US3886976A

US3886976A - Recuperator having a reradiant insert

Info

Publication number: US3886976A
Application number: US409399A
Authority: US
Inventors: Alan Kardas; Dennis H Larson; John D Nesbitt
Original assignee: Institute of Gas Technology
Current assignee: GTI Energy
Priority date: 1973-10-25
Filing date: 1973-10-25
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03

Abstract

The invention relates to a recuperator having a floating extended surface or reradiant insert, that is, an additional area accepting heat by convection and radiation from the hot combustion gases, not integrally connected with the original heat-receiving surface, for enhancing the rate of transfer of heat from the combustion gases to a vehicle flowing through the recuperator.

Description

United States Patent Kardas et al.

[ June 3, 1975 RECUPERATOR HAVING A RERADIANT INSERT Primary Examiner-Charles A. Ruehl [75] Inventors Alan Kardas Chica Dennis H Attorney, Agent, or Firm-Dominik, Knechtel, Godula g & Demeur Larson, Brldgeview; John D. Nesbitt, Chicago, all of Ill.

[73] Assignee: Institute of Gas Technology,

Chicago, Ill. ABSTRACT Filed: 1973 The invention relates to a recuperator having a float- [21] A l N 409,399 ing extended surface or reradiant insert, that is, an adclltional area accepting heat by convection and radiation from the hot combustion gases, not integrally [52] US. Cl 138/38F 1l56d8/1l3; Connected with the original heaweceiving surface, for 65/1 8 enhancing the rate of transfer of heat from the combustion gases to a vehicle flowing through the recuper- 165/183, 141, 185 aton [56] References Cited UNITED STATES PATENTS 2 ohms 8 Drawmg F'gures 2,389,166 11/1945 Seaver 168/185 X q) WITH RERADIATOR WITHOUT RERAOIATOR Ir GAS a TEMPERATURE 4 m AIR gf TEMPERATURE 2 E "'"T l l O 20 4O 6O 80 I00 DISTANCE ft SHEET FIG 1 WITH RERADIATOR WITHOUT RERADIATOR GAS TEMPERATURE AIR TEMPERATURE TEMPERATURE AIR TEMPERATURE DISTANCE ff FIG 2 GAS TEMPERATURE m T W WWR OL EE IL ss WE mm D w NS RMM m om ME Ohm .IM MP2 M mm Pfl O5 INSERT TEMPERATURE TEMPERATURE DISTANCE, f1

SHEET FIG- INSIDE L2 ff INSERT OUTSIDE 0.5 ff INSERT OUTSIDE [.2 ft INSERT INSIDE 2.25 fI INSERT GAS TEMPERATURE INSIDE 6.5 n INSERT oursmz 2.2s n INSERT 2.25 ff INSERT AIR TEMPERATURE NO INSERT INSERT WALL INSERT m w r 535E525 BOO DI STAN CE FIG. 5

P ATE I ETED JIJI: 3 I975 TEMPERATURE,

STREAM 2 STREAM I RERADIATED STREAM 2 STREAM I .F E G. 4 FIG- I 2000 GAS TEMPERATURE UNIT CONDUCTANCE Q I400 Btu/hr sq. ft. F

TEMP DIFFERENTIAL K: G) AcR0ss THE WALL I: WITH UNIT K: 0 I000 CONDUCTANCE K DISTANCE f'I.

1 RECUPERATOR HAVING A RERADIANT INSERT This invention relates to an improved recuperator concept which is based on an annular heat-exchange furnace stack with a reradiant insert.

Flue gas recuperators are generally well-known and, in the past, the majority, if not all, of their designs consist essentially of a furnace stack with a cylindrical inner shell of a smaller diameter so as to provide two annular concentric passages. The combustion air to be heated is passed through the outer annular passage and the flue gases are passed through the inner passage, with the combustion air being heated by convection. These types of flue gas recuperators generally are termed straight-through stacktype recuperators, and are generally large and not too efficient.

In dealing with recuperators, products of combustion can be thought of as an industrial energy carrier that is only partially unloaded" in high-temperature processes. Having served the primary process, the carrier still contains a considerable amount of usable energy, some of which can be recovered outside the process (as in a waste-heat boiler). The present invention deals with the return of the heat to the combustion air by means of radiative-convective recuperation. In particular, the problem is one of effectively returning to the process the thermal energy remaining in the stack gases of a large -10 Btu/hr), high-temperature (up to 2,200F) operation.

Technically, the problem consists of transferring heat from hot gases across a wall to the combustion air flowing in an annulus outside the wall. Ways of enhancing the heat transfer rate on the air side of the shell are well known and are not dwelt upon here. The flow on the air side is likely to be turbulent, and the rate of heat transfer to the air can always be increased by a tradeoff against the horsepower necessary to move the air. The choice between decreasing the width of the annulus or providing the air side with extended surfaces is left to the designer.

An increase in heat transfer on the hot-gas side of the shell is difficult to achieve by ordinary means. There are no practical means of forcing the 2,000F gas through the stack at higher velocities in order to increase the convective heat transfer rate. The upstream conditions in the furnace cannot be changed without interfering with the heating process and the production. Finally, outfitting the hot-gas side of the shell with extended surfaces, such as fins or pins, is not feasible because of the high temperatures involved, nor advantageous if radiation from the carbon dioxide and water Infrared spectra of the products of combustion is to be fully utilized.

The concept of the present invention relates to a floating extended surface, that is, an additional area accepting heat by convection and radiation from the hot gas, not integrally connected with the original heat receiving surface. The heat is retransmitted to the original surface by the continuous-spectrum (the so-called GT, or Stefan-Boltzmann) radiation. Among the advantages of this method are:

l. The absence of finned structures adhering to the original surface and transmitting heat to it by conductance across a (not always perfect) thermal contact at the root of the fin.

2. The original heat-receiving surface is unobstructed and is free to receive heat fluxes by convection and gas radiation as well as by reradiation from the floating extended surface.

The concept is most effective at relatively high temperature levels at which the net radiant fluxes from the reradiator to the original surface are appreciable. This consideration also suggests the use of a co-current gasair stream arrangement.

Accordingly, it is an object of the present invention to provide an improved recuperator.

More particularly, it is an object to provide an improved recuperator concept which is based on an annular heat-exchange furnace stack with a reradiant insert.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

Development work on the improved recuperator concept of the present invention has shown that the inherent advantages of a straight-through stack-type recuperator can be combined with a properly designed reradiative insert positioned in the flue gas stream, to reduce its size or length by 48 percent. For example, a typical stack-type recuperator for an industrial steel mill reheat furnace with an inside diameter of 5 feet and a height of feet (preheating combustion air to 680F and having a heat input from the flue gases of about 48 million Btu/hr) can be reduced to a unit only 52 feet high by using the reradiator concept. This height reduction can significantly reduce plant space requirements, material or equipment costs, and erection costs.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the apparatus embodying features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a graph illustrating the recuperator operating at two different temperature levels;

FIG. 2 is a graph illustrating the temperatures in a radiative recuperator with cylindrical inserts of various diameters;

FIG. 3 is a graph illustrating the separation of gas streams;

FIG. 4 is a graph illustrating heat fluxes at 30 feet from entry in a recuperator with reradiators of various sizes and separated streams;

FIG. 5 are perspective views of recuperators with three-, fourand six-leaf reradiators,

FIG. 6 is a graph illustrating the effectiveness of multi-leaf reradiators;

FIG. 7 is a graph illustrating the effect of finite wall conductances; and

FIG. 8 is a graph illustrating air preheat and energy losses in annuli of various sizes.

FIG. 1 shows the behavior of a recuperator operating at two different temperature levels: In one instance, the hot stream enters at 2200F, and in the other, it enters at 800F. The cold stream, running cocurrently in a 2- inch annulus surrounding the 5-foot diameter shaft, enters at 60F. The mass flow rates are the same in both cases: The hot stream represents the products of combustion of 48,366,000 Btu/hr of methane burned at stoichiometric conditions, and the cold stream represents the combustion air necessary to burn this quantity of fuel.

To make the two cases comparable, the incompleted fractional temperature difference Table I PERCENT REDUCTION IN THE INCOMPLETED FRACTIONAL TEMPERATURE DIFFERENCE Improvement When Reradiator is Used Distance from Gas Enters at Gas Enters at Entry Feet 2200F Percent 800F Percent The convective heat transfer coefficient is virtually the same in both cases (1.3 1.1 on the hot side, 8 7 on the air side); the infrared gas radiation coefficient is about 2 for the hot gas and 0.6 for the cooler gas; and the continuous-spectrum radiation coefficient is 24.7 for the 2200F case and only 3.8 for the 800F case. (All coefficients are averaged over the recuperator length units, Btu/hr-sq. ft.-F). The manner in which these heat transfer modes interact is discussed below.

Reduced to its basic components, the system consists of a long circular, cylindrical shell, thin and highly shell with thermal gradients across the shell need not be considered. I-lot combustion products enter the lower end of the shell and are cooled as they pass upward, losing heat across the shell wall to the ambient or to a stream of air forced through an annulus surrounding the wall on the outside. Three mechanisms of heat transfer come into play; convection, natural and forced; infrared radiation from the carbon dioxide and water bands of the combustion products; and continuous-spectrum radiation between reradiators and the wall and between the wall and the ambient.

The purpose of a reradiator is to extend the area to which heat may be convected and radiated by the hot gas. This heat then is retransmitted to the shell wall by the continuous-spectrum radiation with only a minimal reabsorptance by the intervening gas. Choosing a simple circular-cylindrical geometry with a coaxial reradiating insert and heat fluxes, the behavior of such a simple radiative recuperator can be illustrated. The results, shown graphically in FIG. 2, represent the following set 3f conditions:

Methane is burned with a stoichiometric quantity of air at the rate of 48,366,000 Btu/hr. The combustion products enter the 5-foot-diameter stack at 2200F, and the combustion air enters a 2-inchwide annular ring surrounding the stack wall at 4 60F.

Cylindrical reradiators

1, 2, and 4 feet in diameter are placed, one at a time, coaxially to the stack wall. The reradiators and the wall are assumed to be thermally thin; that is, there is no temperature drop across the wall, and their emissivities are taken to be 0.85.

As expected, the largest reradiative insert produces the best results. The combustion air is preheated to 600F, 42 feet from entry, by using a 4-f0ot-diameter insert. A 2-foot insert requires 53 feet; a l-foot insert, 63 feet; and no insert at all uses 79 feet of the stack length to preheat the air to the 600F temperature. The insert and the wall temperatures plotted in FIG. 2 show, however, that the choice of the largest possible insert diameter is not automatic. Structural and material problems may intervene, forcing the choice of a smaller insert diameter and a lower wall temperature. Other considerations dealing with radial mixing of the products of combustion in the stack are examined below.

A cylindrical coaxial reradiator separates the flow of the combustion products into two streams whose cooling rates may differ, depending on the relative sizes of the reradiator diameter, d, and the shell diameter, D. Accordingly, the stream inside the cylindrical insert can run hotter or colder than the stream outside. The magnitude of this effect and its influence on the overall recuperator performance are as follows.

A quantity of gas moving inside the insert with mass velocity G (lb/s sq. ft.) and losing heat to the insert at the rate ofh (Btu/hr sq. ft.-F), because of the temperature differential AT, decreases its temperature by an ammount, dz, over the length, dx. Calculation indicated that increasing the insert diameter causes the interior stream to run hotter and the exterior stream to run colder. Quantitatively, this effect is shown in FIG. 3, which models the following set of conditions:

Ten million Btu/hr of methane is burned with 10 percent excess air, and the products of combustion at 2200F are introduced into a 2.5-foot-diameter shell. The shell walls are assumed to be thermally thin and cooled by the combustion air, initially at 60F, forced through a 2-inch annulus. Three cylindrical, coaxial reradiator inserts are tested. Their diameters are (d=) 0.5, 1.2, and 2.25 feet.

The results are as predicted, and indicate that the stream inside of 0.5-foot-diameter insert cools much more rapidly than its outside portion. The conditions are reversed for the 2.25-foot insert, while the 12-foot insert nearly balances the gas cooling rates inside and outside the insert. Of more interest is the effect of the temperature nonuniformity on the air preheat temperature. At a distance of 40 feet along the recuperator, the air temperatures are:

Condition Air Temperature, F

No reradiator 580 O.5ft. insert 650 2.25-ft. insert 712 1.2-ft. insert 732 cent of the total heat flux to the cold air is supplied by the reradiator. The inside and the outside streams contribute 22 percent each.

It is convenient to define the effectiveness of a floating extended surface by taking the ratio of the heat transferred to the wall by reradiation, convection, and gas radiation to the heat transferred to the wall by convection and gas radiation along. A formulized definition suggests that in order to operate effectively, a reradiator must have as large an effective radiating area as possible. On the other hand, the cylindrical insert model examined earlier shows that an inordinately large increase in the reradiator diameter is counterproductive because of the split-stream effect, which leads to unequal stream temperatures. Higher air preheat is achieved in a 2.5-foot-diameter shell with a 12-foot cylindrical reradiator than with a 2.25-foot reradiator. However, if perfect radial mixing could have been ensured across the 2.25-foot reradiator wall, the 2.25-foot reradiator would have been superior to the 12-foot one.

Good radial mixing of the hot products of combustion and large effective radiating area can be obtained by using multileaf reradiators of the type shown in FIG. 5. The effective radiating area of a multileaf radiator requires a careful definition for reasons set forth below. Ordinarily, when two surfaces, A and A with emmissivities, e, and 6 are disposed such that the surface A encloses the surface A the net radiative heat flux is given by Christiansens equation. Christiansens equation, however, applies only to convex surfaces A that is, a convex surface being defined as one that cannot see itself. Leaf radiators are essentially concave, with multiple reradiations between the leaves and with shadows on the receiving surface A [The reradiator surface exposed to convection is not the same as the effec tive radiating surface seen by the enclosing wall A,,( =A2)-] The receiver surface, A also is concave. Both surfaces are non-black, so that the intervening radiation is reflected and rereflected, providing additional opportunities for energy absorption. As a result, the original emmissivity (or absorptivity) of the reradiator and of the wall is increased, leading to the so-called cavity or groove effect. Based on these facts, the following values of the ratio A /A in terms of leaf width, '21 (equal to the radius of the circumscribed circle), and the shell diameter, D were determined:

Thus, for a fixed a/D ratio, an increase in the number of leaves tends to improve the system effectiveness. Another improvement in the system effectiveness results when the leaf width, a, is increased. Finally, and most importantly, the greater the number of leaves, the larger the area receiving heat by convection. The area open to convective heat transfer is not the same as the effective area radiating to the inner shell wall. By assuming a constant gas temperature, T and a slowly varying wall temperature, T (a safe assumption if the heat flux on the air side of the wall is high), tests show that a large reradiating area will move T,- closer to T that is, the reradiator temperature rises and increases the system effectiveness.

To illustrate the foregoing and to obtain a quantitative measure of the reradiator effectiveness, the performance of a representative recuperative system was computed for the following set of data:

Shell diameter, D 5 ft.

Leaf width, a 2 ft.

Heat input, 48,366,000 Btu/hr Excess air, 0

Air annulus width, 2 inches Shell emissivity, 0.85

Reradiator effective emissivity, 0.90

Shell wall conductivity, k 10 Btu/hr-ft.-F

Shell wall thickness, 0.5 inch The effectiveness of three-, four-, and six-leaf reradiators is shown in FIG. 6. As defined, the effectiveness of a recuperator without reradiation is set to unity so that the values of 1 measure the improvement due to the various reradiative inserts. Because of the fourth-power radiation law, a reradiator is most effective in the hightemperature region; its effectiveness falls off as the temperature differential diminishes. This suggests that an insert need not run the full length of a recuperator shell.

Also shown in FIG. 6 are the air preheat temperatures for the various inserts, indicating a possible tradeoff of the number of leaves for insert length, and vice versa.

The use of multileaf reradiators calls for a recalculation of mean beam lengths for gas radiation every time the number of leaves is changed. These mean beam lengths are as follows:

Number of Leaves Mean Beam Length amount to much with three-, four-, or six-leaf reradiators. However, a l2-leaf reradiator might show a convective it 30-40 percent lower than the h obtainable on a flat plate under similar flow conditions.

The shell wall transferring heat to the combustion air has a certain thickness and a finite thermal conductivity. An optimum design would call for the highest possible wall conductance consistent with strength and longevity requirements. Again, tests show that the designer can vary the wall conductance parameters at will, introducing high-quality refractories in hightemperature regions and decreasing the wall thickness with height as mechanical and thermal stresses decrease.

An illustrative case using the inputs of FIG. 6 was calculated with unit conductance values of 4.8, 12, 30, and infinity. (Unit conductance is the ratio of the thermal conductivity of the all material, k, to the wall thickness, 8. It has the dimensions of the surface heat transfer coefficient h in Btu/hr-sq. ft.-F). K 4.8 corresponds roughly to a 2.5-inch-thick wall made of firebrick (k l); K 12 corresponds to a 2.5-inch-thick composite wall with an effective conductivity of k 2.5; K 30 corresponds to a l-inch thick wall made of the same composite material; and K to any metallic shell less than 1 inch thick. The results, plotted in FIG. 7, show the gas and the air temperatures along the recuperator and the temperature differential across the shell wall. A four-leaf reradiator was used throughout.

Expressed in terms of recuperator length, a trade-off for a composite steel-ceramic wall in lieu of a thin alloy wall might amount to an additional 4-10 feet on a 60-70 foot recuperator.

As indicated above, the heat transfer rate on the air side of the shell can be increased at the expense of the air horsepower necessary to move the air. In this respect, an increase in the heat transfer rate brought about by narrowing the annulus requires additional blower horsepower to overcome the increased pressure losses.

For example, using the firing rate of FIGS. 6 and 7 in a -foot-diameter recuperator shellw tih a 4-footdiameter cylindrical reradiator, thermally thin wall, split streams, and zero excess air, and varying only one parameter, the air annulus width, the extent to which the recuperator length necessary to reach the desired air preheat level may be traded off for blower horsepower as calculated. The results are shown in FIG. 8. For instance, 700F air is obtainable at 75 feet with a 4-inch annulus or at 55 feet with a 2-inch annulus. The 20-foot reduction in the recuperator length has to be paid for by providing 4.5 air horsepower instead of the 0.8 horsepower required to overcome the frictional and the accelerational losses in the 4-inch annulus.

In the above description, combustion air is described as the material or vehicle to be heated. It should be apparent, however, that other materials or vehicles such as other gases or combination of gases or liquids likewise can be heated. For example, the use of such heated product could be combustion of fuels in air, or oxygen, preheating of work, space heating, drying or others requiring a hot liquid.

It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained and certain changes may be made in carrying out the above method and in the construction set forth. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in'a limiting sense.

Now that the invention has been described, what is claimed as new and desired to be secured by letters Patent is:

1. A recuperator for use in a heat-exchange stack furnace having hot gases at temperatures of approximately 2000F and higher flowing through it and having a diameter exceeding approximately 5 feet and a height of up to approximately feet such that there is no capability of increasing the mass velocity of the hot gases through the stack by taking a higher pressure drop along the flow path through the stack for heating combustion air flowing in an annulus formed outside the recuperator and between the wall of the stack furnace, said recuperator comprising, in combination: a shell wall of tubular construction; a reradiator in the form of a floating extended surface disposed within said tubular shell wall and providing an additional area accepting heat by convection and radiation from said hot gases flowing through said shell wall, said heat being retransmitted to said shell wall by continuous-spectrum radiation, whereby the rate of transfer of heat from said hot gases across the shell wall to the combustion air flowing in the annulus outside the shell wall is enhanced, said reradiator being a cylindrical, coaxial reradiator which separates the flow of said hot gases into two streams whose temperatures differ depending on the relative sizes of the diameters of said cylindrical coaxial reradiator and said shell wall, the diameter of said cylindrical coaxial reradiator being proportioned to balance the temperatures of said two streams.

2. A recuperator for use in a heat-exchange stack furnace having hot gases at temperatures of approximately 2000F and higher flowing through it and having a diameter exceeding approximately 5 feet and a height of up to approximately 100 feet such that there is no capability of increasing the mass velocity of the hot gases through the stack by taking a higher pressure drop along the flow path through the stack for heating combustion air flowing in an annulus formed outside the recuperator and between the wall of the stack furnace, said recuperator comprising, in combination: a shell wall of tubular construction; a reradiator in the form of a floating extended surface disposed within said tubular shell wall and providing an additional area accepting heat by convection and radiation from said hot gases flowing through said shell wall, said heat being retransmitted to said shell wall by continuous-spectrum radiation, whereby the rate of transfer of heat from said hot gases across the shell wall to the combustion air flowing in the annulus outside the shell wall is enhanced, said reradiator being a multi-leaf reradiator to thereby provide a plurality of surfaces providing additional area accepting heat by convection and radiation from said hot gases and being shorter in length than the length of said shell wall, the number of leafs and the length of said reradiator being proportioned to provide the greatest transfer of heat in accordance with the temperature differential in said shell wall.

Claims

1. A recuperator for use in a heat-exchange stack furnace having hot gases at temperatures of approximately 2000*F and higher flowing through it and having a diameter exceeding approximately 5 feet and a height of up to approximately 100 feet such that there is no capability of increasing the mass velocity of the hot gases through the stack by taking a higher pressure drop along the flow path through the stack for heating combustion air flowing in an annulus formed outside the recuperator and between the wall of the stack furnace, said recuperator comprising, in combination: a shell wall of tubular construction; a reradiator in the form of a floating extended surface disposed within said tubular shell wall and providing an additional area accepting heat by convection and radiation from said hot gases flowing through said shell wall, said heat being retransmitted to said shell wall by continuous-spectrum radiation, whereby the rate of transfer of heat from said hot gases across the shell wall to the combustion air flowing in the annulus outside the shell wall is enhanced, said reradiator being a cylindrical, coaxial reradiator which separates the flow of said hot gases into two streams whose temperatures differ depending on the relative sizes of the diameters of said cylindrical coaxial reradiator and said shell wall, the diameter of said cylindrical coaxial reradiator being proportioned to balance the temperatures of said two streams.