LU84353A1 - METHOD FOR DOSING PNEUMATICALLY TRANSPORTED BULK GOODS - Google Patents

Description

»► *. . - 2 - c '· "Method for dosing pneumatically transported bulk goods"

The invention relates to a method for dosing in a. Conveying line using a constant quantity of conveying gas per unit of time of pneumatically transported bulk material by measuring the pressure difference over a given line section, comparing this actual value with a pressure difference setpoint and controlling the quantity of bulk material fed in with the comparison result as a manipulated variable in a control loop.

This method proposed on this side is based on the knowledge that, with a constant conveying gas throughput and thus a constant speed, the pressure loss between two points of the conveying line, based on a specific bulk material, depends only on the loading {p) of the conveying gas stream with the bulk material. In order to dose the quantity of bulk material, i.e. to maintain a certain, predetermined bulk material throughput per unit of time, in other words to maintain a constant load (yü), the difference between the pressure difference setpoint and the measured pressure difference actual value can therefore be used as a manipulated variable for controlling the bulk material feeder , e.g. used for the speed of a rotary valve and thus a control loop can be obtained, which keeps the desired, preset bulk material throughput constant. Maintaining a constant amount of conveying gas per unit of time, which is one of the two necessary prerequisites for the proper functioning of the control loop, can be ensured by feeding the conveying gas from a wind boiler kept at constant pressure through a Laval nozzle into the delivery line, since the conveying gas flow is known to be direct in Laval nozzles from the pre-pressure of the gas. depends on the nozzle. The second essential - 3 - - 3 -

A prerequisite for the correct operation of the control loop is a constant back pressure at the end of the delivery line.

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This requirement is met in most cases because the

Back pressure is equal to atmospheric pressure. However, there is also

Applications in which this requirement is not met, for example in the pneumatic conveying of coal dust into certain types of furnaces or in reactors for coal liquefaction and coal gasification, which has to take place not only against high but above all fluctuating back pressures, whereby the unit of time dosed bulk quantity must be kept at a predetermined setpoint. However, the method of the type specified in the introduction does not succeed because the fluctuating back pressure causes a corresponding fluctuation in the density and thus in the volume of the conveying gas. However, if the gas volume decreases with the same line cross-section, the velocity of the conveying gas flow also decreases. The latter in turn leads to a lower pressure loss over the predetermined line section, given otherwise the same conditions, since the measured pressure difference is a function of the speed of the gas flow. The reduction in the pressure difference is incorrectly interpreted by the system as a reduction in the load, for the compensation of which the quantity of bulk material given is increased, so that the desired constant level of the quantity of bulk material conveyed per unit of time is not achieved thereby.

The invention is therefore based on the object of making the method of the type specified at the outset also useful for applications in which delivery is carried out against a fluctuating back pressure.

The solution to this problem is specified in the characterizing part of claim 1.

The subclaims relate to advantageous types of - k -) a - if -

Realization of this procedure.

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The method proposed by the invention is based on the knowledge that the relationship between the back pressure and the pressure difference measured over a given line section can be represented in the form of a family of curves, the parameters of which are the loading ^ * /. Since a very specific load is normally specified and should be adhered to, the group of curves that applies to the load in question can be selected from this family of curves. Furthermore, the setpoint of the counterpressure against which it is necessary to pump is usually also known. A point of the selected curve corresponds to this back pressure. Since all curves have only a relatively small change in slope, the slope of the curve at the point determined can be used as a constant proportionality factor for determining the correction variable from the difference between the back pressure setpoint and the back pressure actual value.

The method according to the invention is explained with reference to the drawing; It shows:

Fig. 1 shows the scheme of a plant for performing the proposed method and

2 is a diagram illustrating the dependence of the pressure difference Δ p on the counter pressure P.

1, a burner 1 is fed with coal dust from a silo 2 via a pneumatic conveying line 3. For this purpose, a blower generates a conveying gas flow in the conveying line 3 in the manner described in more detail above, the speed of which is constant at least at the beginning of the line. On the silo 2, the coal dust is fed into the delivery line 3 in a manner known per se via a cellular wheel sluice 5> which is driven by a motor 6.

- 5 - - 5 - \

In order to ensure the required constancy of the quantity of bulk material fed to the burner per unit time f, the pressure P1 is measured at point 3a of the delivery line and the pressure P2 at point 3h of the delivery line. In this case, the pressure P2 fluctuates in proportion to the back pressure P prevailing in the burner 1 at the end of the delivery line. The pressure P2 is therefore corrected in an electrical circuit 7 to a pressure P2 ′ corrected for the fluctuating back pressure P, otherwise because of the fluctuating back pressure an actually non-existent change in the load would be faked.

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The correction is made according to the formula P2 "= P2" f (/ i) '-f (P);

For this purpose, the setpoint of the counterpressure P is supplied to the circuit 7 on the one hand, and the setpoint of the loading // from the corresponding setpoint adjusters 8 and 9 on the other.

The corrected pressure difference A p1 is formed in a subtraction element 10 from the pressures P 1 and P 2, which is supplied as an actual value to a comparator 11 which is set by a setpoint. 1er 12 receives the target value A p. The setpoint adjuster 12 in turn receives the setpoint // from the setpoint adjuster 9, since the setpoint of the pressure difference A p in the manner shown in FIG. 2 depends not only on the back pressure P but also on the load //. The comparator 11 provides at its output the manipulated variable n, which changes the speed of the drive motor 6 for the cellular wheel sluice 5 in the direction of reducing the control deviation.

The described method can also be implemented in a simplified form. If the area of loading // within which the system is to be operated with constant loading is not too large, the setpoint can - 6 -. \ -. - 6 - 3 Λ.

one piece 9 are not required. The respective load is then specified solely for the pressure difference.4 p via the setpoint adjuster 12. For the correction of P2 in P'2, the factor f (/ 0 is then regarded as a constant in accordance with the previously given equation.

In the simplest case, especially if the back pressure P fluctuates only within relatively narrow limits, the factor f (P) can also be regarded as a constant, so that the equation given is simplified to P2> = Ρ2 · K; where K is equal to K ^ (/ 0 * K2 (P) according to what has been said.

In principle, it is also possible to arrive at the same result, i.e. keeping the dosage constant regardless of fluctuations in the back pressure P, by correcting either the pressure difference P ^ - P2 = Δ p or by directly correcting the manipulated variable n in n ', however this only shifts the problem to an area in which more variables (eg P1) have to be taken into account and corrected if necessary.

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2 shows the dependency of the pressure difference / ^ p = PI-P2 on the absolute value of the back pressure P for various values of constant loading //. It can be seen that for not too large changes in P around a predetermined setpoint. and for not too far apart values of / 1 with which the system is to be operated, the aforementioned simplification of the correction is permissible, according to which Δ p = P * K.

Claims (3)

1.Procedure for metering bulk material transported pneumatically in a delivery line by means of a constant amount of conveying gas per unit of time by measuring the pressure difference over a given line section, comparing this actual value with a pressure difference setpoint and controlling the quantity of bulk material given with the comparison result as a manipulated variable in a control loop, characterized in that a pressure (P2), which is proportional to the back pressure at the end of the delivery line, is measured as the actual value with a.:; Back pressure setpoint (P) compared and the comparison result is fed into the control loop as a correction variable. î. 2. The method according to claim 1, characterized in that the correction variable is used to correct the pressure difference actual value. 3. The method according to claim 1 or 2, characterized in that the correction variable for correcting one of the two prints (Pl, P2) is used to determine the actual pressure difference. Method according to one of claims 1 to 3a, characterized in that the desired loading of the conveyed material (μ) is applied as a second correction variable to both the pressure difference setpoint and the pressure difference actual value.