Apparatus For Automatic Correction Of The Positioning Control Of A Burner

Abstract

Apparatus for automatically correcting the ratio between the volumetric flows of combustion air and of fuel of a burner comprising an opacimeter which delivers an electric signal proportional to the opacity of the fumes of combustion and a servo mechanism, for example an electric ram controlled by said signal, which automatically varies a dimension, for example the length, of one of the pivoted arms which automatically controls the degree of opening of the combustion air control valve. The apparatus equipped with a reflection opacimeter highly sensitive to fumes which are practically invisible, gives good combustion despite variations of density of the combustion air.

Description

The present invention relates to an apparatus for automatic correction, based on the opacity of the fumes, of the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, which ratio is maintained constant by a positioning control.

The technical field of the invention is that of the construction of burners for pulverulent, liquid or gaseous fuels and more particularly the control apparatus with which these burners are fitted.

It is known that it is very important for the ratio between the quantities of fuel and combustion air delivered to a burner to be properly controlled to prevent the production of fumes.

Prevention of environmental pollution means that constructors must give more attention to this question.

Burners with a very high heating capacity, which are fitted in certain furnaces or steam generating boilers for example, are generally provided with a much improved control sequence known as measuring control, including apparatus for measuring both the fuel flow and the combustion air flow which deliver control signals acting on relays with derived, proportional or integral action, which relays act in turn on servo-motors which control the flow of fuel and the flow of combustion air.

In such control sequences, it is possible to cause correction signals to act on the relays and it has already been proposed to use a signal provided by an opacimeter, which measures the opacity of the combustion fumes, as a correction signal. Such an opacimeter generally consists of a photo-electric receiver which picks up a light bundle which has passed through the combustion fumes.

A control has also been proposed in which the signal provided by an opacimeter acts on a relay which controls a servo-motor acting on the combustion air valve. In such a control, the flow of air is controlled directly as a function of the opacity of the fumes and independently of the fuel flow.

Such a system is very dangerous in practice, for if a fault occurs in the control sequence, the burner operates under very bad conditions.

In addition to measuring controls, there exists a large number of fire-boxes of medium size, the burners of which are fitted with what is known as a positioning control.

A positioning control means a control which has means for simultaneously varying the volumetric fuel and combustion air flows, without measuring these flows, and maintaining a constant ratio between them. Such apparatus have a pressure or temperature probe situated in the circuit of the hot gases, hot water or steam produced by the apparatus to which the burner is fitted, which probe acts on the operating rate of the burner.

They include an air-valve for controlling the combustion air flow, the extent of the opening of which is controlled automatically by motion transmission apparatus having cams and pivoted arms.

The shape of the cams and/or the lengths of the pivoted arms, which consist of rods and levers, are defined experimentally during commissioning trials, so that the ratio between the volumetric combustion air flow distributed by the air valve and the volumetric fuel flow remains constant for all operating rates and equal to the ratio which corresponds to the best quality of combustion. Such a positioning control is much less costly than a measuring control sequence, as it requires only one probe, one relay and mechanical motion transmissions. For this reason it is generally used in fire-boxes having a low or medium heating capacity.

As opposed to its great simplicity and low installation and upkeep costs, such a positioning control does not enable a very good combustion quality to be maintained constantly.

In fact this control does not take into account the differences in density in the combustion air due to temperature variations between night and day or between winter and summer, or possible variations in the temperature, viscosity, calorific power and pressure of the fuel, which result in modifications in the ratio between the mass flow rates of comburant air and fuel, it being possible for this ratio to depart from the limits fixed by regulations for preventing pollution of the atmosphere.

It is only necessary to remember the very great influence of variations in the temperature of the combustion air on the mass flow rate of combustion air to appreciate how the combustion quality can vary with such a positioning control apparatus distributing a determined volumetric flow of combustion air. In fact, for a same volumetric air-flow, the mass flow rate exhibits variations of 3.7 percent for a temperature variation of 10.degree.C; 7.3 percent for a temperature variation of 20.degree.C; 11 percent for a temperature variation of 30.degree.C; 14.6 percent for a temperature variation of 40.degree.C and 18.3 percent for a temperature variation of 50.degree.C.

Temperature variations of the order of 30.degree. between night and day are usual, which fact gives rise to the considerable advantage of the invention.

The object of the present invention is to automatically correct the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, which burner has an air-valve for controlling the combustion air flow, the extent of the opening of which is automatically controlled by a positioning control apparatus having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio.

The solution enabling this object to be achieved consists of providing the said burner with an opacimeter which delivers an electrical signal proportional to the opacity of the combustion fumes and a servo-mechanism, controlled by the said electrical signal, which automatically varies a dimension of at least one of the said arms.

The said servo-motor preferably consists of an electric ram of variable length, forming a portion of the length of one of the said arms.

An apparatus in accordance with the invention includes an apparatus for measuring the electrical signal delivered by the opacimeter, which apparatus has two adjustable thresholds arranged on either side of a median position and determines the extension of the ram when one of the thresholds is exceeded and the retraction of the ram when the other threshold is exceeded.

A cyclic circuit-breaker is inserted between the said measuring apparatus and the said ram to avoid oscillations of the ram.

Opacimeters are known which are used as apparatus for measuring the opacity of fumes and which consist of a light source and a photo-electric receiver which picks up the light bundle emitted by the said light source and reflected by the combustion fumes. Such apparatus deliver an electrical voltage proportional to the opacity of the fumes and they are very sensitive in the almost invisible range of fumes having a Bacharach index of less than 5. In accordance with a feature of the invention, such a reflection opacimeter is used, which offers the advantage of being able to act on the ratio between the volumetric flows of combustion air and fuel, while keeping the combustion fumes practically invisible, the Bacharach index of the latter remaining between 2 and 4 for example, which corresponds to very good combustion.

When the air-valve for controlling the combustion air flow is wide open or completely closed, action of the servo-mechanism, which could cause permanent distortion of the pivoted arms, must be prevented. This result is achieved by means of two circuit-breakers on which the air-valve acts when it reaches its wide open or completely closed positions respectively, which circuit-breakers are inserted in the electric control circuits of the said servo-mechanism.

The result of the invention is an apparatus for automatically correcting a positioning control so that the opacity of the fumes always remains below a threshold and is in constant compliance with combustion quality regulations.

One of the advantages of this apparatus is due to the fact that it acts as a correction apparatus. If a defect occurs in the opacimeter, the boiler continues to function with the non-corrected positioning control.

Another advantage of the apparatus in accordance with the invention results from the fact that it can be adapted easily to known controls and controls which have already been installed.

The following description refers to the attached drawing, which shows schematically and by way of a non-limiting example, an apparatus in accordance with the invention.

The single FIGURE shows a known positioning control consisting of:

1. a probe 1, which measures a value linked to the operation of the firebox, for example the steam pressure or the water temperature in the case of a boiler.

2. a regulator 2. This regulator is a relay which makes an analog conversion of the value measured by the probe into a modulated pressure in the case of a pneumatic control sequence, or a modulated voltage in the case of an electric or electronic control sequence.

3. a relay 3 enabling the operation of the burner to be controlled either manually or automatically.

4. a motorised valve 4 positioned in conduit 4a which carries fuel to the burner (not shown).

Valve 4 is a pneumatic valve, for example, opening of which is modulated by the modulated pressure delivered by regulator 2.

5. a servo-motor 5 which controls the pivoting of a cam 5a.

Servo-motor 5 is a pneumatic ram, for example, movements of which are modulated in parallel to the opening of valve 4, by the modulated pressure delivered by regulator 2.

6. a system of rods and levers 11 connecting cam 5a to a mobile air-valve 6 positioned in the combustion air circuit of the burner.

The lever closest to cam 5a has a roller 11a which bears against the outline of the cam.

Air valve 6 is a double butterfly valve, for example, positioned in the intake or delivery conduit of a fan 6a or an air-flap, or a sleeve sliding around air openings cut out of the side wall of the burner.

The above assembly constitutes a positioning control well known to the man of the art and no purpose will be served by describing it in more detail.

When regulator 2 determines an increase in the fuel flow, i.e. opening of valve 4, it simultaneously determines the pivoting of cam 5a in such a direction that air-valve 6 opens and the combustion air flow varies in the same direction as the fuel flow. Conversely, if regulator 2 determines a reduction in the fuel flow, the rotation of cam 5a causes an attendant reduction in the air flow by means of rod-system 11.

As a modification, cam 5a could be positioned in the control of valve 4, or the apparatus can have two cams, one in the control of air-valve 5 and the other in the control of valve 4.

In accordance with the invention, on the walls of fume conduit 12 is placed an opacimeter consisting of a light source 13a and a photo-electric receiver 13b which picks up a light bundle reflected by the fumes in the case of the figure.

Reference 7 indicates the opacimeter box containing the electronic circuitry and possibly the light source and the photo-electric cell connected to the fume conduit by optical fibres 7a.

In one of rods 11 connecting cam 5a to air-valve 6, is inserted an electric double-acting ram 10 enabling the length of rod 11 to be varied to lengthen or shorten it in accordance with the direction of rotation of the ram.

This electric ram could be replaced by any equivalent apparatus, such as a pneumatic or hydraulic ram, for example.

In the case of an electric ram, the amplified current supplied by opacimeter 7 acts on relays which control the operation of the ram in one direction or the other, depending upon the direction of variation of the current.

In normal operation the burner is supplied with a small excess of air which leads to practically invisible fumes. If the opacity increases, this signifies that the ratio between the quantities of air and fuel is decreasing. In this case, the ram acts on the length of rod 11 in the direction which increases the air flow. If, on the other hand, the opacity of the fumes decreases, this signifies that the air over fuel ratio is increasing, and if no correction were made, there would be a risk of reaching an excess of air which would be prejudicial to efficiency.

In this case, ram 10 acts on rod 11 in the opposite direction, i.e. in the direction which decreases the opening of air-valve 6.

In order that ram 10 should not be acted on continually, in the circuit connecting opacimeter 7 to ram 10, is inserted an apparatus with adjustable thresholds 8, e.g. an indexed milliammeter, so that while the current delivered by the opacimeter is between the two thresholds, it has no effect on the ram.

For example, a lower threshold is set corresponding to a Bacharach index equal to 2, and an upper threshold corresponding to a Bacharach index equal to 4. It is known that while the opacity of the fumes remains between these two thresholds, the fumes remain practically invisible and combustion is very good.

The milliammeter can obviously be replaced by threshold relays or any equivalent electronic comparison apparatus.

References 9a and 9b indicate a double-contact, cyclic circuit-breaker. This can be replaced by a programmer which delivers one or more ram control pulses at each rotation. Oscillations of the ram are avoided by means of this cyclic circuit-breaker.

To prevent ram 10 from acting on the rods when air-valve 6 is wide open or completely closed, extreme position circuit-breakers 14, closed by the air-valve, are employed, which eliminate the action of the ram when they are closed.

Of course, without departing from the scope of the invention, various equivalent modifications could be made by a man skilled in the art to the apparatus described above.

Claims

We claim:

1. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control device having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which apparatus includes a reflection opacimeter comprising a light source and a photoelectric receiver sensing a light bundle emitted by the light source and reflected by the combustion fumes, which opacimeter delivers an electric signal proportional to the opacity of the combustion fumes, and which apparatus includes a servomechanism for automatically varying a dimension of at least one of the said arms as a function of the said electric signal.

2. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control device having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which apparatus includes a reflection opacimeter comprising a light source and a photoelectric receiver sensing a light bundle emitted by the light source and reflected by the combustion fumes, which opacimeter delivers an electric signal proportional to the opacity of the combustion fumes, and which apparatus includes an electric ram of variable length having two operating directions, which ram forms a portion of the length of one of the said arms, and is operated by said electric signal.

3. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control device having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which apparatus includes an opacimeter which delivers an electric signal proportional to the opacity of the combustion fumes and a servomechanism for automatically lengthening or shortening of at least one of the said arms as a function of the said electric signal, which servomechanism comprises an electric ram of variable length having two operating directions, forming a portion of the length of one of the said arms and an apparatus for measuring the said electric signal delivered by the opacimeter, which apparatus has two adjustable thresholds arranged on either side of a median position and determines the lengthening of the said ram when one of the thresholds is exceeded and the shortening of the said ram when the other threshold is exceeded.

4. Apparatus in accordance with claim 3, in which a cyclic circuit breaker is inserted between the said measuring apparatus and the said ram.

5. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control apparatus having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which apparatus includes an opacimeter which delivers an electric signal proportional to the opacity of the combustion fumes, and a servomechanism for automatically varying a dimension of at least one of the said arms as a function of the said electrical signal, and in which the said air-valve for regulating the flow of combustion air has two circuit-breakers on which the said air-valve acts when it reaches its wide open or completely closed positions, respectively, which circuit-breakers are inserted in the electric control circuitry of the said servo-mechanism.

6. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control device having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which apparatus includes a reflection opacimeter comprising a light source and a photoelectric receiver sensing a light bundle emitted by the light source and reflected by the combustion fumes, which opacimeter delivers an electric signal proportional to the opacity of the combustion fumes, and which apparatus includes an electric ram of variable length having two operating directions, which ram forms a portion of the length of one of said arms and is operated by said electric signal, which apparatus includes an apparatus for measuring the said electric signal delivered by the opacimeter which measuring apparatus has two adjustable thresholds arranged on either side of a median position, and determines the extension of the said ram when one of the thresholds is exceeded, and the shortening of the said ram when the other threshold is exceeded.

7. Apparatus in accordance with claim 6, in which a cyclic circuit breaker is inserted between the said measuring apparatus and the said ram.

8. Apparatus for automatically correcting the ratio between the volumetric combustion air flow and the volumetric fuel flow of a burner, including an air-valve for regulating the combustion air flow, the extent of the opening of which valve is controlled automatically by a positioning control device having cams and pivoted arms which simultaneously vary the volumetric flows of fuel and combustion air in a constant ratio, which source and a photoelectric receiver sensing a light bundle emitted by the light source and reflected by the combustion fumes, which opacimeter delivers an electric signal proportional to the opacity of the combustion fumes, and which apparatus includes an electric ram of variable length having two operating directions, which ram forms a portion of the length of one of said arms, and is operated by said electric signal, which apparatus includes an apparatus for measuring the said electric signal delivered by the opacimeter, which measuring apparatus has two adjustable thresholds arranged on either side of a median position, and determines the extension of the said ram when one of the thresholds is exceeded, and the shortening of the said ram when the other threshold is exceeded, in which apparatus the said air-valve for regulating the flow of combustion air has two circuit-breakers on which the said air-valve acts when it reaches its wide open or completely closed positions respectively, which circuit-breakers are inserted in the electric control circuitry of the said ram.