Plant Efficiency

What plant efficiency actually measures

Plant efficiency is not the peak figure on a datasheet. It is the number of saleable tonnes that cross the weighbridge at the end of a shift, divided by everything the shift consumed to produce them: hours, kilowatt-hours, wear metal and labour. A production manager controls four levers only — how many hours the circuit really crushes, how heavily it is fed during those hours, how much energy each tonne draws, and how much material falls outside specification and returns for a second pass. Read together, those four lines usually reveal a double-digit gain that requires no new machine at all.

Why one headline percentage hides the problem

Overall equipment effectiveness is the product of availability, performance and quality. Availability penalises stoppages, performance penalises running below design rate, and quality penalises material that has to be reprocessed. Collapsed into a single number, the three factors cancel each other out and the cause disappears. Recorded separately, they tell you whether the feeder stood empty, whether the chamber ran half full, or whether the fines fraction drifted out of tolerance.

Utilisation: counting hours honestly

Utilisation is actual crushing hours divided by scheduled available hours. Scheduled hours are the hours the plant is staffed and expected to produce; crushing hours are the hours material genuinely passes through the circuit. Waiting for a blast, a broken truck cycle, a plugged chute or a screen media change all belong to the denominator, never the numerator. Plants that blur the two report a utilisation figure that is comfortably wrong.

Split the downtime ledger by cause

A ledger that records every stoppage as "breakdown" cannot locate a bottleneck. Six categories are enough: planned maintenance, wear part replacement, no feed available, blockage, electrical or hydraulic fault, and full product stockpile. Once a week of data is grouped this way, the top two entries are usually no feed and blockage — both organisational problems rather than mechanical ones.

Specific energy in kWh per tonne

Specific energy is total kilowatt-hours drawn over a period divided by tonnes produced in the same period. For hard rock aggregate operations a widely quoted good-practice level is roughly 10.6 kWh per tonne, while sector leaders aim at 8.0 kWh per tonne or better. Whole-plant figures scatter between 2.5 and 40 kWh per tonne depending on rock competence, reduction ratio and auxiliary load. Isolate the crushing stage alone and many applications sit between 0.5 and 2.0 kWh per tonne; the balance is conveyors, screens, pumps and dust suppression.

A whole-plant reading above 20 to 30 kWh per tonne means either the circuit is producing very fine product or a significant loss is hiding in it. To convert the kilowatt-hours into currency per tonne, feed the same period's weighbridge data into the operating cost calculator.

Finding the bottleneck

The bottleneck is not the machine with the smallest rating; it is the machine running closest to full. Three signals identify it: motor current or power draw as a share of rated power, the continuity of the material profile on the belt, and the level trend in surge bins. A bin that keeps filling points downstream of itself; a bin that keeps emptying points upstream.

Log power draw across a full shift

A crusher held steadily near rated power is properly fed. A sawtooth current trace means the feed is intermittent, and no setting change will fix that. If the crusher itself looks healthy, the constraint has moved to screening area. Level and current histories for this kind of diagnosis come from the automation and control system, which is why shift-by-shift logging is worth more than a single spot reading.

Treat a blockage as an outcome

Blockages are usually the last link in a chain that started with intermittent feed or a setting that does not match the feed gradation. Recording the cause of each event, rather than the event itself, is what stops it recurring.

Circulating load in a closed circuit

Circulating load is the oversize returning from the screen to the crusher expressed against fresh feed. In closed-circuit cone applications a band of 20 to 40 per cent is common and manageable; there is no single correct percentage, because it follows from circuit layout and the product curve you sell. Carrying more load than necessary lowers net plant throughput, accelerates liner wear and raises power consumption. Return conveyors and screening area must therefore be sized for the intended load — when the conveyor belt line is undersized, the constraint simply migrates from crushing to handling.

Choke feeding and closed side setting

Choke feeding means keeping the crushing chamber full. With a full chamber, material breaks largely particle against particle, the product leaves more cubical, and the whole liner and concave surface wears together. A starved crusher concentrates wear in one zone and shortens part life measurably. In practice the feed should completely cover the head nut, the feed should be well graded rather than segregated, and the motor should sit near rated power, commonly around ninety per cent of capacity. Holding that by eye is unrealistic; interlocking a level sensor with the feeder makes it continuous.

What closing the setting costs

Reducing the closed side setting raises reduction ratio, increases fines and power draw, and lowers hourly throughput. Sensible reduction per stage is also the measure that decides machine type.

Ratios are calculated from the eighty per cent passing sizes of feed and product, not from the extremes. Which machine suits your secondary stage is worked through in the cone versus impact secondary comparison, and the alternatives can be compared across the full crusher range. Circuits chasing high reduction favour a secondary impact crusher, while abrasive hard rock returns longer part life on a cone crusher.

Screen efficiency and the fines you are giving away

Screen efficiency is the share of undersize present in the feed that actually reports to the underflow. It is derived by mass balance from sieve analyses of three samples — feed, oversize and undersize — using the undersize fractions in each stream. One hundred per cent is unreachable in industrial practice because some fines never contact an opening; most operations target 90 to 95 per cent. Four causes account for most losses: overfeeding, which shortens residence time; blinding and pegging, which reduces effective open area; surface moisture, which agglomerates fines; and a bed deeper than roughly four times the aperture, which prevents stratification. Where moisture dominates, the dry-or-wet decision is examined in the dry versus wet screening comparison, while a twin vibrating screen adds surface area on one chassis and thins the bed.

Cost per tonne, line by line

Cost per tonne is built from six lines: energy, wear parts, oil and filters, labour, planned maintenance, and haulage or rehandling. No line decides on its own, because a setting that lowers energy in one shift can raise wear metal in the next. Capacity step enters the same table directly: a larger step demands less installed power per unit of throughput.

The last column divides optional generator rating by peak capacity and shows the scale effect. Choosing between the three steps depends on quarry reserve, truck fleet and product curve; the detail sits on the 640, 800 and 950 capacity selection page. Where circuit flexibility and stockpiling matter more than mobility, the same arithmetic is applied to a stationary crushing plant.

Where to start measuring

Three numbers logged for one week are enough to see the real plant: actual crushing hours per shift, weighbridge tonnes, and metered kilowatt-hours. Combine them with a downtime ledger split by cause and the bottleneck names itself. Send us that week of figures and our engineering team will review your circuit against the same numbers you measured.