The object of the present invention is a method for controlling a boiler provided with an atmospheric burner according to the preamble of the main claim. Another object of the invention is a device for accomplishing the aforesaid method.

As is known, a common boiler of the type mentioned comprises a valve for controlling the gas sent to a burner, means for detecting the flame in the latter, control means of functional components of the boiler such as actuators present in the boiler, for example, a fan driven by its own electric motor (commonly used in airtight combustion chamber boilers), a circulator, a 3-way diverter valve, temperature probes, etc.

On the "low-end" boilers normally present on the market, it is foreseen that the presence of the necessary supply of combustion air (and thus of optimum and non-polluting combustion) is ensured by components of the mechanical type such as for example a pressure switch placed on the combustion air intake side or on the flue gas exhaust side. This involves, as well as a considerable cost, also limitations such as the possibility of combustion outside the normal parameters in the case of, for example, excessive variation of the mains gas pressure (which even if not provided for in the regulations can still cause a combustion that pollutes and is potentially dangerous to man) or in the case of modification of the mains gas quality or in the presence of particular obstructions to the exhausts or of manufacturing or design or other tolerances.

Furthermore, the pressures of the gas exiting the feed valve can undergo variations also due to tampering or negligence in the calibration of the valve itself by service staff; therefore the operating parameters determined in the design phase of the boiler may not be such as to ensure, during use of the boiler and over time, correct combustion (non-polluting) as previously cited.

Finally, also the presence of water in the heating system and/or in the primary hydraulic circuit, in order to prevent the boiler from operating in the absence of water with the consequent danger of damage to the heat-exchanging parts or even of damage to the boiler itself or to the immediate environment, is verified by mechanical components (single contact) that, in this case too, as well as constituting a significant cost, may be subject to malfunction and therefore to loss of their assigned "safety" functionality.

It is also known that in boilers of the cited type there is a correlation between the flame signal level (or rather its impedance detected in kohm or a current or voltage value proportional to the flame signal) and the combustion quality, i.e. the level of CO, CO ₂ and similar gases, said correlation being defined by a plurality of curves corresponding to the various working capacities on which it is possible to define the relation between the flame and the combustion quality, and to identify the range of values corresponding to a combustion air/gas ratio necessary for a correct operation of the boiler (i.e. with a combustion within such parameters so as not to be polluting).

The purpose of the present invention is that of offering a method and a device for controlling a boiler of the type cited above such that it operates within non-polluting combustion levels.

In particular, the purpose of the invention is that of eliminating the use of mechanical components for controlling the draft of the boiler and ensuring the cleaning of the combustion even in the abnormal working conditions listed above.

Another purpose of the invention is that of being able to obtain the self- adaptability of the control to the length and to the type of the exhausts and/or an increase in efficiency, whilst respecting combustion cleanliness without the aid of further sensors.

Yet another purpose of the invention, in order to optimize the control of the aforesaid boiler, is that of eliminating the mechanical components for controlling the water pressure of the system and controlling the presence of water and its circulation in a dynamic way so as to ensure a safety operation.

These and other purposes that will be obvious to those skilled in the art are achieved by a method and by a device according to the combined claims.

For a better understanding of the present invention, the following drawings, provided merely by way of example but not of limitation, are attached, wherein: figure 1 shows an example graph of possible working curves, corresponding to various working capacities, of a boiler as a function of the combustion (defined by a value Lambda) and of the flame impedance;

figure 2 shows a flow chart of the method according to the invention; figures 3A, 3B and 3C show graphs representing the detected voltage as a function of the time across the motor of the fan, in the moments following its deactivation, of a boiler with an airtight combustion chamber starting from previous conditions of high, low and null rotation speed, respectively;

figure 4 shows a block diagram of a device according to the invention.

With reference to the cited figures, a control of the draft and therefore of the combustion of the boiler will be described in relation to the figures 1 , 2 and 4. As has been known for a long time, in boilers it is common to control, by means of an electrode immersed in the flame and of a defined electronic circuitry that powers it and measures the flame level, a signal coming from the flame itself in order to verify the "quality" of the combustion and thus whether it takes place without generating pollution and within the limits of the specifications.

With the monitoring of the flame signal as feedback it is possible to control the correct progress of the combustion. The flame signal, however, is not easily usable in itself for the purpose as it is influenced by utilization tolerances, by the burner, by the burnt power; furthermore, even for the same application model (same boiler, for example), the variance of the parameter (here, too, due to manufacturing tolerances, types of installation, etc..) is such that the simple setting of an absolute operating level is not sufficient, this is to say, with reference to figure 1 , to consider for example as "out-of-combustion" the detecting a flame level equal to value B' when the correct value in absolute terms is A. Proof of this lies in the fact that the flame as combustion feedback is not correctly used in boilers or atmospheric burner. For this purpose a correct combustion test to be carried out with a predefined timing or at the occurrence of particular working conditions in the boiler has thus been prearranged. The test is based on the flame- combustion correlation, that is obtained anyhow through a component 10, memorized in a suitable memory 12 of means for controlling 13 the operation of the burner 14 (comprising common electrical and/or electronic constituents and preferably a microprocessor and thus defining, with the memory, a programmable control system) of the boiler and that intervenes on a valve 15 for feeding gas to the burner 14.

As is known, this correlation defines a curve that links, for a given working point of the boiler, the values of the flame signal to the varying of lambda (combustion quality index) according to the example of figure 1.

When the typical working curve of a specific application (or boiler), determined in the design and realization phase of the same, is defined, during use of the boiler, at predetermined time intervals or when particular working conditions (as indicated below) are detected, the correct placement of the above mentioned working point occurs by letting the working point itself run along the relative curve. With reference to the particular working conditions cited above, the detecting, under condition of stable working capacity, a variation relating to the flame signal, that shifts for example (ref. Fig. 1) from a starting point A to a different point B', may be deemed a cause for activating the test. This variation is in itself indicative, but not necessarily sufficient for determining a variation of the combustion condition let alone the entity. Another abnormal condition that may require the activation of the combustion test is the detecting of an amplitude of the oscillation of the flame signal (normally present) at levels much higher than is considered normal.

The working point is moved on a given curve by reducing the combustion air quantity sent to the burner; this, for example, by deactivating the fan or reducing the fan speed (for example by acting on a common control system for induction motors 230VAC, for example through phase partialization, acting on the motor of the fan).

All of this while maintaining constant the flow rate of the gas exiting the valve 15 and directed to the burner 14.

Alternatively, it is possible to obtain an analogue result, by modifying the gas quantity directed to the burner 14 (for example by increasing the exit pressure of the gas) by acting on the appropriate control valve 15 maintaining constant the air supply.

In this way, the working point shifts (to the left in the graph) following the curve on which it is placed. The result may be (with reference also to figure 2): a) the starting working point is correct (for example around A) (i.e. it is on the correct working curves for the boiler under control with such air and gas flow rate conditions so as to have an optimum combustion) and in that case the flame signal will drop (considering it to be expressed in value of impedance) by a predefined value until it touches, as maximum possible variation, the lowest point of the curve (X) to then rise again. If the difference in impedance rf _A -rfcu RENT (where rfcuRRENT is the impedance of the instantaneous flame measured at the time tcuRRENT during the test and rf _A is the average flame value detected before the start of the combustion test) reaches at least one predetermined value (it can be reached even before arriving at the lowest point X), the test is considered positive, the fan is restarted and the application continues its normal operation.

b) If the working point is shifted from (A) to an area of bad combustion (B or B'), by varying the ratio between the combustion air and the gas the flame signal will drop less than what is predefined. This results in a negative outcome of the combustion test. This outcome leads to a corrective action in terms of a reduction of the exit gas flow rate of the valve with the aim of returning the application to working in a point (C) of correct combustion in which the execution of the following combustion test will have a positive outcome.

Preferably (non-binding condition) a maximum range of gas exit pressure corrections is defined, after which exhaustion a further combustion test with negative outcome causes a safety shutdown due to bad combustion. According to the method, it is possible (non-binding condition) that the startup of the boiler is re- attempted and if the condition is repeated for "n" attempts a block shutdown follows (the status can be restored by manual reset).

If the conditions that have determined the bad combustion do not apply, with the same method and after combustion tests with positive outcome, the exit pressure can be more or less gradually returned to an intermediate value or even to the initial value.

Hence, one of the advantages of the system is that it is able to work (and thus to ensure comfort to the user) with clean combustion, in the presence of obstructions to the passage of air (normally possible in installation such as for example ice on the air ducts) greater than in traditional systems, simply by working at reduced capacity.

The realized test is configured as a pass-no pass type test according to the logic given in figure 2 given below.

In this figure, that refers to the realized test through reduction of the air supply, 20 defines the beginning of the procedure according to the method indicated above, 21 indicates the initial measurement of the flame value and 22 the action suitable for modifying the ratio between combustion air and gas through deactivation of the fan or the reduction of its speed (or alternatively the variation of the flow rate or of the pressure of the gas to the burner). In block 23 the instantaneous flame value is measured and subsequently it is verified whether the difference in impedance is greater or less than a set value (block 24). If the answer is in the positive, the fan speed is increased again or the fan is reactivated and, if appropriate, the gas flow rate to the burner is increased (block 25) or it is maintained unchanged if it corresponds to a maximum value normally predetermined that defines the maximum capacity. If the answer is negative, in block 26 the difference in impedance is evaluated again and if this evaluation has a negative outcome, in block 27 the gas flow rate is reduced.

In block 28 the reached value of gas flow rate reduction is evaluated, if it is less than the predefined maximum value of reduction, the procedure is terminated with block 30 or the burner is shut down (block 29).

As a further advantage, thanks to operational modes described above, unlike the solutions currently in use in which it is necessary to use additive elements to the basic "mechanical" configuration for the adaptation of the combustion to the different typologies and lengths of the gas exhausts in the environment, or for the recovery (increase) of efficiency of the application wherever allowed by the latter, with the invention (test for correct combustion) it is possible to adapt the fan speed to the length and section of the exhausts or to reduce, wherever possible, the fan speed thereby increasing the combustion efficiency of the boiler. This, together with the control of the fan speed (realized as will be described, for example through phase partialization for 230 VAC fans) and allowing to determine with sufficient approximation the point of correct combustion.

This functionality is currently fulfilled in the systems in use:

- manually, by adding diaphragms (restrictions to the passage of air through increasingly shorter exhausts), or

- automatically by inserting into the boiler an air flow or pressure sensor and adapting the fan speed based on the signal relating to the detected air flow.

According to the invention, the procedure is as follows:

- the above mentioned test is carried out starting from a reduced working speed (lower than the maximum) - the test result is used for confirming or varying the working speed of the fan; in particular:

- if the test detects a correct combustion and within a predefined range, the current fan speed is confirmed for a given working capacity (in that case the system is working with the correct air flow rate);

- if the test detects a poor combustion (C0 ₂ lower than a predetermined value) the maximum working speed is reduced and is used as reference for the following combustion test; and subsequently

- if the test detects non-correct combustion it is possible to proceed according to one of the following possibilities:

- if the present working speed is lower than the maximum one, the working speed is increased; or

- if the present working speed is already the maximum working speed, the procedure according to point b) cited above is followed (in relation to the analysis of figure 1), reducing the gas flow rate and, if appropriate, shutting down the boiler if after "n" adjustment attempts aimed at obtaining a correct combustion the desired value of the latter is not reached.

This option can be used together with the previous one or may not necessarily have to be used for controlling the operation of the boiler.

In order to detect abnormal conditions from the outset, the combustion test described above can be associated (even if not necessarily) with a detection circuit, described above, of the actual activation of the draft component and thus of the fan through measurement of the current or the "alternating" function of the motor itself. For this purpose a circuitry is foreseen suitable for detecting the alternate current signal generated by the motor when turned off and a control algorithm that foresees:

- the activation of the fan

- its turning-off (after a predefined time), and

- the measurement of the current or of the alternate voltage generated in the slowing-down phase.

The developed algorithm allows to obtain information relating to the fact whether the fan is working (rotating) whether it is connected to the network, and a qualitative indication of the rotation speed. With reference to the figures 3A, B and C, they show the detected behaviour of the motor of a fan typically used on gas-fired boilers. During the trial time, the power supply of the fan is shut down (after having previously been started for an order time of 0.5 - 10s). The figures concerned illustrate the course of the voltage across the fan generated by the alternating effect of its motor following its turning-off. The number, the amplitude and the frequency of the voltage generated (detected by the control means 13 and depending on the type and model of the fan) indicate the previous rotation condition of the fan itself.

If absence of rotation is detected (figure 3C), a safety action is performed (for example safety shutdown and restart if the application was already on or starting with reduced capacity or at a failed start if the application was in stand-by mode - burner switched off).

In order to obtain complete control of the boiler, and also reducing its costs, in an apparatus that operates with pressurized water contained in a heat exchanger and heated by a burner, the following methodology is foreseen.

A circulation test is foreseen devised as follows:

- the burner 4 is activated and is operated at a predefined capacity Qn for a predefined time Tn. Capacity and time are defined in the design phase and depend on the weight, on the material of the exchanger itself and on its water content. Time and capacity must be dimensioned such that at least no damage is caused to the exchanger in the case of absence of water and/or circulation.

They must furthermore be dimensioned such as to generate a predefined raising of the temperature of the water contained inside the exchanger subsequently used (as described below) for determining the actual presence of water and active circulation.

During this time the calories stored in the burner are calculated (obtained by the integral of the burner capacity) and the outlet temperature is monitored.

If a temperature rise is not detected with the mutual conductance raised (determined in the phase of definition of the parameters relating to the function in question and dependent on the type of exchanger, temperature probe, etc.) and greater than or equal to a certain value Dtl, expressed in °K/s, the burner 14 is turned off as the absence of water in the exchanger is estimated therefore the temperature is immediately transferred to the probe for thermal conduction of the metal constituting the exchanger and does not attenuate by the presence of water inside it.

In the opposite case, the average outlet temperature is memorized. The circulator or pump (not shown in the figures) is then activated. If the circulator is operating and there is water in the exchanger an instantaneous temperature rise determined by the quantity of heat stored in the exchanger and dependent on the weight, on the material of the same and on its water content should be detected. If the outcome of the test is positive (Dt within a range defined in the design phase) the operation of the boiler to the turned on burner (normal operation) may proceed. In the opposite case the burner is turned off and, if appropriate, one or more retrial phases are carried out.

The dynamic type test obtained supplies information on water/active circulation presence and allows eliminating the absolute pressure switch or the circulation flowmeter normally present in boilers.

A particular embodiment of the invention has been described. Yet others are however possible while remaining within the scope of the combined claims.