Conveyor Capacity: Peak vs Average Feed Rate
Choosing the right conveyor capacity is crucial. Bottom line, a system must be sized according to the highest rate of material fed onto the conveyor belt at any given time – referred to as peak capacity. Like most conveyor manufacturers, we rate our units by this maximum capacity. So if a model is rated at 1,000 tons per hour; that feed rate is its peak, or the point where any of its components (the drive, reducer, pulleys, shafts, bearings, belting) could max out and become a limiting factor. If you exceed capacity, you are compromising the service factor of the conveyor.
A common problem is sizing a conveyor to its average capacity, rather than its peak. Consider that when feeding a conveyor, one rarely has a consistent material flow. Many operators do not have a belt scale that will indicate the output of the conveyor at any moment. But rather, they have a method of measuring output over the course of the day. If the operator is stockpiling 10,000 tons per day over a 10-hour shift, he may “average” out his feed rate at 1,000 tons per hour. But without a consistent material flow, the operation may be running 2,000 tons per hour in a 5-hour period, and zero tonnage for the rest of the shift. This scenario illustrates how using an average feed rate to size a conveyor can result in exceeding rated conveyor capacity by 100-percent.
With this in mind, it’s easy to see why producers, in consultation with their conveyor manufacturers, must identify the true peak capacity figure, which will then dictate the choice of each component – each being chosen due to an ability to meet or exceed capacity. Typically, manufacturers choose horsepower as the limiting factor. In other words, how much horsepower is required to convey material at its peak feed rate? If the feed rate exceeds peak, the service factor of the chosen drive is threatened.
If an operation cannot pinpoint the peak feed rate figure, or if they consistently have erratic changes between peak and average feed rate levels, it is arguably more cost effective to add a surge bin or reclaim tunnel, which will turn any inconsistent flow into an average flow. For example, consider the situation where a loader empties its bucket into a conveyor hopper once every 60 seconds; however, the conveyor transfers that bucket load in 30 seconds. The loader may average 500 tons per hour, but the conveyor sees a peak of 1,000 tons per hour. In this application, a surge bin would be ideal, as the loader could keep the bin full, and the conveyor could just keep pulling from that surge volume at a consistent feed rate. This method is far more efficient, requires far less horsepower, and places far less stress on shafts, idlers and bearings.
But what if you’re dealing with a portable operation where a surge bin may not be practical? Perhaps a truck unloader is being used to transfer material to the feed conveyor, then on to a stacking conveyor. Knowing peak capacity is very important in this case as each of those conveyors will need to match the capacity of that truck unloader – or the system will be undersized.