The present invention refers to an arrangement for controlling the operation of a catalytic burner adapted in particular for use in household-type appliances, and more generally also in commercial and industrial equipment or air-conditioning systems.
Catalytic gas combustion is generally known to consist in the oxidation of a fuel, such as for instance methane gas, with a combustion supporting agent, such as air, said oxidation being sustained at a low temperature (lower than approx. 1000°C) by an appropriate catalyst material which remains unaltered during the entire combustion process. The most widely known and used catalysts are metals or metal compounds, such as oxides, ceramics and the like, based on chemical elements belonging to the group of precious metals, in particular platinum, palladium and rhodium.
Catalysts are usually supported by an inert material having a large surface area and ensuring conditions of great porosity, such as for instance alumina or wire gauze.
Catalytic burners, in particular for domestic cooker applications, are described for instance in JP-A-62 266 317, US-A-3 067 811, US-A-4 189 294 and EP-A-0 469 251. Furthermore, catalytic combustion is getting a firm foothold in the automotive industry, where it is increasingly used in applications aimed at cutting emissions of noxious exhaust gases from internal combustion engines.
One of the major drawbacks with catalytic burners lies in the need for a correct control of the combustion process so as to cut emissions of noxious gases, in particular CO and all those unburnt products that may pollute the environment, down to a minimum. Such a problem is further aggravated by the need in a number of circumstances, such as for instance in the case of application in domestic or commercial cooking appliances, to be able to continuously adjust the heat output of the burners.
Combustion control systems of the closed-loop type for catalytic burners are known in connection with industrial equipment applications. In particular, the heat output of the catalytic burner is controlled by adjusting the flow rate of the fuel gas through a manually operated valve or similar device. The flow rate of the fuel gas, which may for instance be LPG, is measured by an appropriate flow metering sensor. The corresponding information received from such a sensor is used to calculate, through appropriate tables or mathematical functions, the actual amount of air needed to completely oxidize the amount of gas flowing through the metering sensor. Such a computation may be performed by means of a series of functional blocks, which may for instance be provided on a board with an electronic microcontroller.
The air flow required to ensure complete combustion of the fuel gas is produced by a variable-speed fan which is controlled directly by the electronic microcontroller. An air flow metering sensor located in correspondence of the delivery side of the fan measures the amount of air which is actually delivered by the fan and correspondingly drives the microcontroller so as to compensate for any possible variation in the flow rate. Practically, a closed-loop control system acting on the delivery of the amount of air required for a correct combustion is in this way provided. The fuel gas and the combustion air mix in correspondence of the burner, where they are burnt in the presence of the catalyst, which must anyway be preliminarily heated in view of enabling the catalytic reaction to be triggered.
A control system of the above described type would only be able to operate correctly if the quantities measured by both the gas flow-rate sensor and the air flow-rate sensor are not affected by errors and the fuel gas composition is rigorously constant. However, such conditions never occur in the usual practice. Furthermore, it is a largely known fact that inert substances are quite frequently introduced in gas utility lines in view of ensuring an adequate gas supply pressure in peak demand periods. All such factors can easily combine to alterate the exact proportions of the air/gas mixture in the combustion chamber, with a resulting impossibility for a complete oxidation of the fuel gas, and therefore a complete combustion, to be achieved in an optimum manner.
A solution to this particular problem may be obtained by using an oxygen sensor which will then measure the amount of oxygen existing in the flue gases in order to make sure that the combustion is taking place in a really complete manner.
The above described systems ensure good functional results, ie. perform quite satisfactorily as far as both polluting emissions (practically cut to zero) and heat input adjustment capabilities (continuous-type adjustment) are concerned. However, all such systems are very complicated, expensive and poorly reliable, so that they have not been able to find a wide diffusion and cannot practically be proposed for use in any such application as, for instance, home cooking or heating appliances.
On-off type burner control systems are also known from the state of the art. Basically, such systems comprise an atmospheric burner associated to an injector delivering the fuel gas. The jet of gas discharged by said injector in correspondence of the inlet to said burner determines, by the so-called Venturi effect, the suction of an appropriate amount of combustion air from the surrounding environment. The fuel gas and the combustion air get then mixed inside the burner body and are then burnt. By sizing both the injector and the burner inlet in a suitable manner, it is possible to make sure that the amount of air taken in by the gas flow due to such Venturi effect is such as to ensure the complete combustion of the fuel gas. It will of course be appreciated that such a condition can only occur if a pre-determined, unchanging flow rate of the fuel gas is ensured at any moment, so that it is not possible to provide any desirable form of continuous adjustment of the heat output of the burner. As a consequence, a control of the heat output of the burner is in these cases performed through a sequence of alternating on and off periods of the burner. In particular, by appropriately varying the time ratio of the on periods to the off periods of the burner it is possible for the actually desired average burner heat output to be obtained. This type of control arrangement can be implemented by making use of a suitable device that is capable of cycling an electromagnetic valve or similar device in an ON-OFF manner in order to respectively open or shut the supply of gas to the burner in accordance with the average heat output required by the user.
The above solution is simpler, cheaper and more realiable than the afore described industrial-type arrangements. However, such a discontinuous type of heat-output control does not meet, for instance, the typical requirements set for optimum food cooking performance. Said alternating sequence of ignition and extinction periods of the burner is such as to unavoidably bring about corresponding temperature variations in the food contained in the pan placed upon the burner. For instance, this may translate into an intermittent boiling effect which is certainly not desired by the user.
In all control systems for catalytic burners, the catalyst (which is preferably deposited onto a metal wire gauze type of support) can be pre-heated in an indirect manner through an electric heating element welded onto the wire gauze and energized from an appropriate power supply source. As an alternative option, said catalyst can be pre-heated in a direct manner by letting an electric current flow directly through the metal wire gauze.
In the first cited case, the electric heating element can be energized directly from the mains, ie. applying the power supply voltage as such, but this gives rise to undesired problems in connection with the need for the heating element to be insulated electrically with respect to the metal wire gauze. In the case of direct heating, on the contrary, the need arises to provide a low-voltage, high-current (eg. 300 A) transformer for heating up the catalyst-carrying gauze, and the catalyst itself, in a very short time. A transformer of this kind is however undesirably complex, expensive and bulky, so that it simply cannot be considered for use in connection with household-type appliances.
It is therefore a main purpose of the present invention to provide a control arrangement for catalytic burners which is substantially simple, reliable and cost-effective, and is at the same time capable of enabling an optimum combustion to be obtained along with a precise adjustment of the heat output.
In particular, it is a purpose of the present invention to provide a control arrangement of the above cited kind, wherein the catalyst is heated up in a particularly simple and reliable manner.
According to the present invention these aims are reached in a control arrangement for catalytic burners comprising the features and characteristics as recited in the appended claims.
Characteristics and advantages of the present invention will be more clearly understood from the description which is given below by way of non-limiting example with reference to the accompanying drawings, in which:
Referring in particular to Figure 1, this is shown to illustrate how the control arrangement comprises mainly a burner 1, which may be a gas burner of the traditional type, provided with an inlet 2 adapted to receive fuel gas from an injector 3. Such an injector 3 is connected to a gas supply pipe 4 through an electromagnetic valve 5 or similar shut-off device. Both the inlet 2 of the burner and the injector 3 are arranged and sized so as to ensure that an adequate amount of air, such as to enable the gas to be burnt completely, is taken by suction, owing to said Venturi effect, into the burner 1 through the inlet 2 thereof.
Close to the burner 1 there is arranged a catalyst 10, which is preferably formed by a catalytic wire gauze arrangement.
An electronic control arrangement 6 comprises an output 7 by means of which it is adapted to drive the gas control valve 5 in an ON-OFF cycling manner, ie. by letting it switch selectively between a closed condition and an open one. For instance, the arrangement 6 may be formed by an electronic board comprising a Motorola 6805 microcontroller, with a first input 8 (of a potentiometric type, for instance), a second driver input 14 and a further output 9 of the digital type, as well as with a storage memory programmed according to a pre-determined operational cycle (a cooking cycle, for instance).
A control knob 11, or any similar device provided to adjust the heat output of the burner in the desired manner, may be associated to the input 8 of the electronic control arrangement 6.
Between the body of the burner 1 and the catalytic wire gauze 10 there are provided a pair of electrodes 12 that are driven by the output 9 of the electronic control arrangement 6 through an ignition device 13 that may be of the high-voltage type with associated transformer.
A temperature sensor 15, which may for instance consists even of a simple thermocouple, is adapted to measure the temperature of the catalytic wire gauze 10 and to drive the input 14 of the electronic control arrangement 6 accordingly, said electronic control arrangement 6 being programmed so as to cause the electromagnetic gas-supply valve 5 to close, through the output 7, when (in a first operational phase that will be described in a more detailed manner farther on) the temperature sensor 15 detects a temperature of the catalyst 10 exceeding by a certain value A (eg. 20°C) the temperature value T at which the catalytic reaction is triggered. Such a triggering temperature is pre-determined and depends on the characteristics of the catalyst 10, as well as on the type of fuel being used. For instance, such a triggering temperature T will have a value of approx. 300°C when LPG is used as a fuel and a platinum and palladium based alloy as a catalyst.
According to another feature of the present invention, the electronic control arrangement 6 is also programmed so as to cause, through the output 7, the electromagnetic gas-supply valve 5 to open, when (in the above cited first operational phase) the temperature sensor 15 detects a temperature of the catalyst 10 which lies by a pre-determined value B (eg., 5 to 10°C) below the aggregate value T+A, wherein such a temperature value B will in any case be lower than the temperature value A, for reasons that will be more clearly explained farther on).
Referring now also to Figures 2 and 3, these are shown to illustrate the operation or cycling times of the electromagnetic valve 5 and the ignition devices 12, 13, respectively.
An operational cycles begins at the instant t0, when the arrangement causes the gas-supply valve 5 to open. As a result, the gas delivered by the injector 3 flows into the body of the burner 1 while mixing with the air taken in by Venturi effect through the burner inlet 2. At the same time, or immediately thereafter, the output 9 of the control arrangement 6 energizes the ignition device 13 for a pre-set period of time Ta (shown in Fig. 3 and amounting for instance to 2 seconds), so that a series of sparks are generated between the electrodes 12 which ignite the gas/air mixture escaping from the burner 1. The flame which is produced in this manner is essentially a traditional-type, ie. not a catalytic one, and heats up the catalyst wire gauze 10 which initially may for instance be at room temperature.
Through the temperature sensor 15, the electronic control arrangement 6 is able to measure the temperature of the catalyst wire gauze 10 and, when such a temperature is detected to reach the afore cited value T+A, the output 7 causes the electromagnetic gas-supply valve 5 to close, with the resulting extinction of the flame, at an instant t1. In the described example, this initial heating-up phase t0-t1 may for instance have a duration situated anywhere between approx. 5 and 10 seconds.
When (after approx. 5 seconds from the instant t1) the temperature sensor 15 senses that the temperature of the catalytic wire gauze 10 has decreased by the afore cited value B from said value T+A it had previously reached, the output 7 of the electronic control arrangement 6 causes the electromagnetic gas-supply valve 5 to open again in correspondence of an instant t2. The period of time from t0 through to t2 represents the afore mentioned first operational phase.
It appears clearly from the above description that such a re-opening of the electromagnetic gas-supply valve at said instant t2 occurs when the temperature T+A-B of the catalyst 10 is equal to or greater than the temperature T triggering the catalytic reaction. As a consequence, the gas/air mixture escaping from the burner 1 will ignite automatically on the surface of the catalytic wire gauze 10 without any need arising for the ignition device 12, 13 to be operated to that purpose. The catalytic flame being in this manner generated will for instance reach an average temperature of approx. 600 to 700°C, ie. a value at which noxious emissions are minimized.
After this first operational phase t0-t2, which is controlled in accordance with the temperature of the catalytic wire gauze 10, the operation of the arrangement and the entire system goes on in a programmed manner so as to adjust the heat output of the burner according to the corresponding value that has been set in the electronic control arrangement via the knob 11. Such a control action is carried out through a sequence of alternating ignitions and extinctions of the catalytic flame, in a manner that is fully similar to the afore described one. In particular, during this second programmed operational phase t2-tn (Figure 2), the closing and opening times of the electromagnetic gas-supply valve 5 are determined by the electronic control arrangement 6 in such a manner that ignitions to the burner 1 occur in a spontaneous form since, under normal conditions, the temperature of the catalytic wire gauze does never decrease below the triggering value T.
As a result, it is possible for the heat output of the burner 1 to be effectively controlled through a sequence of ignitions and extinctions taking place at a quite high rate so as to correspondingly reduce temperature fluctuations and enable thermal energy to be transmitted in a substantially continuous pattern throughout the time.
Such an advantage is for instance particularly important in household and similar applications, and it is obtained without any need arising for a repeated operation of the ignition devices 12, 13. The entire control arrangement is therefore particularly simple, reliable and accurate. Furthermore, the polluting emissions of the catalytic burner 1 are reduced to a minimum, since the flame is in all cases produced from a gas/air mixture having an ideal stoichiometric ratio.
It will of course be appreciated that the described control arrangement may undergo a number of modifications as considered adequate, without departing from the scope of the present invention.
As illustrated in Figure 2, for example, after the instant t2 the ratio of the ignition to the extinction time can be varied so as to suit any possible requirement. In particular, it is preferable that a relatively long period t2-t3 be provided in view of allowing an adequate amount of thermal energy to reach to food to be cooked, after which the cooking cycle can then go on with an substantially constant sequence of alternating ignitions and extinctions until it comes to its end (tn).