Description
One of the major drawbacks of a flat bottom silo is that they do not unload completely by gravity alone, due to the angle of repose of the grain in the silo.
The larger the diameter of the silo, the more grain gets left behind in the silo.
One of the ways to overcome this problem on flat bottom silos, is to construct – in concrete – a conical bottom for the silo, which allows all the grain to flow to the centre of the silo, and to be unloaded by the silo unload conveyor. We discuss the problems with conical floor concrete foundations below the video.
Sweep augers work on the principle of an auger, that is installed on a pivot point – on the centre well of the silo – and the auger then pivots 360º around the silo, collecting grain from the entire silo surface area, and transporting it to the centre well, where it is discharged into the silo unloading conveyor.
The sweep auger has a backing shield and a maximum angle limiter, and a slipping drive wheel, which allows the sweep auger to run into a heap of grain and slowly work itself into the heap, transport the grain to the centre well.
The sweep auger does not however make a completely clean pass over the grain – because your can’t have the spiral of the auger running into the silo floor (damaging itself against a concrete floor, or damaging the aeration floor, if running in an aerated silo) so you’re always going to have a layer of grain at the bottom of the silo, even after the sweep auger has made multiple passes – that will have to be cleaned manually.
There are a few different types of sweep augers available and the unit you select, will be based on two main factors:
i) The diameter of the silo
Lower diameter silos are able to utilise cheaper, lighter sweep augers, as the distance between the centre well and the sidewall of the silo isn’t very far – thus a sweep auger with a pivot point and a single wheel at the end will suffice – these types of sweep augers are suitable for silos up to 60′ (18m) in diameter
Higher diameter silos 72’/22m and larger would require both support between the pivot point and the end wheel, and will usually require an extra drive unit to push the auger into the grain to ensure capacity, that the single wheel can’t achieve on its own – over the long distance, and the extra support one the auger, stops the spiral from flexing over the span.
ii) The required capacity that you require from the sweep auger.
It isn’t always nececsary to select a sweep auger that has the same of simlar capacity as your unloading conveyor. Usually there will be around 5%-15% of the grain of your silo left that won’t discharge by gravity – and you won’t necesarily be cleaning out the silo completely every time you use the silo – for example in flowthrough silos – you would typically only clean out the silo completely once a season – for mills and more continuous users – you’d perhaps only clean out the silo 4 times a year, so capacity of the sweep auger isn’t as important. Typical sweep auger capacities range between 40 and 80 tons per hour (some large commercial sweep augers can reach up to 150TPH)
See the video below for a typical single wheel sweep auger in action:
There are many drawbacks to this method – most notable being:
i) The cost of constructing is conical floor on a flat bottom silo – is extremely high – for a clean loadout – the walls of the cone need to run at an angle of at least 40º, while still supporting the weight of the grain above it – which is a difficult construction
ii) Typically when constructing the conical floor, it would require the cone of the foundation to extend beneath the natural ground level. Irrespective of who builds this – there is going to be water seepage into the bottom of the cone at some point in the future – especially in areas with high water tables, and water at the bottom of the silo, when it’s full of grain, causes grain to rot in the bin, and may block the unloading conveyor – creating massive headaches for offloading the silos. Constructing the hopper above ground, with the silo placed around the hopper – is extremely costly, and makes installation of the silo very difficult
iii) Installing aeration in conical floor silos is very difficult, and ads to the civil works cost, as there is then a need to construct channels as well in the conical slopes that are running at minimum 40º.
iv) It is near impossible to retrofit aeration systems into conical floor silos.
The benefits are relatively small:
i) The grain flows out – by gravity, for a clean load out of the silo
ii) Some extra tonnage is gained in the cone of the foundation – above what is stored in the grain silo itself
Just based on a cost implication – sweep augers are the better option – the cost of a sweep auger will usually only be about 20% of the extra cost that you’ll have to expend when doing a conical floor instead of a flat floor.
