US1727584A

US1727584A - High-temperature fluid-heating apparatus

Info

Publication number: US1727584A
Application number: US214972A
Authority: US
Inventors: Robert A Carleton
Original assignee: Individual
Current assignee: Individual
Priority date: 1927-08-23
Filing date: 1927-08-23
Publication date: 1929-09-10
Anticipated expiration: 1946-09-10

Description

Sept. 10, 1929. R. A. CARLETON HIGH TEMPERATURE FLUID HEATING APPARATUS Filed Aug. 23, 1927 3 Sheets-Sheet 1 Cur Eei'o n Sept. 10, 1929.

R. A. CARLETON HIGH TEMPERATURE FLUID HEATING APPARATUS Fi g- 1927 3 Sheets-Sheet 2 m m m 0% NN m no 3 WA T u a a 99 A u% M .v Q w 8 2 R W ATTys.

Sept. 10, 1929. R. A. CARLETON HIGH TEMPERATURE FLUID HEATING APPARATUS Filed Aug. 25, 1927 3 Sheets-Sheet 5 mm t 5 ww m m 4 1 T GP N; m i N MA ai Patented Sept. 10, 1929.

UNITED STATES PATENT OFFICE..

HIGH-TEMPERATURE FLUID-HEATHG APPARATUS.

Application filed August 23, 1927. Serial No. 214,972.

This invention relates to improvements in electric fluid heaters for heating oil, water and other fluids, and the Object of the invention is to provide an apparatus by means of which the fluid may be subjected to a high degree of heat under considerable pressure, as, for example, in refining and cracking crude oil, in which case it is desirable, in many'instances, to heat the oil to a temperature of 800 F. or greater, at a pressure of 7 50 pounds or more per square inch.

A further object of the invention is to provide an electric fluid heater in which the fluid is heated by direct contact with the heating element or elements, wherein heat is generated by the passage of an electric current through the said heating element and evenly distributed over the entire surface of the 'heatingelement, thereby avoiding the formation of hot or cold spots, and wherein the heat of the electric heating elementwill present a comparatively large heating surface, which is maintained at a comparatively low temperatlire, thus permitting the heating of certain fluids which are decomposed or altered when subjected to too great a degree of heat, and also avoiding the formation of steam or gas and the pitting or deposit of material upon the surface of the heating element.

This is accomplished, in the present invention, by providing a tubular heating element having a Wall of high resistance material and causing the fluid to flow through said heating element longitudinally thereof. Preferably a plurality of such heating elements are employed and so connected as to form a continuous conduit for the fluid.

A further object of the invention is to provide alternating series of tubular heating elements having walls of high resistance material and arranged in substantially axial parallelism, with means for passing the electric current through said alternating series of heating elements in opposite directions to neutralize to a greater or less extent the effect of self-induction.

A further object of the invention is to provide a fluid-heating apparatus of the character specified in which the fluid is passed progressively through the alternatlng serles of tubular electric heating elements, so that the temperature of the fluid is gradually and progressively increased.

A further object of the invention is to provide a fluid-heating apparatus comprising tubular heating elements having thin walls of relatively high resistance material, with means for equalizing the pressure upon the outer and inner surfaces of the walls of the heating element. Any suitable means may be provided for accomplishing this purpose; preferably, however, the conduit for the heating fluid is immersed within the fluid in a suitable receptacle and communicates therewith. g

A further object of the invention is to provide an electric fluid-heating apparatus of the character specified in which the current supplied to the heating elements is controlled by thermostatic means subject to the temperature of the fluid or vapors thereof which are discharged from the conduit or from the receptacle.

A further object of the invention is to provide an apparatus of the character specified with means operable by the flow of the fluid through the tubular heating elements to cause the electric current to be supplied to the heating elements and to prevent the supply of electric current thereto upon cessation of said flow.

A further object of the invention is to provide an electric fluid-heater of small or large capacity that can be constructed or produced for domestic or industrial purposes in which the heating elements will not burn out or require replacement from over-heating, and which can be arranged to operate upon an alternating or polyphase current of low voltage.

A further object of the invention is to provide an electric fluid-heater of the character described simple and rugged in construction and arrangement, comprising non-corrosive materials or materials not easily affected by chemical or other action of the fluid being treated, that can be produced at a low cost, that will be durable in service and will operate efliciently and economically.

These and other objects of the invention under considerable pressure and velocity. It

will be understood, however, that the invention is not limited to such use or arran ment, and that the same may be readily m ified to provide an eflicient circulating type of heater, evaporator for concentrating sugar Juices and other fluids, or for the generation or so r-heating of steam, and many other useapplications in the industrial processes.

In the drawings, Figure 1 is a view mainly in vertical section of a heating apparatus embodying the invention particularly adapted to heat to high degrees of temperature, such, for example, as are required in refining and cracking oil;

Fig. 2 is a transverse sectional view of the same;

Fig. 3 is an enlarged detail viewin vertical section illustrating particularly the heating elements and the manner in which an electric current is supplied to them;

Fig. 4 is'a 'detail sectional view on line 4-4, Fig. 3;

Fig. 5 is a view mainly in vertical section of amodified form of apparatus embodying the invention;

Fig. 6 is a detail view of one end of the apparatus illustrated'in Fig. 5 showing'the llfiaghnner in which aggw-pohed h h a'm ma 'sup' toteeating elements f rh a tzansfoi mer and caused to pas through alternating elements in oppositedirect'ions; and

Fig. 7 is a detail view of the (ipposite end of the apparatus illustrated in trat' the manner in which the ends of alternatmg heating elements are connected together.

The preferred form of apparatus illustrated in Figs. 1 to 4, inclusive, comprises a receptacle 1,, preferably of cylindrical form, constructed 0 steel or other suitable and-so proportioned as to "withstand the rature and preuure under which crude oil as to be treated, with heads 2 provided with manholes 8 and suitable covers therefor and preferabl surrounded by insulation 4, a por-' tion of w 'ch is shown. g

The means for heating the fluid comprises a tubular heatin elementfiir, as shown herein, preferably ternating series of

elements

5, 6 having preferably relatively thin walls of high resistance material, such as Monel metal, chrome steel alloy, or other high resistance material, the heating elements being supported within the receptacle iipon partitions 7, 8 which preferably are located in tential current of ig. 5, illus- 4 roximity to the ends of the receptacle and rm partitions dividin the receptacle into an inlet section 9, an out et section 10 and an intermediate section 11. The artition 7 desirably is of copper or other ow resistance electric conducting material and serves to support and conduct the current from one heating element to the other. The heating elements may be welded or otherwise electrically and mechanically connected to the partition 7 The partition 8 is formed of insulating material, such as transit, bakelite, or other suitable material, and is arranged to support the other ends of the

heating elements

5 and 6.

A low resistance conductor plate 12 lies against and preferably is secured to the partition 8 and is electrically connected to the heating elements 5, while a similar late 13 is secured upon the opposite face 0 the insulating partition 8 and is electrically connected to the alternating heating elements 6.

Suitable bushings

14, 15 of insulating material surrounding the respective heating elements serve to prevent electrical connection from the conductor late 12 to the heating element 6 and from the conductor plate 13 to the heating element 5, respectively.

By reason of this construction an electric current of relatively low voltage is caused to pass through the alternating series of

heatin elements

5 and 6 in o poslte directions an thereb to substantia y neutralize the efiect of sel -induction which would otherwise re 'sult if the current were passing through alternating heating elements in the same direction.

Any suitable means may be provided for supplying a relatively low voltage electric heating current to the

conductor plates

12 and 13 and from such plates to the respective heaelements.

In t e preferred construction illustrated, the current is supplied from the secondary 16 of a transformerthrough

suitable conductors

17 and 18 to the respective conductor plates 12 and .13.

The primary 19 of the transformer is supplied witlij a preferably alternating current through

conductors

20, 21, a switch 22, an electric cut-out 23 and a voltage regulator 24.

' The transformer desirably is of a step-down type, whereby a high voltage current of relatlvely small am rage wil be transformed into a relatively ow voltage current of high am rage, which is conducted from the secon ary of the transformer to the heating elemnts- Any suitable means may be provided to cause the fluid to flow through the'conduit containing the heating elements. In the preferred construction illustrated, the

alternatin heating elements

5 and 6 are connected by su1table U-shaped bends or

coupling links

25, 26 to provide a continuous conduit of which the heating elements form sections. One end 27 of the conduit communicates with the inner chamber 9 of the receptacle and the remote end 28 of theconduit communicates with the outlet section 10 of the receptacle.

Fluid is introduced, under pressure, into the inlet section 9 of the conduit through a pipe 29 and pump 30, or other suitable pressure-producing means. A current of fluid is thereby caused to flow progressively through the heating sections of the conduit, and will be progressively heated by direct contact with the inner walls of said heating elements during its progress, the heated fluid or vapors thereof being delivered from the end 29 of the conduit to the outlet chamber 10, from which the fluid or its vapors may be delivered through a suitable outlet pipe 31.

-Preferably the lower portion of the partition 7 is provided with a series of ports or apertures 32 through which fluid is forced into the intermediate compartment 11 of the apparatus. Vent ports 33, preferably in the upper art of the partition 8, permit the escape o gases and vapors and, if desired, of the fluid also into-the outlet chamber 10.

By thus enclosing the heating elements within the receptacle 1, which is of a form that can be readily constructed to resist great stresses and internal pressure. Substantially balanced pressure is maintained upon the interior and exterior surfaces of the walls of the heating elements, thus making it possible to use heating elements and fittings of comparatively thin or light material, and eliminating the danger from bursting or leaking tubes and fittings which is a source of great danger to the operators and constitutes aserious fire hazard in usual tube stills or heaters employed for heating or processing crude oil and similar material under high temperature and pressure.

The cylindrical receptacle is provided with a ground connection 34, which eliminates any danger of electric shock to persons comlng in contact with the receptacle or the pipe lead ing thereto. This ground connection also eliminates danger from fire or explosion of oil or gas vapors from electric sparks or arcs that might be formed under certain conditions should accidental electrical connection be formed between the tank or piping and the ground. The ground connection also renders unnecessary the insulating of the receptacle 1 and any piping or metallic connection be tween the ground and the receptacle 1.

Preferably means are provided for maintaining the heating elements at a predetermined temperature. In the present embodiment of the invention this is accomplished by providing regulating means for the electric current controlled by a thermostat which is subject to the maximum temperature of the heated fluid or its vapors.

In the construction illustrated, a thermostat 35 extends into the outlet pipe 31 and is connected by

suitable conductors

36 and 37 to a voltage regulator 24 leading to the primary of the transformer. Should the temperature of the fluid or its vapors flowing from the heater through the outlet pipe 31 exceed a predetermined temperature, the thermostat will close the electric circuit through the

conductors

36 and 37, thereby energizing the controlling mechanism of the Voltage regulator, which will then act to lower or reduce the voltage supplied to the primary of the step-down transformer 19, thus reducing the secondary voltage and current supplied to the

heating elements

5 and 6, and thereby reducing the amount of heat generated in said elements.

Should the temperature of the fluid or its vapors discharge through the outlet pipe 31 be at too low a degree of heat the thermostatic device will open its contacts, causing the voltage regulator 24 to raise the voltage, and consequently the current supplied to the

heating elements

5 and 6, thus causing more heat to be generated therein. By virtue of this construction, therefore, the heating apparatus may be caused to heat the fluid at a substantially constant predetermined temperature. e

Desirably, means are also provided to control the electric circuit in such a manner thzit current will be supplied to the heating elements only when the fluid is flowing through the receptacle and the conduit therein.

In the construction illustrated, an automatic switch or contactor 38 is placed in the inlet pipe 29 and is operated by the flow of fluid through the heater-circulating system. The contactor 38 is so arranged that when the fluid is flowing through the inlet pipe, a circuit is completed through

conductors

39 and 40 and the solenoid or magnet 41 of the circuit breaker 23, causing it to close and complete the electric circuit from the

service switch

20, 22 to the primary 19 of the stepdoWn transformer. Y

Should the flow of fluid through the inlet pipe 29 cease, either accidentally or otherwise, the contacts of the switch 38 would immediately be opened, thus causing the circuit breaker 23 to interrupt the circuit to the primary 19 of the transformer, thus cutting ofi the supply of electric current to the heating elements at all times except when the fluid is being circulated through the heater.

In the operation of the device, therefore, when fluid is introduced, under pressure, into the inner chamber 9 of the receptacle 1. it is caused, by reason gf the partitions 7 and 8, to flow through the conduit, which constitutes the

heatingelements

5 and 6. The fluid is, therefore, progressively heated during its passage through the conduit.

By reason of the fact that the heating ele ments are arranged in substantially axial parthermostat 35, which in turn controls t e voltage of the current supplied to the primary 0 the transformer, and consequently regulates the current supplied to the heating elements, thereby maintainin a substantially constant temperature of t e fluid or its vapors discharged from the heating apparatus.

interrupt the current supplied to the primary of the transformer, so that accidental overheating, which otherwise would occur by reason of the interruption of the flow of the fluid, is wholly obviated.

Inasmuch as the ports in the partition 7 permit the fluid to flow into the interior section of the receptacle and the vent ports permit the escape of air, vapors and liquid from the interior section of the receptacle, a substantially balanced pressure is maintained upon the interior and exterior surfaces of the heating elements, thus enabling heating elements having thin walls to be used under high pressure, and. also enabling relatively small currents of low amperage to be emplayed to raise the heating elements to the desired temperature.

Obviously, other means of producing balanced pressures upon the interior and exterior of the heating elements may be employed within the spirit and scope of the invention.

In Figs. 5, 6 and 7 a modified form of the invention is illustrated in which alternating series of

heating elements

42 and 43, corresponding to the alternatingseries of

heating elements

5 and 6, and connected at their ends by suitable U-shaped bands or

con nectors

44 and 45 are mounted in

piers

46, 47 and 48, preferably enclosed within walls 49. In this construction the fluid is supplied through a suitable ump-50 and pipe 51 to the inlet end 52 of the conduit thus formed by the series of heating elements.

The outlet end 53 of the conduit is desirably provided with a thermostat 54, the conductors 55 and 56 from which are adapted to control the circuit of electrically-actuated contactor 57, arranged to regulate the secondary voltage of transformer 59, which is illustrated herein as a three-phase transformer.

Likewise, an automatic switch or contactor 60, located in the inlet pipe 51, may be employed to actuate a suitable circuit breaker Any interruption in the flow of the fluid into the inlet section of the receptacle 1 will 61 in the conductors leading to the primary of the transformer.

A relatively low tension alternating current is used and such a current may be supplied from the secondary of a transformer of any usual type, either single phase or polyphase.

It will be obvious that various other embodiments of the invention may be .made by those skilled in the art, and it will therefore be understood that the particular devices shown and described herein are of an illustrative character and are not restrictive, and that various changes in form, construction and arrangement of parts may be made within the spirit and scope of the following claims.

Having thus described the invention, what is claimed as new, and desired to be secured by Letters Patent, is:

1. A fluid-heating apparatus comprising a conduit for the fluid having therein a plurality of metallic tubular heating elements having thin walls of relatively high resistance material, arranged in substantially axial parallelism and in proximity to one another, means for pasing an alternating electric heating current of relatively low voltage through adjacent heating elements in opposite directions to neutralize the effect of self-induction, and means for causing the fluid to flow through said conduit.

2. A fluid-heating apparatus comprising a series of tubular heating elements, having thin metallic walls of relatively high resistance material, arranged in substantially axial parallelism and having means c0nnecting the adjacent ends of alternating heating elements to provide a continuous, sinuous conduit, means for passing an alternating electric current of relatively low voltage through said-heatingelements whereby the temperature of the fluid will be progressively increased and means for causing the fluid to flow through said conduit.

3. A fluid-heating apparatus comprising alternating series of tubular heating elements, having thin metallic walls of high resistance material, arranged in substantially axial parallelism and having means connecting the adjacent ends of the members of said alternating series of heating elements to provide a continuous, sinuous conduit, means for passing an alternating electric current of relatively low voltage through the members of the respective series of alternating heating elements in parallel and through the alternating series in opposite directions to neutralize the effect of self-induction. means for introducing fluid at one end of said conduit, and means for delivering the heated fluid or its vapors fromthe other end thereof.

4. A fluid-heating apparatus comprising alternating series of tubular heating elements, having thin metallic walls of high resistance material, arranged in substantially axial parallelism and having means connecting the adjacent ends of the members of said alternating series of heating elements to provide a continuous, sinuous conduit, means for passing an alternating electric current of relatively low voltage through the members of the respective series of heating elements in parallel and through the alternating series in opposite directions to neutralize the effect of self-induction, means for introducing fluid at one end of said conduit, means for delivering the heated fluid or its vapors from'the other end thereof, and thermostatic means subject to the influence'of the temperature of the fluid discharged from said conduit to control the current supplied to said heating elements.

5. A fluid-heating apparatus comprising alternating series of tubular heating elements, having walls of high resistance material, arranged in substantially axial parallelism and'having means connecting the adjacent ends of the members of said alternating series of heating elements to provide a continuous, sinuous conduit, means for passing an electric current through the members of the respective series of heating elements in parallel and through the alternating series in opposite directions to neutralize the effect of self-induction, means for introducing fluid at one end of said conduit, means for delivering the heated fluid or its vapors from the other end thereof, and means operable by the flow of fluid through said tubular heating elements to cause the electric current to be supplied to the heating elements and to prevent the su ply of electric current upon the cessation 0 said flow.

6. A fluid-heating apparatus comprising a conduit for the fluid having a relatively thin metallic wall of high resistance material, means for passing an alternating electric heating current through said high resistance material, means for supplying fluid under pressure to said conduit and means for equalizing the pressures upon the inner and outer walls of said conduit.

7. A fluid-heating apparatus comprising a receptacle for the fluid, a conduit for the fluid comprising a thin metallic wall of high resistance material, means for passing an alternating electric heating current through said high resistance material, means for causing the fluid under pressure to flow through said conduit, and means for maintaining substantially the same fluid pressure in said receptacle as that of the fluid within said conduit whereby the pressure upon the inner and outer walls of said conduit will be equalized.

8. A fluid-heating apparatus comprising a receptacle for the fluid, alternating series of metallic tubular heating elements having thin walls of relatively high resistance material arranged in substantially axial parallelism for passing an alternating electric current of relatively low voltage and high amperage through adjacent series of-heating elements in opposite directions to neutralize the eflect of self-induction, means for supplying fluid to one end of said receptacle, means for causin the fluid in said receptacle to maintain su stantially the same fluid pressure on the outside of said conduit as-the fluid pressure within said conduit, means for causing the fluid to flow through said conduit, and means for delivering the fluid or its vapors from the other end of said receptacle.

9. A fluid-heatin apparatus comprising a receptacle for the uid, alternating series of metallic tubular heating elements having thin walls of relatively high resistance material arranged in substantially axial parallelism with the adjacent ends of members of said alternating seriesof tubular elements connected to-form a continuous, sinuous conduit, with the ends of said conduit communicating with the respective ends of said receptacle, means for passing an alternating electric current of relatively low voltage and high amperage through the members of the respective series of heating elements in parallel and through 'fluid pressure on the outside of said conduit as the fluid pressure within said conduit, means for causing the fluid to flow through said conduit, means for delivering the fluid or its vapors from the other end of said receptacle, thermostatic means subject to the temperature of the fluid discharged from said receptacle, and means operable thereby to regulate the current supplied to said heating elements. 10. A fluid-heating apparatus comprisin a receptacle for the fluid, alternating series of metallic tubular heating elements having thin walls of relatively high resistance material arranged in substantially axial parallelismwith the adjacent ends of members of said alternating series of tubular elements connected to form a continuous, sinuous conduit, with the ends of said conduit communicating with the respective ends of said receptacle, means for passing an alternating electric current of relatively low voltage and high amperage through adjacent series of heating elements in opposite directions to neutralize the effect of self-induction, means for supplying fluid to one end of said receptacle, means for causing the fluid to flow through said conduit, means for delivering the fluid or its vapors from the other end of said receptacle, means operable by the flow of the fluid into said receptacle to cause the electric. current to be supplied to said heating elements and to prevent the supply of electric current thereto upon cessation of said flow.

11. A fluid-heating apparatus comprising a cylindrical receptacle, partitions extending transversely of said receptacle in proximity 10 to the ends thereof and dividing the receptacle into an inlet section, an outlet section and an intermediate section, tubular heating elements having walls of relatively high resistance material mounted in said partitions in substantially axial parallelism with alternating tubularelements connected at their ends to form a continuous conduit, with the respective ends of said conduit communicating with said inlet section and said outlet section, means for passing an alternating electric current through said heating elements to cause the fluid passing through the conduit to be progressively heated by said heating elements and means for delivering the fluid or its vapors from the outlet section of said receptacle.

12. A fluid-heating apparatus comprising.

a cylindrical receptacle, partitions extending transversely of said receptacle in.proximity to the ends thereof and dividing the receptacle into an inlet section, an outlet section and an intermediate section, tubular heating elements having walls of relatively high resistance material mounted in said partitions in substantially axial parallelism with alternating tubular elements connected at their ends to form a continuous conduit, with the respective ends of said conduit communicating with said inlet section and said outlet section, means for passing an electric current through said heating elements to cause the fluid passing through the conduit to be progressively heated by said heating elements, means for delivering the fluid or its vapors means for introducing fluid from said inlet section to said intermediate section, and means for venting the air and vapor accumulating within said intermediate section into said outlet. section.

v 13. A fluid-heating apparatus comprising a cylindrical receptacle, partitions extending transversely of said receptacle in proximity to the ends thereof and dividing the receptacle into an inlet section, an outlet section and an intermediate section, tubular heating elements'having walls of relatively high resistance material mounted in said partitions in substantially axial parallelism with alternating tubular elements connected at their ends to form'a continuous conduit, with the respective ends of said conduit communicating with said inlet section and said outlet section, conductor plates secured to opposite sides of one of said partitions and electrically from the outlet section of said receptacle,

connected, respectively, to alternating heating elements, a source of electric energy, and means connecting opposite terminals of said source of energy to respective conductor plates, whereby the current will be caused to flow through alternating heating elements in opposite directions, means for introducing fluid into the inlet section of said receptacle and means for discharging the fluid or its vapors from the outlet section of said receptacle.

14. A fluid-heating apparatus comprising a cylindrical receptacle, partitions extending transversely of said receptacle in proximity to the ends thereof'and dividing the receptacle tric current. to said heating elements, including a transformer and conductors from the secondary thereof leading to alternating heating elements, thermostatic means sub]ect to the influence of the temperature of the fluid or vapors discharged from the outlet section of said receptacle, and means operable thereby to control the electric current supplied to the heating elements from said transformer.

15. A fluid-heating apparatus comprising a cylindrical receptacle, partitions extending transversely of said receptacle in proximity to the ends thereof and dividing the receptacle into an inletsection, an outlet section and an intermediate section, said partitions having small ports to permit liquid to flow therethrough, tubular heating elements having walls of relatively high resistance material mounted in said partitions in substantially axial parallelism with alternating tubular elements connected at their ends to form a continuous conduit, with the respective ends of said conduit communicating with said inlet section and said outlet section, conductor plates secured to opposite sides of one of said partitions and electrically connected respectively to alternating heating elements, a source of electric energy and means connecting opposite terminals of said source of energy to respective conductor plates, whereby the current will be caused to flow through alter-v nating heating elements in opposite directions, means for introducing fluid under pressure Into the inlet section of said receptacle,

- means for discharging the fluid or its vapors from the outlet section of. said receptacle,-

thermostatic means subject to the influence of the temperature of the fluid or vapors discharged from the outlet section of said receptacle, means operable thereby to control the electric current supplied to the heating elements, and means automatically operable by the flow of fluid supplied to said inlet section to cause the electric currents to be supplied to said heating elements and to prevent the supply 'of electric current upon cessation of flow of fluid to said inlet section.

16. A fluid-heating apparatus comprising a receptacle for the fluid, a plurality of parallel tubular heating elements therein having walls of relatively high resistance material, means for supplying an alternating electric current having terminals connected to like ends of alternating heating elements, means electrically connected to the opposite adjacent ends of said tubular heating elements, means for supplying fluid to said receptacle and for causing it to flow through and. around said heating elements, whereby the fluid will be heated by direct contact with the outer and inner surfaces of the walls of the heating elements, and means electrically grounding said receptacle.

17. A fluid-heating apparatus comprising a receptacle for the fluid, a plurality of tubular heating elements therein having walls of relatively high resistance material arranged in substantially axial parallelism with the adjacent elements, connected to form a continuous, sinuous conduit, a source of electrical energy, means connecting said source of energy to like ends of alternate heating elements to cause the current to pass through said elements in opposite directions, a common conducting means electrically connecting the opposite ends of said elements and said receptacle, means electrically grounding said receptacle, and means for supplying fluid to said receptacle and causing the same to flow through and around said conduit, whereby the fluid will be heated by direct contact with the outer. and inner surfaces of the walls of said heating elements.

18. A fluid-heating apparatus comprising a conduit for the fluid having therein a plurality of tubular heating elements having walls of high resistance material and arranged in substantially axial parallelism and a conduit for the fluid having a plurality of tubular heating elements having walls of relatively high resistance material arranged in substantially axial parallelism and in proximity to one another, an inlet leading to said conduit, means for supplying fluid under pressure to said inlet, means for passing an alternating electric heating current through adjacent heating elements in opposite directions comprising a transformer with conductors leading from the secondary of said transformer to said alternating heating elements, electrically operable means including a switch in the circuit leading to the primary of said transformer operable by the flow of fluid through said inlet to complete the circuit to said primary and operable upon cessation of flow to break said circuit.

In testimony whereof, I have signed my name to this specification.

ROBERT A. CARLETON.

in proximity to one another, means for supplying an alternating electric current to said heating elements comprising a transformer, a circuit leading from the secondary of said transformer to alternating heating elements to cause the electric current to pass in opposite directions through said adjacent heating elements, theremostatic means subject to the temperature of the fluid discharge from said conduit, and means operable by said thermostat for varying the action of the primary of the transformer to control the voltage of said secondary circuit.

19. A fluid-heating apparatus comprising