# Engineering a Small-Batch Tea Roaster

Small-batch tea roasting requires tighter control than its simple hardware suggests. A typical machine combines a rotating stainless-steel drum, an electric heating element, variable-speed airflow, and thermocouples positioned near both the tea bed and exhaust. For batches between 500 grams and 2 kilograms, even minor changes in leaf moisture, ambient humidity, or drum loading can shift the roast profile enough to affect aroma and astringency.

## Thermal Control

The main challenge is managing thermal inertia. The heater and drum continue transferring energy after power is reduced, so a basic thermostat tends to overshoot the target temperature. A PID controller improves stability, but its tuning should favor a restrained response: aggressive proportional or integral settings can cause oscillation as the leaves alternately absorb and release moisture. Logging both inlet and exhaust temperatures helps distinguish heater behavior from changes occurring inside the tea.

```yaml
batch_mass_g: 1200
drum_speed_rpm: 38
preheat_c: 118
exhaust_target_c: 104
roast_time_s: 720
cooling_air_pct: 85
```

## Airflow and Drum Speed

Airflow removes water vapor and volatile compounds while keeping the roast environment consistent. Too little airflow can produce dull, stewed notes; too much can cool the leaf surface and extend the process unnecessarily. Drum speed matters as well: slower rotation increases contact time with the hot metal, while faster rotation produces a more convection-driven roast. For rolled oolong, moderate drum speed usually provides an even finish without breaking the leaves.

## Building a Repeatable Profile

A practical roast profile begins with a preheated drum, followed by a brief temperature drop when the batch is charged. Heater output is then increased gradually until the exhaust temperature recovers. Near the end of the roast, power is reduced before the target temperature is reached to account for stored heat. Operators should record charge temperature, recovery time, exhaust temperature, airflow, and sensory observations for every batch.

## Cooling and Maintenance

Cooling must begin immediately after discharge because the leaves retain enough heat to continue roasting. A perforated tray with strong ambient airflow can bring a one-kilogram batch below 40°C within three minutes. Between sessions, the exhaust path should be cleared of dust and aromatic residue, thermocouples checked for drift, and the drum inspected for oil buildup. These routine checks preserve both flavor consistency and fire safety.