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Inverter in mill fan of power plant
A 100MW mill fan adjusts air flow by inlet damper. In order to reduce tertiary air, inlet of pipeline takes measures of necking down (Ф1200 reducing toФ800) and adding small damper. This way wastes lots of power. In order to lower energy consumption, engineers thought it was feasible to make reconstruction with inverter. Its affection on negative pressure forming and tertiary air is not big. So the company decided to make reconstruction with inverter for 4# mill fan. The company began to prepare materials on 22th November 2004. DCS system adds variable frequency label. Construction began on 8th December 2004. System began to make full commissioning on 1 April, 2005. Frequency inverter put into service on 7th April, 2005. Energy saving is good.
Introduction of reconstruction
1) Parameters
a. mill fan: 2
b. parameters of mill fan
Motor parameters
c. Work parameters: air flow of single fan 48000 m3/h, yearly work time 5850h. Average current is 40A when motor runs in direct on line.
d. Power price: 0.24 Yuan /kWh
2) Equipment after reconstruction
The system is composed of inverter and bypass cabinet. In above figure, QS1, QS2 and QS3 are all in bypass cabinet. QF and M are original equipment.
The system has switch function between line frequency and variable frequency. QS2 and QS3 cannot close together. They are mechanical interlock. During variable frequency operation, QS1 and QS2 close, QS3 opens. During line frequency operation, QS3 closes, QS1 and QS2 open.
3. Mill fan Technics
Mill fan is installed behind of ball mill, raw powder separator and fine powder separator. In ball mill, raw coal mixes with hot air and recycle air from outlet of mill fan. Raw coal is dried and grindered into coal powder. With negative air flow produced by mill fan, fine powder separator separates air from powder. Coal powder drops in warehouse as storage. Residual gas contains 5～10% mixture of air and powder. It is delivered to furnace as tertiary air, or mixes with coal powder left as primary air. Mixture is delivered to furnace. From above processes, we see mill fan provides negative pressure for system. After reconstruction with frequency inverter, outlet gate of mill fan is full open. Meanwhile, adjust outlet gate appropriately. It can meet requirement of providing negative pressure completely, without affection on tertiary air.
Energy saving
a. Power consumption when machine runs in direct on line
Pd: motor efficiency; ηd: motor efficiency; U: input voltage of motor; I: input current of motor;cosФ: power factor
Formula: Pd=1.732×U×I×cosФ
Power consumption: Pd=1.732×U×I×cosФ=1.732×6V×40A×0.87=361.64kW(including necking down and damper throttle loss)
b. Power consumption when machine runs in variable frequency
According to law of fluid mechanics, for load such as fan and pump, speed n, flow Q, pressure H and shaft power P have following relations:
Q∝n, H∝n2 ,p∝n3;
That is, flow is in direct rate to speed, pressure is in direct rate to square of speed, shaft power is in direct rate to cube of speed.
Put data in (Q1/Q0)3=P1/P0
(Q1: actual discharge flow, Q0: rated discharge flow, P0:rated power of fan), (48000/78000)3 we get power consumption: P1＝144.90kW, inverter power gird side consumption: Pb=P1/ηb=144.9/0.96=150.94kW
c. Energy saving rate
(ΔP/Pd)×100%＝(210.7/361.64)×100%=58%
d. Implementation energy saving rate during variable frequency operation, average current of motor is Ig17A, actual energy saving rate: 1-(Ib/Ig) ×100%=1-(17/40)×100%=57.5%
e. Benefits
Yearly power saved=No of mill fan × energy saving rate × power × yearly work time=
2×57.5%×361.64×5850＝2432933.1(kWh)
Power price: 0.24 Yuan /kWh, yearly cost saved is 580000yuan (USD 85294)
Phase II power generating is 1439505600KWh in 2004. Power consumption rate of phase II plant lowers 0.17%.
At present, despite inlet damper is full open, inlet pipe is still necking down. We plan to take advantage of maintenance opportunity to reconstruct for the neck and small damper, to reduce throttle loss and extend variable frequency effect further.
Other benefits
a. Mill fan is load which needs to start frequently. By variable frequency adjustment, medium voltage switch always closes, which prolongs maintenance period greatly.
b. Variable frequency adjustment can realize soft start, prolong lifetime of motor and fan.
Conclusion
It is the first time to make reconstruction with inverter for mill fan in China. Successful inverter application in the Power Plant lays a good foundation for variable frequency control in powder production system of power plants. We hope to exchange experience further with other users in the field and contribute to build an economized society.
Introduction of reconstruction
1) Parameters
a. mill fan: 2
b. parameters of mill fan
| Model | M6－31No20D | Rated flow (qv, max) | 78000 m3/h |
| Rated pressure (Pa) | 13235 | speed (NO) | 1450 r/min |
Motor parameters
| Model | Y5601-4 | Rated voltage (UO) | 6000V |
| Rated power (Pdn) | 630kW | Efficiency (η) | 96% |
| Rated current (IO) | 71A | Power factor (cosФ) | 0.87 |
| speed (NO) | 1485r/min |
|
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c. Work parameters: air flow of single fan 48000 m3/h, yearly work time 5850h. Average current is 40A when motor runs in direct on line.
d. Power price: 0.24 Yuan /kWh
2) Equipment after reconstruction
The system is composed of inverter and bypass cabinet. In above figure, QS1, QS2 and QS3 are all in bypass cabinet. QF and M are original equipment.
The system has switch function between line frequency and variable frequency. QS2 and QS3 cannot close together. They are mechanical interlock. During variable frequency operation, QS1 and QS2 close, QS3 opens. During line frequency operation, QS3 closes, QS1 and QS2 open.
3. Mill fan Technics
Mill fan is installed behind of ball mill, raw powder separator and fine powder separator. In ball mill, raw coal mixes with hot air and recycle air from outlet of mill fan. Raw coal is dried and grindered into coal powder. With negative air flow produced by mill fan, fine powder separator separates air from powder. Coal powder drops in warehouse as storage. Residual gas contains 5～10% mixture of air and powder. It is delivered to furnace as tertiary air, or mixes with coal powder left as primary air. Mixture is delivered to furnace. From above processes, we see mill fan provides negative pressure for system. After reconstruction with frequency inverter, outlet gate of mill fan is full open. Meanwhile, adjust outlet gate appropriately. It can meet requirement of providing negative pressure completely, without affection on tertiary air.
Energy saving
a. Power consumption when machine runs in direct on line
Pd: motor efficiency; ηd: motor efficiency; U: input voltage of motor; I: input current of motor;cosФ: power factor
Formula: Pd=1.732×U×I×cosФ
Power consumption: Pd=1.732×U×I×cosФ=1.732×6V×40A×0.87=361.64kW(including necking down and damper throttle loss)
b. Power consumption when machine runs in variable frequency
According to law of fluid mechanics, for load such as fan and pump, speed n, flow Q, pressure H and shaft power P have following relations:
Q∝n, H∝n2 ,p∝n3;
That is, flow is in direct rate to speed, pressure is in direct rate to square of speed, shaft power is in direct rate to cube of speed.
Put data in (Q1/Q0)3=P1/P0
(Q1: actual discharge flow, Q0: rated discharge flow, P0:rated power of fan), (48000/78000)3 we get power consumption: P1＝144.90kW, inverter power gird side consumption: Pb=P1/ηb=144.9/0.96=150.94kW
c. Energy saving rate
(ΔP/Pd)×100%＝(210.7/361.64)×100%=58%
d. Implementation energy saving rate during variable frequency operation, average current of motor is Ig17A, actual energy saving rate: 1-(Ib/Ig) ×100%=1-(17/40)×100%=57.5%
e. Benefits
Yearly power saved=No of mill fan × energy saving rate × power × yearly work time=
2×57.5%×361.64×5850＝2432933.1(kWh)
Power price: 0.24 Yuan /kWh, yearly cost saved is 580000yuan (USD 85294)
Phase II power generating is 1439505600KWh in 2004. Power consumption rate of phase II plant lowers 0.17%.
At present, despite inlet damper is full open, inlet pipe is still necking down. We plan to take advantage of maintenance opportunity to reconstruct for the neck and small damper, to reduce throttle loss and extend variable frequency effect further.
Other benefits
a. Mill fan is load which needs to start frequently. By variable frequency adjustment, medium voltage switch always closes, which prolongs maintenance period greatly.
b. Variable frequency adjustment can realize soft start, prolong lifetime of motor and fan.
Conclusion
It is the first time to make reconstruction with inverter for mill fan in China. Successful inverter application in the Power Plant lays a good foundation for variable frequency control in powder production system of power plants. We hope to exchange experience further with other users in the field and contribute to build an economized society.
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