Sizing a tunnel dryer — from fresh tonnage to chamber count
Four inputs, one arithmetic, and the number most suppliers do not publish.
Chamber count is derived, not chosen from a range. Four inputs settle it: peak-day fresh intake, the fresh-to-dried yield, the cycle time, and the capacity of one chamber — 1,500 kg for a standard tunnel, 828 kg for a 20 ft container conversion. Everything else follows.
Indicative price of an industrial tunnel dryer, Ex-Works (EXW): USD 6,280–41,885
Shipping, customs clearance and local duties are extra. The final price depends on the configuration and is fixed in the written quotation.
The four inputs
Sizing goes wrong in one of two directions and both are expensive. Undersized, the line turns away fruit in the six weeks that decide the year. Oversized, capital sits idle for the ten months that do not. The way to avoid both is to derive the answer rather than pick a model off a list.
- Peak-day fresh intake, in kilos — not the season average, which is always the smaller and always the wrong number.
- Fresh-to-dried yield for your product, measured on whole fruit at intake rather than on trimmed flesh.
- Cycle time, including turnaround: unloading, cleaning and reloading between batches.
- Chamber capacity, which depends on the format — and the format has to be named alongside the number.
A worked six-chamber line
This is a container-format mango line: six 20 ft conversions, each holding 8 wagons at 46 trays, on a 14-hour cycle. Every figure below is design basis for that configuration and is not a measured result from an operating plant.
| Step | Figure |
|---|---|
| Chamber capacity | 828 kg fresh — 20 ft container conversion, 8 wagons, 368 trays |
| Tray area per chamber | 165.6 m² at a 5 kg/m² mango loading density |
| Line batch | 6 × 828 = 4,968 kg fresh, 2,208 trays, 993.6 m² |
| Dried output | 496.8 kg per batch at a 10:1 whole-fruit yield |
| Water removed | 4,471 kg per batch, at 319 kg/h across 14 hours |
| Evaporation load | 205.8 kW |
| Envelope loss | 9 kW |
| Useful heat | 214.8 kW |
| Fuel input at 40 % overall efficiency | 537 kW, about 462,000 kcal/h |
| Burner selected | 600,000 kcal/h, running at about 77 % of rating |
The efficiency chain, in full
Most suppliers quote combustion efficiency — around 85 % — and let the buyer assume that is system efficiency. It is the first term of five. Published in full, the chain that turns fuel into evaporated water is combustion 0.85 × hot air generator 0.78 × ducting 0.95 × envelope 0.97 × exhaust 0.68.
| Stage | Factor | What it costs, and what it buys |
|---|---|---|
| Combustion | 0.85 | The fraction of the fuel’s energy released as heat in the chamber |
| Hot air generator | 0.78 | Indirect exchange — the price of never letting flue gas touch food |
| Ducting | 0.95 | Heat lost between the generator and the chamber, set by run length and lagging |
| Envelope | 0.97 | What the insulated chamber loses to ambient, 9 kW on the line above |
| Exhaust | 0.68 | Warm, moisture-laden air deliberately thrown away — this is drying, not a defect |
Multiplied through, those five factors give about 0.42. Sizing on this site is carried at 0.40, which is the more conservative of the two and is the figure the worked duty above is divided by. Both are stated because a reader who does the multiplication should find the same answer we did.
Forty per cent reads badly until you notice what the 0.68 exhaust term actually is. Drying works by carrying moisture out of the chamber in warm air, and that air leaves with its heat. Recovering it is possible and adds cost and complexity; a supplier quoting 85 % system efficiency has simply not counted it.
What changes the answer on your site
The arithmetic above is the method, not a quotation. Four site conditions move the result enough to matter, and all four are established at survey rather than assumed.
- Loading density. The 5 kg/m² figure is provisional for mango and is confirmed against your own fruit — variety, ripeness and slice thickness all move it.
- Design ambient. 35 °C is the working assumption for a Sahelian drying season. A humid coastal site changes the evaporation load more than the temperature does.
- Fuel moisture. A residue rated 4,800 kcal/kg dry delivers considerably less if it has been rained on. Covered storage is part of the sizing, not an afterthought.
- Grid supply. Supply is 400 V / 230 V three-phase at 50 Hz and subject to interruption. What that means for the control strategy is decided before the panel is built.
Specifications
| Parameter | Value |
|---|---|
| Standard tunnel chamber | 1,500 kg fresh per batch |
| Container conversion chamber | 828 kg fresh per batch, 8 wagons, 368 trays |
| Mango loading density | 5 kg/m² of tray, provisional pending site confirmation |
| Tray pitch | 70 mm, giving a 45 mm free air channel |
| Overall fuel-to-evaporation efficiency | 0.40 used for sizing; the five-term chain gives 0.42 |
| Burner range | 200,000–900,000 kcal/h |
| Manufacturing lead time | 10–18 weeks from confirmed order and receipt of advance payment |
The equipment itself is on the industrial tunnel dryers page, and the hygienic build on tunnel dryers for food processing. For the crops these numbers are usually run for, see industrial mango drying system and fruit and vegetable drying; for what the burner runs on, the biomass fuel envelope.
Where this equipment is used
The same equipment is specified across several product classes. These are the ones it is configured for most often:
industrial mango drying system, fruit and vegetable drying and tropical fruit drying.
Each of those pages carries the drying parameters, cycle times and quality targets for that product class.
Frequently asked questions
How many chambers do I need for 10 tonnes a day?
On a standard tunnel chamber at 1,500 kg and a mango cycle of 16 to 20 hours, seven chambers take 10,500 kg in a single daily batch. In container format at 828 kg it is twelve. That is orientation, not a quotation: yield and cycle for your own product move it either way.
Why is overall efficiency only about 40 per cent?
Because five losses compound: combustion 0.85, hot air generator 0.78, ducting 0.95, envelope 0.97 and exhaust 0.68. The exhaust term is the largest and is not a fault — carrying moisture out of the chamber in warm air is how drying works. A quoted 85 % is the first term alone.
Should I size on the peak day or the season average?
The peak day, every time. A line sized on a 5 tonne season mean turns fruit away on a 9 tonne peak day, and that is the fortnight generating most of the year’s margin. Idle capacity out of season is the cheaper mistake: an empty chamber costs nothing, and refused fruit is sold at the roadside price.
Can one line dry more than one crop across the year?
That is usually the argument for building it. A mango chamber works about 100 days a year; adding fish, tomato, okra or chilli out of season raises utilisation towards 300. What it costs is a full wet clean between products and, for some pairings, dedicated chambers.
Have the sizing done properly
Send your peak-day tonnage, your product and your target moisture, and we will run this arithmetic against your site rather than against an example.
Request sizing