Air circulation system for industrial food dehydrators

01

How much dehydrator exhaust to send back to the fan

Oklahoma State University's food process engineers explain that warm, moist exhaust air mixed with fresh air gives a mixture warmer than fresh air alone, so less heat is needed to bring it to the set point, while the evaporative capacity of the mix falls only moderately. They cite one study suggesting recirculation of up to 75% with energy savings of 45 to 50%, and another recommending 60 to 85% with savings of 30 to 60%. [1]

Both studies dried fruit. In the first, fruit dried to 24–27% moisture showed no change in drying time with recirculation; the fact sheet adds that reaching a lower final moisture, for example 10 to 15%, may require reduced recirculation, and that the studies used fixed rates where variable rates may be needed for the best result. Savings are offset by a partial reduction in drying capacity. [1]

Recirculated air also adds moisture, which the same sheet links to avoiding case hardening, the hard surface layer formed when a food dries too fast at the start; for products prone to it, recirculation shortens overall drying time, and where case hardening is not an issue researchers found little quality difference with or without it. [1]

Editorial calculation: at 75% recirculation only a quarter of the fan's flow is fresh air; a fan moving 2,250 m³/h would draw about 560 m³/h from outside and exhaust the same. [1]

Give the supplier the product and its target final moisture, batch or continuous operation, the drying temperature, whether case hardening has been a problem, and the recirculation range and control (manual or automatic, on humidity, wet-bulb or dry-bulb temperature) you want the dampers to cover.

02

Where the fresh-air inlet, exhaust and dampers sit relative to the fan

In the recirculating layout Oklahoma State shows, the fresh-air inlet is on the suction side of the fan and the exhaust outlet on the pressure side, with two flow-control damper valves setting the split. The authors note that butterfly dampers are hard to control because airflow is not linear with valve position, and suggest louver or knife dampers for a more linear response. [1]

They report that in one test dehydrator the exit air temperature rose with increasing recirculation, and suggest a control approach of starting at a maximum recirculation rate and reducing it until exhaust temperature begins to drop. Existing cabinet dehydrators can be retrofitted with ducts and valves, or with an enclosure built around them. The sheet also points to air leaks and heat transfer from the dehydrator and ductwork as the main energy losses, found with talcum powder or a smoke stick and then sealed and insulated. [1]

03

What a carrot tray dryer showed at 2 and 3 m/s

Putri and co-authors at Universitas Andalas modelled a tray dryer of 104 × 65.01 × 143.56 cm with five trays of 97.40 × 43 cm spaced 26 cm apart, drying 2 mm carrot slices at 50, 60 and 70 °C and 2 or 3 m/s measured at the outlet. In every case temperature and airflow were unevenly distributed, particularly in the middle and upper sections, with vortices formed by the dryer's internal structure. [2]

At 50 °C and 2 m/s the streamlines reached up to 7.3 m/s locally, and air accelerated upward toward the outlet. At 70 °C and 3 m/s the upper section was reasonably supplied but airflow to the lower trays stayed limited. The authors cite earlier work observing that raising air speed in a tray dryer does not always improve heat distribution, because of recirculation and obstructions. [2]

The study's 0.5–2.41% velocity deviation measures how well the model matched the measurements, not how uniform the air was; the authors say uniformity needs a statistic such as standard deviation or coefficient of variation, which they did not calculate. [2]

Editorial calculation: the five trays give 5 × 0.974 × 0.43 ≈ 2.09 m² of drying area. [2]

Ask the supplier how air speed is measured (at the outlet, between trays, or across the tray face) and for a velocity or temperature map of the loaded chamber with a spread statistic, not only an average.

04

Inlet and outlet placement and dead zones in a continuous tray dryer

Welsh, Kumar and Karim at Queensland University of Technology simulated an empty U-shaped continuous tray dryer, a 500 × 500 mm channel with an overall footprint of 2590 × 2180 mm, at inlet speeds of 0.5, 2.5 and 5 m/s; the flow was turbulent in all three cases. [3]

With one inlet and one outlet at opposite ends of the U, air hit the inside wall and accelerated through the bend, leaving two dead zones, and the faster the inlet air the larger the dead spot or the effect of the bend. With an inlet at each end and one outlet at the top centre of the bend, a single dead zone remained below the outlet, in a region without trays. The authors note the trays and holders were not yet modelled and will change the distribution. [3]

Editorial calculation: the 0.25 m² channel at 2.5 m/s carries 0.25 × 2.5 = 0.625 m³/s, about 2,250 m³/h of air. [3]

05

Send the product, drying and airflow data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Product: [food; slice size; loading per tray; target final moisture]
Dryer: [batch cabinet, tray or tunnel; chamber size; number of trays and spacing]
Air: [temperature; airflow m³/h; air speed and where it is measured]
Recirculation: [range %; damper type; manual or automatic; control signal]
Fan: [motor power; supply voltage and frequency; static pressure]
Hygiene: [materials in the air path; filters; cleaning access]
Tests: [velocity or temperature map of the loaded chamber]

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