The system must distribute heated air evenly across the product, maintain the required drying temperature and carry evaporated moisture away from the product surface.
Part of the return air is usually recirculated to improve energy efficiency, while a controlled fraction is exhausted to prevent humidity from rising excessively.
Fan position is important. A fan installed before the heater handles cooler air, while a fan installed after the heater must be designed for the actual hot-air temperature.
The fan must maintain the required airflow after losses from heat exchangers, filters, ducts, dampers, perforated plates, product racks and return-air paths are included.
Resistance can change as filters load, products block airflow or dampers adjust. The selected operating point should reflect the expected production condition rather than an empty chamber.
The fan is installed before the heater or in a cooler return-air section. This can reduce thermal stress on the fan and motor.
The fan handles heated or moisture-laden air after the heater or drying chamber. Temperature capability and material configuration require confirmation.
For a new system, airflow, heater capacity, recirculation ratio, moisture exhaust and chamber distribution should be considered together.
For an existing oven, compare fan duty with product loading, blocked filters, heater condition, leakage, recirculation ratio and chamber temperature distribution.
The fan moves heated air through the drying chamber, across the product and back through the return-air path. Stable airflow helps maintain temperature uniformity and moisture removal.
Both arrangements are possible. A fan before the heater handles cooler air, while a fan after the heater handles hotter air and requires temperature-capable materials, bearings and drive arrangement.
Airflow depends on chamber size, product loading, moisture evaporation rate, target drying time, air temperature, allowable temperature variation and recirculation ratio.
Higher recirculation can improve energy efficiency, while sufficient fresh-air intake and moisture exhaust are still needed to control humidity and maintain drying performance.
Higher pressure may be required when the system includes long ducts, heat exchangers, filters, narrow air knives, perforated plates or high-resistance product loading.
4-72, 4-79 and 4-68 cover common clean-air circulation directions. 4-2×72 suits large-volume systems, while 9-19 may be reviewed for higher resistance.