A line that runs on containers needs a buffer of them, at both ends and usually in the middle. Too few and the line stops waiting for a container; too many and the buffer occupies floor space that could be doing something else. The right number is not a guess, and it is not the same for fulls and empties – the two have different arrival and departure patterns and need sizing separately.
Two queues, not one
The full-container queue is fed by upstream and drained by consumption. The empty queue is fed by consumption and drained by the return process. These have different variability: consumption is usually steady, while both the upstream supply and the return collection are usually lumpy – a delivery round, a shift change, a vehicle. Where the two buffers are sized the same, one of them is wrong.
What the buffer is protecting against
Variability in supply and variability in collection. A perfectly regular delivery and a perfectly regular collection need almost no buffer; a delivery that arrives late twice a week needs one sized to the lateness rather than to the average. Where a line is sized on averages, it runs well on average days and stops on the others, which is the wrong way round – the buffer exists for the bad days.
Empty buffers are usually the undersized one
Empty containers pile up during production and are collected in batches. The pile between collections is the buffer, and it is usually sized by whatever space happens to be free rather than by the production rate times the collection interval. Where the collection interval is long – once a shift, for example – the pile at the end of the shift is substantial, and if it is in the aisle it is also a safety problem. Nesting or folding empties reduces this by a large factor for the same number of containers.
Where the buffer sits physically
A buffer at the line occupies line-side space, which is the most contested space in a plant. A buffer one step back – in a supermarket or a flow rack behind the line – is cheaper per container and adds a handling movement. The balance depends on how often the buffer turns over: fast-moving buffers belong at the line, slow ones belong behind it. Where a line has both, splitting them is usually better than putting everything at the line.
How many containers are in the fleet
The total fleet size is the buffers plus what is in transit plus what is at the supplier or the customer plus what is being washed or repaired. That sum is usually larger than the number anyone estimated at the start, and the gap shows up as a fleet that is always short. Where a fleet is sized, the in-transit and in-process portions are the ones most often forgotten, and on a long lane they can exceed everything else.
What nesting and folding do to the number
A container that nests when empty takes roughly a third of the stacked height, and one that folds flat takes less than a fifth. Applied to the empty buffer – usually the largest of the two – that converts a large floor area into a small one, and it makes the empty return trip cheaper as well. The trade is a slightly more complex container and a folding operation at each end. Where empties are the dominant buffer, the trade is worth making nearly every time. See also folding empties and storage density.
How to size it in practice
Measure the longest realistic gap in supply and in collection, multiply each by the consumption rate, and add a margin for the variability you have not measured. That gives two numbers, and they are usually larger and more unequal than the ones in use. Where the result does not fit the available space, the answer is to reduce the variability – more frequent, smaller deliveries – rather than to reduce the buffer and hope.