US1390783A - Method of and apparatus for transmitting heat - Google Patents

Description

A c. HERING. METHOD or AND APPARATUS ron TnAusyArTTmG HEAT.

' APPucAnou mm JULY 21.1%.6.

PatentedSept. 13, 1921.

' TEMPE RA TU R E5 OF SURFACE UNITED STATES PA' I'ENT OFFICE,

CARL HERING, OF PHILADELPHIA, PENNSYLVANIA.

METHOD OF AND APPARATUS FOR TRANSMITTING HEAT,

To all whom it may concern Be it known that I, CARL HERING, a citizen of the United States, residing in the city of Philadelphia, county of Philadelphia, and State of Pennsylvania, have in vented certain new and useful "Improvements in Methods of and Apparatus for Transmitting Heat, of which the following is a specification.

My invention relates to the transmission of heat as between flames, hot gases or vapors and liquids, such as water, or other materials, as in steam boilers, condensing ap )aratus, cooking utensils, etc.

t isthe object of my invention to procure such heat transmission at high rate, or with high economy or efficiency, or both.

To these ends I interpose between the hot medium, asthe flames, hot gases or vapors and the medium to be heated, which may be termed the cold medium, a conductor of heat which interposes such resistance to the heat transmission between the hot and cold media that such conductor at its terminal or surface next to or in contact with the hot medium attains a relatively high temperature, a temperature so high that the resistance to transfer of heat between the hot medium and the conductor at such terminal or surface is materially reduced, and to such degree reduced that notwithstanding the interposition of the iesistance of the conductor the total resistance to thermal conduction between the hot and cold media is greatly reduced, indeed to such extent reduced that the rate of heat transfer is increased to a new order of magnitude, that is, several fold or several hundred per cent, and that the efficiency or economy is also increased.

When water is heated in metallic vessels, as in steam boilers or cooking utensils, by means of high temperature gases, like flames or gases resulting from fuel combustion, a thin film of gases offering very high resistance to heat transfer forms on the fire side of the water-containing vessel and greatly retards the transmission of heat and reduces the rate of transmission of heat, since all the heat which is transmitted from the hot gases to the water must pass through this film.

The object of my invention is to break down the resistance of this film to a great extent, thereby making it possible to heat the water at a higher rate with resultant increase in steaming capacity of boilers per Specification of Letters Patent.

Patented Sept. 13, 1921.

Application filed July 17, 1916. Serial No. 109,648.

resultant increase in fuel economy. With the same amount of heat applied during the same length of time, more of the heat will enter the water, whereby the heating eificiency is increased.

In technical terms, the temperature gradient, that is, the fall or drop in temperature through the surface of thefire side, of the water container is exceedingly greatin the present methods and apparatus. If the hot gases have a temperature of, say, 1,000 degrees centigrade and the water 100 degrees centigrade, there is a fall of temperature of 900 degrees in a film of gases of only a few thousandths of an inch thickness, indicating that the film offers a very high resistance to the passage of heat. This is illustrated by the well known fact that a piece of paper pasted on the fire side of a water containing vessel will not even be charredby the flames; and also by the fact that water can be boiled in a vessel of thin paper held directly in a high temperature flame.

This high resistance film of gases is probably due to the sudden chilling of the hotgases striking the relatively very cold surface, these, chilled, gases forming a film which adheres very tenaciously or at any rate continues to be present. It seems to become thinner or less resistant when the hot gases wash over the heat absorbing surface more ra )idly, and it may be greatly reduced in thic ness by a powerful blast of the hot gases, as by a blow pipe. I have discovered that the resistance of this film, or whatever it may he, becomes greatly reduced when the temperature of the metal or intermediate heat conductor on which it forms is forced to become considerably higher than that of the medium to be heated, that is, when the fall of temperature from the hot medium to that of the fire side of the container is reduced, that is, whenthe hot gases are chilled less on striking the metal or conductor surface.

.In my present invention I accomplish this terial, offering a moderate resistance to heat transmission between 1 the hot and cold media, so that the flow of heat through this conductor will cause a moderately high temperature drop between the side or surface exposed to the hot medium and the side or surface exposed to the cold medium. It is found that the resistance of the film which then, forms on e hot side or surface of the conductor will be so greatl reduced that even together with the artificially introduced resistance of'the conductor the total resistance is far less than it was before, thereby enabling heat to travel at higher rate from the hot to the cold medium, which means that more heat will be transmitted in a' given time over the same area of the containing vessel.

For an understandin of my invention and for an illustration 0 some of the forms my invention may take, reference is to be had in the accompanying drawings, in which:

Figure 1 is a graphic representation of relations between thermal resistances and temperatures explanatory of my invention.

Fig.2 illustrates results obtained with different lengths of interposed resistances of equal cross sections.

Fig. 3 illustrates results" from interposed resistances of equal lengths but different cross sections.

Fig. 4 shows the application of the resistances to curved surfaces like those of the water tubes of steam boilers."

In Fig. 5 are shown various shapes the resistances may take. F'gs. 6 and 7 illustrate further modifications of the resistances.

Fig. 8 is a cross sectional view, partly in elevation, of a cooking utensil having the resistances applied thereto. a

Fig. 9 is a bottom plan view of the utensil shown'in Fi 8.

Referring to ig. 1, vertical distances or ordinates represent thermal resistances, while horizontal distances or abscissae represent temperatures ofthe' surface upon which the hot gases impinge. The curves are not exact or accurate, but are explanatory merely, and are sufficiently characteristic of the results determined for the purpose of explaining my invention. 1

The curve A shows the relation between the resistance of the gas film and the different temperatures of the surface upon which the hot gas impinges. This shows that at low temperatures the resistance of the film is very high andthat this resistance diminishes as the temperature increases.

The curve B represents the relation between the resistances and the hot end temperatures of the artificial. resistances, that is,"'the interposed conductors when the heat flows through them. This shows that with increasing resistances'the temperatures at the hot end of the interposed conductor increases.' This resistance is that between the surface heated directly by the hot ases and the water or cold medium. When eat flows through a thermal resistance there is always a difference of temperature between its two ends.

The curve C is the resultant of curves A and B, found by adding their ordinates. The curve C represents the sum of the film resistance and of the artificial resistance interposed by the conductor, and represents the total heat resistance between the hot and cold medium for the various temperatures'. It is this total resistance which governs the flow of heat from the hot into the cold medium, and therefore the less this total resistance the greater will be the flow of heat through it from the hot to the cold medium. The curve C is seen at first to fall rapidly with an increase of the surface temperature of the resistances, and then reaches what may be considered a minimum value substantially near that temperature at which the curves A and B intersect, and thereafter at higher surface temperatures it either rises again slowly or remains nearly horizontal. The important feature, however, is that the total heat resistance at first grows less very rapidly, reaching approximately minimum; beyond this point the nature of the curve is of little impor-' tance.

As stated above, these curves do not represent exactly the values and conditions.

They are given here merely in explanation of the apparent anomaly that an addition of one thermal resistance to another will operate to reduce the total effective thermal resistance, and for this urpose it .suflices that the curve A is a falling curve and-B a rising curve, both of which general facts are true, as observed. Whether the curve C has an actual minimum point or merely continues to fall is not material, the observations show that it falls rapidly at firstv and indicate that it thereafter rises, though much more slowly.

I prefer to produce the artifical thermal resistance by means of long thin lugs of metal or other suitable heat conductor fastened to the fire side of the water container and projecting into the hot gases or flames.

Fig. 2 shows a vessel V containing water and having three, lugs of equal circular cross sections but of different lengths and all of the same material, soldered, welded or otherwise autogenously attached to the bottom of the vessel V and projecting into the hot gases or flames. It is found that the water will boil far more rapidly over the lugs than over .the rest-of the bottom of the container and approximately in the proportions reparesented by the lengths of the vertical lines a, b, c and d, the length a representing the corresponding rate of evaporation for the same area of a similar vessel without lugs,

and therefore it represents ordinary practice, as with cooking utensils. The line b represents proportionally the rate of evaporation over the cold ends of the lugs procured with anexactly similar vessel having on its bottom lugs of the length and cross section of lug e; similarly the lines 0 and d represent the relative rate of evaportion over the lugs of exactly similar vessels supplied with lugs like the lugs f and g, respectively. By way of example, merely, it may be stated that with similar tinned iron vessels, supplied respectively with round lugs about} inch diameter, of which one vessel was supplied with lugs e practically 1 g of an inch long, another with lugs f 1% inches long, and another with lugs g substantially 21- inches long, the evaporation was far greater over the shortest lug than over an equal amount of the same surface which has no lugs; the evaporation was about twice as rapid over the moderatelvlong lugs f than over the short lugs e, and over the lugs g a little less than over the lugs f, even though the ends of the lugs g were red hot while the ends of the lugs e and 7 were still black. About 12 times as much heat was transmitted per second through the lugs f as through the same area of the vessel itself without any lug, which latter condition represents "common practice.

These results show that with lugs of the same. diameter and of the same material there is anadvantage in making them long, approximately, about five to seven times their diameter, yet there is no further gain,

and there may be-even a loss, in making them much longer.

These observations show further that the additional thermal resistance introduced by these lugs causes much more heat to enter their hot ends and to pass through them than will pass through an equal area of the normal vessel bottom, and it appears that the resistance of the gas film at their hotter ends is reduced by the resultant higher temperature of those ends. as though they pierced through the high resisting film to allow the heat to pass through it. These observations show also that as the length of these lugs, and therefore their resistance, increases, the amount of heat transmitted by them increases, but

that a limit is reached beyondwhich there is no further increase, but even in some cases a decrease. This corresponds with the disclosure by the curves in Fig. 1. There are certain intermediate conditions in which i The lugs operate lines m, 'n, 0, and p represent in proportion the rates of evaporation procured, respectively, by the lugs h, z, j, and 7s per unit of area of lug section exposed to the water. They show that the smaller the diameter the greater the rate of evaporation. The lug k became-red hot at its lower end while the others continued black. These observations show that for the same length of lugs the smaller the diameter the better, but since the cost per unit area of the vessel increases as the lugs become more numerous when made smaller, there is a practical limit beyond which their diameter should not be decreased. I

It is further observed that with all the lugs of the same material and diameter and length and all parallel to each other, secured perpendlcularly to .the flat bottom of the vessel, the farther they are spaced apart from each other the greater is the heat transmitted through each, but when arranged closer to each other, the greater number of lugs per unit area of the vessel bottom causes a greater total transmission of heat per unit area of the bottom of the vessel, though when the lugs are spaced too close together or are too crowded, this fact nolonger holds true. When the lugs are farther apart the flames and hot gases have better access to them.

This latter observation shows that the lugs should be numerous, but not too crowded to prevent free access to the gases. There is therefore an advantage in having the surface of the vessel curved, like the outside of the tubes of a water tube boiler, or the hemispherical bottom of a cooking dish, as the water or cold ends of the lugs ma be )laced close together while the fire en s will be farther apart.

This is illustrated in Fig. 4;, where T may represent a part of the clrcumference of a water tube of a boiler or the hemispherical bottom of a cooking or other utensil having applied thereto in radial relatio'ns'the various lugs, as kl.

Surfaces having small radii of curvature, as small wires, points, sharp edges, etc., will both emit and absorb heat more rapidly than the same extent of surface arranged with greater radius of curvature. For this reason also lugs of relatively small diameter are more effective than those of larger diameter. And I may therefore form the hot ends of the lugs as illustrated in Fig. 5, wherethe lug q is in the form of a point, the lug r in the form of a sharp edge, and the lug s has a sharp edged head similar to that of a tack.

a It is further observed that numerous lugs of small diameter distributed over a given surface produced better results than thin flat strips of the same total cross section and projecting the same distance from the sur-- face of the vessel, again proving the ad- Vantage of small radii of curvature. Tacks that they had too low a thermal resistance,

being much too short and thick to occasion that rise of temperature of the hot side sufii-v 'ciently to reduce the resistance. of the film of chilled gases. Their principle I isdifferent from that of my lugs or projections, and their proportions were such that the principle underlyingmy lugs is absent.

The marked advantages which are obtained in accordance with my invention are not due merely to an increased surface exposed to the flame. I have found that the increase in evaporation bears no relation whatsoever to theincreased surface of the lugs. For example, doubling the length of the lugs of the same diameter and spaced 1 the same distance apart, thereby doubling their surface, other things being e ual, actually produced less evaporation. I n another case with lugs of the same total external surface distributed over the same area of the vessel, those lugs with the smaller diameters gave far better results, nearly four times as great per square inch of lug section-as obtained by lugs of about four times the diameter; the thermal resistance of the latter was far too low, and it was this and not the surface that determined the heat flow. The ends'of the smaller lugs became red hot, showing that the resistance of the film had been very greatly reduced by their higher The maximum of surface for a given cross section of the lugs is reached when instead of round lugs thin flat strips of the same length and cross section are used; in a case in which the surface of the strips was double that of the lugs, the round lugs produced about twice the evaporation for thesame area of vessel,, although the surface was only half as great.

Nevertheless, in case the thermal resistance of the lugs has been properly proportioned to break down the resistance of the film, the greater the surface exposed to the flame orzhot gasesrtthe better willglbe the total result. An increase of surface is desirable, but is not alone the controlling factor, which latter is the breaking down of the resistance of the film by a material increase of temperature of the surface receiving heat from the hot gases. 4

The total film resistance of a given vessel is obviously reduced by increasing the surface which receives the heat from the flames.

' The lower this film resistance the lower need be the artificially introduced resistance of the lugs, and therefore the lower will be the sum of the two and the greater the re sultant flow of heat. It is therefore of advantage to increase the surface of the film as, for example, by using numerous long small diameter lugs, but their resistance must nevertheless be correspondingly proportioned or else the full advantage of the larger surface will not be gained. It would be possible to form a thermal resist 'ance on the fire side of a container by means of a very'thick metal wall or a very thick coating of enamel, but aside from the im racticability of so doing, theheat absor bing surface would not be greatly increased, and the film resistance at best would still be-high and therefore also the artificial resistance, making the sum of the two high; and it is this sum that controls the heat flow. The smaller the flow of heat the greater must be the artificial resistance necessary to raisethe temperature of the surface at the hot .end to the desired degree.

The desired high drop or fall of temperature between the fire ends and the water ends of the lugs is a function of the flow of heat through them as well as a function of their thermal resistance. This relation is quite analogous to that in electric circuits in which the fall of potential due to a current through a resistance'is a function of both the resistance and the current flowing through it. The purpose of introducing the artificlal thermal or heat resistance of the lugs or interposed conductors is to reduce the resistance of the surface film by increasing the tem erature of that surface. The artificial resistance should therefore be as low aswill make the total resistance from flame to water as low as possible. The flow of heat from flame to wateris governed by this total resistance, namely the sum of that of the film and that of the interposed lugs to procure about 27 times the evaporation per unit area of the lugs as compared with {he evaporation per unit area without the ugs. I

A further advantage accruing from the use of the lugs or thermalresistances resides in the fact that since these resistances are of such magnitude that their ends exposed to the hot gases are raised to high temperature, they do not chill the hot gases or flames impinging thereon to such extent as to cause deposit of carbon or soot upon the lugs or resistances, such carbon or soot deposits in ordinary practice being due to the too great chilling of the flame or gases, and the presence of such carbon, or soot further interferes with heat transfer.

The increase in heat flow through properly proportioned lugs or interposed artificial thermal resistances is so great relative to the heat flow through the ordinary and usual walls of the water containing vessel, that it may not always be safe to take full advantage of the principle on account of the possible danger of reaching what is known as the spheroidal state of the water. The danger of reaching this spheroidal state may, however, be greatly lessened by similar y adding a thermal resistance in the form of a lug, point, edge, etc., on the water side of the container. This is illustrated, for example, in Fig. 6, where the lug, as It, has on its inner end within the water a pointed lug k.

lVith increase of velocity of the hot gases impinging upon the heat absorbing surfaces better results are produced; and the increased advantage due to the velocity of the hot gases is greater in the case of lugs than in the case of the plain heat absorbing surface. With slowly moving hot gases I have evaporated water two, three and nearly four times as rapidly per unit area of the total water exposed surface of the vessel, in vessels supplied with the properly proportioned lugs than without, the vessels themselves being otherwise identical. The flame being the same for all, the fuel economy was increased in about the same proportion. In good steam boilers in which the heat economy is already quite high, the advantage of these lugs will reside more in increasing the steam output, that is, rate of steaming, or in a reduction of the size of the boilers, rather than in any very marked fuel economy.

As the thermal resistance at the contact between two surfaces even when close together is generally quite high, I prefer to solder, weld or cast the lugs on to the surface of the vessel, as distinguished from attaching them by means of screw threads or by rivetin For steam boilers I prefer to attach the lugs by means of electric weldmg.

The lugs need not be of the same metal v as the Water container, but may be of any suitable metal or material, as for example brass, copper,'iron or steel.

In Fig. 7 a lug t is shown flared at its water contact end in order to decrease the heat resistance between that end and the Water. I

The principle of my invention may also be applied to cooking utensils by making the bottom a casting, as shown at u, Figs. 8 and 9, with a large number of small lugs thereon as indicated at e, which may be arranged as indicated in Fig. 9, or in any other suitable manner.

As a suitable guide in practically proportioning the interposed lugs or thermal resistances it may be stated that they should be of such material, length, and "cross section that the flow of heat through them causes in them a temperature fall or drop which is substantially one-half of the total temperature fall or drop between the hot medium and the cold medium.

I have herein used the term thermal or heat resistance as convenient in referring to that property of the surface film by which it resists, obstructs or retards the passage of heat through it. Whether it be a true thermal resistance or not is not material. There is a retardation of the flow of heat through the absorbing surface, and the effect is therefore like that of a true resistance and may therefore be considered as such for the purpose of describing my invention.

It will further be understood that I do not wish to be limited to the theory or explanation of the phenomena hereinbefore given, recourse being had to the same simply for purposes of better explaining my invention.

What I claim is:

1. The method of heating a liquid to desired temperature by a hot gas, which consists in transmitting the heat from the as to the liquid through solid thermal resistances of such magnitudes that they assume at their fire ends a temperature approximately mid way between the-temperature of the hot gas and the desired temperature of said liquid, substantially as described.

2. The method of heating a liquid consisting wholly or largely of water, to a desired temperature by a hot gas, which consists in transmitting heat from the gas to the liquid through thermal resistances of such magnitudes that they attain, at their fire ends a temperature corresponding approximately to a red heat, said temperature being approximately mid way between the temperature of said gas and the desired temperature of said liquid, substantially as described.

3. A container adapted for heating a liquid by a hot gas having on its fire side 'a plurality of lugs constituting thermal reslstances of such magnitudes that their fire ends assume a temperature approximately 'mid way between that. of the hot gas and the transmitting wall, and a plurality of In on the fire side of said wall constituting thermal resistances of such magnitudes that the fall in temperature between their op 0- site ends is-approximately equal to the all in temperature between the hot gas and the hot ends of said lugs, substantially as described.

' 5. Apparatus for heating a liquid by a hot gas comprising a container havin a bottom adapted to be presented to the at gas, a plurality of slender lugs decreasing in cross section from said bottom toward the hot gas and distributed over substantially the entire extent of said bottom, said lugs constituting thermal resistances of such magnitudes that the fall in temperature between their opposite ends is approximately equal to the fall in temperature between the hot gas and the hot ends of said lugs, substantially as described.

In testimony whereof I have hereunto aflixed my signature this th day of July, 1916.

CARL HERING.

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