A tea factory is a drying building that also stores fuel
Tea processing is a sequence of moisture changes: fresh leaf is withered to reduce water, rolled, fired to stop oxidation and dry, then graded and stored.
Each stage has a different relationship to water and heat, and the building has to hold all of them without letting one condition leak into the next.
Withering needs air, not heat
Withering removes moisture slowly with moving air. The troughs need a large volume of air at controlled humidity, which means the withering floor is essentially a ventilation hall with troughs in it.
For the building, that means large air inlets and outlets, and a structure that does not obstruct the airflow. It also means the roof underside will be at the dew point much of the time, so a liner or a ventilated detail is worth considering before the purlins are ordered rather than after the first drip lands on a trough.
Firing is a hot, dry room next to a wet one
The dryer or firing machine produces heat and dry air, and it sits next to a withering floor that is deliberately humid. Two adjacent rooms with opposing humidity conditions will exchange moisture through any opening between them.
That is a planning question before it is an envelope question: a wall and a door between the two zones, rather than one large open hall, keeps both conditions stable and keeps the building simpler.
Finished tea is a fire load that stays
Once the leaf is dry, it burns readily. Storage of finished tea, packing materials and any fuel for the dryer are all combustible loads, and in most factories they end up adjacent to the production area because that is where the space is.
A fire compartment between the store and the production hall is the cheapest fire engineering in the project. It costs a wall; it saves a building. The same principle applies to any fuel store serving the dryer.
Dust from grading is fine and it travels
Grading and packing produce fine dust that settles on every horizontal surface. On a steel roof, that dust holds moisture and it also settles into the profile where it cannot be cleaned.
Smooth, accessible surfaces and a roof pitch that sheds rather than traps are worth more here than a heavier frame. The detail that matters is the one at the eave, where dust accumulates and water follows it.
What I want before drawings
- The process stages and the humidity condition each one needs.
- Where finished tea and packing materials are stored.
- Whether the dryer is fired by wood, oil, gas or electricity, and where that fuel is stored.
- The airflow requirement for the withering floor.
- Whether any part of the building needs fire separation.
- The wind speed for the site in km/h from the applicable local code, not a round number.
Tea factories are usually built in places where the climate is already humid and the site is already sloping. Both of those facts should reach the structural design, rather than being handled on site after the frame is up.
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: heat and thermal movement · a plant that carries its own fuel · separating a fire load · a store that needs air, not heat
