Patent Publication Number: US-2011053099-A1

Title: Radiant heating apparatus

Description:
The invention relates to a radiant heating apparatus and a process for monitoring the radiant heating apparatus. In particular, the invention relates to a radiant heating apparatus for a furnace system, for example, an industrial furnace, as well as to a process for monitoring such a radiant heating apparatus with at least one burner that is arranged inside a ceramic radiant heating tube and is connected via a combustion gas line to a gas collector line and via a combustion air line to an air collector line. 
     In many industrial applications in the area of industrial furnace construction a thermal transfer to a material to be treated in a protective- or reactive gas is required. The indirect heating of the furnace systems used takes place here by electrical heating or by radiant heating pipe apparatuses in which customarily several radiant heating pipes are fired with at least one gas burner. The radiant heating pipe, that consists of metal or a ceramic material, is constructed in such a manner that it transfers the heat by radiation to the material during operation. Such heating systems for industrial furnaces that heat the inner furnace chamber indirectly via radiant heat are sufficiently known and can be used for furnaces into which a material to be treated with heat for a certain time is introduced or is continuously moved from a furnace entrance to a furnace exit arranged at a distance from the latter. In order to generate the radiant heat, radiant heating pipes are often used that are constructed, for example, as a pipe open on one end or as a U-shaped pipe and in the inner hollow chamber of which a fuel or combustion gas is combusted with combustion air by a burner. The inner chamber of the furnace can be uniformly heated by an arrangement in which several such radiant heating pipes are arranged adjacent to each other in the inner chamber of the furnace. 
     Furthermore, so-called recuperator burners are used in which the heat from exhaust gases is used for the pre-heating of combustion air. In such recuperator burners at least one burner and a recuperator is associated with a radiant heating pipe. In order to achieve a good efficiency the attempt is made to transfer the largest possible part of the exhaust heat onto the air. Part of the exhaust gas flowing back in the radiant heating pipe is used to preheat the combustion air or fresh air, whereby the transfer of heat or the heat exchange takes place in the recuperator. Such burners are used in industrial furnaces indirectly heated with radiant heating pipes in which furnaces the material introduced into the furnace chamber and the atmosphere present in the furnace chamber must not be loaded with exhaust gases and/or must not be contaminated. Significant energy savings can be realized by using recuperator burners as a function of the burner model and manner of operation. 
     The known radiant heating pipes consist of at least one burner with burner nozzle, a combustion air supply and a burner pipe as well as of an outer pipe. The outer pipe or jacket pipe is closed by a bottom so that the exhaust gases are returned into an annular space between burner pipe and outer pipe. Then, a recuperator is built in at the end of this return stretch by means of which the combustion air necessary for the operation of the burner is preheated in countercurrent. The exhaust gases leave the outer pipe via an annular slot and are connected directly or via an exhaust gas collector head into a pipe system. 
     In such radiant heating apparatuses metallic or ceramic radiant heating pipes are used but ceramic radiant heating pipes are preferred nowadays because they have a plurality of advantages over metallic pipes such as, for example, a higher resistance to temperature. 
     The significant disadvantage of ceramic radiant heating pipes compared to metallic radiant heating pipes is the brittleness of the ceramic material. Whereas damage to metallic radiant heating pipes usually occurs slowly, damage to a ceramic radiant heating pipe results in an immediate break of the pipe, as a result of which a large free cross section is present to the furnace chamber so that the exhaust gases produced during combustion pass from the radiant heating pipe into the furnace chamber. Damage to a ceramic radiant heating pipe can not be readily recognized by the operator of the furnace system. This can result in significant disadvantages during the operation of the furnace system. If the furnace system is operated under protective gas or reaction gas, the furnace atmosphere is changed and contaminated by the exhaust gases exiting from the damaged radiant heating pipe, which generally results in a qualitative adverse effect on the product to be produced. 
     In order to avoid production losses or product devaluations by a damaged radiant heating pipe, it is necessary to recognize the damage to a radiant heating apparatus immediately and to cut out or turn off the burner of the broken radiant heating pipe. Since the furnace system usually comprises a plurality of ceramic radiant heating pipes, each individual radiant heating pipe must be monitored for potential damage. 
     Since the amount of exhaust gas of the radiant heating pipe is generally small in comparison to the volumetric flow of protective- or reaction gas supplied to the furnace chamber, no rise in pressure occurs upon damage to or a break of a radiant heating pipe so that the damage is usually not immediately recognized. In an operation of the furnace system only with air, damage to a radiant heating apparatus can hardly be determined, whereas, in contrast to which in an operation of the furnace system with a protective- or reaction gas the atmosphere of the inner furnace chamber is changed, which can only be determined by constant analyses due to the slow process. Furthermore, it cannot be immediately determined in a plurality of furnace systems comprising radiant heating apparatuses which radiant pipe is damaged. Either the entire furnace system must be turned off and each individual radiant heating apparatus checked for damage or a limitation must be performed by cutting off each radiant heating apparatus individually. Both procedures are cost-intensive and time-intensive, so that very different monitoring possibilities were viewed for avoiding this disadvantage. For example, measures are known from the state of the art in which the operation of a burner is monitored with the aid of a flame detector. Furthermore, monitorings regarding the density of radiant heating pipes are known in which the exhaust gas current of the burner is monitored. Usually, measuring apparatuses or sensor apparatuses are arranged in the exhaust gas current for this purpose that bring about the pressure losses so that this can be appropriately taken into consideration and compensated during the supplying of the combustion air and of the combustion gas to the burner. On the whole, the known measures are expensive, complicated and costly monitoring possibilities. 
     The invention is based on the problem of creating a solution that makes available a simplified and improved possibility of monitoring a radiant heating apparatus in a constructively simple and economical manner. 
     This problem is solved in accordance with the invention by a process for monitoring a radiant heating apparatus with the features in accordance with Claim  1 . 
     The process in accordance with the invention is suitable for monitoring a radiant heating apparatus that is used for the indirect heating or cooling of a furnace system, especially of an industrial furnace, whereby the radiant heating apparatus comprises at least one burner that is arranged inside a ceramic radiant heating pipe and is connected via a combustion gas line to a gas collector line and via a combustion air line to an air collector line. The at least one burner is supplied with combustion air and combustion gas for the indirect heating of the furnace system. In the instance in which the radiant heating apparatus is used for the indirect cooling of the furnace system, the at least one burner is supplied exclusively with cooling air. Furthermore, in the indirect heating of the furnace system the pressure of combustion air conducted in the combustion air line and the pressure of combustion gas conducted in the combustion gas line are lowered until being supplied to at least one burner to a predetermined pressure. The supplying of the combustion air and/or of the combustion gas to at least one burner is monitored by a monitoring apparatus arranged in the combustion air line and/or in the combustion gas line. During the monitoring a current pressure of the combustion air and/or of the combustion gas is determined by the monitoring apparatus immediately before they are supplied to at least one burner. A signal indicating a leak in the ceramic radiant heating pipe is then emitted when the pressure currently determined by the monitoring apparatus drops below a predetermined limit value and/or a pressure difference Δp formed from currently determined pressure and a pressure of a corresponding collector line exceeds a limit value. 
     The problem forming the basis of the invention is also solved by a radiant heating apparatus with the features according to Claim  4 . 
     The radiant heating apparatus in accordance with the invention for a furnace system, especially for an industrial furnace, comprises at least one burner that is arranged inside a ceramic radiant heating pipe and is connected via a combustion gas line to a gas collector line and via a combustion air line to an air collector line. A monitoring apparatus for determining a current pressure is provided in the combustion air line and/or in the combustion gas line directly in front of the entrance into the housing of the at least one burner and outputs the currently determined pressure to a burner control. The monitoring apparatus and/or the burner control is designed in such a manner here that they compare the currently determined pressure with a predetermined limit value and/or they compare a pressure difference Δp formed from a currently determined pressure and a pressure of a corresponding collector line with a predetermined limit value. The monitoring apparatus and/or the burner control furthermore output a signal indicating a leakage of the ceramic radiant heating pipe if the currently determined pressure drops below the predetermined limit value and/or if the pressure difference Δp formed from currently determined pressure and a pressure of a corresponding collector line exceeds a predetermined limit value. 
     Advantageous and purposeful embodiments and further developments of the invention result from the corresponding subclaims. 
     The invention makes a possibility available with which damage to a radiant heating pipe can be reliably recognized and in addition it is prevented that in case of damage a material introduced during a heat treatment into a furnace system is loaded and contaminated with exhaust gases. The monitoring takes place just as reliably for a furnace system operated with protective gas or reaction gas. This is achieved in accordance with the invention in that a characteristic pressure value is continuously monitored in the combustion air line and/or in the combustion gas line. As soon as the currently determined value drops below a limit value and/or exceeds a pressure difference determined from the currently determined pressure and a pressure value prevailing at the start of the supply lines to the burner, measures are initiated by the monitoring apparatus or the burner control that turn off the burner of the damaged radiant heating apparatus and immediately prevent a supply of combustion air and combustion gas in order that no exhaust gases and/or no combustion air and combustion gases pass into the inner furnace chamber. By virtue of the invention no construction changes of the burner are necessary. Furthermore, in contrast to comparable monitoring apparatuses no additional pressure losses are generated in the current of exhaust gas that would have to be overcome by an elevation of pressure in the supply lines to the burner. 
     In the process and in the apparatus of the invention a value, for example, of 10 mbar, preferably of 5 mbar, more preferably 3 mbar and in particular 2 mbar can be used as limit value for the currently determined pressure. 
     In an alternative or additional embodiment of the process and of the apparatus the limit value for the formed pressure difference Δp can be 70 mbar, preferably 50 mbar and more preferably 30 mbar. 
     However, even limit values deviating from the above ones for the currently determined pressure and for the formed pressure difference Δp that are a function of the geometry and the operating conditions of the radiant heating apparatus are conceivable. 
     It is furthermore provided in an embodiment of the radiant heating apparatus of the invention that the monitoring apparatus comprises a pressure switch. As a result, an immediate prevention of the supplying of combustion air and/or of combustion gas and in addition an immediate cutting off of the burner is possible without a delay in time of the cutting off occurring as a consequence of a signal transmission from the monitoring apparatus to the burner control and from the latter back to, for example, closing valves in the combustion air line and in the combustion gas line. 
     In order to achieve lesser NOx emissions with a high efficiency of the radiant heating apparatus at the same time, a further development of the invention provides that a recuperator is present in the ceramic radiant heating pipe which recuperator uses part of the exhaust gas flowing in the ceramic radiant heating pipe for preheating the combustion air. 
     Finally, the invention provides the use of at least one radiant heating apparatus according to one of Claims  4  to  8  using the process according to one of Claims  1  to  3  for heating or cooling a furnace system. 
     It is understood that the previously named features and features still to be explained in the following can be used not only in the particular indicated combination but also in other combinations or by themselves without departing from the framework of the present invention. The framework of the invention is defined only by the claims. 
     Further details, features and advantages of the subject matter of the invention result from the following description in conjunction with the drawing in which a preferred exemplary embodiment of the invention is shown by way of example. 
    
    
     In the drawing the single  FIG. 1  shows a schematic view of a radiant heating apparatus  1  in accordance with the invention. Radiant heating apparatus  1  comprises the burner  2 , not shown in detail, of the type of a recuperator burner. Burner  2  is arranged inside a hollow jacket pipe, the so-called radiant heating pipe  3  of radiant heating apparatus  1 , and comprises a flame monitoring apparatus or a flame detector (for example, a UV monitoring) for its monitoring that is coupled to a burner control  5 . Radiant heating pipe  3  customarily consists of a ceramic material; however, a metallic radiant heating pipe  3  is also conceivable. 
     Burner control  5  constructed as an automatic gas firing apparatus is configured in such a manner that it can display operating states and disturbance states of radiant heating apparatus  1 . In addition to the monitoring of radiant heating apparatus  1 , burner control  5  serves to ignite and control burner  2 , to adjust different modes of operation of radiant heating apparatus  1  and to control the supply of combustion air and of combustion gas. The various modes of operation of burner control  5  can be inputted via a memory-programmable control (SPS)  6 . However, it is also conceivable that the memory-programmable control (SPS)  6  realizes the control and regulation of burner  2  via burner control  5  and assumes the flexible part for converting different modes of operation. Also, the connection to a central calculator or computer can take place via memory-programmable control (SPS)  6 , which calculator or computer assumes the control and regulation of a plurality of burners  2  and/or radiant heating apparatuses  1  of a furnace system. 
     Burner  2  is supplied with combustion air and combustion gas, during which the combustion air is taken from an air collector line  7  and passes via a combustion air line  8  to burner  2 . Furthermore, the combustion gas required for the combustion is taken from a gas collector line  9  and passes via a combustion gas line  10  to the burner. 
     A manual blocking valve  11  for manually opening or closing the supply of combustion air, a compensator  12  for compensating movements in combustion air line  8  as a result of vibrations or thermal expansions or shortenings, an orifice measuring restrictor  13  with a differential switch  14 , a dosing valve  15  for the rapid adjustment and precise dosing of the combustion air, and a magnetic valve  16  for the automated opening or closing of combustion air line  8  are arranged in combustion air line  8 . Furthermore, a pressure switch  17  is arranged downstream from magnetic valve  16  and upstream from burner  2 . Pressure switch  17  is coupled to burner control  5  and designed in such a manner that it emits a signal and/or a pressure value of the combustion air to burner control  5  when a predetermined minimal pressure of the combustion air is dropped below. 
     Just as in combustion air line  8 , a manual blocking valve  18  for manually opening or closing the supply of combustion gas, a compensator  19  for compensating movements in combustion air line  8  as a result of vibrations or thermal expansions or shortenings, an orifice measuring restrictor  20  with a differential switch  21 , a dosing valve  22  provided for the rapid adjustment and precise dosing of the combustion gas, and a magnetic valve  23  coupled to burner control  5  are arranged in combustion gas line  8 . A pressure switch  24  is provided downstream from magnetic valve  23  and upstream from burner  2 , is coupled to burner control  5  and emits a signal and/or a pressure value of the combustion gas to burner control  5  when a predetermined minimal pressure of the combustion gas is dropped below. 
     As an alternative to the differential pressure measuring by orifice measuring restrictors  13 ,  20  functioning in accordance with the differential pressure principle, a volumetric measuring, a flow rate measuring with an inductive or ultrasonic process or a pressure probe measuring can also be used. 
     Radiant heating apparatus  1  is built in part into a wall (not shown in  FIG. 1 ) of a furnace system, in particular of an industrial furnace, whereby at least radiant heating pipe  3  projects into the inner furnace chamber  25  in order to heat it by radiant heat. Of course, several radiant heating apparatuses are provided for the indirect heating that are built in located adjacent to each other in the furnace wall and ensure a uniform heating of inner furnace chamber  25  as well as of material introduced into it. The combustion gases and/or exhaust gases returned via radiant heating pipe  3  pass via a discharge of radiant heating pipe  3  and a tension interrupter  27  into an exhaust gas line  26 . 
     An exemplary operation of radiant heating apparatus  1  for the indirect heating of inner furnace chamber  25  is described in the following. The indicated values are selected solely by way of example and must be adapted as a function of the burner geometry and of the concrete use. 
     During indirect heating of furnace chamber  25  a superpressure of approximately 10 mbar (ca. 100 mmWS) prevails in radiant heating pipe  3 . This superpressure is substantially identical with the pressure loss on the exhaust-gas side in burner  2 , i.e., with the pressure loss on the exhaust-gas side in the recuperator of burner  2 . 
     The combustion air pressure on manual blocking valve  11  of burner  2  is at least 40 mbar here (ca. 400 mmWS), but at the most 60 or 80 mbar (ca. 600 or 800 mmWS). During operation the combustion air pressure on manual blocking valve  11  of burner  2  is maintained constant with a tolerance of +/−5%. The lowering of the combustion air pressure down to the pressure in radiant heating pipe  3  of approximately 10 mbar (ca. 100 mmWS) at full-load operation of burner  2  takes place in the fittings of combustion air line  8 , i.e., in manual blocking valve  11 , orifice measuring restrictor  13  and dosing valve  15 , and on the air side in the recuperator of burner  2 , in the part of burner  2  through which the combustion air flows to the heat exchange with the hot exhaust gas. The air-side pressure loss in the recuperator, i.e., the pressure loss of the combustion air preheated in burner  2  is also on the order of magnitude of 10 mbar (ca. 100 mmWS). 
     The pressure of the combustion gas in combustion gas line  10  is at least 40 mbar (ca. 400 mmWS) on manual blocking valve  18 , but at the most 60 mbar (ca. 600 mmWS). This pressure of the combustion gas on manual blocking valve  18  of burner  2  is also maintained constant with a tolerance of +/−5%. Under full-load operation of burner  2  the pressure of the combustion gas in combustion gas line  10  is reduced down to the pressure in radiant heating pipe  3  of approximately 10 mbar (ca. 100 mmWS), whereby the pressure reduction takes place in manual blocking valve  18 , orifice measuring restrictor  20 , dosing valve  22  and in the gas nozzles in the burner mouth of burner  2 . The pressure loss in the gas nozzles in the burner mouth is on the order of magnitude of 10 to 20 mbar (ca. 100 to 200 mmWS). 
     Thus, during an operation of radiant heating apparatus  1  for the indirect heating of the furnace system the pressure of combustion air conducted in combustion air line  8  and the pressure of combustion gas conducted in combustion gas line  10  is lowered to a predetermined pressure until the supplying to burner  2 . The supplying of the combustion air and of the combustion gas is monitored by a monitoring apparatus in the form of pressure switch  17  and  24 , whereby a current pressure of the combustion air and of the combustion gas is determined by the two pressure switches  17 ,  24  immediately upstream from burner  2  before they are supplied to burner  2 . 
     If radiant heating pipe  3  is damaged or broken, the combustion exhaust gases no longer take the path via the recuperator to exhaust gas line  26  but rather exit without great loss of pressure into inner furnace chamber  25 . The superpressure in inner furnace chamber  25  is customarily less than 1 mbar, for example 0.1 to 0.5 mbar (ca. 1 to 5 mmWS). The pressure rises slightly due to the combustion exhaust gases additionally entering into inner furnace chamber  25 . 
     Upon a break of radiant heating pipe  3  the exhaust-gas-side pressure losses (approximately 10 mbar or 100 mmWS) in the recuperator, i.e., the pressure losses of the combustion gas conducted out of burner  2  is replaced by the superpressure in inner furnace chamber  25  of a few mbar. For this reason the amount and the mass flow or volume flow of the combustion air and of the combustion gas that are supplied to the burner rise slightly, as a result of which the air-side pressure loss in the recuperator, i.e., the pressure loss of the combustion air preheated in the burner and the pressure loss in the gas nozzles are elevated. In spite of the elevation of the air-side pressure loss in the recuperator, i.e., in spite of the elevation of the pressure losses of the combustion air preheated in the recuperator, upon a break in radiant heating pipe  3  the pressure of the combustion air in combustion air line  8  behind magnetic valve  16  drops significantly immediately before entering into the housing of burner  2 . An analogous development can be determined upon a break of radiant heating pipe  3  for the pressure of the combustion gas immediately before entering into burner  2 , i.e., upstream from burner  2 , downstream from magnetic valve  23 . 
     According to the invention the pressure of the combustion air and/or the pressure of the combustion gas is monitored immediately before entering into burner  2  with a monitoring apparatus in the form of pressure switch  17  and  24 . If the pressure of the combustion air and/or the pressure of the combustion gas on pressure switches  17 ,  24  drops below a given limit value during full-load operation of burner  2 , there is damage to radiant heating pipe  3 , which is indicated by an appropriate signal emitted by pressure switch  17  to burner control  5 . The use of only one pressure switch  17  or  24  is sufficient for determining damage to radiant heating pipe  3  and/or to monitor the tightness of radiant heating pipe  3 . Preferably, only one pressure switch  17  is used in combustion air line  8 . The ratio of the air-side pressure loss in the recuperator to the exhaust-gas-side pressure loss in the radiant heating apparatus is less than the ratio of the pressure loss in the gas nozzles to the exhaust-gas-side pressure loss in the radiant heating apparatus, so that instead of using a pressure switch in combustion gas line  10  the use of a pressure switch  17  in the combustion air line  8  is more suitable on account of the higher sensitivity. 
     Alternatively or additionally, the pressure drop in combustion air line  8  and/or in combustion gas line  10  indicating damage to radiant heating pipe  3  can also be detected if a pressure difference Δp drops below a limit value. Pressure difference Δp is determined here by a currently determined pressure value, for example, at the position of pressure meters  17 ,  24 , and a corresponding pressure prevailing in the collector line, i.e., air collector line  7  or gas collector line  9  at the transition to combustion air line  8  or to combustion gas line  10 . 
     Therefore, in the case of leakiness of radiant heating pipe  3  an appropriate signal is emitted that can be emitted directly from one or both pressure switches  17 ,  24  or from burner control  5 . This signal is emitted when the pressure currently determined from at least one of the two pressure switches  17 ,  24  drops below a predetermined limit value. In the present instance, for example, a value between 2 mbar and 10 mbar can be used as limit value for the currently determined pressure. Additionally or alternatively, this signal can also be emitted if a pressure difference Δp formed from currently determined pressure and from the pressure of a corresponding collector line (air collector line  7  or gas collector line  9 ) exceeds the limit value. A value between 30 mbar and 70 mbar can be considered as limit value for the formed pressure difference Δp. 
     However, radiant heating apparatus  1  can be used not only to heat inner furnace chamber  25 . The usage for the (indirect) cooling of inner furnace chamber  25 , whereby only cooling air is supplied to burner  2  and radiant heating pipe  3  serves as a heat exchanger by cooling off the hot inner furnace chamber  25  by a heat exchange with cooler air flowing through radiant heating pipe  3  is also conceivable. To this end not only the air from a combustion air ventilator but also air from a separate cooling ventilator can be used. Possible damage or a possible break of radiant heating pipe  3  will then be recognized, just as previously during the operation for heating inner furnace chamber  25 , in that the pressure drop is detected by pressure switch  17  and outputted to burner control  5 , which then further processes this signal in order to prevent the supply of air to radiant heating pipe  3  and/or to take measures to turn off radiant heating pipe  3 . 
     Furthermore, in order to increase a reliable monitoring of radiant heating apparatus  1  during indirect heating as well as cooling of a furnace system, the current pressure and/or the pressure difference Δp and a signal indicating the operating state of the burner can be linked to each other so that upon an inadmissible combination of the signal indicating the operating state of the radiant heating apparatus (heating or cooling) and the current pressure or the current pressure difference Δp a signal indicating damage to radiant heating pipe  3  is emitted from at least one of the two pressure switches  17 ,  24  and measures are taken to turn off radiant heating apparatus  1  and/or burner  2 .