US3435806A - Heat exchangers - Google Patents

Description

April 1 1969 H. N. SHAW 3,435,806

HEAT EXCHANGERS Filed March 14, 1968 Sheet of 5 3 I I v 4/ 5 7 46 Z10 42a 3| 6 17b 17 48 Z/b 42 30 35 i li 3/ 24 2/? L z m 3 V H L 1 37 1 H T zz 4o INVENTOR HAROLD N. SHAW ATTORNEYS April 1, 1969 Filed March 14, 1968 H. N. SHAW HEAT EXCHANGERS Sheet 2 of 3 INVENTOR HAROLD N- SHAW ATTORNEYS United States Patent Oflice 3,435,806 Patented Apr. 1, 1969 3,435,806 HEAT EXCHANGERS Harold N. Shaw, P.O. Box 173, Everett, Mass. 02149 Continuation-impart of application Ser. No. 614,125,

Feb. 6, 1967. This application Mar. 14, 1968, Ser.

Int. Cl. F22b 27/12 US. Cl. 122-41 19 Claims ABSTRACT OF THE DISCLOSURE In gas heated steam generators useful in supplying steam to steam cookers or other appliances, a high rate of heat transfer is obtained by the use of inwardly projecting fins from a surrounding stepped steam generating surface, the steps providing surrounding superimposed shelves, there being means for heating the fins to a sufficient temperature to cause generation of steam by heat conducted from the fins to the outer side of said stepped wall, there also being means for causing the feed of water at substantially the rate at which it is evaporated, with the water flowing by gravity from the upper shelves to collect on a lowermost shelf, and there being a thermostat which controls the heat input for maintaining the lowest shelf near the critical temperature for maximum steam generation to provide violent spattering of water from thelowest shelf to upper shelves for recirculation purposes.

This application is a continuation-in-part of co-pending application, Ser. No. 614,125, filed Feb. 6, 1967, now abandoned.

BACKGROUND OF THE INVENTION Field pf the invention The present invention is particularly adapted for use in gas heated steam generators useful in supplying steam to steam cookers or other appliances. The present invention is also applicable to boilers and to forced circulation water heaters used in heating buildings.

Description of the prior art The present invention is an improvement upon the heat exchangers employed in the stored heat generators of my prior patents, -Re. 24,861, issued Aug. 23, 1960, and No. 3,082,312, issued Mar. 19, 1963, and is designed to provide for greater efficiency of operation.

Summary of the invention In the present invention fins are employed to conduct heat from the flue gases to the steam generating surfaces. For a high rate of heating transfer these fins must have a heavy section in order to operate at practical temperatures. Inasmuch as these temperatures are relatively high the fins store a substantial amount of heat. If the unit of the present invention were operated with a high water level, as is customary in conventional boilers, the volume of water should be small, because of the compact design of the unit, so the heat storage capacity of the water would be low compared to the heat storage capacity of the fins. Thus, if gas were turned off by a conventional control arrangement, steam would continue to be generated by the hot fins causing the usual safety valve to open and blow continually until cooling of the fins took place. This would be an undesirable and inefiicient arrangement.

One of the objects of this present invention is to provide a fin-type heat exchanger, particularly adapted to operate as a steam generator with only a thin layer of water on the heating surface, there being an improved arrangement whereby the water is fed at substantially the same rate that it is evaporated, and there being a control to turn off the water supply rather than to turn off the source of heat, as is conventional, thereby causing the steam generation to stop as soon as the small quantity of water remaining after water shut-off has been evaporated. With this arrangement the steam generating surface will operate dry at a relatively high temperature when not generating steam, there being a resulting expansion which serves to crack off any scale which may have formed on the heating surface.

It is a further object of the invention to provide, in that form of the invention utilizing cast aluminum for the heating surfaces and fins, means for minimizing stress on the cast parts by the use of rolled aluminum parts in conjunction with said cast parts.

A further object of the invention is to provide an improved arrangement whereby water which runs off of the upper heating rings is retained on the lowest ring where it boils violently, with the result that it splashes up onto the upper rings to provide automatic recirculation.

A further, more specific object of the invention is to provide relatively heavy, horizontal, superimposed, surrounding heating surfaces which are joined by relatively flexible vertical bands or rings, said steam generating surfaces being heated by heat conduction fins cast integrally therewith or joined thereto by means providing for good thermal conductivity.

A=further object of the invention is to provide a device as above described wherein the heat exchanger forms a coincally-shaped flue above the gas flames, there being fins projecting horizontally into the flue which have vertical surfaces exposed to the gases to provide efiicient means to absorb heat therefrom.

A further object is to provide in one form of the invention'an arrangement wherein the ring-shaped heat transfer surfaces are cast integral with inwardly-extending heat-absorbing fins in such a manner as to insure relatively flexible casting to thereby reduce stresses from unequal heating or cooling and, at the same time, provide a path through each fin which is of relatively heavy section for eflicient heat conduction.

A further object of the invention is to provide a steam generator wherein the necessity of having a steam outlet valve is eliminated.

A further object of the invention is to provide a steam generator wherein there is a pulse-type water feed controlling the flow to a water distribution trough, which trough has outlet means so arranged as to insure a uniform flow of water over all of the heating surfaces at a relatively low rate of flow, as is required for steam generation.

A further object of the invention is to provide a steam generator having a water pre-heating coil in the flue through which the water passes before it enters the feed trough, there being a condensate cup positioned to collect and evaporate condensate from the flue gases which may be formed on the cold coil.

A further object of the invention is to provide means on one of the heating rings, such as the lowest ring, to accommodate a long thermostat bulb which is arranged to control the heating input, said bulb preventing overheating of any section which may run dry.

A further object of the invention is to provide an arrangement wherein there is an inner liner on the outside shell arranged to keep the shell from cooling too rapidly as a result of spattering water, and to eliminate stresses at the welds between the shell and the bottom ring, said liner sloping upwardly and inwardly to direct spattering water onto the upper heating rings.

A further object of the invention is to provide a heat exchanger which is compact and which has a low height, making it practical for installation under counters.

With the above and other objects in view, the invention consists of the improved heat exchanger, and all of its parts and combinations, as set forth in the claims, and all equivalents thereof.

Brief description of the drawings In the accompanying drawings, illustrating several embodiments of the invention:

FIG. 1 is a front elevational view of a gas steam generator, part of the generator being broken away and shown in vertical section;

FIG. 2 is a fragmentary sectional view taken approximately on the line 2-2 of FIG. 1;

FIG. 3 is a partially diagrammatic top view of the steam generator with the control and wiring hook-up;

FIG. 4 is a fragmentary vertical sectional view showing portions of the bottommost ring together with the one thereabove to illustrate a modified type of flexible connection between the superimposed rings, which connection is formed by thin cast parts, the view also illustrating how electric heating elements may be employed in lieu of the gas heating means;

FIG. 5 is a fragmentary perspective view showing a modified arrangement wherein the fins may be copper and may be brazed to a surrounding steel wall which is stepped to provide evaporating surfaces, this construction being suitable as a steam generator for high pressure uses for higher temperatures;

FIG. 6 is a fragmentary vertical sectional view through the stack and the upper portion of the device, showing a modification wherein the pre-heating coil extends upwardly into the stack rather than downwardly; and

FIG. 7 is a front elevational view of another modified construction, part of the generator being broken away and show in vertical section.

Description of the preferred embodiment Referring more particularly to the drawings, first to FIG. 1, the numeral 10 designates a suitable base which supports an apron 11, the latter being preferably square. In a combustion space 12 below the apron is a series of gas burners 13, the arrangement being such that flue gases travel upwardly through the top 14 of the apron.

Supported on top of the apron rim are superimposed heat exchanger rings 15, 16, 17 and 18. In the preferred form of the invention shown in FIG. 1 these rings are of cast aluminum. Each of the rings includes a relatively heavy outer portion, preferably circular, to provide relatively horizontal, surrounding top shelves or ledges 19, 19a, 19b and 190. Inwardly of the shelves are upwardlyprojecting annular beads 20, 20a, 20b and 200. On the lower side of the rings below the shelves are grooves 21, 21a, 21b and 210. The outer portions of the rings are of relatively heavy cross-section, as shown in FIG. 1.

Referring now more particularly to FIG. 2, the bottommost ring has inwardly-projecting radial fins 22 which are of relatively heavy section to serve etficiently in conducting heat to the outer steam generating surfaces. These fins are capable of storing a substantial amount of heat. The fins 22 project horizontally inwardly but have vertical surfaces 23 which may be wiped by the flue gases. The ring 16 is of less diameter than the ring 15 and it has radially-inwardly-projecting fins 24 whose tips project farther inwardly than the tips of the fins 22, as is clear from FIG. 2. The fins 24 have vertical surfaces 25. The ring 17 is of less diameter than the ring 16 and it has inwardly-projecting radial fins 26, as shown in FIG. 2, which have vertical surfaces 27. The top ring 18, in turn, is of less diameter than the ring 17 and it has radially-inwardly-projecting fins 28 with vertical surfaces 29. It is to be noted from FIG. 2 that the fins 26 and 28 project farther than the fins 24.

In the preferred form of the invention of FIGS. 1 and 2 the superimposed horizontal rings are connected to one another and held in vertically-spaced relationship, as shown in FIG. 1, by flexible vertical circular bands 30. The lower edges of the bands fit in grooves 31 formed behind the shoulders 20, 20a, and 20b. The upper edges of the bands fit in the grooves 21a, 21b and 21c, respectively. In the preferred form of the invention of FIG. 1 the bands are formed of rolled aluminum, and the upper and lower edges are welded in the grooves. The rings are assembled with the fins staggered, as shown in FIG. 2, to provide a zigzag passage for the flue gases.

Seated in the groove 31 of the top ring 18 is the lower edge of a cap 32, preferably formed of rolled aluminum, which cap has a top opening with its margin 33' welded around the lower end portion of a flue collar 34. An outer shell 35 has its lower end welded to the outer periphery of the lowermost ring 15 and has at top opening, the margin of which is welded as at 36 to the exterior of the upper portion of the flue collar 34. In the principal form of the invention this shell is formed of rolled aluminum and is provided with a drain opening 37 communicating with a valve controlled drain line as illustrated, with a steam outlet 38, with a safety valve 39, with a safety thermostat 40 of the anti-flood type, and with a .water inlet fitting 41.

A thin stainless steel liner '42 is positioned against the lower portion of the inner side of the outer shell. The upper portion 42a of the liner is tapered inwardly away from the outer shell, as illustrated in FIG. 1, to provide a splash batfie to deflect spattering water onto the upper rings. The splash bafl'le portion also protects the steam outlet from the spattering water while allowing steam to flow over the top of the bafile 42a to the outlet. Extending downwardly through the flue collar and within the heat exchanger is a water pre-heating coil 43 having an inlet end 44 for receiving water from a supply (see FIG. 3), and having an outlet end '45 which enters the inlet fitting 41 to supply feed water to a surrounding trough 46 having holes 47 spaced at intervals around its lower portion to allow water to drain out onto the cap 32 for distribution uniformly around the circumference of the latter. Suspended from a lower portion of the coil is a condensate cup 48 for the purpose of catching condensate from the coil. The bottom of the cup is heated by the flue gases so the condensate evaporates and passes up the stack with the flue gases. Some, of course, re-condenses on the coil to further pre-heat the water. The pre-heating coil may run 150 cooler than the boiler. The cup 48 also forms a flue bafile and causes the flue gases to be directed between the fins, as they pass upwardly, to wipe the vertical fin surfaces 23, 25, 27 and 29. In some applications it is neces sary to have the pre-heating coil above the inlet '41 of FIG. 1, so as to drain completely between operations for instantaneous steam generation. Such alternate arrangement is illustrated in FIG. 6, wherein the pre-heating coil is indicated by the numeral 243, which coil has an inlet end 244 and an outlet end 245 which enter the inlet fitting 41.

In the modification of FIG. 4, instead of having the separate circular bands 30 for connecting the super-imposed rings in vertically-spaced relation, the rings have cast upper and lower extensions 130 which are welded together as at 131, the lower extension being omitted from the bottommost ring 115. Thus, in the form of the invention of FIG. 4, all portions of the rings, such as the rings and 116, are formed of cast aluminum. In FIG. 4 all parts not specifically described are designated by the same numerals used in FIG. 1 preceded by the digit 1. FIG. 4 also illustrates how electric heating elements 113 may be interposed between the rings and used in lieu of the gas heating means of FIG. 1.

The form of the invention of FIG. 5 illustrates another modification wherein the stepped heating surface is formed by a surrounding steel wall 50 having steps 51 which correspond to the shelf portions 19, 19a, 19b and 190 of FIG. 1, the steps being integrally connected by vertical portions 52 which take the place of the connecting bands 30 of FIG. 1. In this form of the invention the fins are provided by brazing inwardly tapered copper fin elements 53 to the undersides of the shelves as shown in FIG. 5. The fins may be arranged and disposed in the same general manner shown in FIG. 2. In FIG. 5 the heat transfer is along solid bars (the copper fins) with one end exposed to the flue gases and with the other end engaging the stepped wall over which the water falls. This is a very eflicient method for heat conduction.

In the modification of FIG. 7, all parts not specifically described correspond to like parts of FIG. 1 and are designated by the same numerals used in FIG. 1 preceded by the digit 2. In FIG. 7 the rings 215 and 216 are formed of cast aluminum, just as in the modification of FIG. 4. FIG. 4, however, shows the rings welded together as at 131. In the form of the invention of FIG. 7 there are annular gaskets 231 at the joints between rings, and the lowermost ring is seated on a gasket 231' supported on an annular shelf 90 which may be bolted in place to the outer shell as illustrated. The flue collar 234 extends slidably through the opening in the top of the generator instead of being welded thereto as in FIG. 1, and there is a suitable seal 91 Within a retainer 92 to provide a slip joint. In the embodiment illustrated, the seal is an O- ring. This could also be any stufling box construction with a high temperature asbestos packing. The lower end of the flue collar is welded to the inner head 232 as at 93 and the lower end of 232 seats on the top gasket 231. Between the inner head 232 and the outer head of the generator shell 235 is a spring 94. This spring serves to keep the gaskets 231 tight while steam pressure builds up, the steam pressure holding the gaskets tight thereafter. The slip joint movement of the flue collar 234 within the annular seal 91 allows for expansion of the aluminum rings and for shrinkage of the gaskets. The gaskets 231 and 231' may be formed of any suitable material which is resistant to high temperatures and the heavy rings can slip laterally on the gaskets 231 and 231' a suflicient amount to compensate for any differences in expansion. Either the electric heating elements of FIG. 4 or the gas heater of FIG. 1 may be used in FIG. 7.

Referring more particularly to FIG. 3, which shows a top view of a steam generator with the necessary controls and wiring, the manifold 54 for the gas burners leads from a gas valve 55 in the gas supply line 65. The valve 55 is controlled by a normally closed thermostat 57 and by the normlly closed safety thermostat 62 at the top. The thermostat 57 has a long sensing element 59 which extends around the full circumference of the device as shown in FIGS. 1 and 2, and is positioned in the groove 21 in the lower side of the bottom ring. This thermostat sensing element may be divided into two sections to operate two thermostats, one of which is set at a higher temperature to act as a safety cut-off. The solenoid valve 60 in the water feed line 44 is electrically connected in series with a steam pressure switch 61 at the top of the unit, there being a normally open thermostat 40 at the bottom of the unit which prevents flooding. Also electrically connected in the series is a control thermostat 63.

A motor 64 is arranged to drive a rotary pulsing cam 65 i which acts on a pulse switch 66. In the power supply lines 67 and 68 is a line switch 69 which is adapted to turn the unit on or off. Flow control orifice 71 (see FIG. 3) provides a constant flow regardless of the Water pressure. This ensures a controlled rate of water feed. This flow control is in general use and makes the pulse feed unnecessary for larger steam generators where the water feed is above the minimum rate of available flow control orifices.

In order to remove any loose scale or other deposits at flush button 70 is shown in the electrical circuit. When this button is pushed while the unit is hot and while the drain is open, water will flow continuously, and all deposits will be blown or flushed out through the drain 37. The double throw line switch 69 prevents flushing when the gas is turned on.

When the device is to be used as a hot water heater the normally closed thermostat bulb 59 is located in the water to control the water temperature, and the other controls are not required.

Operation In operation, when the line switch 69 is closed the gas valve 55 opens and the unit starts to heat. When the lower ring 15 heats to a steam generating temperature the normally open thermostat 40 closes and the water valve 60 opens to feed water to the unit in pulses controlled by the motor operated pulse switch 66. When the desired pressure builds up, the pressure switch 61 opens the circuit and the water feed valve closes. Soon thereafter the normally closed thermostat 57 opens and the gas valve closes to control the temperture of the lower ring.

When steam is called for by the pressure switch, or by the control thermostat, water cools off the unit and the thermostat turns on the gas. For certain applications the control thermostat may be replaced by other controls and, in some cases, the pressure switch 61 is not required. A safety pilot control (not shown) may be provided to prevent gas flow if the gas pilot is not burning. The safety thermostat 62 prevents overheating.

During operation, the water in the trough 46 comes out of the bottom holes 47 to provide a thin layer of water over all of the heating surfaces. This water is fed at the rate at which it is evaporated, with the automatic controls turning off the water supply rather than turning off the supply of heat. The pulse feed is timed to operate at predetermined intervals so as to keep the trough filled Without overflowing. Normally it is preferable to adjust the pulse feed and the water trough drain holes so as to provide a uniform flow of steam, but in some cases it may be desirable to provide steam flow in intermittent high rate pulses having a total flow equal to the normal uniform flow. The pulse rate may be made double the uniform rate by using twice as many holes in the bottom of the water distribution trough. Steam generation at the maximum pulse rate must occur below the critical temperature at approximately 300,000 B.t.u./sq. ft./hr. Therefore, steam generation at the uniform rate must be well below this temperature, at about 150,000 B.t.u./sq. ft. hr. Inasmuch as the pulses may be at about ten seconds apart the heavy rings store the heat between pulses. Steam generation stops after the water is turned off and as soon as the small layer of water is evaporated. It is to be noted that whenever the steam generating surface is operating dry, as it does when it is not generating steam, it is operating at a relatively high temperature and will cause sufiicient expansion to crack off any scale which may have formed on the heating surfaces.

When the unit is idling under pressure there is no water present and the steam generating rings 15-18 may run as much as hotter than the saturated steam. This is not objectionable as the steam merely takes on super heat and the safety valve will relieve any excess pressure. By operating the rings in a condition hotter than the saturated steam, considerable stored heat is available from the heavy casting portions at the periphery. The Water pre-heat coil 43 serves to reduce the flue gas temperature sufficiently to make possible an efliciency of over 75% in this relatively small unit. The steam space is outside of the vertical walls 30 (referring to FIG. 1) but the largest steam space is at the top because of the conical shape of the steam generating unit, whereas the largest flue space is at the bottom. This makes it possible to design a compact unit within a limited height. The upwardly contracting flue space increases the gas velocity to compensate for falling temperature as the gases flow upwardly to the stack. This also provides for a high heat transfer to the upper rings as heat transfer is proportional to both temperature differences and velocity.

In the form of the invention of FIG. 1, where the device is made of aluminum, in order to obtain good heat conductivity and lightness in weight, the aluminum will rapidly lose strength at temperatures above 400 F. It is, therefore, desirable to keep the operating temperatures below this point. If higher temperatures are desired, a steam generator with stepped heating services 51 of the type shown in FIG. may be used.

In the form of the invention of FIG. 1, it is desirable to minimize stresses on the cast parts. For this reason the rolled aluminum bands or walls 30 have been used, and these bands take most of the stress produced by nonuniform heating and cooling.

In any heat exchanger it is desirable to have the hot flue gases flow over the heating surfaces with considerable velocity. The superimposed vertical surfaces of the present invention which are directly over the combustion chamber 12 provide such high rate of flow.

With the present invention some water is evaporated from each shelf and water which runs off of the upper rings is retained on the shelf 19 of the lowermost ring 15 where it boils so violently that it splashes up through the narrow space between the steam generating unit and the inclined baffle portion 42a. This splashing helps to distribute the water to insure that no part will run dry when it is not supposed to. The violent boiling is due to the very high heat conduction from the radially-inwardly-projecting fins (shown in FIG. 2) to the heating rings, there being no drain of water from the rings when the unit is in operation and when the drain valve is closed, and the thermostat bulb 59 keeps the shelf 19 near the critical temperature for maximum steam generation (about 40 above the boiling temperature).

The heat input may be such that the B.t.u. per square foot of heating surface is close to that of the critical temperature discussed in Reissue Patent No. Re. 24,861, or about 300,000 B.t.u. per square foot per hour. In com mercial steam generators this heating input is usually less than 30,000 B.t.u.s per square foot per hour.

In applications where a range of steam flow from full steam to no steam is required, the drain valve is opened, either manually or by suitable automatic means, to blow out excess water.

A feature of the present invention is that it can be directly connected, without a valve in the steam line, because pressure builds up in a few seconds following opening of the water valve and pressure drops to zero in a few seconds following the opening of the drain.

With the present invention the fins provide effective heat transfer surfaces for the full height of the unit, and the fins are vertically staggered to increase the length of the path of travel for the hot gases. This makes efiiciencies as high as 75% possible. This is to be distinguished from vertical fire tube boilers where the heat transfer is limited to the parts of the tubes which are below the water level. In low boilers the water level is usually so low that the effective tube length is short and the path for the hot gases is straight, limiting the efiiciencies to about 60%.

When the heat exchanger of the present invention is used in a hot water heating system having a system circulation pump, the latter will force the water flow over the heating surfaces fast enough to prevent steam formation. When the gas is turned off, the large volume of water in the system absorbs the heat stored in the fins without excesrve temperature rise.

It is to be noted that the present heat exchanger has a high ratio over :1) between the fin surface and the ring surface. This helps in making the relatively small size possible.

The heat exchanger of the present invention may be employed as an evapoartor. When evaporators are used with sea water they must be blown down at frequent intervals to remove the salt which is being continually concentrated. This requires a shut-down and there is considerable loss of heat when the brine is discharged. With the present invention, any salt would build up on the heating surfaces as a scale which could be removed in a few minutes by allowing the unit to run dry to loosen the scale and by then flushing it out with sea water. For an application of this type, copper alloys would be used in place of aluminum. Any scale still adhering to the heating surfaces would be broken off by the violent boiling beneath them during the next operation. The reason for this is that water would get under the scales through the cracks formed when the unit was running dry. This is similar to the action which takes place on the bottom of a teakettle used to heat water in the kitchen.

In areas where the water is very hard, a boiler used as a steam supply operates in a manner similar to the opera tion of an evaporator. The water which is used to form steam leaves a deposit of its salt content, causing the boiler water to eventually become a brine if the boiler is not blown down and refilled with fresh water at intervals. With the present invention, such blowing down is not necessary because of the above-mentioned automatic removal of scale when the unit runs dry.

While in the preferred form of the invention the rings are circular, it is to be understood that various other surrounding forms can be employed. Various other changes, modifications and adaptations may be made without departing from the spirit of the invention, and all of such variations are contemplated as may come within the scope of the claims.

What I claim is:

1. In a stem generator having a surrounding upright wall with its outer side constituting a steam generating surface and having inwardly projecting fins, said surface being of stepped formation to provide superimposed shelves whose outline is of decreasing size upwardly, having means including a feed water inlet for directing water simultaneously onto substantially all portions of the top of the outer side of said wall to flow down said steam generating surface by gravity from one shelf to the other, and having means for heating the fins of said wall to a sufiicient temperature to cause generation of steam by heat conducted from said fins to the outer side of said wall, the improvement comprising a water distribution means to which the feed water inlet is connected, said distribution means having distribution holes and be ing so shaped and positioned over the uppermost shelf as to distribute water uniformly thereon, which Water will flow by gravity from one shelf to another with some Water being evaporated on each shelf, and means for maintaining the lowermost shelf near the critical temperature for maximum steam generation for causing violent spattering of boiling water from said lowermost shelf onto upper shelves.

2. A steam generator as claimed in claim 1 in which the supply of generated steam is controlled by controlling the feed of water.

3. A steam generator as claimed in claim 1 in which flue gases from a source of heat are caused to travel upwardly within the surrounding upright wall in contact with the inwardly projecting fins.

=4. A steam generator as claimed in claim 3 in which there are superimposed tiers of fins and in which the fins of one tier are staggered with respect to the fins of adjacent tiers to provide circuitous paths for the upwardly traveling flue gases.

5. A steam generator as claimed in claim 1 to which the superimposed shelves are formed by horizontal surfaces of superimposed rings which are of heavy crosssection, and in which the rings are vertically spaced from one another by upright wall portions of the heating surfaces.

6. A steam generator as claimed in claim 5 in which the rings are of cast material and the upright wall portions are bands of rolled material.

7. A steam generator as claimed in claim 1 in which there is a water coil within the surrounding upright wall having one end connected to the water inlet and having its other end positioned to supply preheated water for steam generation.

8. A steam generator as claimed in claim 7 in which there is a condensate receiving cup supported below the preheating coil which also forms a bafi'le for flue gases to cause the latter to efiiciently wipe the fins when traveling upwardly.

9. A steam generator as claimed in claim 1 in which there is means for supplying the water with a pulse feed.

10. A steam generator as claimed in claim 1 in which there is an outer shell surrounding the steam generating surface and confining the generated steam, said shell having a steam outlet therein, and in which there is bafile means in said shell positioned to protect the steam outlet from the spattering water and to cause the latter to be directed onto upper shelves for recirculation.

11. A steam generator as claimed in claim 1 in which the wall material below the lowermost shelf has a surrounding groove, and in which there is a surrounding thermostat bulb in said groove and in which there is means responsive to said bulb for controlling the input of heat.

12. A steam generator as claimed in claim 1 in which said stepped surface is formed of steel to provide the superimposed shelves and in which the inwardly projecting fins are in the form of cupric lugs brazed to the under side of the shelves.

13. A steam generator as claimed in claim 12 in which each cupric lug has a portion extending angularly downwardly and inwardly from below the shelf and has a portion projecting inwardly therefrom with an upper surface which is in a plane above the shelf.

14. A steam generator as claimed in claim 12 in which the brazed joint extends throughout the width of the shelf.

15. In a steam generator having a surrounding upright wall with its outer side constituting a steam generating surface and having inwardly projecting fins, said surface being of stepped formation to provide superimposed shelves whose outline is of decreasing size upwardly, having means including a feed water inlet, and having means for heating the fins of said wall to a sufficient temperature to cause generation of steam by heat conducted from said fins to the outer side of said wall, the improvement comprising a water distribution trough to which the feed water inlet is connected, said trough having distribution holes and being so shaped and positioned over the uppermost shelf that the holes distribute water uniformly thereon with the water flowing by gravity from one shelf to another while some water is being evaporated on each shelf, and means for maintaining the lowermost shelf near the critical temperature for maximum steam generation for causing violent spattering of boiling water from said lowermost shelf for recirculation purposes onto the upper shelves.

16. A steam generator as set forth in claim 15 in which there is means for directing said spattering water onto said upper shelves.

17. A steam generator as claimed in claim 1 in which there is a flue stack and in which there is a water coil within the fiue stack having one end connected to the water inlet and having its other end positioned to supply preheated water for steam generation.

18. A steam generator as claimed in claim 1 in which the superimposed shelves are formed by horizontal surfaces of superimposed rings which are of heavy crosssection, there being a surrounding wall portion of one ring seated on a surrounding wall portion of a ring therebelow to provide a joint and to support the rings in vertically-spaced relation, and there being an annular gasket for each of said joints.

19. A steam generator as claimed in claim 18 in which there is spring means normally urging said rings into tight engagement at the gasketed joints.

References Cited UNITED STATES PATENTS 377,228 1/ 1888 Bartlett 122-40 1,586,761 6/1926 Shepherd. 2,920,179 1/1960 Shaw 122-40 XR KENNETH W. SPRAGUE, Primary Examiner.

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