Creating Velocity

Creating Velocity

Overview

Some spray booth providers allege that 14,000 and 16,000 CFM are what the user needs for today’s new paints. Results from some recent testing debunk that allegation.

In May 2002, a well-known manufacturer spent some time at our Tech Center with a new paint product, trying to reduce the drying times even lower than he had previously achieved with our SmartCure™ process. Our Ultra cabin at the Tech Center can access either a 2-motor, 10 HP, 12,000 CFM generating group (air make-up system) or a 2-motor, 15 HP, 15,000 CFM Ultra generating group. Both units were used in the testing to see which one would cure the fastest.

The evidence was conclusive, and astonishing! The 12,000 CFM unit achieved a skin temperature of 140°F (60°C) faster than the 15,000 CFM unit, leading the paint company to conclude that there is / would be no advantage in using 15,000 CFM. In fact, the cost of operation would be greater, in view of higher-demand 15 HP motors and a larger burner.

Creating Velocity

In order to achieve balance, a design must have a means of creating velocity. This is usually accomplished with a fan or blower on either the exhaust or supply or both.

The air velocity or ventilation rate must be sufficient to insure that the solid particles and flammable vapors are confined to the inside of the spray booth. The configuration of the object being sprayed plays an important role in determining velocity requirements.

For example:

  • Manually finishing the interior of file cabinets would require higher air velocities to insure that the overspray is removed from the area between the finisher and the cabinet interior. This “capture” velocity can often be as high as 150 FPM with a conveyorized production system.
  • Another example could be the finishing of large flat sheets. A high velocity spray booth would be necessary to ensure that the air movement around the edges of the large sheets would be adequate to prevent the overspray from rebounding and escaping from the inside of the booth.
  • The finishing of small objects with a lot of open spaces will allow the overspray to be captured with velocities of 125 FPM or sometimes less.
  • Manual electrostatic spray guns, which are used to coat objects with open areas and objects that do not block the air flow, will allow overspray to be captured at velocities as low as 100 FPM.

In NFPA-33 (section 5-2) air velocity requirements are defined. According to the guidelines, a booth needs to “provide adequate ventilation to maintain the concentration of flammable vapors or combustible vapors or mists in the exhaust stream below 25% of the lower flammable limit (LFL) of the paint”. Lower flammable limit is defined as the concentration level at which a particular atomized solvent will ignite.

The volume of air needed to move through the booth and into the exhaust chamber is measured in cubic feet per minute (CFM).

This is the formula for determining the volume of exhaust air: Area x Velocity = CFM (cubic feet per minute of air)

Area is the cross-sectional area in square feet of all openings in the spray booth. When air input plenums are used the conveyor openings may be ignored. When connecting vestibules are used, the opening between adjacent booths may be ignored. Velocity is the speed or velocity of air required by code. Speed of air movement is measured in feet per minute (fpm). Cubic feet per minute (CFM) is the volume of air moving through the booth.

To calculate the requirements for the booth in cubic feet per minute: Multiply the cross-sectional area of the booth in square feet by the velocity of the air through the booth in feet per minute.

10 ft x 12 ft = 120 ft2 x 100 fpm = 12,000 CFM

Spray Gun Painting