01 Process Status Quo
In response to the national requirement of eliminating backward production capacity, a company reconstructed a set of combined grinding system consisting of CLM150-100 roller press and 3.8m×13m cement mill, with an annual output of 1.5 million tons of cement, which was put into operation in September 2014. The power of the cement mill is 2500kW, and the roller press is driven by two high voltage motors, and the power is 2×800kW. When designing the platform, the output is 140 ~ 150t/h. See Figure 1 for the process flow chart.
Fig. 1 Process flow of cement grinding system
In the past year, due to the poor cement market and declining demand, in order to reduce the electricity consumption cost of cement, the company adopts peak avoiding production. The peak avoiding time is 8h every day, and the production operation time is divided into two periods, namely, from 11:00 to 16:00 and from 21:00 to 8:00 the next day. That is, it starts and stops twice a day.
02 Improvement Measures
At the initial stage of the design, the starting sequence (as shown in below) is: finished product system → oil mill station system → main motor of ball mill → roller press system → main motor of power roller and fixed roller → raw material feeding system.
It can be seen from the analysis of Figure 2 that during the starting process, the main motor of the ball mill was idle for 10min, which wasted electricity. We proposed to turn on the main motor of the ball mill at the end, so as to save some power consumption. Because of the process protection in the program, when the main motor trips accidentally, it is necessary to stop feeding the equipment such as the grinding chute fan. Therefore, it is necessary to remove the interlocking protection of the main motor of the mill at the start of the machine, so as to complete the purpose of turning on the main motor of the mill after all the equipment is turned on. So, we through the modify the program, and on the computer monitor screen made a switch button, operators need to control in the process of startup to start state, at this time will remove the main motor chain protection, when all the devices are normally open and feeding, central operator again to hit the button to the running state, so it will be put into operation mill chain safeguard.
03 Analysis of electricity saving
Compared with before and after the improvement, the current of the mill during idling was 160A, and the power factor was 0.93. Two turns on the main motor of the ball mill could reduce the idling time by 20min, which could save electricity per day: 1.732×160×10×0.93×0.33=859kWh. Based on 0.75 yuan per kilowatt hour, 859×0.75×30=19329 yuan per month could be saved. Note: electric quantity =√ 3UABIABCOS φ×t, where: √3=1.732, UAB is line voltage, IAB is line current, COSφ is power factor, t is time (h); Main motor rated line voltage is 10kV.
It can be seen that by changing the process starting process and modifying the control program, the idling consumption of the high-power motor is effectively prevented and the purpose of energy saving and consumption reduction is achieved.