Air recirculation in drying: Recovering energy without compromising quality.

In a conventional system, much of that air is discharged into the environment. However, although it leaves the dryer with higher humidity, it still retains a significant amount of thermal energy. In fact, previous work at UC Davis had identified that a considerable portion of the energy supplied to the dryer ended up escaping with the exhaust air.

Recirculate to take advantage of the heat

The idea behind recirculation is relatively simple: Take a portion of the air coming out of the dryer, mix it with ambient air, and reintroduce it into the system..

Since the air is already hot, less energy is needed to bring the mixture back to operating temperature. This reduces the generator's heat consumption.

The UC Davis trials analyzed recirculation levels of 0, 25, 50, 75 and 90%The results showed that increasing recirculation reduced energy consumption, although at the same time the time required to complete drying began to increase.

With approximately 75% recirculationThe study obtained energy reductions on the order of 31% under September conditions and 45% under November conditions, with an increase in drying time of approximately three hours.

Why not recirculate 100% of the air?

Because the air we recover has two characteristics: It's hot, but it's also humid.

As the air passes through the nuts, it absorbs some of the water extracted from the product. If we recirculate too much, we begin to introduce air with a higher moisture content back into the dryer.

Thus emerges a fundamental commitment:

More recirculation → greater heat utilizationbut at the same time More recirculation → less capacity of the air to continue removing moisture.

In the trials, reaching 90% recirculation significantly increased drying time. UC Davis concluded that operating above 75% was impractical under the studied conditions due to the increased processing time.

The true potential: intelligent recirculation

This point is especially interesting for the development of an intelligent drying system.

The conclusion shouldn't simply be “75% needs to be recirculated”That percentage corresponds to the particular conditions of those tests.

What's really important is understanding that The advisability of recirculating depends on the conditions of the air we are recovering..

If we can measure temperature and humidity in both the incoming and outgoing air, we can understand how the process is progressing and decide how much air to recover and how much humid air we need to expel.

Thus, instead of using a fixed damper with a constant percentage of recirculation, we can think of a modulating recirculation, capable of varying during the cycle.

Initially, during periods of high water extraction, a higher proportion of air may need to be renewed. Under other conditions, when energy-efficient, hot air recovery can be increased. This dynamic strategy is a proposed design for our systems; the UC Davis study demonstrates the trade-off between recirculation, energy, and time, but does not specifically validate this control algorithm.

From controlling variables to controlling the process

The three analyses then begin to connect.

Temperature, airflow, and recirculation should not be understood as isolated parameters. Each one modifies the drying behavior and also interacts with the others.

The next step for HANUTEC is precisely to work on that interaction: measuring the actual conditions inside the dryer and using that information to act on the heat generator and, progressively, on the different variables of the system.

The ultimate goal is not simply to automate a dryer. It's to move towards a system capable of adapt its operation to the actual evolution of the productseeking to achieve the target humidity with greater uniformity and making more efficient use of energy.