Wood-fired and solar dryers, step by step
Interactive lesson: how a dryer works, building a small wood-fired dryer with a heat exchanger, firing in stages, fire and CO, the solar dryer and maintenance.
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The whole lesson as text
1. How a dryer works
Every dryer, from a kitchen model to a brick one, works the same way.
- Three parts — A heat source, a chamber with trays and a path for the air.
- Air from bottom to top — Cool air comes in through openings near the bottom, warms up, rises through the trays and leaves with the moisture through an opening near the top.
- Indirect heat — The smoke goes through the heat-exchanger pipe into the chimney, not through the food: no soot, tar or CO in the chamber.
- Swap the trays — The bottom tray is the hottest. Every 2–4 hours the top trays go down, the bottom ones up, and they are turned front to back.
Why: Cool air comes in at the bottom, warms up, passes through the trays, takes up moisture from the food and leaves at the top. If you close the outlet, the moisture stays inside and the food stews and turns sour.
- Wood smoke contains tar and PAHs, which settle on the food. For a new dryer, choose indirect heat.
- In larger dryers the fresh batch goes at the top and the half-dried batch moves down.
2. Building a wood-fired dryer
A proposal for a small dryer with 10 trays of 50 × 100 cm: 20–30 kg of fresh fruit per batch.
- Slab and lower section — A concrete slab 150 × 120 cm, 10 cm thick. On it, a lower section of solid brick, inside 110 × 60 cm, 70–80 cm high.
- Stove and heat exchanger — A small sheet-steel or cast-iron stove, door on the outside. Flue pipe Ø 120–150 mm (5–6 in), 3–4 m long, U-shaped, then into the chimney. A metal shield above it.
- Chamber and trays — Chamber inside 110 × 60 cm, 150–160 cm high. Tray runners every 12–15 cm; the first tray at least 40–50 cm (16–20 in) above the pipe.
- Chimney and vents — Chimney 1 m above the dryer roof, with a spark arrestor. Two inlets 20 × 10 cm at the bottom and an outlet 20 × 20 cm in the roof, with a flap and mesh.
Why: The sizes are approximate; adapt them to the materials you have. What matters most is that the smoke stays in the pipe and the timber stays well away from the hot pipe.
- Before building, check whether the structure needs planning permission or building regulations approval (UK: Planning Portal and council building control; IE: local authority; US and Canada: county or municipal building department).
- Flue pipe joints must be tight and sealed, and the pipe is swept of soot before every season.
3. Firing · Locked
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4. Fire and carbon monoxide · Locked
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5. Solar dryer · Locked
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6. Maintenance · Locked
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