The case of the no-flow airflow

April 1, 2005
Engineers at a consulting company used fluid-flow software to discover why the air in a paint booth was just swirling around instead of exiting with the fumes.

The 40 X 85 X 70-ft paint booth modeled in Flovent is at Robins Air Force Base in Georgia. Six Ductsox fabric plenums are to the lower right.

Clint Hardie, principal at Technicon Engineering, Macon, Ga., analyzed flow in the air-supply duct, plenum, and paint booth with CFD software.

Software supplier Flomerics Ltd., Southborough, Mass., created a CFD model of the airflow through the plenum and paint booth. A critical design feature, as found from flow models in Flovent CFD software, is a ratio of hole diameter to duct-wall thickness. Only by controlling these ratios would designers get air to turn 90° and flow evenly across the plenum length. "Without computer simulation, it could have taken months of expensive experiments to understand what was causing the turbulence," said Hardie. "Simulation results let us quickly specify a duct material that solved the problem."

Technicon engineers determined that air entered the plenum too quickly. Experiments showed that a delivery flow rate of 58,000 cfm produced a duct velocity about 567 fpm, too high for uniform distribution in the paint booth. Hence, the swirling. Simulations also showed that a cross-sectional velocity below 65 fpm across the paint bay would solve the problem.

Flomerics engineers modeled ducts and used an optimization capability in the software to adjust design variables and converge on a design goal. Models showed hole size-to-wall thickness ratio had to be 4:1 or greater. Technicon engineers found a fabric called Microbe-X LT from Ductsox, Dubuque, Iowa, would work. The fabric contains thousands of holes with size-to-wall thickness ratios of 10 to 12:1. Exit velocities from the fabric approach 50 fpm, so stray eddy currents aren't a problem.

Ductsox Inc.
(563) 589-2777
Flomerics Ltd.
(508) 357-2012

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