LT5972B - Method for control of air supply to solid fuel combustion chamber - Google Patents

Abstract

Method for control air supply to solid fuel combustion chamber provides measurements of smoke temperature, chimney flue wall temperature and environmental temperature of chimney. New is the fact that these measurements treatment of the present invention follows the signal, which is generally by functional connection and in partial cases by linear dependence is associated with a real supply air flow to combustion chamber. This according to the results of measurements formed signal used as a feedback signal in the regulation of the air supply to the combustion chamber.

Description

The invention relates to heat engineering, more particularly to heating with solid fuels, e.g. firewood.

The proposed method can be used to control the combustion process in heating units using an intermediate heat carrier, e.g. water, or in direct heating installations, for example. stoves, fireplaces. This method is best suited for batch charging units where the full charge of the fuel burns and the combustion air is supplied to the bottom of the combustion chamber. The combustion intensity of these units can be adjusted by varying the draft or by changing the flow of air to the combustion chamber.

A known method of regulating the supply of air to the combustion chamber, with the aid of a dedicated lever, maintains an approximately constant supply air flow rate (see GBI162304, 1969). , as it does not estimate the actual burning intensity.

A known method of regulating the supply of air to the combustion chamber by a predetermined program is dependent on the burning time (see US20090260553, 2009). This method is of limited accuracy as it does not estimate the actual airflow to the combustion chamber and does not estimate the actual combustion intensity.

The closest embodiment of the invention is a method for controlling the supply of air to a solid fuel combustion chamber by measuring the temperature of the flue gas leaving the combustion chamber, measuring the ambient temperature of the combustion chamber and adjusting the flow to the combustion chamber depending on these temperatures (see AU779808, 2005). This method estimates the combustion intensity based on the smoke temperature, but does not estimate the actual supply air flow, which makes it difficult to achieve high control accuracy. Without estimating the actual supply air flow, it is difficult to estimate the heating power solely on the basis of the smoke temperature, which limits the versatility of the known method as it is difficult to apply in variable-power heating units.

The object of the invention is to increase the accuracy and versatility of regulation.

To solve this problem, the method of regulating the air supply to the solid fuel combustion chamber by measuring the temperature of the smoke leaving the combustion chamber, measuring the ambient temperature of the combustion chamber, regulating the air flow to the combustion chamber, additionally measuring the flue wall temperature. temperature and ambient air temperature, calculates the second temperature difference between the flue temperature and the flue wall temperature and regulates the flow of air to the combustion chamber depending on the ratio of the first and second temperature differences.

The ability to solve the problem in this way is demonstrated by the fact that the calculated temperature difference ratio is functionally related to the actual air flow to the combustion chamber (the proof of which is given below). Adjusting the air supply to this ratio achieves greater control accuracy by estimating the actual supply air flow, and by selecting the target value for this ratio, the heating power can be varied and thereby increase the versatility of the control mode, enabling it to be applied to variable power heating units.

As the flue passes through the flue, heat is transferred from the flue to the flue wall and from the flue wall to the ambient air. These thermal processes are illustrated in Figs. Scheme 1.

The heat balance per unit area of the flue wall is described by the equation:

(td ts) hds ~ (t d ta) hsa>

(1) where: t _d , t _Sl t _a is the smoke, flue wall and ambient air temperatures, respectively; h _ds - coefficient of heat transfer from the smoke to the flue wall; h _sa is the coefficient of heat transfer from the flue wall to the environment.

Equation (1) expresses the temperature ratio S above:

(2)

The coefficient of heat transfer from the smoke to the flue wall hds can be calculated from the well-known heat transfer patterns of gases or liquids flowing through a pipe (see, e.g., βρβίίτ Φ., Βηοκ y. 1983 - 512c):

h _ds = 0.023 Re ^{0 8} Pr ^{0 33} , where: k is the heat transfer coefficient of the gas (smoke); D is the inside diameter of the pipe (flue); Re - Reynolds criterion;

Pr - Prandtl criterion.

The Reynolds criterion is expressed as:

PdDVd

Here: pd - density of gas (smoke);

v _d - velocity of the gas (smoke) in the pipe;

pd-gas (smoke) dynamic viscosity coefficient.

Since the Prandtl criterion for gas is approximately constant and the Reynolds criterion is directly proportional to the velocity of the gas (smoke) in the pipe, equations (3) and (4) suggest that the coefficient of heat transfer from the smoke to the flue wall is _ds and velocity v _d is a defined functional relation, ie- h _ds = f {v _d y

Heat transfer from the wall of the flue into the air _sa coefficient h is determined by the well known heat transfer in natural convection patterns depending on the flue position (vertical, sloping, horizontal). The specific value of this factor may take into account the specific temperatures of the flue wall and ambient air (both of which are measured in the proposed method).

From the equation (2) and the above dependencies we get that there is a defined functional relation between the temperature difference S and the velocity v _{d of the} flue gas, which is calculated according to the proposed method, ie S = f (v _d ). It is obvious that the smoke velocity v _d is directly proportional to the airflow rate L to the combustion chamber. Thus, in the proposed method, the calculated temperature difference ratio S is functionally related to the airflow rate L to the combustion chamber, i.e. S = f (L).

By applying a method to a particular heating appliance, this functional dependence can be replaced by a linear proportional relationship. This possibility is illustrated in Fig. 2, where the characteristic relationship between the calculated temperature difference ratio S and the air flow rate L to the combustion chamber is shown, where the difference between the smoke and ambient air temperatures is constant. This functional dependence, as can be seen from the graph of Fig. 2, is close to linear and may be reversed in specific cases, especially at a narrower adjustment range.

Since the calculated temperature difference ratio in the proposed way is generally functionally related and in some cases linearly related to the actual air flow to the combustion chamber, by setting the target value of this ratio and entering this target value into the automatic control loop, the flow of the supply air is, to a certain accuracy, equal to the task.

It is known that changing the air flow to the combustion chamber changes the burning intensity of the solid fuel. Thus, by varying the setpoint of the temperature difference ratio, the power of the heating unit can be adjusted within certain limits.

By adjusting the set value of this ratio over time in the combustion cycle, the supply air flow rate can be better adapted to the combustion phase of the fuel. The characteristic graph of the thermal power P during the combustion cycle τ of a batch combustion unit is shown in Fig.3. After ignition, the gaseous fraction of fuel (eg wood) is intensively released and burned. A higher air flow rate is required during this combustion phase. Subsequently, the combustion of the solid fraction (carbon) is less intense and requires less air. By adjusting the supply air flow to the combustion phase, more fuel-efficient and less polluting fuel combustion is achieved.

Fig. 1 shows a diagram of thermal processes in the flue segment. In this scheme: 2 - flue; t _s , t _a - smoke, flue wall and ambient air temperatures, respectively: h _ds , h _sa - heat transfer coefficients from smoke to flue wall and from flue wall to environment, respectively: v _d - smoke flow rate.

Fig. 2 is a graph showing the characteristic functional relationship between the temperature difference ratio 5 calculated according to the proposed method and the airflow L to the combustion chamber.

Fig. 3 shows a graph of the thermal power P characteristic of batch charging solid fuel combustion plants at time τ of the combustion cycle.

FIG. 4 is a block diagram of an example of a device implementing the proposed method. The possibility of implementing the proposed method by known technical means is shown in Figs. Device Example 4. the unit consists of a solid fuel combustion chamber 1, flue 2, chimney 3, air control valve or valve 4, flue temperature sensor 5, flue wall temperature sensor 6, flue ambient air temperature sensor 7, first subtraction unit 8, second subtraction unit node 9, dividing node 10, request signal generating node 11, regulator 12, execution mechanism 13.

By implementing the proposed method, the temperature sensors 5, 6 and 7 measure the temperatures of the smoke, flue wall and ambient air respectively and receive signals proportional to the respective temperatures at the outputs of these sensors. At the first subtraction unit 8, the output signal of the temperature sensor 6 proportional to the temperature of the flue wall subtracts the signal of the temperature sensor 7 proportional to the ambient air temperature, and at the output of this unit receives a signal proportional to said temperature difference. In the second subtraction unit 9, the output signal of the temperature sensor 5 proportional to the smoke temperature subtracts the output signal of the temperature sensor 6 proportional to the temperature of the flue wall, and receives an output proportional to said temperature difference at the output of this unit. Divides node 10 by dividing the output signal of the first subtraction node 8 from the output of the second subtraction node 9 and receiving a signal proportional to the temperature difference ratio S mentioned in this method description at the output of this node, generating a signal proportional to the target value of this ratio. This unit may generate either a freely selectable signal, or a signal dependent on the desired heating power, or a time-varying signal on the combustion cycle, or an on-demand function dependent on the desired heating power.

The output signal of the task node 11 is fed to the request input of the controller 12, the output signal of the divider node 10 is proportional to the temperature difference calculated from the measurement results. The controller 12 may be a well-known proportional integral (PI), proportional integral differential (PID), or other suitable controller. The output signal of the regulator 12 through the actuator 13 changes the position of the valve or valve 4 and thereby regulates the flow of air to the combustion chamber. The air damper or valve 4 and actuator 13 may be of any known suitable mutually compatible design, such as a shutter-type damper with an electromechanical actuator. The first subtraction unit 8 and the second subtraction unit 9, the partition unit 10, the task unit 11, the controller 12 may be implemented as a single programmable controller 14. The use of a programmable controller increases the versatility of the device, facilitates its tuning and enables a functional or simplified linear relationship between calculated temperature difference S and air flow L to the combustion chamber

The device described does not limit the possibility of implementing the proposed method by other suitable means, but merely illustrates one embodiment.

Claims (5)

DEFINITION OF INVENTION 1. A method of controlling the supply of air to a solid fuel combustion chamber, which measures the temperature of the flue gas leaving the combustion chamber, measures the ambient temperature of the combustion chamber, regulates the flow of air to the combustion chamber by measuring the temperature of the flue wall. flue wall temperature and ambient air temperature, calculates the second temperature difference between the flue temperature and the flue wall temperature and adjusts the flow of air to the combustion chamber depending on the ratio of the first and second temperature differences. 2. A method according to claim 1, wherein the relationship between the air flow to the combustion chamber and the temperature difference calculated according to claim 1 is linear. 3. The method of claim 1, further comprising forming a target value of the temperature difference ratio referred to in claim 1 and adjusting the flow of air to the combustion chamber so that the temperature difference ratio calculated from the measurement results is equal to the target value. 4. A method according to claim 3, characterized in that the set ratio value is formed depending on the required power of the combustion chamber. 5. The method of claim 3 or claim 4, further comprising forming a time-varying target ratio value for the combustion cycle.