Patent Description:
<CIT> discloses the operation of burners with flameless oxidation. This operation is based on injecting a fuel and pre-heated air at high speed into a waste gas recirculation eddy. A burner is used to heat the combustion chamber and is operated with a flame. Once the operating temperature has been reached, it is possible to switch over to flameless oxidation.

Furthermore, <CIT> discloses the heating of radiant tubes by means of burners which operate with flameless oxidation. Burners of this kind are also designed to assume an operating mode with flame in the heating phase.

<CIT> discloses a burner for heating furnace chambers by means of flameless oxidation, in which the operating temperature is below a limit temperature necessary for the flameless oxidation. To this end, hot gases are generated by means of a steadily burning flame and are used to assist and maintain the otherwise flameless oxidation.

<CIT> discloses a method of heating a furnace using radiant tubes with flameless operation below oven temperature of <NUM> and flameless operation with furnace temperature monitoring by means of two safety temperature monitors for two temperature limits such as <NUM> and <NUM>. This method extends the temperature range for flameless operation to temperatures below <NUM> such as above <NUM> instead of only above <NUM>.

Radiant tubes are typically used for indirect heating of industrial furnaces for temperatures up to approximately <NUM>° C. In order to dissipate radiant heat, the radiant tubes are heated from the inside, wherein the radiant tube can be heated by flameless oxidation, which significantly reduces the nitrogen oxides in the waste gas.

If the temperature of the industrial high temperature application is controlled by switching individual radiant tubes or entire radiant tube groups on and off, the radiant tubes must be switched on and off whilst still warm from operation. However, if the radiant tubes are heated by burners with a flame and if the process is performed with high air preheating (in particular preheating to more than <NUM>° C) for energy conservation reasons, the resultant nitrogen oxide values are unacceptably high. The operation of the burners with flameless oxidation by contrast leads to lower nitrogen oxide values, wherein according to experience, in the case of furnaces heated by means of radiant tube, a furnace operating temperature of at least <NUM>° C is necessary in order to be able to reliably put a burner into operation with flameless oxidation. If the furnace temperature is lower, however, deflagrations can occur in the event of intermittent operation, i.e. switching on and off of burners for output regulation. In this regard, furnace operating temperatures below <NUM>° C can be considered critical for intermittent flameless burner operation (flameless pulsed burning).

In high-temperature furnaces, which are gaining larger markets due to increasing low-emission requirements, the furnace system switches from flame monitoring to temperature monitoring at a central position after the ignition temperature has been reached. The temperature monitoring must be carried out with an approved complex fail-safe double thermocouple. In furnaces with indirect heating, i.e. the exhaust gases from the burners do not enter the furnace chamber, the central position is not the coolest position under all conditions. After a cooling mode, the temperature in each individual radiant tube can fall below the monitored furnace chamber temperature. As a consequence, a thermocouple would need to be integrated in each radiant tube to reliably monitor a temperature above a critical temperature.

Examples of currently used systems can be found in the following documents:.

<CIT> which discloses a burner for heating a heating space, with a reduction of NOx emissions. The burner comprises a mixing and combustion chamber, a mixing and igniting device that is arranged in the mixing and combustion chamber, and a fuel supply that is connected to the mixing and igniting device and is designed to supply fuel to the mixing and igniting device. Furthermore, an air supply is provided, which is designed to supply at least one partial airflow to the mixing and combustion chamber. A combustion chamber opening opens the mixing and combustion chamber toward a heating space to be heated. In addition, control means are designed to control a fuel flow via the fuel supply and to control at least one partial airflow via the air supply, the burner, and the control means is designed for the operation of the burner with a stable flame that extends from the mixing and igniting device into the heating space through the combustion chamber opening.

A furnace heating device is provided for the heating a furnace chamber in accordance with claim <NUM>.

Another embodiment provides a method for controlling a furnace heating device in accordance with claim <NUM>.

The core of the present invention is that in the start-up in central temperature monitored flameless operation the flameless mode cannot be started until the combustion zone inside of a radiant tube is first brought above a critical temperature. This causes the combustion chamber temperature in the radiant tube to rise to a safe temperature above a critical temperature. In comparison to the state of the art, the invention solves the safety problem described above by means of software in the fail-safe burner control. Having a temperature monitoring configuration for each individual radiant tube burner would be complex and the costs for a double thermocouple with fail-safe evaluation per burner would be prohibitive, especially with the number of burners found in a typical furnace. With this innovation there is a higher safety and at the same time a competitive advantage over existing systems. The cooling of the radiant tube interior below a critical temperature and below the furnace chamber temperature detected by the usual temperature monitoring at a central point can only be achieved by control processes, e.g. cooling or purging by the user. This control process is performed by a burner control unit. In accordance with the solution, this burner control system memorizes the process. In the case of a previously used cooling process, the start-up must not follow a high-temperature start-up afterwards, as this can lead to an explosion.

In such a situation, the temperature in the radiant tube must first be brought to a value above a critical temperature and only then may the start-up or switchover of the burner occur in high-temperature operation follow. The time needed to reach the required temperature in the radiant tube is given by the mechanical construction and can be defined depending on the size of the radiant tube and the burner capacity.

By using the burner control with the software extension, the furnace operator can use the burners for cooling and heating according to his control without any risk. In the burner control unit, the determined time for reaching the critical temperature must be parameterized. Depending on the capacity of the burner and the radiant tube volume, the time to be determined for this system must be defined.

In a simplified operation of the burner control with the software extension, the burner is started in high temperature mode. Flame operation is activated after cool down after one of the following conditions have occurred: The device was switched on, there was a purge of gases in the burner tubes or there was a cooling process. A start attempt or a restart of the burner does not cause this start-up in flame operation to be aborted. The start in flame operation is repeated until the required burning time for heating up the burner is reached.

In a typical furnace there may be <NUM>-<NUM> burners per zone. There may be one or two or as many as <NUM> zones within a furnace. In the operation of a radiant tube burner, it is desirable to control the burner without having a thermocouple on each burner tube. In starting up the burner, it is in flame mode and then switched to flameless mode. The flame continues to burn in the burner tube until the temperature is high enough so that flame monitoring may be turned off.

The furnace heating device according to the invention comprises at least one radiant tube, preferably a plurality of radiant tubes, which can be heated in each case by means of a burner (at least one burner), which in a first operating mode can be operated with flame and in a second operating mode can be operated without flame, i.e. with flameless oxidation. The interior of the radiant tube is preferably sealed off with respect to the furnace chamber. At least one control device is provided, by means of which the burner or the burners of the radiant tubes can be switched on and off and can be switched over between the first and the second operating mode. The temperature of the interior of the radiant tube in the case of an active burner is greater than the temperature of the furnace chamber. During breaks in operation, which occur repeatedly in the case of pulsed burning, the temperature in the interiors of the radiant tubes approaches the furnace temperature from above.

In accordance with the invention the control device is designed to operate the burner temporarily in the first operating mode and then in the second operating mode in the event of a warm start. If the burner of the radiant tube is clocked, i.e., is operated with what is known as pulsed burning, the burner must be reliably started again and again.

The furnace heating device can comprise a device for at least local detection of the furnace temperature, for example in the form of one or more switching temperature sensors. If a temperature sensor of this kind is arranged at a point of an industrial furnace, it determines the switching on and off of burners on the basis of the furnace temperature, but not on the basis of the radiant tube temperature.

<FIG> shows three radiant tubes <NUM>, <NUM> and <NUM> that are connected to burners <NUM>, <NUM> and <NUM> that are connected with burner control units <NUM>, <NUM> and <NUM>. A sensor <NUM> is shown that sends data including temperature data to a control unit <NUM> that is configured to communicate through line <NUM> to each of burner control units <NUM>, <NUM> and <NUM>.

<FIG> shows the steps involved in the operation of the process of the invention. At box <NUM> is shown the start of the startup of the burner in high temperature mode. Before the gas is released for the flame, it is checked on a safety monitor as shown in box <NUM> if one of the following conditions was present before starting the burner:.

If one of those conditions is underway, then the burner flame is operated as shown in box <NUM> which continues for a period of time, such as <NUM> to <NUM> seconds as shown in box <NUM> where the flame operation period is checked to have finished. Box <NUM> checks whether the time has already elapsed; if not, the burner remains in flame operation (box <NUM>). If yes, it switches to flameless operation according to box <NUM>. If none of the conditions is present in box <NUM>, then flameless operation takes place as the next step in box <NUM> followed by the end of flameless operation in step <NUM>.

<FIG> illustrates an example device <NUM> for detecting a safe point to start up according to the invention. The device <NUM> could, for example, denote any of the controllers, operator stations, or other devices in or used in conjunction with the system <NUM> in <FIG>. The device <NUM> could also represent the computing device that implements part or all of the control approach in <FIG>. However, the device <NUM> could be used in any other suitable system.

As shown in <FIG>, the device <NUM> includes at least one processor system <NUM>, at least one storage device <NUM>, at least one communications unit <NUM>, and at least one input/output (I/O) unit <NUM>. Each processor system <NUM> can execute instructions, such as those that may be loaded into a memory <NUM>. The instructions could implement the safe point functionality described herein. Each processor system <NUM> denotes any suitable processing device, such as one or more microprocessors, microcontrollers, digital signal processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or discrete circuitry.

The memory <NUM> and a persistent storage <NUM> are examples of storage devices <NUM>, which represent any structure(s) capable of storing and facilitating retrieval of information (such as data, program code, and/or other suitable information on a temporary or permanent basis). The memory <NUM> may represent a random-access memory or any other suitable volatile or non-volatile storage device(s). The persistent storage <NUM> may contain one or more components or devices supporting longer-term storage of data, such as a read only memory, hard drive, EEPROM, Flash memory, or optical disc.

The communications unit <NUM> supports communications with other systems or devices. For example, the communications unit <NUM> could include a network interface card or a wireless transceiver facilitating communications over a wired or wireless network. The communications unit <NUM> may support communications through any suitable physical or wireless communication link(s). The I/O unit <NUM> allows for input and output of data. For example, the I/O unit <NUM> may provide a connection for user input through a keyboard, mouse, keypad, touchscreen, or other suitable input device. The I/O unit <NUM> may also send output to a display, printer, or other suitable output device.

As explained above, after a cooling process, the burner must first be brought above the ignition temperature. The calculation of the time required to reheat the radiant heating tube can be done for either SI units or imperial units as explained in the following Example.

In an example where the radiant tube has D = <NUM>, L = <NUM>, P = <NUM> kW <MAT> reheating time = <NUM> seconds.

Radiant tube with D = <NUM> inch, L = <NUM> inch, P = <NUM> BTU/hr <MAT> reheating time = <NUM> seconds.

In some embodiments, various functions described in this patent document are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase "computer readable medium" includes any type of medium capable of being accessed by a computer or controller, such as read only memory (ROM), random access memory (RAM), a hard disk drive, or any other type of memory. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable storage device.

It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms "application" and "program" refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer code (including source code, object code, or executable code). The term "communicate," as well as derivatives thereof, encompasses both direct and indirect communication. The phrase "associated with," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like.

The description in the present application should not be read as implying that any particular element, step, or function is an essential or critical element that must be included in the claim scope. The scope of patented subject matter is defined only by the allowed claims.

Claim 1:
A furnace heating device, for heating a furnace chamber, comprising:
at least one radiant tube (<NUM>) having a burner (<NUM>) and a combustion chamber, and configured to heat the furnace chamber, wherein said at least one radiant tube (<NUM>) is heated using said burner (<NUM>), wherein said burner (<NUM>) is operable in a first operating mode with a flame and in a second operating mode with flameless combustion, and wherein the temperature in the combustion chamber must be above a critical temperature for safe operation of flameless combustion;
a burner control device (<NUM>) configured to control on and off states and operating mode setting for said burner (<NUM>) of said at least one radiant tube (<NUM>); and
at least one single safety monitor for monitoring a temperature of said furnace chamber and communicating said temperature to said burner control device (<NUM>),
wherein said burner control device (<NUM>) is configured to send a signal to prevent the start of said flameless combustion when it is determined that said temperature of the furnace chamber is above said critical temperature and a cooling process or a purging process or a control device switch on procedure has occurred.