How to Size a Tunnel Dryer

The four inputs, and where each one is usually wrong

Peak-day fresh intake is the first and the most often mis-stated. Buyers quote a season tonnage, divide by the number of days and size against the result. That average is always smaller than the peak, and the peak is the fortnight that generates most of the year’s margin. A line matched to the mean turns fruit away exactly when it is worth most.

The second is yield, and the trap there is measuring it on trimmed flesh rather than on whole fruit at intake. Ten to one for mango is a whole-fruit ratio: ten tonnes through the gate for one tonne out of the packing room. Quote it on flesh and the chamber count comes out low by roughly the peel and stone fraction.

The third is cycle time including turnaround — unloading, cleaning, reloading. A sixteen-hour drying cycle is not one batch every sixteen hours. The fourth is chamber capacity, which depends entirely on format and must be named alongside the number.

The arithmetic

With those four, the calculation is one line: peak-day intake, divided by chamber capacity, divided by batches per chamber per day. A 9,000 kg peak day on 1,500 kg standard tunnel chambers at one batch a day is six chambers. The same 9,000 kg in container format at 828 kg per chamber is eleven.

That difference is the whole reason format has to be stated. Two suppliers quoting "six chambers" can be offering 9,000 kg and 4,968 kg of daily intake, and a buyer comparing chamber counts rather than tonnages will pick the wrong one.

Product	Cycle	Batches per chamber per day
Mango	16–20 h	One
Fish	24–30 h	One every day and a half
Tomato, onion, chilli	Product dependent, 40–100 °C range	One, occasionally two on thin slices

Nothing in that table is a promise about your product. It is the shape of the answer, and the shape is what most sizing conversations are missing.

What a supplier should be able to show you

Ask for the working, not the conclusion. A supplier who can only produce a chamber count has either done the arithmetic and is hiding it or has not done it. The chain from tonnage to burner duty runs through water removed per hour, evaporation load in kilowatts, envelope loss, and an overall fuel-to-evaporation efficiency — and that last term is where quotations diverge most.

Most published efficiency figures are combustion efficiency alone, around 85 %. The full chain multiplies combustion by the hot air generator, the ducting, the envelope and the exhaust, and lands around 0.40. A supplier quoting 85 % as system efficiency is quoting the first of five terms.

Size for the peak, not for the average

Undersized, a line refuses fruit in the weeks that decide the year, and refused fruit is sold at the roadside price. Oversized, capital sits idle for the ten months that do not matter. Both are expensive, but they are not equally expensive: an empty chamber costs nothing to leave empty.

The way out of the trade is usually not a bigger line but a longer season. A mango chamber works about 100 days a year; adding tomato, okra, chilli or fish out of campaign raises utilisation towards 300 without adding a chamber. What it costs is a full wet clean between products, which is a shift rather than an hour.

Frequently asked questions

How many tunnel chambers for 10 tonnes a day?

On standard 1,500 kg chambers with a mango cycle of 16 to 20 hours, seven chambers take 10,500 kg in one daily batch. In 20 ft container format at 828 kg it is twelve. That is orientation rather than a quotation: your own yield and cycle move it in either direction.

Does a longer cycle always mean fewer batches?

Below 24 hours, no — a 16 and a 20 hour cycle both give one batch a day. Above it, yes: fish at 24 to 30 hours gives one batch every day and a half, so the same tonnage needs roughly half as many chambers again. The step is at 24 hours, not proportional to cycle length.

Can I add chambers later instead of buying them now?

Only against burner headroom, so the phasing decision is really a burner-sizing decision made on day one. A burner running at 60 % of rating has room for expansion; one at 90 % does not, and phase two then means replacing the heat plant rather than adding to it.