EP2639506A1

EP2639506A1 - Apparatus for the combustion of liquid fuels and related method for modulating the power of such an apparatus

Info

Publication number: EP2639506A1
Application number: EP13159077.0A
Authority: EP
Inventors: Nicola Bianchini; Emil Calzolari; Ruben Cattaneo; Simona Marchiori
Original assignee: Riello SpA
Current assignee: Riello SpA
Priority date: 2012-03-13
Filing date: 2013-03-13
Publication date: 2013-09-18
Also published as: ITBO20120128A1

Abstract

"An apparatus (10) for the combustion of a liquid fuel. The apparatus (10) comprises the following components:
- a combustion head (30), which is fed with liquid fuel by a pump (11) and with oxidizing air by a ventilator (12); and
- an adjustment device (14, 16, 17, 19) for adjusting the flow rate of the liquid fuel and of the oxidizing air. The apparatus (10) is characterized in that the adjustment device (14, 16, 17, 19) comprises an inverter (14), a modulating valve (16) for modulating the liquid fuel flow rate delivered by the pump (11), an electric gearmotor (17) for adjusting the modulating valve (16), and a pressure transducer (19) arranged downstream of the pump (11). Furthermore, the ventilator (12), the electric gearmotor (17), and the pressure transducer (19) are electrically connected to the inverter (14)."

Description

The present invention relates to an apparatus for the combustion of liquid fuels and a related method for modulating the power of the apparatus.
As is well known that an apparatus for the combustion of liquid fuel (fuel oil) is called "modulating" if the thermal power thereof is continuously variable as a function of the fuel flow rate sent to the combustion head. In residential-type combustion apparatuses, such a flow rate is usually provided by an oil pump.
In order to obtain the desired modulation of the thermal power delivered by the combustion apparatus, there is a need for the flow rate of the oil pump to be continuously varied. The flow rate of the oil pump is also known to be proportional to the number of revolutions per minute (rpm), or equivalently to the frequency (revolutions per second, Hz) at which the shaft rotates about the pump, which normally is of the gear type or of the lobe type.
The power modulation of a combustion apparatus also depends on the quantity of oxidizing air, hence on the number of revolutions of the ventilator system, which is a component designed to draw oxidizing air and to send it towards the combustion head.
In order to obtain a stoichiometric combustion, i.e. with the proper fuel/comburent ratio and with the smallest possible quantity of emissions, the pump is to feed the combustion head with the proper liquid fuel flow rate value for each value of air flow rate emitted by the ventilator system.
Essentially two techniques currently exist for modulating the power in a combustion apparatus.

1) The use of an electric motor (servomotor) splined on the same rotating shaft as the liquid fuel pump (refer for example to EP-B1-0943056 ) so as to vary, when there is a need, the rotation frequency of the shaft and accordingly the flow rate of the pump; nevertheless, such a technique has certain significant drawbacks:
1. a. the cost of the electric motor;
2. b. the increased size volume due to the presence of the electric motor itself;
3. c. the increased consumption of electric power due to the motor;
4. d. the need to use a rotating shaft with larger physical-geometrical features than the original one of the pump alone.
2) The use of an electromechanical mechanism (servomechanism) (refer for example to W02004040192 and W02009044016 ) usually provided with a small piston or diaphragm, arranged in series between the pump and the combustion head, which allows the continuous variation of the liquid flow rate sent to the combustion head itself by means of electric control; nevertheless, such a technique has the following drawbacks:
- e. the cost of the electromechanical mechanism;
- f. the increased size volume due to the presence of the additional component;
- g. a decreased degree of reliability, and hence of the time value of average life, of the pumping assembly due to the criticality resulting from the use of an electromechanical component.

Therefore, the object of the present invention is to provide an apparatus for the combustion of liquid fuels, which is free from the above-described drawbacks while being easy and affordable to be implemented.
A further object of the present invention is to provide a related method for modulating the power of the combustion apparatus.
The desired modulation of a liquid fuel pump is simply and affordably obtained by means of this invention, so as to modulate the thermal power delivered by the whole combustion apparatus.
Therefore, in accordance with the present invention, a combustion apparatus and a related method for modulating the power of the apparatus are provided as claimed in the appended independent claims, and preferably, in any one of the claims directly or indirectly depending on the above-mentioned independent claims.
To better understand the present invention, a preferred embodiment thereof is now described, by way of a mere, non-limiting example and with reference to the accompanying drawings, in which:

figure 1 shows a diagram of the combustion apparatus according to the present invention;
figure 2 shows an enlargement of a detail of the apparatus in figure 1; and
figure 3 shows two curves referring to two Cartesian axes in which the motor shaft speed (in revolutions per second) is indicated along the abscissa points, while the related pressures referring to said two curves are indicated along the ordinate points.

In figure 1, reference numeral 10 indicates a combustion apparatus as a whole, provided according to the dictates of the present invention.
Incidentally, the mechanical connections in figure 1 are depicted with solid lines, while broken lines are used for the electrical ones.
The combustion apparatus 10 comprises the following elements:

a pump 11 (fed with liquid fuel by a feed line 111) which has the rotating mechanism (with gears or lobes) alone for drawing and delivering the pumped liquid, but not the adjustment mechanism, usually inside the pump itself and adapted to keep the flow rate of the liquid exiting from the pump, and hence the corresponding pressure, constant at the calibration value;
a ventilator 12, which impeller is splined on the same rotating shaft 13 of pump 11;
an inverter 14 usually present in all combustion apparatuses equipped with electric motors with variable revolutions; inverter 14 controls the change in the number of revolutions of the impeller of ventilator 12 by means of an electrical connection 15;
a modulating valve 16 (see figure 2), the structure of which will be better seen below, fed by the feed line 111 and which serves to adjust the flow rate of fuel oil towards pump 11 by means of a conduit 25; such a modulating valve 16 is modulated by an electric gearmotor 17, controlled in turn by the aforesaid inverter 14 by means of an electrical connection 18; a shaft 17A is located between the electric gearmotor 17 and the modulating valve 16;
a pressure transducer 19 arranged downstream of pump 11 and adapted to detect the instantaneous pressures of the liquid fuel exiting from the delivery aperture of pump 11; the pressure transducer 19 is also adapted to transfer data to inverter 14 by means of an electrical connection 20; and
a combustion head 30 hydraulically connected to pump 11 by means of a conduit 21 (in which the pressure transducer 19 is located) and to ventilator 12 by means of a conduit 22 for feeding the oxidizing air; in other words, the combustion head 30 is fed with liquid fuel pumped from pump 11 by means of conduit 21 and with the air sent by the ventilator 12 through conduit 22.

In the apparatus 10 object of the present invention, the usual adjustment mechanism (usually inside the pump itself and adapted to keep the flow rate of the liquid exiting from the pump, and hence the corresponding pressure, constant at the calibration value) is replaced by the modulating valve 16 illustrated in greater detail in figure 2.
Such a modulating valve 16 (figure 2) comprises a main body 16A, which is provided with a longitudinal through hole 16B having an axis (Y).
A portion 16B* of the wall of said longitudinal through hole 16B is threaded.
An end 16C of the main body 14A is provided with an annular element 16D provided with a central shaped opening 16E (also having an axis (Y)).
The main body 16A further has at least two lateral apertures 16F for purposes which will be explained below.
The longitudinal through hole 16B houses a plug 16G (also having an axis (Y)), which has a conical tip 16G*, which is complementary to a central shaped opening 16E.
A portion 16G** of the surface of plug 16G is threaded and, in use, couples with the aforesaid portion 16B* of the wall of the longitudinal through hole 16B.
Plug 16G is therefore screwed on the lateral surface of the cylindrical seat thereof so as to axially move according to a double-ended arrow (F1) (figure 2). The screwing/unscrewing thereof is allowed by the rotation imposed on said plug 16G by the shaft 17A of the mentioned electric gearmotor 17 (figure 1).
In use, the conical end 16G* is adapted, by means of a longitudinal movement given by the double-ended arrow (F1), to partialize the central shaped opening 16E and accordingly, the quantity of fuel exiting from the two lateral apertures 16F.
In other words, the liquid fuel from the feed line 111 enters plug 16 through the central shaped opening 16E and exits plug 16 from the two lateral apertures 16F to feed pump 11 once it has crossed conduit 25 (figure 1).
As said above, the pressure of the exiting liquid fuel, which varies according to the flow rate based on the characteristic pressure-flow rate curve of the pump (see below) is measured by the mentioned pressure transducer 19 according to a fixed time frequency divided by uniform time intervals lasting Δt seconds (usually, a fraction of a second).
A method will now be described, with the aid of figures 1, 2 and 3, for modulating the power of apparatus 10, which method, as mentioned above, is a further object of the present invention.
Let us suppose that a request for a variation of the fire thermal power corresponding to the liquid fuel flow rate value Q_0 is made at the time instant t_0.
To obtain the new power level requested, the liquid fuel flow rate is to be updated to a new value Q*, according to the heating power of the liquid used.
Inverter 14 controls, by means of the electrical connection 15, the ventilator 12 to vary the number of revolutions of the impeller from the value N_0 to a new suitable value N_1, which is calculated by means of pre-coded piloting instructions in the logic of the inverter 14 itself, so that the air flow rate goes from an initial value A_0 to a new value A_1, which is required for the combustion of the liquid fuel flow rate Q to be stoichiometric. Accordingly, pump 11, which is splined on the same rotating shaft 13 of ventilator 12, changes the flow rate value, exiting from conduit 21, of the fuel drawn by the feed line 111, from Q_0 to Q_1.
Based on the characteristic curve of the pump (CCP) (figure 3), the pressure of the liquid fuel along conduit 21 changes from the initial value p_0 (for example, 8 bar) to the new value p_1 (for example, 10 bar). Value p_1 is read, at time instant t_1 = t_0 + Δt, by the pressure transducer 19 and then sent, by means of the electrical connection 20, to inverter 14.
Generally, the new value Q_1 of the liquid flow rate, which corresponds to the pressure value p_1, is not equal to the fuel flow rate value Q* necessary to have a stoichiometric combustion, which corresponds to the flow rate value A_1 of the air provided by the ventilator at the number of revolutions N_1.
This depends on the fact that the characteristic variation curves (CCP) (figure 3) of the flow rate of pump 11 and of the flow rate of ventilator 12 might not have the same shape as the number of revolutions of the common rotating shaft (see axis of the abscissa points in the diagram in figure 3) varies; in mathematical terms, the two curves could have different derivatives (slopes in the graphs), with subsequent phase displacement of the comburent and fuel flow rates with respect to the stoichiometric needs.
In other words, there should be an outlet pressure of the liquid fuel of p*_0 (i.e. about 5 bar, not 8 bar, as the curve (CCP) would indicate) when pump 11 and ventilator 12 are at the number of revolutions N_0 in order to have a stoichiometric combustion.
Similarly, there should be an outlet pressure of the liquid fuel of p*_1 (i.e. about 7.5 bar, not 10 bar, as the curve (CCP) would indicate) when pump 11 and ventilator 12 are at the number of revolutions N_1 in order to have a stoichiometric combustion.
Such a phenomenon is shown in the example depicted in figure 3, where the characteristic curve (CCP) (outlet pressure of the liquid fuel versus number of revolutions of pump and ventilator) is depicted, by a single dark line, of a standard pump used in residential burners, and a corresponding curve (CCS) is depicted, by a dotted line, of the fuel pressure values which ensure a stoichiometric combustion.
In the example shown in figure 3, pump 11 is calibrated to ensure a stoichiometric pressure of 15 bar at 60 revolutions per second.
By decreasing the number of revolutions of pump 11 (and thus also of ventilator 12, both of which operate with the same number of revolutions), and therefore the thermal power delivered by the combustion head 30, it should be noted that pump 11 tends to have higher delivery pressure values, and therefore fuel flow rate values, than those required for a proper combustion.
Likewise, it should be noted that the progressive decrease in the number of revolutions involves an increase in the phase displacement of the effective delivery pressure with the stoichiometric one.
Therefore, if Q_1 differs from the requested flow rate value Q* necessary to have a stoichiometric combustion, inverter 14 is programmed to send the electric gearmotor 17, by means of the electrical connection 18, the command to rotate (in one direction if Q* > Q_1, in the opposite direction if Q* < Q_1) plug 16G so that such a plug 16G moves according to one of the directions given by the two-ended arrow (F1).
Thereby, the conical tip 16G* more or less closes the central shaped opening 16E thus correcting the flow rate value of the liquid fuel exiting from the two lateral apertures 16F.
Thereby, by regulating the exiting flow rate, the pressure of the liquid fuel exiting from the pump is also regulated.
If the new pressure value p**_1 read by the pressure transducer 19 at the time instant t_2 = t_1 + Δt = t_0 + 2 Δt is to be further corrected, the process is repeated until the pressure value coincides with the stoichiometric one (an acceptable preset deviation is allowed).
Experimental tests show that the optimal pressure value is achieved at most in a few seconds, hence the power variation is carried out by the combustion apparatus object of the invention in a very small time interval as compared to the time duration (usually a few hours) of an operating cycle at constant operating speed.
The advantages of the solutions proposed are substantially as follows:

1. use of components which results in simple technology that is already marketed;
2. very small geometrical dimensions of the modulating valve and of the pressure transducer, to the benefit of the overall containment of the physical dimensions of the whole assembly of a standard combustion apparatus for residential purposes;
3. the model may have a 24 V supply and accordingly the absorption of electric power is greatly reduced in size (watts absorbed) and operating time (the few seconds required to achieve the new power value); and
4. there is no need to have electromagnetic components, which usually have snap mechanisms having dynamic behaviors which are not easy to be managed - in the mathematical and mechanical sense - in an iterative process like that described in this application.

Claims

An apparatus (10) for the combustion of a liquid fuel; said apparatus comprising the following components:
- a combustion head (30), which is fed with liquid fuel by pumping means (11) and with oxidizing air by ventilating means (12); and

- adjustment means (14, 16, 17, 19) for adjusting the flow rate of the liquid fuel and of the oxidizing air;
said apparatus is characterised in that said adjustment means (14, 16, 17, 19) comprise an inverter (14), a modulating valve (16) for modulating the liquid fuel flow rate delivered by said pumping means (11), an electric gearmotor (17) for adjusting said modulating valve (16), and a pressure transducer (19), which is arranged downstream of said pumping means (11), said ventilating means (12), said electric gearmotor (17), and said pressure transducer (19) being electrically connected to said inverter (14).
An apparatus (10), according to Claim 1, characterised in that said modulating valve (16) comprises a main body (16A), which is provided with a longitudinal through hole (16B) having an axis (Y).
An apparatus (10), according to Claim 2, characterised in that a portion (16B*) of the wall of said longitudinal through hole (16B) is threaded.
An apparatus (10), according to Claim 2 or to Claim 3, characterised in that an end (16C) of said main body (16A) is provided with a central shaped opening (16E); said longitudinal through hole (16B) housing a plug (16G), which is at least partially threaded and presents a conical tip (16G*), which is complementary to a central shaped opening (16E) for the inlet of liquid fuel into said longitudinal through hole (16B).
An apparatus (10), according to any of the Claims 2, 3, 4, characterised in that said main body (16A) presents, furthermore, at least one lateral aperture (16F) for the outlet of the liquid fuel from said longitudinal through hole (16B).
An apparatus (10), according to Claim 4 or Claim 5, characterised in that the plug (16G) is at least partially screwed on a threaded portion (16B*) of the wall of said longitudinal through hole (16B), so as to move in an axial direction (F1); its screwing/unscrewing being actuated by the rotation that is imposed on said plug (16G) by a shaft (17A) of said electric gearmotor (17).
A method for modulating the power of an apparatus for the combustion of liquid fuels; said method comprising the following steps:
(f1) updating to a new value (Q_1) the liquid fuel flow rate towards a combustion head, if, in the time instant (t_0), there is a request for a variation of the fire thermal power corresponding to the liquid fuel flow rate (Q_0), so as to obtain a new power level requested;

(f2) causing ventilating means to reach a new suitable level (N_1), which is calculated by means of pre-coded controlling instructions, so that the air flow rate passes from an initial value (A_0) to a new value (A_1), which is necessary for the combustion of the liquid fuel flow rate (Q) to be stoichiometric;

(f3) changing from (Q_0) to (Q_1) the value of the liquid fuel flow rate pumped by pumping means, which are splined on the same rotating shaft as the ventilating means;

(f4) varying the pressure of the liquid fuel coming out of said pumping means, based on the characteristic curve of said pumping means themselves, from an initial value (p_0) to a new value (p_1);

(f5) reading the pressure value (p_1) in the time instant (t_1 = t_0 + Δt) and send this value to electronic control means;

(f6) checking whether the new value (Q_1) of the liquid fuel flow rate, which corresponds to the pressure value (P_1), is equal or not to the fuel flow rate value (Q*) that is necessary to have a stoichiometric combustion corresponding to the value (A_1) of the air flow rate delivered by said ventilating means at number of revolutions (N_1); and

(f7) ending the operations, if the result of the check is positive; or,

if the result is negative, changing, if necessary in a feed-back manner, the flow rate and the pressure of the liquid fuel fed to the combustion head, until substantially stoichiometric combustion conditions are reached for the power value requested.