A grain dryer is hot, tall and bolted to something cooler
A grain dryer is a tall, narrow box or column through which grain falls against a flow of hot air. It is often the tallest thing on a cooperative site after the silos, and it is the only thing on the site that is routinely hot.
Both of those properties — height and heat — have structural consequences that are easy to draw around and expensive to discover later.
A hot tower is a longer tower
Steel expands with temperature. A tower that is several storeys high and heated through a drying season grows measurably, and it grows from a fixed point that somebody has to choose.
If the tower is welded or rigidly bolted to a cooler supporting frame at every level, the growth has nowhere to go. It goes into the connections, into the supporting frame, and into the ductwork that joins the two.
The conventional answer is a fixed support at one level and sliding or slotted supports elsewhere, with the fixed point chosen so that the important connection — usually the inlet or outlet duct — sits near it and moves least.
Ducts are the part that tears
Hot air ducting between a burner, a plenum and the drying column is large, thin-walled and often insulated. It is also the most flexible thing in the assembly, which means it is where movement shows up first.
Ducts need their own supports, their own expansion provision, and — critically — they should not be used to brace the tower. A duct that has become a structural member is a duct that will crack at its flange.
The dryer is not usually the problem; the interface is
A dryer supplied as a package arrives with its own frame and its own support points, and the supplier will say where it wants to sit. The building designer's job is the interface: a foundation or a steel platform that takes those points, at the right levels, with the right allowance for movement.
Where that interface is left to site, the result is shimming and welding under a hot tower, which is the worst place to improvise.
Tall and narrow is a wind case of its own
A dryer tower presents a small plan area and a large vertical face. Its wind load goes into a compact foundation, and its own overturning resistance has to come from that foundation rather than from the building it stands beside.
If the tower relies on the adjacent structure for stability, then the adjacent structure has been given a job it was not designed for. The tie can exist, but it should be a tie with a defined purpose and a defined load.
Grain dust does not stop at the dryer
A dryer moves a lot of air and a lot of grain, and it produces dust and fines at both ends. The area around it — the wet pit, the discharge, the elevator boot — is the dirtiest part of a grain site.
For the steel, that means the base details, the stairs and the platforms around a dryer deserve more attention than the same details elsewhere on site. It is the same argument as for a wet corner in a pump house: the environment is not uniform, so the specification should not be either.
What I want before drawings
- The dryer package drawings with support points and their loads.
- Operating temperature of the structure, not just the air — the steel temperature is what drives expansion.
- Where the fixed point is expected to be, and which duct or chute cannot move.
- Whether the tower is free-standing or tied, and the load the tie is meant to carry.
- The maintenance access arrangement: platforms, stairs, and where a person will stand.
- The wind speed for the site in km/h from the applicable local code, not a round number.
Dryers are bought as equipment and installed into a building. The building is where the heat, the height and the access all have to be resolved at once.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: silos and the mill settle differently · expansion joints in a long building · a rice mill carries its own fuel · erection sequence
