Airflow: Why more air doesn't always mean faster drying?

Air is the medium responsible for transporting the moisture extracted from the walnuts. Therefore, it might initially seem logical to think that the greater the airflow, the faster the drying process. However, tests conducted by UC Davis show that this relationship has its limits.

The tests analyzed air speeds approximately equivalent to 0.25; 0.51; 0.76; 1.02; 1.27 and 1.52 m/sBy increasing the flow, the drying time effectively decreased, going from approximately 8:00 PM to 9:75 AM between the extremes studied.

But the most interesting fact emerges when observing how that reduction evolves.

The initial increases in airflow produce significant improvements in drying time. As we continue to increase the air velocity, the reduction achieved becomes progressively smaller. The study itself concludes that above approximately 0.76 m/s (150 ft/min)Increasing the flow had relatively little effect on drying time, while increasing energy requirements.
Let's look at that data in the following graph:

Initially, increasing the airflow greatly improves the process. After a certain point, we continue to move and heat more and more air to achieve progressively smaller reductions in drying time.

Reducing the drying time by approximately half resulted in an increase in total energy consumption of around 123%, i.e., consuming about 2.2 times more energy per ton processed.

Furthermore, separately, the effect is even more illustrative: the theoretical propane increases by approximately 108% (22.4 → 46.7 kg/t), while the fan electricity increases by around 1,000% (4.9 → 54.2 kWh/t).

It's not just about moving more air

This behavior is especially important when designing a drying system.

Increasing the airflow means the fan must move a greater volume of air through the product mass. And that air must also be brought to the necessary temperature for the process.

That's why there's a point beyond which we're using higher capacity fans and heating larger quantities of air, but obtaining a progressively smaller improvement in the total drying time.

In the UC Davis trials, at the maximum condition studied, roughly equivalent to 1.52 m/sThe energy used by the fan came to represent around 30% of the total energy cost of the process.

How we bring this concept to the development of our heat generators

This principle has been especially important in the development and testing of our HANUTEC heat generators.

We are not simply trying to generate heat. We aim to deliver temperature and airflow under conditions that are truly beneficial to the drying process.

Therefore, the development of our equipment includes tests designed to analyze the combined behavior of the generator, the fan, and the air distribution system. The goal is to achieve sufficient airflow to properly penetrate the product mass and transport the extracted moisture, avoiding unnecessary oversizing of the air movement.

Towards an intelligent drying system.

And once again a question arises similar to the one we posed when we analyzed the temperature:

Does it make sense to work with maximum airflow throughout the entire process?

If increasing the flow above a certain point produces less and less time reduction, but increases energy consumption, then controlling only the temperature leaves out an important part of the problem.

Hanutec dryers are manufactured with proprietary, exclusive fans that generate airflow and pressure sufficient to maintain a continuous, highly efficient airflow within a specified range. The only requirement is to fill the bin to the indicated level; otherwise, the airflow will deviate from its optimal operating point.