that in Figs. 7 and 8 the chamber 90 is omitted, the sheaths I04 being secured to a plate IIB welded over an aperture in the tank 8 92, with the terminal ends of the resistance elements I00 extending into outlet box 96. This arrangement permits some loss of heat through the outlet box, but is cheaper to manufacture and install than the other forms of the invention disclosed herein.

Either of the forms of the invention disclosed in Figs. 5 to 8 is suitable for use in fixed installations in which the tank 92 serves as a jacket around the vessel containing the product, and the jacket-space contains a heating medium of the nature referred to above. The tank 92 may be hermetically sealed, and designed to transmit heat from the heating medium to the material to be processed, in the manner described in my co-pending application Serial No. 603,776, filed July 9, 1945, now Patent No. 2,437,453, issued March 9, 1948.

The forms of the invention shown in Figs. 5 to 8 may also be used where the material to be processed will not be injured by overheating, or where the danger of overheating is slight, and where the heater tubes will be always submerged in the said material.

Although the invention has been described with'particular reference to certain embodiments thereof, it may be embodied in other forms and is therefore not limited to the forms disclosed, nor otherwise, except in accordance with the language of the appended claims.

I claim:

1. Apparatus for heating material in a container, comprising a tank, liquid heating medium therein, electrical resistance elements extending within said tank for heating said medium, pressure responsive means operative upon the occurrence of a predetermined maximum pressure to deenergize said resistance elements, radiating elements connected to the interior of said tank for transmitting heat from the vaporized heating medium to the material in the container, a conduit leading from said resistance elements to a point above the normal level of material in the container, electrical connections extending through said conduit to. said resistance elements, and means associated with said conduit for suspending the heating apparatus in the container in spaced relation from the walls thereof.

2. Portable heating apparatus for material in a container, comprising a sealed evacuated tank, a vaporizable heating medium in said tank, said heating medium having such va-por tension relative to the degree of evacuation as to produce subatmospheric pressures in the tank at room temperatures and superatmospheric pressures at normal operating temperatures of the apparatus, reentran't members extending into the interior ofthe tank, electrical heating means removably mounted in said reentrant members for heating the heating medium, means for transmitting heat from the heating medium to the material in the container, a conduit fixed to said tank and extending above the container, a pressure-responsive device mounted on said conduit, means in said conduit for transmitting pressures from the interior of the tank to the pressure responsive means, and electrical connections extending from said heatingmeansthrough said conduit and controlled by the pressure-responsive means.

3. Heating apparatus comprising a container for material to be heated, means in a wall of said container forming a reentrant recess, a plurality of reentrant members communicating with said recess and extending into the interior of the container, a plurality of bafile members interposed between said reentrant members, electrical resistance elements removably mounted in said reentrant members for transmitting heat therethrough to the material to be heated, electrical terminals for said resistance elements in said recess, an electrical outlet box mounted exteriorly of said container in juxtaposition to said recess, and electrical connections extending from said terminals through said outlet box.

4. The invention defined in claim 3, wherein the baffle members are formed as a grid and secured in spaced relation by means of transverse members Welded thereto at intervals along their length.

5. Heating apparatus for material in a container, comprising a sealed tank adapted to be submerged in said material, means for suspending said tank from the container walls in spaced relation thereto, a vaporizable liquid heating medium occupying a portion of the space in said tank, said tank being exhausted of gases to produce subatmo-spheric pressures therein at room temperatures, electrical heating means in heatexchanging relation to said heating medium and spaced from the walls of said tank, means responsive to pressures generated by said heating medium for controlling said heating means, and radiating tubes connected to the interior of said tank and extending within said material for receiving vaporized heating medium therefrom and transmitting heat to said material, said suspending means including a rigid conduit fixed to said tank and extending above the normal level of material in the container, and means associated with said conduit for engaging the upper edge of said container.

6. The invention defined in claim 5, wherein said radiating tubes are provided with inlets connected to the interior of said tank above the normal level of heating medium therein, and outlets connected below the normal level of heating medium therein.

7. Apparatus for heating material in a container, comprising a sealed tank adapted to be immersed in said material, means for suspending said tank from the container walls in spaced relation thereto, a vaporizable liquid heating medium in said tank, a plurality of horizontally extending electrical heating elements in heat-exchanging relation to the heating medium in said tank, said elements being arranged in staggered relation and spaced from each other and from the walls of said tank, a plurality of vertical bailles interposed between said heating elements to facilitate circulation of heating medium past said heating elements, and radiating elements connected to the interior of said tank and extending within said material for receiving vaporized heating medium therefrom and transmitting heat to said material;

8. The invention defined in claim 7, comprising in addition means responsive to pressures generated by said heating medium for controlling said heating elements.

9. The invention defined in claim 7, wherein the heating medium occupies but a portion of the free space within said tank, and said tank is provided with an aperture for evacuating gases from the tank, and a check valve in said aperture for preventing reentry of gases therethrough.

10. Heating apparatus comprising a cylindrical tank having a convex end wall and a recessed end wall, a cover plate closing the recess in the recessed end wall, reentrant members extending from said recess to the interior of the tank, electrical heating elements removably mounted in said reentrant members, liquid heating medium in said tank covering said reentrant members to receive heat therefrom, a conduit leading from said recess through the cylindrical wall of said tank, electrical connections extending through said conduit and connected to said heating elements, a pressure-responsive device mounted on said conduit and controlling said electrical connections, a duct connecting the interior of said tank with said pressure-responsive device, saidduct including a liquid trap, and a radiating element comprising a tube having one end connected to the interior of the tank at a point above the liquid level therein and its other end connected to the tank at a point below the liquid level therein, to form a closed circulatory system.

11. Heating apparatus comprising a cylindrical tank having a convex end wall and a recessed end wall, a cover plate closing the recess in the recessed end wall, reentrant members extending from said recess to the interior of the tank, electrical heating elements removably mounted in said reentrant members, liquid heating medium in said tank covering said reentrant members to receive heat therefrom, a plurality of baffle members in said tank for inducing movement of said liquid heating medium past said reentrant members by thermo-syphon action, a conduit leading from said recess through the cylindrical wall of said tank, electrical connections extending through said conduit and connected to said heating elements, a pressure-responsive device mounted on said conduit and controlling said electrical connections, and a duct connecting the interior of said tank with said pressure-responsive device, said duct including a liquid trap.

12. Heating apparatus comprising a cylindrical tank having a recessed end wall, a cover plate closing the recess in the recessed end wall, re-" entrant members extending from said recess to the interior of the tank, electrical heating elements mounted in said reentrant members, liquid heating medium in said tank covering said reentrant members to receive heat therefrom, a conduit leading from said recess, electrical connections extending through said conduit and connected to said heating elements, a device connected to and responsive to conditions within the interior of said tank and controlling said electrical connections, and a radiating element comprising a tube having one end connected to one end of the tank at a point above the liquid level therein to form a closed circulatory system.

ROBERT C. BELGAU.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 945,904 Barrett Jan. 11, 1910 1,471,913 Otis Oct. 23, 1923 2,000,438 Dougherty May '7, 1935 2,225,850 Wright Dec. 24, 1940 2,230,085 Ortgiesen Jan. 28, 1941 2,373,084 Morgan et a1 Apr. 3, 1945