Temperature and airflow management in tunnel dryers
More air beats hotter air more often than people expect.
In a tunnel dryer, temperature supplies the energy to evaporate water and airflow carries that water away. Both have to be matched to the product: SahelDry chambers hold 1,500 kg per batch between 40 and 100 °C at 2.0–3.0 m/s, and the common failure is raising the first while leaving the second alone.
Two variables doing different jobs
Temperature provides the energy that turns liquid water into vapour at the product surface. Air velocity removes that vapour once it is there. If the air is not moving fast enough, the layer immediately around the product saturates, and no amount of additional temperature will speed things up — the water has nowhere to go.
That is why the instinct to raise temperature when a cycle runs long so often fails. If the constraint is vapour removal rather than energy supply, more heat adds nothing but risk: the surface dries faster relative to migration from the centre, and you get case hardening on top of a cycle that is still slow.
What the numbers mean in practice
| Parameter | Value | What it governs |
|---|---|---|
| Chamber temperature | 40–100 °C | Energy available for evaporation, and the product’s heat tolerance |
| Air velocity | 2.0–3.0 m/s | Rate of vapour removal from the product surface |
| Batch size | 1,500 kg per chamber | Total evaporative load per cycle |
| Panel construction | 100 mm rockwool-core sandwich | How much heat leaves through the wall rather than into the product |
| Fire rating | 120 minutes | Panel classification, not a process parameter |
| Control | Full PLC with recipe management | Repeatability of the profile, batch to batch |
Uniformity across the chamber matters as much as the setpoint. If one rack sees 2.0 m/s and another sees 3.0, the batch does not finish together, and operators end up running the whole load to the slowest rack — which over-dries everything else. That is what volume-control dampers are for.
Staging beats a flat setpoint
A flat temperature for a whole cycle is the wrong profile for most products, because the constraint changes as the cycle progresses. Early on there is free surface moisture and plenty of it: the limit is vapour removal, so airflow matters most and temperature can be gentle. Later, the surface is dry and the limit is migration from the centre: temperature can rise without risk because there is no longer a fast-drying surface to seal.
This is why PLC recipe management is a process feature rather than a convenience. A staged profile executed identically on every batch is what makes moisture and breakage predictable, and predictability is what a buyer is actually paying for.
Where the heat comes from
The chamber is fed by a hot air generator delivering 2,500–3,000 m³/h at a controlled 50–150 °C, which is above the chamber range because the delivered temperature at the product is set by the process, not by the generator. Heat transfer is indirect, so combustion gas never enters the air that touches food — a requirement rather than a refinement in any food application.
Frequently asked questions
My cycle is running long. Should I raise the temperature?
Check airflow first. If the air around the product is saturating, extra temperature buys nothing and risks case hardening. Confirm you are getting 2.0–3.0 m/s across the bed, check the dampers are balanced to that chamber, and check loading density — an overloaded rack blocks its own airflow.
How do I tell whether temperature or airflow is my constraint?
Measure the air leaving the chamber. If it is close to saturation, vapour removal is the limit and more heat will not help — increase airflow or reduce loading density. If it leaves dry and warm, the chamber has spare capacity to carry moisture and temperature is what is holding the cycle back. Design air velocity is 2.0 to 3.0 m/s, so there is usually room to move within it.
Why do the racks nearest the inlet finish first?
Because the air reaching them is both hotter and drier than the air reaching the far end, which has already picked up moisture. Balanced volume-control dampers reduce the gradient; rotating wagons through the cycle removes what is left. A batch that finishes unevenly is nearly always a distribution problem rather than a setpoint one — the target is an even 2.0 to 3.0 m/s across every rack, not a higher number at the inlet.
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