1. Field of the Invention
This invention relates to a method and apparatus for heating flat or curved surfaces such as those found in paper, textile and food drying equipment. More particularly, this invention relates to a method and apparatus for heating flat or curved surfaces employing indirect firing means to increase the temperature uniformity and heat flux of equipment employing such surfaces, to reduce NOx emissions formation utilizing flue gas recirculation, and to increase the overall system thermal efficiency.
2. Description of Related Art
A significant amount of heating/drying, such as paper and textile drying, is carried out in indirectly fired equipment that is heated by steam. This approach limits the maximum heating surface temperatures because of the increasing steam pressure requirements at higher temperatures. As a result, equipment such as steam paper dryers are generally limited to temperatures below about 400° F. Several gas-fired heater/dryer concepts have been developed for providing higher heating surface temperatures, but implementations of these are generally complex in design.
One approach to addressing these issues has been the use of fuel combustion within a drum dryer to heat the surface of the drum, either through the use of radiant burners or open flame burners that direct combustion gases at the drum surface. U.S. Pat. No. 5,416,979 teaches a paper drying apparatus consisting of a rotatable dryer drum with a plurality of burners located in a hood partially surrounding the drum and means for directing hot combustion products towards the wet paper. U.S. Pat. No. 5,791,065 teaches a rotating drying cylinder with a gas-fired assembly employing segmented burners mounted in the interior of the cylinder to transmit heat to the cylinder by convection and infrared radiation. U.S. Pat. No. 5,842,285 teaches a drying apparatus having a number of independently controllable radiant gas burners located inside the drum to heat the drum. PCT International Publication WO 99/45196 teaches a rotating drying cylinder having a ribbon burner facing towards and adjacent to an inner surface of the cylinder and an acoustic baffle mounted behind the burner. In addition, a hollow roller heated by burning fuel internally and directing hot products of combustion against the inner surface of the roller and a baffle means disposed behind the burner for absorbing sound is also taught by this publication.
It is one object of this invention to provide a method and apparatus for heating flat or curved surfaces which provides for more uniform temperature control over conventional methods and apparatuses.
It is one object of this invention to provide a method and apparatus for heating flat and curved surfaces that provides reduced NOx emissions formation compared to conventional equipment and methods.
It is another object of this invention to provide a method and apparatus for heating flat and curved surfaces that provides increased convective heat transfer as well as greater thermal efficiency over conventional equipment and methods.
These and other objects are addressed by a method and apparatus in which a hydrocarbon fuel and oxidant at ambient temperature, or preheated to temperatures up to about 1500° F., are injected through one or more slots, holes, or porous matrices, premixed or separately, into and along a thin space, typically 0.5 to about 6 inches thick, referred to as the combustion space, formed between two flat or curved plate surfaces. The fuel and oxidant burn to form a substantially sheet-shaped flame within the combustion space. The kinetic energy in the flame induces substantial amounts of recirculation, greater than about 20%, preferably in the range of about 20% to about 200%, of cooled products of combustion into the root of the flame, thereby cooling the flame and reducing NOx formation. In addition, the recirculated gases increase the flow rates and velocities, thereby improving temperature uniformity of the heating surface and increasing the convective heat transfer rates. The method of this invention is applicable to a wide range of heaters and dryers, such as drum dryers, heating, curing, drying and baking ovens and cooking griddles.
a and 5b show an application of the method of this invention in which the heated plate surface incorporates heat-transfer enhancement features, e.g. dimples, on the flame side of the surface to further increase the heat transfer rates, resulting in higher heating/drying rates as well as further reduced NOx formation as a result of the recirculation of cooler gases; and
As shown in accordance with one embodiment of this invention in
The flame resulting from the combustion of the fuel and oxidant is a substantially flat sheet flame, designated by arrow 15 in FIG. 1 and arrow 23 in
In addition to fuel and oxidant inlets 13, cylindrically-shaped surface 12 forms a combustion products or flue gas outlet 14 through which the products of combustion may be exhausted from annular space 16. In accordance with one particularly preferred embodiment of this invention, at least a portion of the products of combustion are diverted from the flue gas outlet 14 and recirculated to the base portion or root of the flame. By “base portion” or “root” of the flame, we mean that portion of the flame proximate the initiation of combustion of the fuel and oxidant.
An alternative embodiment of the apparatus of this invention is shown in FIG. 2. The apparatus 20 comprises two concentrically disposed, cylindrically-shaped surfaces 21 and 22, which form annular space 26 therebetween. In contrast to the embodiment shown in
In accordance with one embodiment of this invention, the combustion products exiting the combustion space are passed through a heat exchanger to preheat the combustion oxidant and/or fuel for heat recovery and efficiency improvement. In accordance with another embodiment, the heating surface plate moves in the same direction as the flame gases, for example as in drum/hollow roller dryers, whereby the amount of combustion gas recirculation is further increased, thereby further reducing NOx formation and increasing convective heat transfer rates.
In accordance with one embodiment of this invention as shown in
In accordance with the method of this invention, a fuel and oxidant are injected along the length, width or longitudinal side of a combustion space formed between two correspondingly-shaped surfaces. In accordance with one embodiment of this invention, the correspondingly-shaped surfaces are concentrically disposed, cylindrically-shaped surfaces, one of which constitutes a heating surface for providing the heating/drying function of the apparatus and the other of which provides a means for introducing the fuel and oxidant into the combustion space. In accordance with another embodiment of this invention, the correspondingly-shaped surfaces are in the form of flat plates. Heat from the combustion products is transferred by convective and radiant heat transfer mechanisms to the surface being heated and on to the material being dried/heated. At least a portion of the combustion products are recirculated to the root of the flame. In accordance with one preferred embodiment of this invention, at least about 20% up to about 200% of the combustion products are recirculated back to the root of the flame.
As previously indicated, the fuel and oxidant may be injected into the combustion space through separate openings in one of the correspondingly-shaped surfaces. Alternatively, the fuel and oxidant may be premixed and the mixture introduced through the same opening(s). To provide the possibility of providing temperature profiling across the surfaces, the fuel and/or oxidant may be segmented into individually controlled sections.
To avoid direct impingement of the flame against the heating surface, the fuel and oxidant are injected into the combustion space at an angle less than about 45 degrees to the surfaces at the plane of injection. In accordance with a particularly preferred embodiment, the fuel and oxidant are injected substantially parallel to the surfaces.
In accordance with one embodiment of this invention, at least a portion of the products of combustion exiting the combustion space are passed through a heat exchanger for preheating the combustion oxidant and/or fuel. In accordance with one preferred embodiment of this invention, the stationary surface of the correspondingly-shaped surfaces is incorporated into the heat exchanger to preheat the incoming oxidant and/or fuel.
To test the method of this invention, an apparatus as shown in
The temperature of the heated dimpled flat wall reached levels of 800-1000° F. without any heating/drying material. Rough temperature uniformity measured manually by contact thermocouple was about ±50° F. The above data show significant NOx reductions with recirculation without any attempts at system optimization.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.
This application claims the benefit of an earlier filed provisional application having Ser. No. 60/426,484 and a Filing Date of 14 Nov. 2002.
This invention was made with Government support under DOE Contract No. DE-FC36-01GO10621 awarded by the Department of Energy. The Government has certain rights in this invention.
Number | Name | Date | Kind |
---|---|---|---|
3452967 | Durand | Jul 1969 | A |
4562655 | Bosshart et al. | Jan 1986 | A |
4677773 | Kamberg | Jul 1987 | A |
4688335 | Krill et al. | Aug 1987 | A |
4693015 | Hemsath et al. | Sep 1987 | A |
5416979 | Joiner | May 1995 | A |
5553391 | Bakalar | Sep 1996 | A |
5791065 | Gamble et al. | Aug 1998 | A |
5842285 | van der Veen | Dec 1998 | A |
5966835 | Bakalar | Oct 1999 | A |
6560893 | Bakalar | May 2003 | B1 |
Number | Date | Country |
---|---|---|
WO 9945196 | Oct 1999 | WO |
Number | Date | Country | |
---|---|---|---|
20040096795 A1 | May 2004 | US |
Number | Date | Country | |
---|---|---|---|
60426484 | Nov 2002 | US |