The present invention relates generally to heating systems, and more particularly, to thermal processing plants in which streams of fluids or other products are heated indirectly in guide tubes.
In thermal processing plants, fluids that are often under pressure are indirectly heated, wherein thermal energy is used for heating, a phase change, as reaction heat (endothermic reaction), or for several of these effects. The fluids are guided in heating tubes that are usually heated to their thermal limit. In this way, there are no local points of overheating.
Thermal processing plants 1 of the type illustrated in FIGS. 7 and 8 are known in the art. The thermal processing plants 1 each has an interior space enclosed by an oven wall 2, e.g., made from bricks, in which heating tubes 4 are arranged. For heating these tubes, burners 6 are positioned in openings in the oven wall 2. In the embodiment according to FIG. 7, the heating tubes 4 are arranged in line vertically and one after the other, so that an alley 8 is formed between pairs of tube lines 7. The burners 6 impart an axial output momentum to the fuel and to the air and fire into the alleys 8, wherein the heat is transferred predominantly through flame and gas radiation to the heating tubes 4. The radiation originating from the oven wall 2 meets the heating tubes 4 non-uniformly. This arrangement results in a relatively minimal heat transfer effect. In addition, this arrangement is not suitable for the use with flameless-oxidation burners, which form relatively large areas of reducing atmosphere that is harmful to the heating tubes 4.
Thermal processing plants 1 that are known from practice are illustrated in FIGS. 7 and 8. The thermal processing plants 1 each have an interior space enclosed by an oven wall 2, e.g., made from bricks, in which heating tubes 4 are arranged. For heating these tubes, burners 6 are positioned in openings in the oven wall 2. In the embodiment according to FIG. 7, the heating tubes 4 are arranged in line vertically and one after the other, so that an alley 8 is formed between pairs of tube lines 7. The burners 6 impart an axial output momentum to the fuel and to the air and fire into the alleys 8, wherein the heat is transferred predominantly through flame and gas radiation to the heating tubes 4. The radiation originating from the oven wall 2 meets the heating tubes 4 non-uniformly.
This results in a relatively minimal heat transfer effect. In addition, this arrangement is not suitable for the use of flameless-oxidation burners, which form relatively large areas of reducing atmosphere that is harmful to the heating tubes 4.
With the thermal processing plant 1 according to FIG. 8, the heating tubes 4 are arranged in a line approximately in the center of the interior space 3. Swirl burners 6xe2x80x2 are positioned in openings in the oven wall 2 and they impart an essentially radial output momentum to the fuel and to the supplied combustion air. The flames and heated regions of the oven wall 2 generate a hot spot that transfers the major portion of the energy from the flame to the heating tubes 4 through radiation. The zones of higher radiation intensity are small, and for this reason, a plurality of swirl burners 6xe2x80x2 must be installed.
It is an object of the present invention to provide a thermal processing heating system that allows for uniform tube heating while preventing points of overheating in the tubes.
Another object is to provide a thermal processing heating system as characterized above which is compact in design.
Through the arrangement of burners and heating tubes according to the present invention, the burners generate a large zone of increased temperature at the opposite wall and this zone is in radiation exchange with the heating tubes with no shadow formation. The energy is transferred uniformly through solid body radiation (wall-to-tube), gas radiation (reaction zone-to-tube), and convection (completely reacted combustion products-to-tube).
The burners preferably impart a large axial output momentum to the output fuel-air stream. Preferably, this momentum is so large that the stream reaches the opposite wall and generates a hot spot/region at that location. The burners can have fuel-air-mixed output openings or separate output openings for the fuel and air. This releases radiation heat to the heating tubes.
If burners that operate according to the principle of flameless oxidation are used, then no temperature spikes are formed near the burner so that tube line sections opposite in angle can be assembled relatively close in front of the burner. The large-area recirculation of the combustion gases enables, particularly for flameless oxidation, an optimum reduction of the NOx emission even for highest pre-warming of the air. Relatively large, self-forming reducing areas do not contact the heating tubes due to the tube arrangement. In addition, the tube arrangement enables a compact construction and high heat-transfer effects without temperature spikes at the heating tubes. In addition, burners suitable for flameless oxidation also allow the use of problematic fuels or compositions of changing fuels that would conventionally result in stability problems for the formation of a flame.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:
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