US3267910A

US3267910A - Process heater

Info

Publication number: US3267910A
Application number: US393989A
Authority: US
Inventors: Salvatore A Guerrieri
Original assignee: Lummus Co
Current assignee: CB&I Technology Inc
Priority date: 1964-09-02
Filing date: 1964-09-02
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23

Description

S. A. GUERRIERI PROCESS HEATER Filed Sept. 2, 1964 HEAT FLUX TO FRONT OF TUBE D Q) Q 5 SheecsShee1; 2

v I l 'w/o =20 I ,3 I I [I w/o 1.5 I

W/D=0 I", z I

FIG. 5

INVENTOR. SALVATORE A. GUERR/ER/ A T TORNE V5 1966 s. A. GUERRIERI 3,267,910

PROCESS HEATER Filed Sept. 2, 1964 5 S eets-S eet 5 INVENTOR. SALVATORE A. GUERR/ER/ A T TORNE VS United States Patent 3,267,916 PROQESS HEATER Salvatore A. Guerrieri, Rowayton, Conn, assignor to The Lurnmns Company, New York, N.Y., a corporation of Delaware Filed Sept. 2, 1964, Ser. No. 393,989 4 Claims. (Cl. 122-356) This invention relates, in general, to a new and improved process furnace utilizing finned radiant tubes and, more particularly, it relates to a process furnace in which finned radiant tubes are employed to control the distribution of the heat flux about the periphery of process tubes.

Process furnaces comprise tubes heated in a chamber either by the combustion of fuel within the chamber, by a radiator, by hot combustion products entering the chamber, or by a combination of these means. These furnaces differ from boilers (i.e., steam generators) because of the difierence in the physical properties of the fluids flowing in the tubes and because of the nature of the process, at least for boilers operating below the critical temperature of water.

In the case of boilers, the fluid inside the tubes is inert and has a high thermal conductivity. Therefore, heat transfer coefficients inside the tubes are high, and high heat fluxes are possible with moderate tube-to-water temperature dilferences. Because of this, local hot spots are no problem. In the case of process heaters, on the other hand, the fluids inside the tubes are temperature sensitive and have relatively low thermal conductivities. Therefore, the heat transfer coefficients are also relatively low.

Further, in many modern process furnaces, chemical reactions are intended to take place inside the tubes, and these reactions are temperature sensitive as to yield and quality of product. It is usually necessary to design into the furnace a definite heat flux pattern and temperature patttern for the process fluid in the tubes. These process requirements call for very careful design of process furnaces and the arrangement of the tubes. Many designs of furnaces and burners have been developed to achieve these ends. In all of these designs, the attempt is made to control the temperature and heat flux along the tube as well as around the tube. However, the independent control of temperature and heat flux along the tube and around the tube has not yet been obtained economically. Circumferential heat flux may be improved somewhat if the tubes in the radiant section of the furnace are arranged in a single row, and the plane of the tubes is irradiated from planes on both sides of the tube plane. However, even when the two radiating surfaces approach the ideal of being uniform in temperature and heat flux over the entire area, circumferential heat flux around the tubes is far from uniform, heat flux at the sides of the tubes often being as little as 0.5 to 0.6 of the heat flux at the points directly opposite to the radiating planes. Thus, ideal radiating planes may provide a substantially uniform heat flux and temperature along the length of a given process tube, but will have little effect on the uniformity of heat flux around the periphery of the tube. On a practical level, of course, it is much more expensive to provide a heater with two radiating surfaces, and with only a single row of process tubes, throughput is kept low unless the furnace is very large.

Thus, it would be advantageous to be able to achieve a uniform or controlled heat flux about the periphery of process tubes in a process furnace when radiated from a source located at one side only.

Accordingly, it is the general object of this invention to provide a new and improved process heater which is more economical to manufacture and which can be designed for substantially uniform peripheral heat flux to the tubes.

Another object of this invention is the provision of a new and improved process heater capable of utilizing direct and indirect radiation from a single source to equalize the heat flux about the tubes.

Still another object of the invention is the provision of a new and improved process heater capable of greater throughput per unit of heat energy, and in which heat flux both around the tubes and along their length is subject to positive control.

Various other objects and advantages of the invention will become clear from the following description of several embodiments of the invention, and the novel features will be particularly pointed out in connection with the appended claims. A better understanding of the invention will be gained by referring to the following description in conjunction with the accompanying drawings, which are illustrative only and are not to be interpreted in a limiting sense, and in which:

FIGURE 1, is a cross sectional view of a process tube built in accordance with the principles of the present invention;

FIGURE 2 is a cross sectional view of a second type of process tube built in accordance with the principles of the present invention;

FIGURE 3 is a schematic representation of a conventional tube in aprocess furnace, with the radiant source at the bottom of the drawing;

FIGURE 4 is a schematic representative of process tubes of the present invention in the same environment as the process tubes of FIGURES 3;

FIGURE 5 is a graph showing the effect of the tube spacing ratio of the direct heat flux to the front face of the tube relative to the heat flux to the back face of the tube, for plain tubes and tubes with various fins;

FIGURE 6 is a cross sectional elevation of a pyrolysis heater built in accordance with the principles of the present invention;

FIGURE 7 is a partial cross sectional plan view of the heater of FIGURE 6 taken along lines 7-7;

FIGURE 8 is a cross sectional elevation of another furnace built in accordance with the principles of the present invention; and

FIGURE 9 is a cross sectional elevation of a process heater with a central downdraft convection section built in accordance with the principles of the present invention.

In FIGURE 1, there is shown a process tube 10 built in accordance with the principles of the present invention. The process tube 10 has a tubular main body portion 12, the front face 14 thereof being adapted to face a source of radiant heat energy. A fin 16 is welded to the b ack face 18 of the main body 12. Fin In is shown as a flat piece, but under certain circumstances a curved fin may be preferred. The fin 16 extends the length of the tube 14 and has a width which varies in accordance with the diameter of the tube body 12. The fin 16 is preferably made of a material having high thermal conductivity and an emissivity approaching unity. This is because heat flux to the fin 16 llS by radiation whereas heat flow from the fin 16 to the tube is mostly by conduction, although in some instances a significant fraction of the fin t o tube heat flow may be by reradiation. In the alternative, in order to obtain maximum fin efilciency, the fin can be coated with a material of high emissivity so that substantially all of the radiant heat striking the fin will be absorbed with little reflected.

In FIGURE 2, there is shown a sec-0nd embodiment 10' of the present invention in which a-tubular main body 12' is utilized having a front face 14'. Two fins 24 and 26 are welded to the back face 18 of the tube 12' along suitable

welded joints

27 and 28. The fins 24 and 26 have their overall width (from tip to tip) varied in accordance with the diameter of the tube 12' in the same manner as the width of fin 16 is varied with respect to the 3 tube 12. The fins 24 and 26 extend the length of the tube 12', and are cut transversely at intervals to avoid high temperature stresses and warpage.

In both FIGURES 1 and 2, the radiating source is illustrated as being below the tubes. Both the tubes and the fins absorb radiation incident upon them and transmit the heat received to the fluid flowing within the main bodies 12 and 12'. In the case of the

main bodies

12 and 12, the transfer is directly through the walls of the tubes, whereas in the case of the

fins

16, 24 and 26 the heat received first flows along the fins and then into the tubes through the welded areas.

In FIGURE 3, the mechanism of radiant heat transfer to the face of a tube away from the flame and to the face of the tube adjacent the flame is shown for a standard plain tube. In FIGURE 3, two tubes 30 and 30' are shown having a face 32 and 32' closest to a source 34 of radiant heat. Behind the tubes 30 and 30' there is a refractory wall 36 which reflects or reradiates heat from the source 34. It can be easily seen from the arrows denoting the direction of the radiated heat from the source 34 that some of the heat flux passes between the

tubes

30 and 30 and is reflected back between the

tubes

30 and 30 as well as to the back faces 38 and 38' of the tubes.

Some of the thermal rays from the source 3-4 are interrupted and absorbed by the tubes 30 and 30'. If there is no heat loss through the refractory wall 3 6, all of the heat from the source 34 is either reradiated or reflected. As in the case of the direct radiation, this reradiation and refleotion is partly intercepted by the

tubes

30 and 30 and a portion of this reradia-tion and reflected energy passes through the spaces between the tubes. It is obvious that the heat received by the

portions

38 and 38 of the tubes facing the reradiating wall is primarily by reradiation. The relative intensity of the heat received by the front and rear faces 32, 32 and 38, 38' of the tubes 30 and 30' respectively depends primarily on the spacing of the tubes. The closer the tube spacing, the greater the ratio of the heat flux on the front relative to the rear. These characteristics are more clearly brought out in FIGURE 5.

In FIGURE 5, Curve 1 shows the ratio of the direct heat flux to the front face 32 of the tube 30 relative to the indirect, reradiated and reflected heat flux to the back face 38 of the tube 30. It is to be observed that for the normal tube spacing of two tube diameters, the heat flux to the back face is only about one third of that received by the front face. Curve 1 and the other curves of FIGURE neglect the effect of the tube wall as a redistributor of heat which tends to reduce the disparity in heat flow of the front face relative to the back face of the tube.

In FIGURE 4, there are shown two finned tubes 40 and 40' built in accordance with the principles of the present invention and generally similar to the tube shown in FIGURE 1. The tubes 40 and 40 have a

front face

42 and 42 and a back face 44 and 44' respectively. Pins 46 and 46' are welded to the back faces 44 and 44 respectively of tubes 40 and 40. A source of thermal energy 47 directs thermal rays at the tubes 40 and 40. Only three rays from a single point are shown, it being understood that rays will go in all directions from all points on the plane. A refractory wall 49 is provided behind the tubes 40 and 40' which reradiates or reflects the heat from the source 47. It will be noted that the tubes 40 and 40' are on a common center line and are spaced a given number of tube diameters from center to center thereof. The

fins

46 and 46 have a width W which is greater than the diameter D of the tubes 40 and 40. Thus, the ratio W/D is greater than unity.

Curves

2, 3 and 4 of FIGURE 5 show the ratio of direct and indirect heat flux to the

fins

44, 44 relative to the direct heat fiux ltO the front faces 42., 42' of the tubes 40, 40 for different ratios of total fin width to tube diameter. As it is to be expected, for a given tube spacing the ratio of the heat received by the fins to that received by the tube increases with the fin width. It is of interest to observe that for a given value of W/D, the relative heat flux to the fins increases with tube spacing and above certain values the fins receive more heat than the front face of the tubes. In other words, it is possible by the use of suitable fin widths and tube spacing to reverse the normal conditions of heat flux and to find the back face of the tubes to be receiving more heat (via fins) than the front face. However, it is usually the aim to equalize the heat flux front and back.

Curve 5 of FIGURE 5 is a cross plot of

Curves

2, 3 and 4 for the case when the ratio of the front-to-back heat interchange factor is unity and shows the variation of tube spacing with W/D. Curve -6 is a similar cross plot for the case when the ratio is .8. As an example, if W/D=2.0, the tube spacing as read from Curve 5 should be 2.9 diameters, in order to obtain equal heat flux to the front and back faces of the tube.

The utility of the invention may be illustrated by considering a specific example of the advantages thereof when applied to a furnace having a single row of tubes spaced on two diameter centers, irradiated from one side and backed up by a refractory wall on the other, wherein the average heat flux is 12,000 Btu. (hr.) (sq. ft.). From Curve 1, the heat flux to the back of the tubes for a plain tube without the fins of the present invention is one third of that to the front half. Hence the rate to the back half is /z of 12,000 or 6,000 B.t.u. (hr.) (sq. ft.), whereas the rate to the rate to the front face is 18,000 B.=t.u./ (hr.) (sq. ft.). If this row of tubes were replaced by a row of finned tubes at 2.9 diameters spacing and W/D=2.0 (to give equal flux front and back) and if the maximum rate of the plain tubes was acceptable, then each square foot of tube of the finned tubes would be capable of receiving 18,000 Btu/(hr.) (sq. ft.). But since these tubes occupy 2.9 times as much wall space as the plain tubes the flux for equivalent wall space is 1 8,000 (2) /-(2.9) equals 12,400 B.t.u./ (hr.) (sq. ft.). Thus there will be only a slight saving in refractory material, but a considerable saving in tubes, since only two thirds as many finned tubes and return bends will be required as plain tubes. If allowance is made for the effect of the tube walls as redistributors of heat, somewhat closer spacing of the fintned tubes may be used than was used in the example and therefore, the finned tubes will show up to even better advantage. In the example of finned tubes shown, the direct heat flux on the tube and iliu is about eighty four percent, whereas the indirect heat flux is about thirteen percent of the total. It is obvious therefore, that the finned tubes 40, 40 can be arranged in two parallel rows along the center lane of a combustion chamber fired from both sides and the heat of each of the rows could be substantially independently controlled.

In FIGURES 6 and 7 there is shown a process heater built in accordanc with the principles of the present invention. The heater 50 has a cylindrical outer wall 52 and a cylindrical inner wall 54 defining an annular chamber '56 therebetween, with a refractory bottom ring 58 and a refractory top ring 60 enclosing the chamber 56. The inner and outer walls '54 and 52 have refractory linings on the portion thereof exposed within the chamber 56. Combustion gases from chamber 56 pass through duct 80 to convection unit 82 and thence to stack 84. The process heater 50 is supported on suitable structural steel members '66.

Within th chamber 56 there are a plurality of groups of process tubes each group comprising two rows of tubes 68 and 70. As seen in FIGURE 7, each group of tubes '68, 70 is on a radius of the annular chamber 56. Such an arrangement has certain advantages with respect to even heating by burners 7-8 mounted in both the inner and outer walls. With the finned process tubes of the invention this advantage is even greater, due to the fact that the two rows of tubes heated by any given rows of burners form a uniquely controllable chamber, in

that radiation between adjoining rows of tubes, which are back-toback with respect to fin placement, is negligible.

As shown in FIGURE 6, process tubes 68, 70 are connected to

headers

62, 64 for passing process fiuids therethrough.

As noted hereinabove, it is desirable to have the fins 16 cut transversely at regular intervals along their length to prevent adverse thermal effects such as warping and the like, and this feature is also illustrated in FIGURE 6. The cutting may be done either before or after the fins 16 are installed on the process tubes 68, 70, depending on ease of fabrication and cost. The fins 16 will generally extend over the entire length of process tubes 68, 70 within chamber 5 6, though in certain applications only partial use of the fins may be desired.

Opposing burners 78 in any given segment of heater 50 are staggered so that burners do not directly oppose one another. Valves 79 on each burner and valves (not shown) for controlling the flow of fuel to groups of burners provide a complete measure of control of the heat flux over the length of the tubes. In operation, opposing burners 78 radiate heat into a segment of chamber 56 and directly onto th front of process tube group 68 of one radial group and group 70' of another radial group. The fins 1-6' keep this radiated heat substantially contained in a given segment of chamber 56. Pins 16 insure that the circumferential heat flux is substantially even around the periphery of the tubes, with the result that process fluids react (-or are heated) uniformly and completely during passage through the tubes, production is increased and recycle loads are decreased.

A more complete description of the heater illustrated in \FIGURES 6 and 7 and described hereinabove will be found in my copending U.S. Patent application Serial No. 384,706, filed July 23, 1964, and entitled, APPA- RATUS.

In FIGURE 8, there is shown a cabin type furnace in vertical cross section which utilizes the principles of the present invention. Here, too, the need for two, spaced radiating surfaces having a plurality of burners therein, so as to more nearly achieve a uniform radiating plane on opposite sides of process tubes, has been eliminated by the use of the finned tubes of the present invention. That is, the cabin furnace '86 of FIGURE t3 has two groups of

burners

88 and 90 positioned in parallel paths along the side edges of the bottom wall 92 of the furnace 86. The groups of

burners

88 and 90 are positioned along lines adjacent the side walls 94 and 95 of the furnace. A suitable opening 98 is provided adjacent the top of the furnace, which opening leads to the convection section of the furnace. Of course, it is also possible to position the burners along the

side walls

94, 96, for firing directly at the process tubes. In the furnace '86 there are positioned two

groups

100 and 102 of horizontally positioned process tubes. The group 100 includes process tubes 104 each having a fin 106. The group of process tubes 102 includes process tubes 105 having fins 107 on the side thereof furthest from side wall 96. The group of tubes 100 has the fins 106 thereof on the side of tube 104 furthest from side wall 94. Tubes 104 are closest to side wall 94 and tubes 105 are closest to side wall 96. The tubes 105 are staggered with respect to the tubes 104. In some instances, fins 107 may overlap th space between the fins 106. Similarly, the fins 106 may overlap the space between adjacent fins 107 in group 102. It can thus be seen that heat radiating from the burners 88 which passes between the fins 106 of group 100 will be reflected and/ or reradiated by the back sur faces of fins 107 of group 102. Similarly, heat radiating from burners 90 which passes between adjacent fins 107 of group 102 will be reflected and/or reradiated by the fins 106 of the group 100. Although

th tubes

104 and 105 have been shown as horizontally disposed, it can easily be understood that in some instances, the tubes may be better arranged vertically, as in FIGURES 6 and 7.

It will also be easily understood that in accordance with the teachings of the present invention, the W/D ratio will determine the ratio of heat fiux supplied to the finned surface of tubes 104 and with respect to the diametrically opposite part of

tubes

104 and 105.

FIGURE 9 is a cross sectional elevation of another furnace incorporating the principles of the present invention, which furnace includes a central downdraft convection section.

In FIGURE 9, the furnace 108 is shown having two spaced parallel heatingchambers 1110 and 112 therein. The chamber is defined by a bottom wall 114, an outside side wall 116, and one vertical side wall 118 of a central downdraft convection section 120. The top wall 122 defining the chamber 110 is inclined upwardly toward the center of the furnace 108 so that combustion gases from

burners

124, 126, i128 and mounted in the bottom wall 114 will be led to the upper opening 132 of the convection section .120. Burners 1-24, 126, 12 8 and 130 are arranged in parallel paths parallel to side walls 116 and '1-18 and are spaced in parallel vertical planes intermediate three horizontally disposed process tubes 1'34, 166 and 13 8 spaced above the bottom wall 114 and having

fins

140, 142 and 14-4 on the side thereof closest to top wall 122. The spaced horizontal tubes '134, 136, and 138 are thus not immediately opposite any of the

burners

124, 12 6, 1 28 and 1-30, but receive the beat fiux therefrom as uniformly as possible. The

fins

140, 142 and 144 have a width greater than the diameter of th tubes 1-34, 136 and 1 3 8 and are designed so that the heat flux on the side of tubes 1'34, 136 and 1 3 8 associated with

fins

140, 142 and 144 is equal to the flux on the diametrically opposite side of the tubes facing the burners.

Chamber 112 is .a mirror image of the chamber 110 and has a bottom wall 144, side walls 116' and 118', and a top wall 122'.

Burners

124, 126, 128' and 130' are mounted in the bottom wall 144' to direct the heat at suitable horizontally disposed tubes 134, 136' and 138' having

fins

140', 142 and 144 on the side thereof closest to top wall 122. Top wall 122' is inclined so that combustion gases from the

burners

124', 126, 128, and 130 will be directed to the downdraft convection section 120. Although a central downdraft convection section has been shown, it will easily be understood than an updraft 'or sidedraft convection section may also be utilized.

It will be understood that various changes in the details, steps, materials and arrangements of parts de scribed hereinabolve for purposes of illustrating the invention can be made by those skilled in the art without departing from the scop of the invention as set forth in the appended claims.

What is claimed is:

I. A process heater comprising,

a housing defining a chamber therein, two walls of said chamber being parallel, refractory heat radiating and reflecting walls;

heating means along said two walls for heating said chamber;

a first group of process tubes spaced from each other and arranged in a path parallel to one of said two Walls;

a heat conductive fin on each of said tubes extending the length thereof on the side of said tubes furthest from said wall;

a second group of process tubes spaced from each other and arranged in a path parallel to the second of said two walls, said second group of tubes also having fins, the fins on said second group being on the side of said tubes furthest from said second wall, said first and second groups of tubes thereby having fins on facing sides;

the width of said fins being greater than the diameter of said tubes and being controlled with respect to said diameter and to the spacing of said tubes so as to equalize the heat flux from said heating means to the point on said tubes nearest said heating means and the heat flux to the fin side of said tubes.

2. The process heater as claimed in claim 1, wherein said first and second groups of tubes are spaced so as to be in staggered relation.

3. A radiant process heater comprising,

a housing defining an annular chamber having coaxial inner and outer Walls;

a plurality of burners in said inner and outer walls in spaced segments thereof;

a plurality of groups of process tubes with-in said chamber in spaced segments between the segments of said burners, said tubes thus being adapted to receive radiant heat from said burners;

a heat conductive tfin on each of said tubes extending the length thereof;

the tubes in each said group comprising two rows, with said tins being on the common side of said tubes between said two rows; and

means for passing process fluids through said tubes.

4. A radiant process heater comprising a housing defining a chamber;

a plurality of burners in the walls of said around the periphery thereof;

chamber References Cited by the Examiner UNITED STATES PATENTS 7/ 1948 De Lorenzo 122356 3/1954 :Huet 122-667 6/1956 lPermann 1223 35 X 3/1960 Wallis et al. 122333 6/1962 Dwyer 122-356 3/1965 Koniewiez 122-240 X FOREIGN PATENTS 4/1952 France.

CHARLES I. MYHRE, Primary Examiner.

Claims

4. A RADIANT PROCESS HEATER COMPRISING A HOUSING DEFINING A CHAMBR; A PLURALITY OF BURNERS IN THE WALLS OF SAID CHAMBER AROUND THE PERIPHERY THEREOF; A PLURALITY OF PROCESS TUBES ARRANGED IN TWO ROWS WITHIN SAID CHAMBER, SAID TUBES BEING ADAPTED TO RECEIVE RADIANT HEAT FROM SAID BURNERS; A HEAT CONDUCTIVE FIN ON EACH OF SAID TUBES EXTENDING THE LENGTH THEREOF ON THE SIDE OF THE TUBE AWAY FROM THE SOURCE OF RADIANT HEAT; AND MEANS FOR PASSING PROCESS FLUID THROUGH SAID TUBES.