EP0772010B1 - Burner-head air heater - Google Patents

Abstract

The air is indirectly heated by the heater. It is separated into two streams (B,C). One stream (B) is heated in cross-counterflow with flue gas in a heat exchanger (22). The other stream (C) is heated by passing uniformly around a combustion chamber (16) jacket. The two streams are passed through the heating plant without intermixing. Preferably, the combustion chamber is rotationally-symmetrical and the main dimension of the chamber is in the direction of the axis of rotation. The second air stream passes through an enclosed flow duct (34) which surrounds the combustion chamber.

Description

The invention relates to an air heating system for indirect heating of air for drying plants after the Preamble of claim 1 and a method for indirect heating of air for drying systems.

The task of such air heating systems for indirect Air heating for drying plants is that the air required for a drying process to a predetermined temperature is heated without this directly is heated by a burner flame. Due to the in Flue gases contain pollutants that exceed the Burner exhaust air can be transferred to the material to be dried could, in the case of drying of food, such as for example, brewing malt, powdered milk or coffee Drying air can be heated indirectly.

This task of burner-heated air heaters is carried out in the DE 43 08 522 described in detail. To solve this Task is presented in DE 43 08 522, a device which the furnace with the help of a liquid Intermediate medium, for example water, is cooled. By the cooling of the furnace is the Surface temperature of the air to be heated in Touching surfaces, especially the surface of the furnace, to a temperature below 700 ° C limited. This can lead to contamination of the warming air due to nitrogen oxides or other undesirable Combustion residues can be prevented effectively. Of further leads the use of a liquid Intermediate medium to a good utilization of the waste heat Own power generation plants, for example CHP plants.

Technologically for drying processes to achieve the required air temperatures are high Temperatures in the liquid intermediate medium necessary in in practice are above 120 ° C. This means that in If water is used as liquid Intermediate medium, all devices installed in the circuit and Devices fall under the Pressure Vessel Ordinance. This Circumstances increase the investment cost of such Plant and complicates the operation due to the need official requirements or approvals.

In applications where there is no waste heat in the form of liquid intermediate medium is available for recycling, it therefore makes sense to use a liquid Dispense with the intermediate medium and the cooling of the Furnace directly through the air to be heated bring about.

DE 30 39 065 describes a burner heated Air heater without the use of an intermediate medium. On Partial air flow of the air to be heated flows around the Combustion chamber and through a preheater in which the in the flue gases generated in the combustion chamber are cooled. On second partial air flow is in a separate Reheated heat exchanger, which is already in the Pre-heat exchanger is flowed through cooled flue gases. The two partial air flows of different temperatures are mixed into a total air flow in the air heater and leave the air heater together.

Since the first partial air flow flows across the combustion chamber, forms on the entry of the first partial air flow facing away from the combustion chamber with a lee area insufficient air movement on the lateral surface of the Combustion chamber, so that in this area a limitation of Surface temperature not to a maximum of 700 ° C can be ensured. A good flow around the Combustion chamber is essential because the flame is in the Combustion chamber has a temperature above 1000 ° C and therefore Wall temperatures of over 700 ° C can occur.

An improved influence on the surface temperature of the Combustion chamber is described in DE-OS 23 29 305. Here the surface of the combustion chamber is parallel to Flows against the longitudinal axis of the combustion chamber, so that no lee area with insufficient air movement and consequently reduced cooling effect can arise. However, are along of the flue gas duct adjoining the combustion chamber such surfaces exist that are inadequate to be cooled and therefore undesirable Contamination of the drying air to be heated with Can lead to nitrogen oxides. Furthermore, it is Fuel utilization is low because the drying air in DC to flue gas from the combustion chamber to one Tube bundle heat exchanger flows in which the drying air is led to the flue gas in cross-direct current. Through the serial arrangement of the individual heat exchange elements as well the essentially direct current flow of drying air and flue gas in these is an intensive flue gas cooling, which is a prerequisite for optimal energy utilization represents, not given.

This disadvantage is with that in DE 33 30 924 described device fixed. With this air heater the air flows in counterflow to the flue gas and along the Shell surface of the combustion chamber. The inflow of flue gas flow of cold air through the tube bundle leads to an intensive flue gas cooling and thus a good one Energy utilization; however, this is already preheated Air for cooling the outer surface of the combustion chamber used. This leads to the cooling of the Lateral surface of the combustion chamber has deteriorated significantly, because that's proportional in the determination of heat transfer incoming driving temperature drops significantly reduced is. For this reason, adequate heat dissipation from the outer surface of the combustion chamber is disproportionate large combustion chamber diameter are made possible because thereby the heat transfer surface, which also is proportional to the amount of heat given off becomes.

FR-A-1.138.276 describes a burner whose exhaust gases be passed through a heat exchanger. The combustion chamber is with helically arranged rib plates as well an outer jacket around the rib plates, so that a helical flow channel around the Combustion chamber forms around. A first airflow to warming air hits the end of the helical Rib up and flow around the combustion chamber while a second Airflow of the air to be heated in counterflow to the Tubes of the heat exchanger is performed.

Starting from FR-A-1.138.276, the invention lies in the The task is based on a single, compact device ensure intensive flue gas cooling and at the same time the surface temperature of all with the drying air in To keep the coming outer surfaces as small as possible.

This technical problem is caused by an air heating system with the features of claim 1 and a method for indirect heating of air solved according to claim 7.

Effective cooling both in the combustion chamber generated flue gas in a flue gas heat exchanger, as the outer surface of the combustion chamber can also be achieved because drying air is in contact with them occurs that has not yet been upstream Heat exchange step has been preheated. This stands for the cooling of the outer surface of the combustion chamber greatest possible driving temperature gradient available, so that an effective cooling of the outer surface is possible. Of further flows the second partial air stream to be heated Drying air along the outer surface of the Combustion chamber, so that there are no areas with insufficient Form overflow that can overheat, causing Wall temperatures can arise that are above the maximum tolerable lie. By the two partial air flows without mutual mixing within the air heating system can be performed, each of them specifically for the two Partial tasks, intensive flue gas cooling with the first Partial air flow and sufficient cooling with the Airflow contacting outer surfaces with the second partial air flow can be coordinated.

By the first partial air flow and the second partial air flow the drying air to be heated without mutual Mixing can be performed in the air heating system in a simple and convenient way, for example by Adjust the pressure drop of one or both Partial air flows, the mass fraction of the two partial air flows be regulated

The air heating system has one Mass flow ratio adjusting device of the two Partial air flows of the drying air to be heated. Hereby the air heating system can be targeted to different Operating areas to be adjusted. If, for example Drying air at a higher or lower temperature should be generated, this can be done by setting the Mass flow ratio of the two partial air flows reached be without causing unwanted overheating can come inside the air heating system.

The adjustment device is an axial one sliding baffle. The axially movable Baffle plate narrows or widens the outlet cross section of the flow channel one of the two partial air flows on Leaves the air heating system and thus controls the Pressure loss of the respective partial air flow. This is possible because the first and the second partial air flow the drying air to be heated without mutual Mixing and thus without the possibility of one mutual pressure equalization in the air heating system be performed.

Preferred embodiments of the invention are characterized by the other claims marked.

The mass fraction of the second is advantageous Partial airflow less than the mass fraction of the first Partial air flow in the air to be heated.

The combustion chamber is advantageously essentially the main extension is rotationally symmetrical the combustion chamber in the direction of the axis of rotation of the Combustion chamber. Due to the rotationally symmetrical shape of the Combustion chamber is flow around in the longitudinal direction, ie. H. in Direction of the axis of rotation easier.

The second partial air flow is preferably the warming drying air in a closed Flow channel guided, which surrounds the combustion chamber, wherein the closed flow channel ribs or fins has that on the outer surface of the combustion chamber are formed and in the main flow direction of the second Extend partial air flow within the flow channel.

The provision of a closed flow channel as well as the ribs arranged therein in the main flow direction or Slats provide a controlled and even Overflow of the outer surface of the combustion chamber for sure. This will reduce the risk of uneven and locally insufficient overflow with the resulting resulting local overheating avoided. The one on the Air side, d. H. outer surface, the combustion chamber applied, heat-conducting fins cause a significant increase in the heat exchange area, which the Cooling effect is greatly improved. This allows the Volume flow of the second partial air flow at the same Heat transfer from the combustion chamber to the second Partial air flow can be reduced.

Preferably the flue gas heat exchanger comprises Pipe bundle in which the flue gases flow, the first Partial air flow of the drying air to be heated in cross-counterflow is guided around the tube bundle. Through the cross-countercurrent flow of the drying air to the flue gas the best possible use of those used in the burner Primary energy can be achieved by not yet preheated air in heat exchange with the outlet side End of the flue gas pipes is brought.

Fig. 1

ein Längsschnitt durch die Lufterhitzungsanlage mit schematisch eingezeichneten Pfeilen zur Verdeutlichung der Strömungsrichtung der Rauchgassowie Teilluftströme; und

Fig. 2

ein Querschnitt durch die Lufterhitzungsanlage gemäß Fig. 1 darstellt.

The invention is described below purely by way of example with reference to the attached figures, in which:

Fig. 1

a longitudinal section through the air heating system with schematically drawn arrows to illustrate the flow direction of the flue gas as well as partial air flows; and

Fig. 2

a cross section through the air heating system shown in FIG. 1.

The air heating system shown in Fig. 1 is generally referred to by reference number 10. The Air heating system 10 has a suitable housing 12, its construction with different housing covers, flanges and load-bearing components both ensure that the Air heating system 10 on the stand area 14 as well allows easy access to the individual components, wait for the air heating system 10 and if necessary to be able to clean.

An essential component of the air heating system 10 represents the combustion chamber 16 with a burner 18 connected by the combustion of a liquid or gaseous fuel produces flue gases. The operation of the burner 18, the burner flame 19 schematically in the Figures is shown, is not described in more detail below explained, since this is a common in technology Connection of a burner to a combustion chamber.

It is not for convection drying of food permissible, the air used for drying directly through to heat the burner flame. This is because the im Flue gas contained pollutants in the drying air the goods to be dried, e.g. brewing malt, milk powder, Coffee or other foods that can be transferred. Therefore, the heating must be done indirectly, by the Combustion, hot flue gases in heat exchange be brought to the air to be heated.

The smoke gases A arising in the combustion chamber 16, the Direction of movement schematically through the "A" marked arrows, leave the Combustion chamber 16 at the combustion chamber outlet 20, through a Flue gas heat exchanger 22 located on the combustion chamber connects.

The tubes of the tube bundle of the flue gas heat exchanger 22 are exposed to the flue gases A and end in one Flue gas outlet 24, from which the flue gases are suitable Way to be dissipated. In that shown in Fig. 1 Example are the flue gases in a tube bundle loop led and the flow direction of the flue gases on the one hand through a specific bending of the individual pipes in the Deflection area 25 and on the other hand by interposing a deflection bell 26 changed.

The tube bundle of the flue gas heat exchanger 22 runs ahead the exit from the air heating system 10 along a Air inlet 28 through which a partial air flow to warming air enters the air heating system 10.

The one entering through the air inlet 28, not yet preheated air is divided into two partial air flows B and C split up independently of each other by the Air heating system 10 flow and only after the outlet mixed together from the air heating system 10 become. The separation of the two partial air flows can be reduced to one done in any way, for example by installation of separating and air guiding elements, which are in the housing 12 are arranged.

In the embodiment shown in Fig. 1 forms for example a wall of the deflection bell 26 of the Flue gas heat exchanger 22 is part of this required separating elements.

The first partial air flow B, the flow course of which is shown in FIG. 1 is represented by the arrows marked "B", flows around the tube bundle of the Flue gas heat exchanger 22. As already explained above is in a multi-pass flue gas heat exchanger the last pass before the exit of the Smoke gas from the air heating system in the immediate vicinity Proximity to the first entering through the air inlet 28 Partial airflow B. This creates a cross-counterflow between the flue gas to be cooled and the partial air flow B to be heated of the air to be heated, thereby making the best possible use of the burner primary fuel energy is achieved.

As best seen in Fig. 2, the first leaves Partial air flow B after flowing around the tube bundle Flue gas heat exchanger 22 through the air heating system 10 Exit openings 30 to which suitable facilities can connect to the from the outlet openings 30th emerging, heated first partial air stream B. and continue to transport them as intended.

The second partial air flow C of the air to be heated is completely separate from the first partial air flow B through the Air heater and is used in a suitable manner, for example via air guide plates 32 to the combustion chamber 16 passed, whereupon the second partial air flow C into one Chamber jacket 34 occurs, which surrounds the combustion chamber 16.

The chamber jacket 34 is designed so that the second Partial air flow C the outer surface of the combustion chamber 16 completely flows around. As shown in Fig. 1, are preferably ribs or slats 36 on the outer Shell surface of the combustion chamber 16 is formed. These slats 36 enlarge the heat-emitting area of the combustion chamber 16 and thus increase the cooling effect by the im Chamber jacket 34 guided second partial air flow C. Die Dimensioning of the laminated combustion chamber size is aimed according to the requirement that the flue gases in the Combustion chamber to be cooled down so far that Combustion chamber outlet 20, at which the flue gases enter the Pipe bundles of the flue gas heat exchanger 22 enter Pipe surface temperature not a temperature of 700 ° C exceeds. The second partial air flow C is parallel to Longitudinal axis of the preferably substantially cylindrical formed combustion chamber 16 through the chamber jacket 34 performed and leaves the air heating system 10 on the end of the combustion chamber 16 remote from the burner.

The second partial air flow C is preferably at the outlet from the air heating system in two partial flows split, with a first substream Air heating system essentially in the direction of Flow through the chamber jacket 34 leaves, the other stream however, is deflected and the distant, preferably bell-shaped front end of the Combustion chamber 16 flows around before it reaches the air heating system leaves.

The outlet-side division of the second partial air flow C in which two partial flows are made with a baffle plate 38 reached, the same time the setting of Mass flow ratio of the two partial air flows B and C serves the air to be heated. For this, the baffle plate 38 in the axial direction, d. H. in the direction of the longitudinal axis of the essentially cylindrical combustion chamber 16, shifted and be fixed in position.

Moving the baffle plate 38 regulates the Flow resistance at the outlet of the second partial air flow C from the air heating system 10. Because of the inside the air heating system 10 completely separate Flow control of the two partial air streams B and C acts the outlet pressure loss of the second Partial air flow C on the distribution of the two Partial air flows at the air inlet 28. The stronger with help the baffle plate 38 the outlet cross section for the second Partial air flow C is narrowed, the smaller the second Partial air flow C, because the pressure drop with decreasing Flow velocity of the second partial air flow C decreases and the air to be dried still at the air inlet 28 exists together, d. H. the pressure at the air inlet 28 for both partial air flows B and C are the same size.

After exiting the air heating system 10, too the second partial air flow C is taken up in a suitable manner and promoted as intended.

By dividing the two partial air flows B and C. on the one hand, optimal use of the primary fuel energy used achieved by the first, not yet heated, partial air flow B in cross-counterflow to the Flue gases in the flue gas heat exchanger 22 is led to other sufficient cooling of the combustion chamber 16 the complete flow around the same through the second, also not yet heated, partial air flow C ensured. Since the second partial air flow C also at Entry into the air heating system 10 is cold, the Combustion chamber surface effectively cooled. The Design of the outer surface of the combustion chamber, which sufficient over all surfaces Ensures air circulation, but especially that Flow in the longitudinal direction of the combustion chamber. Since the Dimensioning of the laminated combustion chamber size according to the specification directs what the flue gases in the combustion chamber so far be cooled that also at the flue gas inlet in the Tube bundle the tube surface temperature is not 700 ° C by using the also cold, second partial air flow C for cooling the combustion chamber 16 these are dimensioned smaller and thus the entire Air heating system despite the optimal use of the used primary fuel energy more compact become.

Claims (9)

1. Air-heating unit for the indirect heating of air for drying installations having:

   a splitting device which divides the stream of air to be heated into two branch streams of air, a first branch stream of air (B) and a second branch stream of air (C), which are conveyed inside the air-heating unit without being mixed with one another;

   a flue gas heat exchanger (22) which brings the flue gases (A) produced in a combustion chamber (16) into heat-exchanging contact with the air to be heated, the flue gas heat exchanger (22) being supplied with the first branch stream of air (B) which has not yet been heated in cross-countercurrent flow; and

   a combustion chamber (16) around whose outer jacket surface the second branch stream of air (C) which has not yet been heated flows completely and uniformly;

   characterised in that

   the air-heating unit possesses an adjusting device for the adjustment of the mass flow ratio of the two branch streams of air,

   the adjusting device being an axially displaceable baffle disk.
2. Air-heating unit according to Claim 1, characterised in that the combustion chamber (16) has a substantially rotationally symmetrical shape and the principle dimension of the combustion chamber is aligned in the direction of the axis of rotation of the combustion chamber.
3. Air-heating unit according to Claim 2, characterised in that the second branch stream of air (C) flowing around the combustion chamber is conveyed in the direction of the axis of rotation of the combustion chamber.
4. Air-heating unit according to one of the preceding claims, characterised in that the second branch stream of air (C) is conveyed in a closed flow channel (34) surrounding the combustion chamber (16).
5. Air-heating unit according to Claim 4, characterised in that the closed flow channel possesses fins or blades (36) which are formed on the outer jacket surface of the combustion chamber and extend inside the flow channel (34) in the principle direction of flow of the second branch stream of air (C).
6. Air-heating unit according to one of the preceding claims, characterised in that the flue gas heat exchanger (22) comprises a tube bundle in which the flue gases (A) flow.
7. Method for the indirect heating of air for drying installations comprising the following steps:

   splitting of the stream of air to be heated into two branch streams of air, a first branch stream of air and a second branch stream of air;

   heating of the first branch stream of air which has not yet been heated by exchange of heat with a flue gas heat exchanger, the first branch stream of air being conveyed in cross-countercurrent flow with respect to the hot flue gas;

   heating of the second branch stream of air which has not yet been heated by exchange of heat with the jacket surface of a combustion chamber, the second branch stream of air flowing around the combustion chamber completely;

   adjustment of the mass flow ratio between a first branch stream of air and second branch stream of air by changing the pressure loss of one of the two branch streams of air which the latter undergoes on passing through the air-heating unit; and

   separate extraction of the first branch stream of air and of the second branch stream of air from the air-heating unit.
8. Method for heating air in an air-heating unit according to Claim 7, characterised in that the second branch stream of air is conveyed along heat-emitting blades in a flow channel surrounding the combustion chamber.
9. Method for heating air in an air-heating unit according to Claim 7 or 8, characterised in that the mass fraction of drying air in the second branch stream of air (C) is set to be smaller than the mass fraction of drying air in the first branch stream of air (B).

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