US2081972A - Process and apparatus for heating fluids - Google Patents

Description

June l, 1937- J. G. ALTHER PROCESS ANDl APPARATUS FOR HEATING FLUIDS f 'original Filed June 4. 1952 2 Sheets-Sheet 1 D D DCI June l? 1937.

J. G. ALTHER l v 2,081,972

PROCESS AND APPARATUS FOR HEATING FLUIDS Original Filed June, 4, 1952 2 Sheets-Sheet 2 Patented June 1, 1937 UNITED STATES PATENT oFFicE Joseph G. Alther, Chicago, Ill., assignor, by mesne assignments, to pany, Chicago, Ill.,

Universal Oil Products Coma corporation of Delaware Application June 4, 1932, Serial No. 615,359 Renewed October 7, 1936 4 claims. (ci. 12a- 356i This invention particularly relates to an improved method and means of heating fluids. substantially by convection or fluid heat during their passage in a continuous stream through a l More particularly the invention refers to an improved method and means for heating fluids wherein a heat carrying medium, e. g. hot combustion gases, are directed by positive means to opposite sides of the conduit through which the fluid is passing, whereby they are uniformly distributed over the surface of the fluid conduit in such a manner as to `effect an average rate of heating around the entire periphery of the conduit approaching the maximum rate of heating and whereby direct control for any desired rate of heat input is obtained.

While the invention may be applied to the heating of fluids generally, its preferred use is in the heating of hydrocarbon oils and it is especially useful in the cracking of hydrocarbon oils.

Among the principal advantages of the present invention are included (a) high average heat input over the entire surface of the fluid conduit and uniform application of heat to opposite sides of the conduit; (b) flexibility of control of heat application to obtain any desired rate of heat input by direct control of the firing conditions and direct control of heat distribution to obtain any desired heating curve for the material undergoing treatment; (c) high tube efficiency, as compared with convection heating imethods of conventional character. Maximum capacity for a iuid conduit of given size or, vice versa, minimum heat absorbing surface for a given capacity; (d) quick heating of the fluid to a high temperature in a minimum of time. As applied to the conversion of hydrocarbon oils, this feature reduces excessive cracking and gas formation as well 'as the formation and deposition of coke in the heating zone or fluid conduit, thereby increasing the yield and antiknock rating of the motor fuel product as compared with the same cracking process employing heating methods which necessitate lower rates of heating and longer time in the heating zone. The use of the invention may also reduce installation, operating and maintenance costs and increases the safety factor. In the heating of fluids substantially by convection, itis common practice to pass hot combustion gases over the uid conduit, comprising a plurality of tubes through which the fluid flows in a continuous stream. When the tubes are arran-ged in rows parallel to the flow of combustion gases there is a tendency for the gases to channel through the space between the rows of tubes, permitting a large volume of hot gases to pass through the tube bank without coming into intimate contact with the tubes and transmitting heat to the fluid. To overcome this channeling of the hot combustion products it has become common practice to place the tubes of the iiuid conduit in staggered formation so that the path of flow of the hot gases is continually changed in an attempt to distribute them more uniformly around 'the tubes and obtain more intimate and uniform contact between the combustion gases and the tube surfaces. However, such methods do not secure positive uniform distribution of the gases about the tubes, different tubes in the same row perpendicular to the general direction of ilow of the combustion gases often being heated at different rates and the Iuniform distribution of hot gases around the circumference of each tube being the exception rather than the rule. The staggered arrangement of tubes which breaks up the stream of gases, changing their uni-directional flow, offers increased resistance to the flow of gases which, with a given draft condition, decreases the volume of gases which may pass through the tube bank in a given time, thereby decreasing the velocity and heating efliciency of the gases which come into actual contact with the tube surfaces.

In the operation of the process of the present invention, the hot combustion gases, which furnish heat substantially by convection to fluid passing in a continuous stream through the tubes of a fluid conduit, are directed against opposite sides of the tubes in such a manner that a substantially uniform distribution of gases and substantially uniform heating is obtained on opposite sides of each individual tube in the fluid conduit. This insures a high average heat input per unit area of tube surface for any given maximum rate of heating, which means high tube efliciency and maximum capacity for a fluid conduit of given size or, vice versa, a minimum of heating surface for a given capacity. Also, by employing the features of the present invention, wide differences in heat intensity at vdifferent points around the circumference of the same tube are avoided, thereby eliminating stresses. due to unequal expansion, and increasing the safety of a tube of given diameter and wall thicktubes of the same diameter. This reduces the cost of installation. Also, as a result oi' increasing the average rate of heat input, the time of treatment may be reduced and over-cracking minimized, which results in smaller gas production per barrel of oil being treated, greater gasoline yield and higher octane number, as stated previously. Furthermore, due to the decreased length of fluid conduit required, there may be a considerable reduction in the pressure drop, due to friction through the fluid conduit, resulting in smaller pumping costs, a reduction in the cost of cleaning the heating coil and in furnace maintenance generally, as Well as in decreased initial cost.

The attached diagrammatic drawings illustrate a specific form of apparatus designed to incorporate the features and advantages of the present invention. It should be understood that a specific form of apparatus is shown only for the purpose of more clearly illustrating the ease of incorporating the features of the invention in a simple furnace structure and that the invention is not limited to this nor to any other specific form of apparatus. For example, the invention may be used in conjunction with the usual method and means of heat application as top or bottom ring, the use of tube ns, all types of fuels and improved means of utilizing them.

The accompanying drawings illustrate one specic form of furnace structure which is especially adapted to uniform heating of each tube in the heating coil regardless of its position in the heating zone. l Y

Figure 1 is a sectional plan view of the furnace;

Figure 2 is a sectional elevation; and

Figure 3 is a sectional end View. y

The main furnace structure comprises side walls 2| and 2|', end walls 22 and 22', a roof 23 and a floor 24. In this furnace, the fluid conduit comprises a vertical row of tubes horizontally disposed, extending between the end walls 22 and 22' and connected in series by means of suitable headers or return bends 26, inlet and outlet lines 21 being provided through either of which the oil or other fluid to be heated may be introduced or discharged so that the flow of fluid through the heating coil may be either from top to bottom or from bottom to top, as desired.

Firing tunnels 28 and 28' are provided, extending beneath the main furnace structure, to which any desired fuel'such as oil, gas or pulverized solid fuel is supplied through firing ports 29 and by means of suitable burners, not shown in the drawings. The hot combustion gases pass upward from the firing compartments 28 and 28' through openings 30 in the iioor 24 of the furnace into zones 3| and 3|. Zones 3| and 3|' are located on opposite sides of the uid conduit and are separated from the zone in which the uid conduit is located by means of prforated walls 32 and 32. The hot combustion gases are directed from zones 3| and 3| through openings 33 in walls 32 and 32' to opposite sides of the tubes 25 in equal quantities on each side of the uid conduit, bathing the outside of each tube in a stream of hot combustion gases which supply heat by convection to the tube walls and to the fluid flowing through ,the tubes. The spent combustion gases having passed over the tubes of the fluid conduit are discharged through openings 34 in the roof of the furnace to ue 35, from which they may pass t0 a Suitable stack, not shown.

2,081,972 ness or permitting the use of a thinner Wall in The size of openings 33 in walls 32 and 32' may be varied to suit requirements, from top to bottom of the tube bank, so that each tube of the heating coil may receive substantially the same amount of heat or so that diiferent degrees of heating may be employed for tubes in different sections of the heating coil, thus permitting a substantially uniform rate of heating throughout the heating coil, an increasing rate throughout the heating coil, a decreasing rate throughout the heating coil or a maximum or minimum rate at any desired intermediate point in the heating coil. In the case illustrated, the openings increase in size from bottom to top of the tube bank so that a smaller quantity of the relatively hot gases from the lower portions of zones 3| and '3|' are introduced around the tubes in the lower portion of the heating coil, as compared with the quantity of gases discharged from progressively higher points in zones 3| and 3| around the tubes in higher portions of the heating coil. This design permits heating the tubes in the upper portion of the tube bank at a higher rate than the tubes in its lower portion. By reversing the arrangement of openings 33 in walls 32 and 32' the opposite eiect may be secured or the same thing may be accomplished by reversing the flow of oil through the uid conduit. When openings of substantially uniform size and spacing from top to bottom of walls 32 and 32' are employed, a substantially uniform rate of heating may be secured through the tube bank.

Preferably vthe firing conditions and firing compartments 28 and 28' are equalized and the perforations 33 are the same on opposite sides of the uid conduit in order to insure substantially uniform heating on opposite sides of each individual tube.

Firing conditions may be so controlled that combustion is substantially completed in the ring compartments 28 and 28 or some combustion may be allowed in compartments 3| and 3| Particularly in the latter case, however, the walls 32 and 32' are constructed of suitable material of low thermal conductivity, such as fused or molded alumina or silica, natural or artificial mullite, etc. (i. e. material which will transmit only a minor amount of radiant h'eat from compartments 3| and 3|' to the tubes) in order that the tubes may be heated primarily by convection or fluid heat from the hot combustion gases which come into contact therewith.

As a specific example of some of the advantages obtainable by use of the features of the present invention, as compared with a conventional bank of tubes heated substantially by convection without provision for directing the hot combustion gases equally on opposite sides oi each vertical row of tubes, the emciency of the furnace measured in terms of the B. t. u. input per square foot of heating tube surface, per hour, may be increased by 20 percent, or more, due to intimate contact of hot combustion gases with a greater portion of the circumference of the tube surfaces at a higher velocity and to the more uniform distribution of heat around the entire circumference oi' each tube and a consequently higher average heat input for any given condition.

In a furnace structure of the type illustrated in the attached drawings wherein fresh incre-V ments of hot combustion gases are supplied to the iiuid conduit along its entire length, the drop in the temperature of the hot combustion gases between the bottom and top of the tube bank will 4be greatly diminished, thereby increasing the velocity,

around the upper portion of the tube bank, mayaverage rate of heating throughout the tube bank. When desired, the heat input into the tubes in the upper portion of the tube bank may be maintained at a high rate by increasing the proportion of fresh gases entering the upper portion of the heating zone, as illustrated in the drawings. In this case the increased volume and due to the increased volume 'of gases compensate or more than compensate for the somewhat decreased temperature o-f -the gases. To further illustrate this improvement we may first assume two cases wherein a hydrocarbon oil is heated from a temperature of 500 F. at the inlet to the heating coil to an outlet temperature of 900 F. In the first case, wherein the heating is accomplished by convection heating of the co-nventional type, the hot combustion gases enter the heating zone at a temperature of about 1650* F. and leave at a temperature of about 900 F.

- This gives us a logarithmic mean temperature difference of approximately 615 F. and assuming an average rate of heat absorption of 9 B. t. u. per square foot of tube surface per 1 F. log. mean temperature difference, this method of heating gives a rate of about 5550 B. t. u. per square foot per hour. In the second case, wherein the features of the present invention are employed, combustion gases being directed to opposite sides of each tube and fresh increments of hot combustion gases being introduced all along the path of travelA of the oil through the fluid conduit, the outlet temperature i of the combustion gases is increased to about l400 F. for example. This increases the log. mean temperature difference to about 830 F. and assuming a figure of 10.8 B. t. u. per square foot of tube surface per 1 F. log. mean temperature difference or an increase of 20 percent in the heat transfer coefficient because of more intimate contact between the gases and the tube surfaces, as mentioned in the first of the foregoing examples, the rate of heating is increased to approximately 9 000 B. t. u, per square foot per hour. This is a 62 percent increase over` the rate of heating obtained by conventional meth- 'ods in the comparable case above given. It will be understood, of course, that even higher combustion gas outlet temperature may be utilized whereby much higher increases in the rate of heating are obtainable.

It is to be understood that by the term convection, as used in the above specification and appended claims, I mean heat carried by the gases, which may also be termed fluid hea 1 claim as my invention:

l. A method of heating fluids wherein the fluid is passed through a plurality of substantially horizontal restricted conduits superimposed one above the other in a common vertical plane within a heating zone, the improvement which consists in supplying hot combustion products from independent sources of combustion to the Yheating zone on oppos'te sides of the vertical l=plane of the conduits, and dividing the combustion products on each side of said plane into a plurality of streams directed against the conduits.

. 2. A method of heating fluids wherein the fluid is passed through a plurality of substantially horizontal restricted conduits superimposed one above the other in a common vertical plane within a heating zone, the improvement which oo nsists in supplying hot combustion products from independent sources of combustion to the heating zone on opposite sides of the vertical plane of the conduits, anddividing the combustion procl-` ucts on `each side of said plane into a plurality of streams directed against the conduits, and regulating said streams of combustion products to obtain approximately equal heat transfer to. v, each of the conduits.

3. A fluidA heating apparatus comprising a furnace, a pair of spaced perforated walls parallelly disposed within the furnace and forming a heating zone and a distributing zone on each side of the heating zone, a plurality of heating conduits within the heating zone and disposed in a common plane parallel to and spaced from said walls, and means for supplying combustion gases Ito the distributing zones to pass through the perforated walls into. the heating zone on opposite sides of the conduits.

4. yA fluid heating apparatus comprising a furnace, a pair of spaced perforated walls vertically disposed within the furnace, a plurality of heating conduits positioned one above the other in a vertical plane between and spaced from said walls, means for supplying combustion gases to the outer side of each of said walls to pass through the perfcrations and heat the conduits by convection, and means for removing the combustion gases from the space between said walls.

JosEPH G. Aurmsm..l

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