Application Case in Chemical Industry
Case I
Application of Senlan Inverter on Fan of Chemical Plant
1.
Introduction
Technology of frequency-conversion speed regulation is widely applied because of its
obvious power saving result, convenient speed regulation mode, wide speed regulation
range, reliable operation and perfect protection function as well as other advantages.
2.
Frequency Conversion Transformation of the Blowing Fan
2.1 Conditions Prior to Transformation
This air supply system employs 350kW 380V fan which controls the air supply volume
by adjusting the air damper, so as to fit the change of production load. Because the fan
with big power is selected when the project is put into construction, which results in the
conditions of “large horse pulling a small cart”, most of electric energy is consumed by
the air damper, causing the efficiency of fan reduced.
This system employs self-coupling step-down starting mode during operation, however,
the startup current is still great, which causes great mechanical and electrical shocks on
the motor. Motor squirrel-cage bar is always broken. The control system of blowing fan
shall be altered to solve the above problems. Frequency-conversion speed regulation
method of motor is utilized to realize control of air supply volume and soft start of motor,
so as to achieve the aim of energy saving and reliable control.
2.2 Brief Introduction on Transformation Scheme
A frequency-conversion device shall be added between the previous motor and the
switch, and the previous power frequency circuit is reserved as the by pass, and its
circuit diagram is as shown in Fig. 1.
L1
L2
L3
DZ1
DZ2
FU1
R S T
Previous startup
system
SB61P375KW
U V W
KM1
KM2
M
Fig. 1
Circuit Diagram
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3.
Effect after Transformation of Inverter
The fan motor of this system is 350kW and two-pole, with the rated current of 629A.
When operated under the previous power frequency, the average running current is
530~590A. The power consumption per hour is about 317kWh (active power) if taking
550A for calculation; when operated after the frequency-conversion system is put into
operation, the average power consumption per hour is 207kWh (active power). The
voltage is still 380V and the running current is 280~ 320A when tested at the input side
(power end) of the inverter if taking 300A for calculation; the electricity price is RMB
0.5 /kWh if taking the power factor of frequency-conversion system of 0.93.
(1) Power saving per hour
40%×0.93≈37%
If calculated as per active power, the result of the above formula shall be multiplied
with the power factor of 0.93.
Power saving per hour: 317－207＝110(kWh)
(2) Annual economic benefits
If the fan is working continuously without shutdown for 365 days per year, the
annual economic benefits on power saving is:
365×24×0.9×110×0.5≈43.4 (RMB 10,000)
(3) Investment recovery period
The total investment on this system is more than RMB 200,000, and the annual
profit is RMB 434,000, thus the investment recovery period of this project is only
half a year; Meanwhile, the production efficiency is improved and the production
cost is reduced, thus benefit is significant. The system is running stably until now,
and no fault is ever occurred.
Case II
Application of Senlan Inverter on Air Compressor of Chemical Industry
I
Foreword
The compressed gas is widely used in industrial production. If several air compressors
are installed in a place within the plant, a compressed air station is constituted. When
selecting the motor capacity of hydrogen compressor, too much consideration is given to
the difference required by long-term process before and after construction, which results
in too much surplus capacity. Besides, accurate calculation is difficult to be made during
design. Considering various problems which may be occurred in long term operation, the
long term full load operation of the hydrogen compressor is always taken as the basis for
type selection. However, the series of hydrogen compressor motor is limited, no
appropriate motor type can be selected usually, and the power which is 20%~30%
greater than the rated power always selected. The period of light running of hydrogen
compressor will be obviously increased in the actual production due to the above
reasons.
Moreover, the gas consumption is dynamically changed due to the irregularity of
hydrogen use in production, and several hydrogen compressors are required to be
operated at same time, while the gas of a hydrogen compressor is sufficient at some
times, while under such condition, the hydrogen compressors are still full speed running.
The discharge pressure regulation unit provided with the hydrogen compressor when
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leaving the factory is intake pipe closed type, and its operating principle is when the air
pressure inside the air reservoir (air receiver) reaches or exceeds the setting pressure
(0.82MPa), the butterfly valve on the inlet pipe of the compressor will be automatically
closed, and the compressor will enter into idle unloading status. When the air pressure
inside the air reservoir is less than the setting pressure (0.77MPa), the butterfly valve on
the inlet pipe of the compressor will be automatically opened, and the compressor will
enter into full load operating status.
The air discharge and pressure of the hydrogen compressor are not always unchanged,
but the requirements on the process are dynamically changed, thus the hydrogen
compressor is always in the repetitious operation mode of full-load and unloading. The
operating current under full load condition is in proximity to the rated current of the
motor, while the no-load running current under unloading condition is about 35~50% of
the rated current of the motor, and under such condition, the current is not doing useful
work but produces idling loss made by the machinery under rated rotation speed. Though
this type of mechanical regulation unit can regulate the pressure, its pressure regulation
accuracy is low, and the pressure fluctuation is great. If the hydrogen compressor is
always working under rated rotation speed, the mechanical wear will be great, the
operation efficiency is always in low-level, and the power consumption is high.
If frequency-conversion speed regulation is employed, the rotation speed of the motor
can be regulated according to the demand, and the operating power of the motor can be
reduced under high efficiency, which can realize the purpose of energy saving when the
requirements on production process are met. In view of operating quality, most of
hydrogen compressor systems cannot be continuously regulated according to the load.
After frequency-conversion speed regulation is employed, continuous regulation can be
made conveniently and effectively, and the parameters such as pressure and flow can be
maintained stable, thus greatly improving the working efficiency and performance of the
compressor.
II
Energy Saving Principle of Hydrogen Compressor
According to air compression theory, the shaft power PZ(kW), air discharge QD(m3/min)
and shaft speed n(r/min) of the hydrogen compressor meet the following formulae:
Mrn
(kW )
9550
QD  K  V  n2 (r / min)
PZ 
Where: M r --average torque of hydrogen compressor input (N·m)
K—Coefficient relating to volume, pressure, temperature and leakage of the
cylinder
n2—rotation speed of the compressor after regulation (r/min)
V --volume of cylinder (m3)
According to the above theoretical analysis, the rotation speed of the hydrogen
compressor can only be regulated by changing the air discharge when the cylinder
volume of hydrogen compressor cannot be changed. The torque load of the hydrogen
compressor is constant, i.e. the shaft power of the compressor is in direct proportion to
the first power of the rotation speed; when the total air discharge of the hydrogen
compressor is greater than the air supply volume, the air supply pressure can be adjusted
by reducing the rotation speed of the compressor, and it is an effective method for
economic operation of the compressor.
III Air Supply under Frequency Conversion and Constant Pressure
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The inverter and pressure transmitter constitute a pressure closed-loop system, under
which the air supply pressure will be adjusted automatically by reducing the rotation
speed of hydrogen compressor according to the demand, so as to realize economic
operation of the compressor. A pressure transmitter is considered to be installed on the
air reservoir to feed back the pressure signal to the inverter terminal, which constitutes a
constant pressure air supply system, with the air supply pressure of 0.8MPa.
The rated current of the hydrogen compressor motor is equal to or less than the rated
current of the inverter with constant torque. A PID regulator and 4~20mA analog signal
interface shall be built in the inverter. A Senlan SB61G110KW inverter is selected in
this case, and Senex DG13W=BZ-A (1.6MPa) is selected for pressure transmitter. The
schematic diagram of air supply under constant pressure is as shown in Fig. 2:
In the figure: the pressure feedback signal is taken from air reservoir by pressure sensor
PT and then sent to input end of built-in PID regulator of the inverter, which shall be
compared with the preset pressure demand signal, at last the working frequency and
rotation speed of the motor shall be determined after processed by PID regulator. Under
such control mode, the rotation speed of the motor will be automatically regulated when
the air consumption is changed under the premise that the pressure of the hydrogen in the
air reservoir is always constant, so as to maintain highly efficient operation and realize
energy saving.
1
2
L1
L2
3
L3
4
L1
L2
L3
5
L1
L2
6
L3
D
DZ1
DZ2
DZ3
FU1
R
PT
S
T
IPF
4--20mA
GND
SB61G132
Auto-tran
sformer
Starter
Auto-transfor
mer Starter
M1
M2
M3
110KW
110KW
110KW
SB12S132KW
C
Startup/Stop
Æð¶¯/Í£Ö¹K
FWD
y1
y2
CM
CM
U
V
W
B
Fig. 2
Schematic Diagram of Air Supply under Constant Pressure
The inverter controls the first hydrogen compressor, and the operation keyboard on the
inverter is used for the setting control. Multifunctional outputs Y1 and Y2 of the inverter
are connected to the startup/stop circuit of auto-transformer starter of the second and
third hydrogen compressors, thus the operation or shutdown of other two hydrogen
compressors can be controlled by the output of the inverter. Under manual operation, the
first hydrogen compressor is controlled by the inverter, and the second and third
hydrogen compressors can be manually started/stopped with auto-transformer starter.
A
1
2
3
4
4
5
Title
Size
A3
Date:
File:
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Under automatic operation, the first hydrogen compressor will be operated under
frequency conversion, and when output frequency of the inverter reaches 50Hz, the air
supply volume is still insufficient, the output nodes Y1 and Y2 of the inverter will be
activated to start the second and third hydrogen compressors; if the air supply volume is
greater than the setting value, the system will automatically stop the second and third
hydrogen compressors working, so as to realize air supply under constant pressure by
means of closed loop regulation.
Hydrogen is combustible gas, thus the work site of hydrogen compressor has explosion
hazard. The level of protection of SB61G series inverter is IP21, and obviously, it shall
not be used in the site where the explosion hazard exists. The inverter shall be installed
in a switching house where no explosion hazard exists, and a remote control box shall be
used nearby the compressor for operation. The remote control box shall be intrinsically
safe.
IV Precautions
V
1.
Because the moment of inertia of air compressor is great, the inverter capacity shall
be determined according to the actual operating conditions and running current of
the site when the inverter is selected.
2.
The pressure sensor shall be installed at the place where the pressure change is not
intense, and it may be installed on the air reservoir for the best. 4~20mA electric
current signal shall be used as the signal of pressure sensor to avoid interference,
and the transmission line shall select dual core shield line.
3.
Open loop and closed loop control modes shall be set in the control system of
inverter, so as for commissioning and use under special conditions.
4.
Because the air compressor is not allowed to be working under low frequency
condition for long term, under which condition, the stability becomes inferior,
surging is easy to be occurred, the lubrication of cylinder body becomes inferior,
which accelerates abrasion, thus appropriate, effective and safe lower limit of
working frequency shall be set. The actual setting shall be subject to aborative
regulation according to different operating conditions and use conditions and
requirements.
Effect after Transformation of Energy Saving
The compressor is altered in Aug. 2005 and realizes the anticipated purpose after three
months’ operation. The hydrogen pressure is maintained about 0.8MPa no matter how
the production or air supply volume is changed, and the gas supply quality is greatly
improved.
The air compressor is smoothly started at zero velocity after frequency-conversion speed
regulation is employed, which greatly improves the safety in production. The hydrogen
compressor is not full speed running any longer at any time. When the rotation speed is
reduced, the noise of working environment is reduced correspondingly; when the
rotation speed is reduced, the mechanical wear and tear is reduced correspondingly,
which is good for prolonging the life of the compressor and reducing the maintenance
cost.
In view of energy saving, it can be seen from the formula of the compressor, the shaft
power PZ(kW) consumed by the compressor is in direct proportion to shaft speed
n(r/min), while gas discharge of the compressor QD(m3/min) is in direct proportion to
shaft speed, thus the shaft power PZ(kW) consumed by the compressor is in direct
proportion to gas discharge of the compressor QD(m3/min). The reduction in rotation
speed can save shaft power, and the power saving ratio is actually measured to be 26%,
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thus significant economic benefit is obtained.
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