Energy Transfer in Crushing Process: Power Consumption

The short-time transfer of energy, especially when crushing large block s, is very high; so high in fact that it would not be economically feasible to provide sufficient motive power to deliver it. The usual practice, when these rolls are driven electrically, is to drive the slugger roll with a 250-hp motor, and the regular roll with 200 hp -a total of 450 hp. As compared to this motive power, instantaneous energy delivery may run as high as 4000 hp –obviously far beyond the capacity of the motive equipment. But the rolls themselves, when running at normal no-load speed, have a stored kinetic energy of upwards of 4,000,000 ft -lb, and it is this stored energy that does much of the actual crushing, the motors serving to bring the rolls back to normal speed between crushing periods. In crushing a skip-load of stone the rolls may lose anywhere from 30 to 60 r p m in speed; this loss occurs partly through slowing down of the motive equipment, and partly through belt slippage. It requires from 5 to 10 sec. to bring the machine back to speed, during which time the power input will vary from 400 to 600 hp. The power required to run the rolls empty is something less than 100 hp. The average power consumption, when crushing from 3000 to 4000 tons per 10 hr. day will run in the neighborhood of 150 hp on medium limestone.

While the average power consumption of this machine compares favorably with that of other types, the rather violent fluctuation outlined and the relatively high connected horsepower are unfavorable features.

It is also natural to expect that the belt slippage we have noted would constitute something of a problem over a period of time. Performance records indicate that belt trouble accounts for about 50 percent of the total lost time on a set of these rolls, and about 25 percent of the total maintenance expense.