A tobacco plant is a humidity machine with a fire risk
Tobacco arrives at a processing plant with a moisture content and leaves with a moisture content, and almost everything the plant does is about moving one to the other without overshooting.
That makes it an unusual industrial building: the product is not damaged by weight, impact or temperature so much as by a slow change in humidity.
The envelope is the humidity control
Redrying and conditioning equipment does the active work, but the building has to hold the result. A hall with a large uncontrolled air change rate will fight the equipment all day and lose at the edges.
In practice this means the walls and roof of the conditioned area need to be treated as an insulated, relatively airtight envelope, and the steel frame behind it needs to be protected from the condensation that follows.
The detail that fails first is the junction between the insulated lining and the structure: a cold bridge there produces a wet patch, the wet patch produces mould, and the mould is found during a quality audit rather than during construction.
Leaf is a combustible load with a low ignition threshold
Dry leaf burns readily and it is stored in large volumes. A leaf store is a fire compartment by nature, not by preference.
The structural implication is separation: the store should not share a roof plane with the processing hall, and any opening between them should be a fire-rated door rather than an opening left for forklift convenience.
Where relief venting is required for the store, the panels providing it must be designed to release and must not carry services or lining.
Dust from handling accumulates in the roof
Leaf handling produces dust and fine leaf fragments. On a steel roof these collect in the profile, hold moisture and, in a dry season, present a surface that will carry a flame along the underside of a roof.
A smooth soffit and a roof detail that can be cleaned matter more than the fire resistance of the frame, because the fuel is the problem rather than the structure.
Floor loads are modest, but the layout is not
Compared with a mill or a foundry, tobacco handling is light. Forklifts, pallets and containers set the floor loading, not the process.
What is demanding is the layout: receival, conditioning, redrying, cooling, packing and storage are sequential, and each one has a different environment. A single open hall forces compromises at every boundary.
What I want before drawings
- The process stages and the moisture condition each requires.
- Whether the conditioned areas need an insulated envelope, and to what standard.
- The leaf store volume and whether it is a separate compartment.
- The dust extraction concept, and who is designing it.
- The vehicle and container movement at receival and dispatch.
- The wind speed for the site in km/h from the applicable local code, not a round number.
A tobacco plant is a building where the HVAC and the envelope do most of the work. Getting the structure right is necessary; getting the boundary between the conditioned and unconditioned zones right is what decides whether the plant performs.
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: another dried-leaf process · a store that needs air movement · separating a fire load · adding a floor inside a store
