A coffee or cocoa primary processing line has three stages that want three different buildings: washing and drying, which want air; hulling and grading, which want dust control; and storage, which wants stability.
Putting them under one roof is normal, and it is where most of the trouble starts. The three stages fight each other over humidity, and the building is the referee. If it is designed as a single volume, the drying stage humidifies the storage stage, and the storage stage loses the quality the drying stage was trying to protect.
1. Three stages, three sets of requirements
The stages are not equally demanding, and knowing which is which decides where the money goes.
Washing and drying needs large air volumes and a way for moisture to leave. It is tolerant of temperature and intolerant of stagnation: a drying floor without air movement does not dry, it just gets warm. Hulling and grading is a dust and fines problem, and the dust is both a nuisance and a fire load. Storage wants a stable, dry, moderate environment, and it is the stage most easily damaged by the other two.
The design consequence is that the building should be conceived as three zones with defined boundaries rather than one hall with equipment in it. Where those boundaries fall is a plan decision, and it is much cheaper to make on paper than to build a partition later.
2. The drying floor: air in, moisture out, and a roof that does not drip
A drying floor is the part of the building that most resembles a machine.
Air has to enter, pass through or over the product, and leave carrying moisture. That means inlet area low down, outlet area high up, and a path between them that the air actually follows rather than short-circuiting at the first opening. In a naturally ventilated building, the roof shape and the ridge opening do most of that work; a ridge that is sealed to save on flashing is a ridge that stops the drying floor from working.
The other requirement is that the roof does not condense onto the product. Warm, moist air under a cool roof sheet will condense, and it will drip — onto the drying beds, which is worse than rain because it arrives continuously.
3. Humidity is the boundary that matters
Between the drying zone and the storage zone, the important barrier is not a wall — it is a humidity barrier.
A partition that is not sealed at the top, at the base and around every penetration will let the drying air migrate into the store, and the store will drift to the humidity of the drier rather than staying at its own. The detail that matters is continuity: a vapour-tight layer that runs from the floor, past the junction with the roof, and is not interrupted by a service penetration that nobody sealed.
A sealed boundary is also the only way to run the two zones at different air-change rates without one draining the other. Two zones sharing one volume will equalise, whatever the design says, and the equalising point is somewhere neither process wants to be.
Where the two zones share a roof, that continuity is difficult. The practical answer is to separate them with a full-height wall that runs to the underside of the roof, and to treat the roof above each zone as part of that zone's environment — which usually means a different roof detail on each side, and a joint between them.
4. Dust, and the fire load it creates
Hulling, grading and roasting all produce dust and fines, and in a steel building the dust settles where the structure is horizontal.
The cleaning argument is the fire argument. Fines on a purlin, on the top flange of a rafter, or on the underside of a roof sheet are a fuel bed in exactly the place where a fire would spread along the building. The design answer is a structure that sheds: no ledges above the process, no inaccessible pockets, and access to clean whatever cannot be avoided.
The extraction is the second half. Dust collection needs to be sized to the process and positioned so that the duct runs are short and do not pass over the storage zone, because a duct full of fines passing through a store is a fire route with a straight line to the most valuable material on site.
5. Storage: the zone that pays for the building
The storage zone is where the product waits, and it is where value is lost quietly.
Two things matter more than the structure. The first is that the floor stays dry: a slab that wicks moisture from the ground will damage bagged product from below, which means a vapour layer under the slab and a floor detail that does not rely on the bag's own packaging. The second is that the roof does not drip, which brings the insulation and the ventilation back into the argument.
Bag stacks also load the floor heavily and unevenly, and they need height — a store with 4 m (13 ft) of clear stack height under the eaves is a different building from one with 6 m, and the difference is a few rows of bags, not a few percent of area.
6. What I want before drawings
- The three stages, their areas, and which ones produce or need moisture.
- The drying method — sun, forced air, or mechanical — and the air volumes involved.
- The product's target moisture at each stage, and how far it may drift.
- Dust and fines volumes, and where the collection plant can stand.
- Stack heights and bag weights in the store.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the ridge and open-side detail — a drying floor needs large openings, and large openings are where wind pressure enters the building and where rain arrives sideways.
Coffee and cocoa are decided by moisture, not by span. Treat the three stages as three climates, and the steel becomes the simple part of the project.