Abstract: this paper introduces VFD feasibility and necessity in cement plant. It also introduces energy saving effect.

Key words: cementmedium voltage variable frequency energy saving

I. Introduction

At present, competition is very intense. The key is cost. Power consumption of motor accounts for 30% of cost. Motor driving fan is a big part. So it is very important to reduce consumption of motor. Today, phenomenon of “large horse drives small dolly” is serious. If technology of variable frequency is adopted, it not only satisfies production requirement but also saves energy. Meanwhile, VFD saves a lot of energy source and improves efficiency for plant. After innovation with VFD, energy saving rate is in 30-50%. The investment usually recovers in one or two years.

II Problems in baffle control mode

Conventionally, air flow is controlled by valve opening. This is a behindhand method with inferior economic benefit, much consumption, serious damage, difficult maintaince and high cost. Fan performance is shown following:

III Work in direct on line state
It indicates pump character curve keeps unchanged, change network curve. Usually, keep fan curve unchanged (not change fan speed), change outlet by valve to change air flow. As the drawing:

When fan works in A, air flow is Q₁，pressure is H₁. Keep fan speed constant, adjust air flow to Q₂ by baffle. Pressure will rise to H₂, fan work point is B. Because of baffle throttle, reisitance curve of duct is changed to OB.

When fan works in A, its power is P_A＝H₁×Q₁/102；when in B, its power is P_B＝H₂×Q₂ /102.

Q₂＜Q₁， but H₃＞H₁，so P_A P_B do not change a lot. It indicates that in direct on line state,

shaft power of fan reduces little when air flow is changed.

IV Work in variable frequency state

It indicates network curve keeps unchanged, change fan curve. Usually, do not change fan outlet, change fan curve(change fan speed) to change air flow.

When fan works in A, its power is P_A＝H₁×Q₁/102；when in B, its power is P_B＝H₂×Q₂/102.

Q₂＜Q₁，H₂＞H₁，so P_A P_B change a lot. It indicats that in variable frequency state, shaft power of fan reduces a lot when air flow is changed. Energy saving is considerable.

The relations:

Output flow Q and speed n：Q₁/Q₂=n₁/n₂……（1）

Output pressure H and speed n：H₁/H₂=(n₁/n₂)² ……（2）

Output shaft power P and speed n:P₁/P₂=(n₁/n₂)³……（3）

If 100% speed -100% flow -100% pressure-100% output power，there is: 80% speed- 80%flow- 64% pressure- 51% output power. Namely, in variable frequency state, when flow reduces 20%, shaft power reduces 49%. That is the reason why VFD has a considerable energy saving effect.

V Application in Tianrui Cement Plant

1 Instruction

A high temperature fan in 5000t/d production line needs reform with VFD. At present, air flow and pressure are controlled by valve and liquid coupling. But there are following problems in site:

(1) Disadvantage of liquid coupling: efficiency is lower with speed reducing. Installation needs cutting off connection of motor and load. Site is dirty and needs much maintaince.

(2) With water resistance, there is still 3 times of rated current to impact grid. And water resistance starts slowly, which makes water boiling.

(3) It has large throttle loss and low inlet pressure, low efficiency with valve control.

Therefore, take account of variable frequency technology to control. It not only solves problem of low efficiency of valve but also improves economic index. What is more, it reduces throttle loss and wear and tear on valve, lowers temperature rise, prolongs equipment lifetime, saves energy and lowers power consumption.

2. Site data

(1) motor and liquid coupling
(2) average price :0.5yuan/kWh
(3) annual time: T=8000h

3. Comparison

VI. Advantages

(1) By soft start, start current is smaller than rated current. Start is more smooth.

(2) Improve efficiency, eliminate throttle loss.

(3) Process technology is improved and equipment lifetime is prolonged.

(4) After reform with VFD, 41 down harmonic can be eliminated because of cell-cascade. Total voltage harmonic is controlled in 2%. Total current harmonic is less than 4%.

(5) PWM control makes output voltage waveform near sine. Total harmonic is less than 1%. Above indexes all satisfy IEEE-519 standard.

(6) Voltage source instruce, power factor can be 0.95.

(7) Noise lowers a lot.

VII Concluding remarks

Baffle control has disadvantages of much consumption, inferior economic benefit and bad damage. Technology of variable frequency can reduce consumption and heighten economic benefit. The practice proves that VFD is necessary, feasible in cement plant and has considerable economic benefit.