I Preface

In refining process of shale retorting, in order to supply saturated air to redox reaction of shale semicoke from gas retort, centrifugal fan (main fan) sends air flow to underneath of gas retort. Motor of main fan is 6kV, 250kW. In actual operation, air flow should be adjusted according to demand of each process index. Air flow is adjusted manually via valve opening and outlet opening. Excessive flow is discharged from outlet. Motor current is 20～22A，and motor output is about 70%. According to requirement of production process, worker adjusts air flow manually once or twice every hour. Lots of air is discharged through outlet. This adjustment mode is irrational. It increases work labour, wastes energy. So It should be improved.

Variable frequency technology on main fan can save energy as well as satisfy process requirement. After VFD reform, it reduces noise and damage on equipment, prolongs equipment lifetime and saves maintaince cost.

II. Drive selection

In early term of reform, we made demonstration for VFD reform on main fan. In term of production process, air flow is required to be adjusted frequently everyday, which increases worker labour. Furthermore, manual adjustment has big deviation. With VFD, it can adjust speed of motor in high precision and save work labour. What is more, during fan operation, less angle valve opens, more energy wastes on damper. Therefore, VFD has great potential in energy saving.

According to fan characters, there are 3 schemes:

1.      High-high mode: purchase high voltage VFD, no need to change motor.

2.      High-low-high mode: converter high voltage to low voltage, adopt low voltage VFD, then rise the voltage.

3.      High-low mode: lower voltage from 6kV to 380V by an independent transformer, adopt low voltage VFD and motor.

Make comparison between 3 schemes:

High-high mode: no change on motor; motor can start directly when VFD is broken; high cost.

High-low-high mode: design circuit again; circuit is complicated; add 2 transformers; high cost

High- low mode: need a reducing transformer. In the case of transformer with small capacity, if VFD is broken, motor can not start directly; in the case of transformer with excessive capacity, there will be cost added and power wasted. By comprehensive comparison, we select high-high mode.

After long term of inspection and demonstration, we choose HARSVERT－A06/030 VFD.

HARSVER-A series VFD adopts technology of multi-level PWM. Several low voltage PWM cells connect in series to output high voltage directly. There are 7 power cells of 480V in each phase. Output phase voltage is 3450V, line voltage is about 6kV. The drawings is following:

III. Energy saving effect

High voltage VFD has been running stably without fault since it was installed in May 2004. Input current of motor is about 10A, input power is 85kW. Compared with other modes of 20A, energy saving is considerable. Daily power saved is 2040kWh, monthly power saved is 61200kWh, corresponding to 25700yuan. We infer that at least 670000kWh can be saved every year, corresponding to 280000yuan(USD 41176)

IV Concluding

The oil refinery began to use VFD on material feed machine in 1997, and reformed with VFD on boiler, ID fan, pump in 2003, on main fan this year. We have accumulated much experience. There is great potential for technology reform. As long as insisting on serious attitude and strict research, we are bound to gain pay back.