CROSS REFERENCE TO RELATED APPLICATION
The present invention is related to the invention covered by the inventors' prior U.S. patent application Ser. No. 239,433, filed Mar. 30, 1972 now Pat. No. 3,777,717.
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for heating or evaporating liquids or gases, in particular water, intended for example for the heating of buildings, providing heat for industrial purposes, for power engineering and the like. In the new apparatus, which preferably may have the shape of a boiler, the heat released by the combustion process is transmitted to heat exchange surfaces nearly exclusively by radiation, the proportion of heat transmitted by convection being negligible.
Boilers in which radiation is the main and substantially the only way of heat transfer, and among which also the boiler according to the present invention is included, are still in the first stages of their development and therefore cannot be compared either directly or indirectly with known boiler types conventionally used for the above mentioned purposes, since not only the manner in which the combustion proceeds but also the design and construction of such conventional boilers do not permit transfer of the major part of the heat unless it takes place through convection surfaces. It has to be admitted, however, that in connection with these classic boiler type it has already been proposed to equip them with means for flameless combustion, which, due to the higher radiation effect would result in more favorable specific performance of the heat exchange surfaces. The expected increase in boiler performance has, however, materialized only to a limited degree, because the attained temperature of the ceramic surface remained low due to the impossibility of releasing a relatively large amount of heat, and furthermore the choice of flameless burner types, as well as their positioning relative to the heat exchange surfaces, has been found unsuitable.
Trials have already been made with an entirely nonconventional design of new type of pure radiation boilers operating with the aid of heat exchange surfaces embedded in refractory clay. This apparatus had the character of a fire-tube or water-tube boiler; the design was, however, based entirely on known general principles of the transfer of heat released by surface combustion, and the desired intensification effect which was expected from the introduction of radiation heat transmission elements into boiler technique did not materialize. This is why these radiation boilers have never found their way into practical use.
Recently, a basic change in radiation boiler design has taken place, since for the first time the favorable proportion of some ceramic or other materials at higher thermic levels have been made use of; this was made possible by an intensified combustion process on the active surface of such materials. The proportion of heat transferred by radiation has entirely outweighed other kinds of heat transmission, and this has opened the way to a conspicuous increase in the thermal load of heat exchange surfaces.
These new boiler types, however, still show some disadvantages of design, which do not allow a full use of the method for heating liquids according to our above referred to copending U.S. patent application Ser. No. 239,433, which will be summarized later in this specification.
Single-chamber boilers, representing the smallest operative unit used according to said copending application, have smooth walls, or walls provided with ribs which follow the flow direction of the combustion gases. Such boilers resemble fire-tube boilers, even if their cast-iron sections are assembled in known ways to form high-performance units, or if they are made as assembled steel reactor units submerged in the heated liquid. The smooth plane surfaces, shallow ribs or longitudinally oriented ribs, and in particular reactor chambers having the shape of an elongated rectangle, have their heat exchange surfaces less suitably arranged for an optimum impinging angle of the radiated heat rays and, moreover, they do not prevent the undesirable escape of unburned gases to higher regions of the reaction space along the heat exchange surfaces, whereby the spectral radiation effect is reduced.
A considerable part of the hollow heat exchange surfaces which enclose the heated liquid, are irradiated from one side only. As a result of all these disadvantages, the volume and weight of the boiler cannot be reduced further, and its manufacture places high demands on labor and machinery.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome the disadvantages set forth above.
The invention relates to a type of boiler adapted for flameless combustion of a mixture of fuel and oxidizing agent, with a bed of a gas-permeable radiation substance, such as zirconium silicate, adapted to be brought into a thermal condition in which it radiates substantially the entire thermal energy liberated at its surface, said substance being in intimate contact with heat exchange surfaces by which it is surrounded and enclosed.
The essential feature of the present invention is that the heat exchange surfaces comprise hollow jackets of similar formation, through which the heated liquid flows, the jackets being placed one inside the other around a common axis of symmetry, regular and continuous annular gaps or spaces of a constant width being formed therebetween. The gaps as well as the space within the innermost jacket are filled with said radiation substance, while at the bottom of said jackets there are provided means for homogenizing and distributing the mixture of fuel and oxidizing agent are provided such means being accommodated in a common casing.
In the boiler of the above mentioned type the radiation substance which fills the gaps between the heat exchange surfaces, is capable of producing a combustion process, which may be termed a "contact-kinetic" combustion process and has been disclosed in detail in our aforementioned prior U.S. patent application Ser. No. 239,433. It is only this combustion process which is capable of producing a thermal condition in which the radiation substance, due to its properties, permits radiation of substantially the entire thermal energy released at its surface in the wavelength range of 0.5 to 6 micrometers.
According to a further feature of the invention, the various coaxial heat exchange jackets are provided with hollow radial ribs of semicircular, triangular or prismatic shape; the jackets are preferably wound from metal tubes of any required profile as a single or multiple coil helix or spiral with a constant radius of curvature through which the liquid flows and whose coils are preferably in close contact with one another.
In a modified embodiment in which the system of coaxial jackets has a horizontal axis os symmetry, the jackets may be arranged in such a way that, instead of a continuous helix or spiral, each jacket consists of a plurality of individual closely adjoining coils whose inner spaces are at their lowest points connected in parallel to a common supply and at their highest points are connected to a common discharge of the heated liquid.
The boiler according to the invention therefore comprises one, two or more hollow jackets of similar formation, preferably of equal height, having any of the above described shapes; the heat exchange surfaces of the jackets may be either entirely smooth, but preferably are provided with ribs of any desired profile along their entire height, said ribs lying in parallel planes perpendicular to the axis of symmetry, which means that the ribs are positioned across the flow direction of the combustion gases. In jackets which are wound from circular tubes to the shape of a single or multiple coil helix or a spiral with series-connected, parallel-connected or series-parallel connected coils, the tube surfaces themselves represent a natural ribbing. Tubes of other than circular profiles have to be wound so as to produce a transversely undulated heat exchange surface on the jacket. In an alternative construction, the jackets may be produced by pressing or by other suitable shaping of their sheet metal walls, or may be cast as a unit, or may consist of sections with a suitably shaped surface, which after assembly of the sections, produce the surface ribbing.
According to a further feature of the invention, a common casing housing means for homogenization and distribution of the fuel mixture into the various spaces between the jackets and into the innermost jacket is arranged in the bottom part of the jackets. The arrangement of a common casing is highly advantageous, since it results in uniform and equal thermal conditions existing at the same time in all gaps and spaces filled with the radiation substance so as to bring about the highest thermal effect. Such an arrangement represents an advance as compared with previous arrangements, in which separate homogenizing and distributing means had to be provided for each chamber.
In the new boiler, the hollow jackets are assembled according to the required performance to larger boiler units by placing the individual jackets into one another; this is why the heat exchange jackets have to be dimensioned so that the jacket lying closest to the axis of symmetry has the smallest diameter or the smallest distance from the axis of symmetry, and thus represents the smallest basic unit. Each further jacket has a larger diameter or a larger radial distance from the axis of symmetry. This radial distance is so chosen as to provide between each two neighboring heat exchange jackets along their entire periphery a regular and continuous space or gap of constant dimensions, whose width corresponds to double the radial distance of the axis of symmetry from the innermost heat exchange surface of the jacket of the smallest basic unit.
Combustion and simultaneous radiation of the liberated heat takes place in a reaction space formed by the gaps produced between the various jackets and the space within the innermost jacket, all of which are filled with a gas permeable radiation substance. The jackets enclose the flowing liquid to be heated and, as already said, the gaps are formed by suitably stepping off the radial distance of the jackets from the axis of symmetry. The radiation substance is able to insure a highly intense process of contact-kinetic combustion on its surface of a perfectly homogeneous mixture of fuel and oxidizing agent under extreme conditions, i.e., at such a flow velocity of the combustible mixture as to exceed the forward speed of flame propagation, while the temperature of this mixture in the main streams passing through the layer of radiation substance remains below the ignition point. It is only this method of surface combustion that is capable of stabilizing such thermal conditions within a narrowly defined layer of radiation substance, which, in consequence of the properties of said substance, is able to transmit by radiation practically the entire usable heat, liberated by the combustion, to the metal walls of the heat exchange jackets in spectral wavelengths from 0.5 to 6 micrometers. Under thermal conditions lying at a lower level, such as are currently obtained by other known methods of flameless surface combustion, the proportion of the heat flux transmitted by radiation is smaller, whereas the proportion of heat to be transmitted by convection is greatly increased; for this reason such a known combustion process is unsuitable for radiation boilers.
Due to the fact that the radiation substance is in intimate contact with the heat exchange surfaces, primarily at points where the aforementioned transverse ribs are located, a two-fold advantage is obtained. On the one hand, the angles under which the heat rays impinge on the heat exchange surfaces are favorable; this results in an increased flow of heat onto the heat exchange surfaces. On the other hand, the transverse ribs are an obstacle to the undesirable free escape of the combustible mixture along the heat exchange surfaces to the space above the main combustion zone; the undesirable expansion of the combustible mixture is thereby prevented. The combustion is thus concentrated into a small space at an increased pyrometric temperature, thereby favorably influencing the intensity of the heat flux in the most effective range of wavelengths.
Owing to the grouping of the concentric boiler jackets according to the invention, all inner jackets have their heat exchange surfaces irradiated from both sides. The only exception is the largest peripheral jacket, which is irradiated from one side (the inside) only. It follows that the larger the number of concentric jackets, the better the utilization of its surfaces for the exchange of heat. All this leads to an advantageous miniaturization of the boiler, and to wider possibilities of application, reduction of weight, costs and labor.
The smallest basic unit, consisting of a single hollow jacket having the smallest radial distance of its periphery from the axis of symmetry, can operate as a self-sufficient boiler with relatively lowest performance. Due to the arrangement of its heat exchange jacket, produced by any of the aforementioned methods, this unit achieves about a 30% saving in weight as compared with radiation boilers provided with longitudinal ribbings, as known at present and, in addition, shows an increased efficiency of performance.
When the performance of a boiler according to the present invention having a single heat exchange jacket wound from a tube to the shape of a circular cylinder is compared with a known boiler having a peripheral jacket in the shape of a smooth circular cylinder of the same inner diameter, or is compared with a boiler whose smooth peripheral jacket has the shape of a prism with a square cross-section, whose side has a length equal to the diameter of said cylindrical jacket, it will be found that the performance of the boiler wound according to the invention is 84% higher than that of the boiler in the shape of a smooth circular cylinder, and is 44% higher than that of the boiler in the shape of a four-sided prism. The heights of the above-compared boilers were equal. If the new boiler is equipped with more jackets, the same advantages will be obtained.
BRIEF DESCRIPTION OF THE DRAWING
An exemplary embodiment of the apparatus embodying the invention, and having the shape of a boiler, intended for the heating of water, is shown diagrammatically in the accompanying drawing, in which
FIG. 1 is a view in vertical axial section through the boiler;
FIG. 2 is the corresponding plan view, with the combustion gas collecting hood removed;
FIG. 3 is a schematic view showing a series connection between successive hollw jackets;
FIG. 4 is a schematic view showing a series-parallel connection between the successive hollow jackets; and
FIG. 5 is a fragmentary view in perspective illustrating the parallel connection of the partial coils which constitute one of said jackets.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawing, the illustrative boiler there shown comprises three coaxial cylindrical jackets 1, 2 and 3 of circular cross-section, the jackets having been produced by winding three metal tubes of circular cross-section into three respective self-contained single coil helices of the same height. The inner diameter of the jacket 1 equals the width of the annular space or gap formed by the annular space between jackets 1 and 2 as well as the annular space between jackets 2 and 3. The axis of symmetry 4, around which the jackets 1, 2, and 3 are coaxially arranged, is vertical in the embodiment shown in the drawing. The lower ends of the tubular helices of jackets 1, 2, and 3 are connected to a common cold water supply pipe 5, and the upper ends are connected to a common discharge pipe 6 for the heated water.
The bottom of jackets 1, 2, and 3 is closed by a common gas distribution grate 7 comprising a plate with a system of circular openings or slots 8. Mounted below the grate 7 is a circular casing 9 with a spiral bottom 10 and a tangential supply conduit 11 for a homogeneous mixture of fuel gas with air. The upper part of the jackets 1, 2, and 3 is covered by a common collecting hood 12 with a chimney 13 for the discharge of combustion gases. All gaps or spaces 15 between the jackets 1, 2, and 3, and the inner space 15 of the innermost jacket 1 are filled with a gas permeable radiation substance 14, e.g., zirconium silicate in granular form.
The boiler according to the invention operates as follows.
A homogeneous fuel-air mixture, preferably in a stoichiometric proportion, is fed from a mixing and homogenizing apparatus 16 through the supply conduit 11 to the circular casing 9, whose spiral bottom 10 further homogenizes and directs the stream of combustible mixture towards the distribution grate 7, through whose openings 8 the gaseous mixture flows into the spaces or gaps 15 above the grate 7, such space being filled with particles of the radiation substance 14. The dimensions, shape and properties of these particles of the radiation substance 14 are chosen according to the width of its layer, size of the boiler, the type of fuel used, etc. The dynamic equilibrium between the flow velocity of the combustible mixture through the channels between the particles of the substance 14 and the speed of the combustion process at their surface having been stabilized, conditions are created in which a combustion zone is formed at a distance of about 10 millimeters above the grate 7; such combustion zone occupies about one third of the height of said radiation substance layer 14.
The combustion proceeds with an intensity of 60 to 110 million k cal/hour in 1 m3 of space filled with the radiation substance 14. At such concentration of liberated heat and at a pyrometric temperature above 1600°C, and optionally also with the use of a radiation substance capable of radiation in selected parts of the spectrum, the thermal energy is radiated primarily in spectral wavelengths of 0.5 to 6 micrometers. The radiated heat impinges on the semicircular ribs of jackets 1, 2, and 3, which as we have seen consist of tubes through which water flows from the common supply pipe 5; the water having been heated, the water leaves through the common discharge pipe 6.
The ribs formed by the outer faces of the tubes cause the heat rays to impinge on the heat exchange surfaces in a wide range almost at right angles, so as to increase the thermal flux upon a unit of the heat exchange surface. The largest proportion of liberated heat is transmitted in the combustion zone. A smaller part of the heat is radiated in larger wavelengths in the remaining two thirds of the height of the radiation substance layer 14, which begins to be heated by combustion gases, which in this zone have a lower temperature than in the combustion zone.
The overall heat transmission is so intense that the temperature of the combustion products, when they leave the upper surface of the radiation substance layer 14 (which is only about 200 to 300 millimeters high) amounts to 180° to 250°C, so that there is no need to equip the boiler with further heat exchange surfaces for heat transmission by convection; in spite of this, the efficiency of the boiler is about 90%. The heat transmission by convection, on the other hand, is as low as about 5% of the total heat transmitted to the jackets 1, 2, and 3 in the spaces 15 filled with the radiation substance 14.
Combustion products, which have already been cooled in the upper layer of the radiation substance 14, are accumulated under the collecting hood 12 and leave through the chimney 13. The combustion process in the boiler may proceed either by subjecting the combustible gas mixture to an overpressure or by subjecting hood 12 to an underpressure. Such a high rate of radiation can be achieved only if the combustion zone is as narrow as possible (considered in the direction of flow of combustion gases) and remains limited to the lower part of the boiler. The transverse ribs of the jackets 1, 2, and 3 act as a resistance against the flow of an unburnt combustible mixture along the heat exchange surfaces, so that the combustion zone is not transferred to a higher level, which would reduce the intensity of the combustion process.
Tests have been carried out with a boiler of the new type containing three jackets. The average performance was 274,700 k cal/h, the average load on the heat exchange surfaces being 203,000 k cal/m2, and the efficiency amounted to 90.8%. The weight of the boiler itself without fan and control instruments amounted to 42 kg, the weight of the zirconium silicate bed was 48 kg. The specific weight of the boiler without bed was thus reduced to 0.153 kg/1000 k cal installed capacity. Still better results are achieved, if the boiler is adapted for the production of saturated steam.
The miniaturization, made possible by the present invention, is important in practical use and the new boilers may find application as radiation boilers in dwelling houses or industrial buildings, in power engineering, and in the future as a source of pressure steam for steam traction. The new boilers are able to meet up-to-date requirements regarding the prevention of air pollution and protection of the environment. From the manufacturer's point of view, the new boiler is labor saving, the saving in weight amounts to about 90% as compared with conventional boilers, and no intricate machiner is required for its production.
The new apparatus has been described as intended for heating or evaporating of liquids. It will, however, be clear to those skilled in the art, that after a suitable adjustment the new apparatus can equally well be used for heating gases, in particular, air.
Although the invention is illustrated and described with respect to a single preferred embodiment thereof, it is in no way limited to the disclosure of such a single embodiment, but is capable of numerous modifications within the scope of the appended claims.