Heating – Having condition responsive control – Of or by pressure of fluid work or work chamber atmosphere
Reexamination Certificate
1998-09-02
2001-08-21
Lu, Jiping (Department: 3742)
Heating
Having condition responsive control
Of or by pressure of fluid work or work chamber atmosphere
C432S024000, C432S037000, C110S162000, C110S163000, C431S019000, C431S020000
Reexamination Certificate
active
06276929
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a method for controlling furnace pressure of a shuttle kiln, and, more particularly, to an improvement of method for controlling pressure in a shuttle kiln to a target value by controlling the number of revolutions of an exhaust gas fan to adjust an exhaust gas flow rate of combustion a exhaust gas or the like.
To maintain a combustion state in a shuttle kiln at the most desirable state, it is one of requirement to control a kiln pressure to within a certain range. The kiln pressure is obtained by providing suitable resistance in exhausting combustion air or fuel supplied into the kiln from burners of the shuttle kiln or other feed air such as cooling air in order to cause a pressure difference between inside and outside of the kiln.
In a shuttle kiln, particularly, since a series of heating operations such as raising of temperature, soaking of the maximum temperature or cooling are performed, supply operations significantly vary for combustion air or cooling air as time passes, so that control of kiln pressure is one of automatic control factors essential to the operation of shuttle kiln.
Herein, is described an example of conventional method for controlling kiln pressure of a shuttle kiln with reference to FIG.
4
. In the kiln body
11
of the shuttle kiln, fuel, combustion air or the like fed from outside become exhaust gas after combustion, the exhaust gas being discharged to an exhaust flue
12
, fed to a chimney
15
from an exhaust gas damper
3
through a plenum
13
, into which outside air is introduced, as required, by an exhaust gas fan
14
, and finally discharged into the air.
In this case, to also control the kiln pressure in the kiln body
11
, there is formed a PID control loop which uses the kiln pressure as an input element and the number of revolution of the exhaust gas fan
14
as an output element. Referring to
FIG. 4
, the kiln pressure in the kiln body
11
is sent by the pressure transmitter
21
to a control arithmetic unit
22
, which in turn instructs an inverter controller
23
driving the exhaust gas fan
14
to issue a variable speed drive output so that the exhaust gas fan
14
rotates at a number of revolution depending on a deviation from a target value.
Thus, the kiln pressure can be stably maintained by automatically adjusting flow rate of the exhaust gas in such manner that the number of revolution of the exhaust gas fan
14
is decreased under control of the variable speed drive to raise the kiln pressure, and increased to reduce the kiln pressure.
In addition, the flow rate of exhaust gas may be controlled by adjusting opening of the exhaust gas damper
3
with an exhaust gas damper program
31
which is programmed according to elapsed time. However, in operation of an actual kiln, it is unavoidable that offset from the program is caused by disturbance such as a shape of a product, changes in dimensions, pack weight, or variation of outside air temperature. Accommodation to such situation requires corrective operation by an operator, or safety measures to maintain the number of revolution at a value higher than necessary, which causes disadvantages in promotion of automation or energy saving measures.
On the other hand, when the number of revolution of the variable speed drive is controlled for the exhaust gas fan
14
, there are disadvantages that, in a range between 1/10 and 1/20 or lower of the rated number of revolution, a motor for driving the fan would have insufficient continuous allowable torque, and could not handle load torque, leading to unstable rotation, and thus difficulty in control, and that cooling air flow becomes insufficient due to insufficient number of revolutions causing insufficient cooling in the motor. Thus, the control range for the number of revolutions of the exhaust gas fan would be values in a range between 1:10 to 1:20 in terms of the rated number of revolution, leading to a problem that the flow rate of exhaust gas can be controlled only for values in a range between 1:10 and 1:20 corresponding to the control range.
SUMMARY OF THE INVENTION
As described, in the kiln pressure control exemplified in
FIG. 4
, the lower limit of control for the number of revolutions of the exhaust gas fan is 1/10 to 1/20 of the rated number of revolution, and has a control range of 1:10 to 1:20, and this causes no particular trouble in the conventional shuttle kiln because its control range for an exhaust gas flow rate is about 1:20. However, a control range as wide as 1:30 to 1:50 of the exhaust gas flow rate is demanded for a recent energy saving combustion system intended for reducing excess air in the low temperature range represented by pulse firing or the like.
The present invention has been made to satisfy such new requirements, and provides a method for controlling kiln pressure for a shuttle kiln, the kiln comprising an exhaust gas fan and an exhaust gas damper, both of which are conventional, which can expand a control range, and enables it to enhance an energy saving effect.
To solve the above problems, the method for controlling kiln pressure for a shuttle kiln according to the present invention comprises an exhaust gas fan and an exhaust gas damper in an exhaust gas passage, the method comprising the steps of performing a control operation for controlling the number of revolutions of the exhaust gas fan to control the flow rate of the exhaust gas to hold the kiln pressure at a predetermined pressure, and performing a control operation for adjusting opening of the exhaust gas damper based on the number of revolutions of the exhaust gas fan.
In addition, the present invention can be embodied in a form in which opening of the exhaust gas damper is controlled to a closing direction when the number of revolutions of the exhaust gas fan lowers less than a predetermined set value, and to an opening direction when it recovers to the set value or more. Then, it is particularly preferable to set the predetermined set value of the number of revolutions to 1/5 to 1/10 of the rated number of revolution.
Furthermore, it is preferable to prepare a control method for stopping the control of opening of the exhaust gas damper based on the number of revolution of the exhaust gas fan, and automatically or manually operating the opening to an opening separately and arbitrarily set, and to allow it to freely change over to this method.
REFERENCES:
patent: 5257574 (1993-11-01), Hiromichi
patent: 56-162320 (1981-12-01), None
patent: 59-177329 (1984-10-01), None
patent: 63-118520 (1988-05-01), None
patent: 63-134245 (1988-09-01), None
patent: 7-280256 (1995-10-01), None
Ito Yasuhiro
Kawaguchi Kenji
Kuroda Tsuyoshi
Lu Jiping
NGK Insulators Ltd.
Parkhurst & Wendel LLP
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