Nowadays electric charge has become the third highest cost in cement industry, only next to raw material cost and labor cost. Since the short supply of electric energy is becoming critical, all manufacturers attach great importance to electric energy conservation. The shaft kiln is one of the most important equipments in cement plant, and it is also the large user of electric energy. The selection of shaft kiln depends on cement output target, while a certain surplus capacity is usually taken into conservation. The shaft kiln usually keeps rotating at the maximum speed for a long time, and air quantity that the shaft kiln actually needs varies with seasons, daytime or nighttime and operating modes, which in most of cases is lower than anticipated value during the design; when the rotational speed of shaft kiln is fixed, the surplus air quantity, if any, often exhausts via the ventilation duct (or control the inlet air quantity at ventilation door), in this case, the draught fan of shaft kiln has a great waste of power, therefore, the transformation of draught fan of shaft kiln is of great practical significance.
II. Power Saving Principle by Frequency Inverter
Regarding the draught fan, the air quantity “Q” increases directly with rotational speed “n”, the air pressure “F” increases directly with the square of rotational speed “n”, and the shaft power “P” increases directly with the cube of rotational speed “n” (please refer to Table 1). The functional relationships between these variables are as follows:
Rotational Speed n% Air Quantity Q% Frequency H% Air Pressure F% Shaft Power P% 100 100 50 100 100 90 90 45 81 72.9 80 80 40 64 51.2 70 70 35 49 34.3 60 60 30 36 21.6 50 50 25 25 12.5
Apparently, air quantity control system may achieve good energy-saving effect. The curves of air pressure “H” and air quantity “Q” are shown in Figure 1.
n1: show the characteristic of draught fan at rated rotational speed;
n1: show the characteristic of draught fan at low rotational speed;
R1: show the resistance characteristic when the pipe resistance is reduced to the minimum;
R2: show the resistance characteristic when the pipe resistance is increased to a certain value;
When the draught fan works according to pipe resistance characteristic curve R1, the operating point is A, the air quantity and air pressure are Q1 and H1, in this case, the power that the draught fan needs is in the direct ratio to the product of H1 &Q1, i.e. in the direct ratio to the area of AH10Q1. Due to technical requirement, the air quantity needs to reduce to Q2, which can be realized by increasing the pipe resistance, the operating point moves to point B on curve R2, the air pressure increases to H2, in this case, the power that the draught fan needs is in the direct ratio to the product of H2 &Q2, i.e. in the direct ratio to the area of BH20Q2, which indicates the power that the draught fan needs has increased. This control method is simple, but it causes high power consumption and goes against with power conservation purpose. It achieves simple control by increase operation cost.
When using variable frequency speed control technology, the rotational speed decreases from n1 to n2, the operating point moves from point A to point C, the air quantity remains Q2, and the air pressure decrease from H1 to H3, in this case, the power that the draught fan needs is in the direct ratio to the product of H3 &Q2, i.e. in the direct ratio to the area of CH30Q2, which indicates the power has greatly decreased.
III. Main Parameters and Working Principle of Shaft Kiln
- Basic Parameters of Shaft Kiln
(1) Mechanical Shaft Kiln Parameters:
Sizes: (kiln diameter * height) 2.0*8.5
Annual output: 50,000~70,000 tons of common (Portland cement)
Feed method: single-tube dispersion chute 11KW
Chute rotational speed: 6.48 R/Min
Tower grate model: double eccentric
Tower grate rotational speed: 1.2~12.2 R/Min
Driving mode: center drive, power: 15KW
Material sealing mode: vertical sealing pipe
Discharge grain size: ≤100mm
(2)Draught Fan Parameters
rated voltage: 380V
rated current: 245A
rotational speed: 2,980R/Min
running current: 120A~180A
air pressure: 21,995 Pa
start mode: △/Y
- Working Principle of Shaft Kiln
After being added with a certain amount of water, raw materials are delivered by the hopper hoister to ball granulator for nodulizing, and then the globate raw materials are evenly distributed in the kiln by material distributing device. The globate raw materials undergo complete combustion in the kiln depending on the air quantity provided by centrifugal fan at the bottom. After cooling at the bottom of the kiln, the tower grate keeps rotating to send the clinkers into the hopper. After being sprayed with some water, the clinkers enter the clinker crusher and then move to the next procedure. This technical process shows that the draught fan quality directly affects cement output, therefore, many cement plants are seeking for improvement on the draught fan, such as increasing motor power, air quantity or air pressure, however, these methods usually achieve opposite effect.
Among all equipments in shaft kiln, the power of Roots blower motor is 132KW, which accounts for a half of the power of all equipments in shaft kiln, therefore, the transformation of Root blower power is of great significance to cement plants.
IV. Functional Requirement after Transformation
(a) Good performance in following, stable quality and low failure rate.
(b) The frequency inverter should support operation at two places (two operation panels required), one operation panel is installed on kiln surface via extended line so as to replace original operating system; this operation panel should enable operators to set or adjust parameters, adjust air quantity according to production status, and start or stop the frequency inverter. The other operation panel should be installed in the low-voltage distribution room, and it should enable operators to monitor operating parameters, start or stop frequency inverter instead of setting or adjusting parameters or adjusting air quantity.
© Emergency stop switches should be installed at both on the kiln surface and in low-voltage distribution room. In case of low air quantity, the temperature rise and noise of motor should not exceed the given limits.
V. Transformation of Draught Fan of Shaft Kiln
(a) According to on-site working condition and customer’s requirements, INVT has designed a square torque frequency inverter featuring high quality and reliable performance. The model No. is Goodrive100-132G/160P-4, and the capacity is 132KW. It has parallel connection to the main loop circuit of original draught fan starting cabinet, and interlock with such main circuit; it realizes the speed control of shaft kiln for energy conservation purpose, and the control frequency can be set by operators according the actual production process and the calcinations status in the kiln. The air pressure in the kiln can be adjusted by pressing the upper or lower buttons on the frequency panel; therefore, it gets rid of the traditional control mode applied to shaft kiln, and achieves the best effect in speed control and energy conservation.
(b) The frequency inverter can be connected with two control panels depending on an external keyboard via 485 communication interface; the keyboard can be connected to the mainboard via a shielding line within 1,200 meters. The frequency inverter keeps the emergency stop switches on the kiln surface and in the distribution room, and adds an AC contactor between air switch and the main circuit. The original snort valve is sealed with rolled steel. After transformation, the opening in the middle of kiln need to need adjusted one by one.
Features of Variable Frequency Energy Conservation System
Goodrive100 frequency inverter has wide speed range, therefore, it is applicable to various speed control equipments, and the frequency range is 0.00~400.00Hz;
High-precision control. The digital resolution ratio of frequency inverter is +/-0.01%, and analog resolution ratio is +/-0. 1%;
Strong control functions and applicable to different control systems. The frequency inverter can be connected to a wide range of set signals via terminals, such as 0~10V, 4~20mA. Multiple operations via terminal control, such as forward rotation and reverse rotation;
Parameter adjustment for torque increase, torque restriction, current limitation, so as to achieve great starting torque and avoid tripping operation or other failures due to sudden load change;
Comprehensive protection functions. The frequency inverter offers 23 protection functions, and adopts CPU operation against over voltage, under voltage, over current, over load, over temperature. It also records all failure causes;
Simple PLC based on 16-point input, multistage speed control, PID control, wobbulator function;
Non-stop function against sudden power failure, automatic voltage adjustment function to maintain constant output voltage in case of voltage change of the power system;
Speed tracking and restart function to realize smooth start during motor rotation;
Offer RS485 communication interface, support MODBUS protocol, support RTU and ASCII transmission formats, easy to arrange network, support monitoring via host computer;
Creative quick debugging mode, enabling users to debug the power frequency.
VI. Frequency Inverter Parameters
Parameter setting is required after the installation of frequency inverter. Some parameters may directly affect the operation of frequency inverter or cause dead start of motor, in which case the transformation project would fail, therefore, the parameters need to be set carefully so as to ensure these parameters well match those of the motor. Here are some parameter values which have been confirmed based on the calcinations process and the result of test run by kiln operators. Please be reminded these parameters are for reference only.
Serial Number Functions Set Values Remarks 1 Control Mode Via keyboard P0.03=0/1 2 Upper Frequency Limit 50HZ P0.05=50 3 Acceleration Time 110S P0.07=110 4 Deceleration Time 110S P0.08=110 5 Torque Increase 2% P0.10=7 6 Carrier Frequency 8KHZ P0.03=8 7 Stop Mode Free stop P0.03=1 8 Running Frequency Presetting 0/1 P3.01=0/1
VII. Transformation Precautions
Date August, 31 (24H) September, 1 (24H) September, 2 (24H) September, 3 (24H) September, 4 (24H) September, 5 (24H) September, 6 (24H) Power Frequency Mode 2640 2720 2680 2688 2656 2704 2712 Variable Frequency Mode 2240 2280 2264 2248 2224 2256 2232 Electricity Saved 400 kilowatt-hour 440 kilowatt-hour 416 kilowatt-hour 440 kilowatt-hour 432 kilowatt-hour 448 kilowatt-hour 480 kilowatt-hour
The stop mode for frequency inverter is set to be free stop. In consideration of the large inertia load of draught fan, if the acceleration/deceleration time is set to be short, it may cause over current or overload during acceleration or cause over voltage protection during deceleration.
However, if the acceleration/deceleration time is set to be long, it may cause sluggish air quantity adjustment and poor following performance, in which case the system might have short instable status and might not be able to meet the production demand. Therefore, on the premise of ensuring the normal operation (without sudden breakdown) of frequency inverter, it is necessary to properly set the acceleration/deceleration time.
VIII. Energy-Saving Effect after Transformation
In order to be clear about the energy-saving effect after adopting frequency inverter, we made a test in cooperation with the cement plant. The original data are shown as follows:
Based on above data, we know the electric charge saved every day is 436.5 kilowatt-hour; as per the information provided by the cement plant, the shaft kiln keeps operation for 10 months (i.e. 300 days) in a year, and the electricity charge is RMB0.5/kilowatt-hour, so the electric energy saved in a year should be:
After the transformation, the cement plant will save around RMB 65,000 in terms of electric charge each year, which greatly facilitates the increase in economic benefits.
IX. System Features
(1)This system adopts energy-saving standalone control equipment. Through adjusting the rotational speed of draught fan, it is able to control air quantity and realize energy conservation while meeting production demand.
(2)It features high efficiency, stable performance and high precision during draught fan speed control and low loss during rotation mode switching. Depending on rotation deceleration and soft start, it helps to reduce the starting current of the motor, low loss of stator and rotor when starting operation, and prolongs the service life of the motor.
(3)Depending on Goodrive rotational speed tracking function, the motor achieves quick start during rotation.
(4)The system offers multiple protection functions against over load, over current, over voltage, short circuit and under voltage, which ensures the safe operation, high efficiency and reliable performance of the system, and also effectively avoids kiln spurt.