Belt-driven tube axial exhaust fan (in-line axial fan with the motor and belts outside the airstream, for spray booth and fume exhaust)

01

What OSHA 1910.107 asks of a spray booth exhaust fan, its motor and its belts

OSHA's spray finishing standard, 29 CFR 1910.107, sets rules that bear directly on the fan. The rotating element must be nonferrous or nonsparking, or the casing must be made of or lined with such material, with enough clearance between rotor and casing to avoid a fire by friction. Motors driving exhaust fans must not be placed inside booths or ducts, and belts must not enter the duct or booth unless belt and pulley there are thoroughly enclosed. The shaft is preferably to have bearings outside the duct and booth. [1]

The same section sets the air the fan has to move in a dry spray booth: an average velocity over the open face of the booth of not less than 100 linear feet per minute, or not less than 60 ft/min for electrostatic spraying, with a gauge, alarm or pressure device to show the velocity is maintained. Where more than one fan serves a booth, the fans must be interconnected so that one cannot run without the others. [1]

Editorial calculation: a booth with an open face 12 ft wide and 8 ft high has 96 ft²; at 100 ft/min it needs at least 9,600 cfm (about 16,300 m³/h), and at 60 ft/min for electrostatic spraying 5,760 cfm. Filters, ducts and any roof cap add the static pressure the fan must overcome at that flow.

Aerovent's BTABD, a belt-driven tube axial fan made for paint spray booth exhaust, answers these points with a spark-resistant die-cast aluminium propeller and hub, a motor located out of the airstream, cast-iron sheaves with non-static conducting belts isolated from the airstream, and OSHA belt guards as standard. [2] Greenheck lists paint booth exhaust among the uses of its TBI-CA belt-drive tube axial fan, with spark-resistant construction and motor and drives out of the airstream as standard features. [4]

Give the fan supplier the booth face area and the required face velocity, the filter type and its clean and loaded pressure drop, the duct run and discharge (in-line or roof upblast), the solvent or powder being sprayed, and the motor enclosure your electrical classification calls for.

02

Arrangement 9 belt drive: motor on the tube, belts in a belt tube, bearings purged by inlet air

Aerovent's installation instructions describe the belt-driven tube axial as a duct fan built to become part of the duct, at its entrance, middle or discharge. The motor is outside the airstream; the belt drive and bearings are enclosed in a tube and cover on the suction side of the impeller. For normal and high-temperature air the bearing cover is left open at the impeller end, so air is drawn through the belt tube and bearing enclosure as a purge that carries contaminants away and cools drive and bearings. For wet air heavy with corrosive chemicals, the bearing housing can be fully enclosed with a seal around the shaft. [3]

The BTABD specification calls this Arrangement 9, V-belt driven: a NEMA foot-mounted TEFC or ODP motor on a base plate of at least 3/16 in steel welded to the casing, a motor base adjusted with two jack-bolts to tension the belts, and a variable-pitch drive sheave on motors up to 5 hp so the fan speed can be trimmed during booth balancing; larger motors get a fixed sheave. Access to the fan sheave is through a cover plate on the inner cylinder. Bearings are non-relubricable sealed ball bearings in a cast aluminium monoblock, specified for an L-10 life of at least 60,000 hours. [2]

Greenheck's TBI-CA instead uses grease-lubricated cast-iron pillow block bearings selected for an L10 life over 80,000 hours in horizontal service and 40,000 hours in vertical service, with the shaft sized so its first critical speed is at least 25% above the maximum operating speed. [4] Editorial note: sealed-for-life bearings remove the need for grease lines into the tube; greased pillow blocks can be relubricated but need a lube path from outside the duct, which is the arrangement 1910.107 prefers for bearings. [1]

Aerovent's BTABD casing gauge steps with size: 12-gauge hot-rolled steel through 20 in diameter, 10-gauge from 24 to 28 in, and 7-gauge above 30 in, with inlet and outlet flanges rolled from the casing sheet. [2]

Ask for the arrangement number, motor frame and enclosure, sheave type (variable or fixed pitch), belt type, bearing type and rated L10 life, and whether the bearing enclosure is open (air-purged) or sealed for corrosive wet air.

03

Tube axial or vane axial: pressure range, adjustable propeller pitch and the roof upblast allowance

A spray booth tube axial and a medium-pressure axial fan from the same family differ mainly in how much pressure they are built to develop. The two catalogues below state these limits for belt-driven, cast-aluminium-propeller models; the pressure column is what decides whether a booth with loaded filters and a long duct can be served.

Greenheck gives a correction for roof upblast mounting: the extra static pressure is a size factor K times the flow squared. Its worked example takes a size 36 fan at 25,000 cfm against 2 in wg: with K = 4.78×10⁻¹⁰ the upblast adds 0.299 in wg, so the fan is selected at 25,000 cfm against 2.299 in wg. [4] Editorial calculation: with the size 24 factor (4.16×10⁻⁹), the same 25,000 cfm would add about 2.6 in wg, which is why the upblast loss is checked size by size rather than as a fixed allowance.

Aerovent's installation notes add that the rating tables assume the fan sits in a duct of the same size; a tapered discharge regains part of the velocity pressure as static pressure, and converging inlet and discharge sections lower duct velocities, either of which can let the fan be selected at a lower pressure at a higher installation cost. [3]

04

Overspray on the propeller, balance limits and flexible duct connections

Aerovent notes that deposits build up evenly on an axial impeller and do no harm until they break away in crust-like pieces; the impeller can then be thrown out of balance and the vibration can be serious. Fans exhausting process air should be cleaned as required, by solvent or scraping, taking care not to cut through any protective coating. [3]

The BTABD propeller is balanced to ANSI/AMCA 204-05, category BV-3, grade G6.3, and belt-driven units are trim-balanced in the casing to peak vibration of 0.15 in/s rigidly mounted or 0.20 in/s flexibly mounted, with test-room readings recorded in three planes and available on request. [2] Editorial calculation: 0.15 in/s is about 3.8 mm/s and 0.20 in/s about 5.1 mm/s, values that can be used as the acceptance reading after cleaning or re-pitching the blades.

Flexible connections between the fan and the duct reduce vibration passed to the duct and lower noise, and Aerovent states that vibration isolators under the fan are also needed for noise dampening. [3]

Ask for the factory balance record in the three planes, the propeller coating if any, the cleaning access (inner cylinder cover, bolt-on vane section) and the isolator and flexible connector arrangement.

05

Data for a belt-driven tube axial exhaust fan quotation

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

Duty: [airflow cfm or m³/h; static pressure at that flow; air temperature]
Application: [spray booth face area and face velocity; filter loading; solvent or powder; corrosive vapour]
Mounting: [in-line horizontal base or ceiling hung; vertical up or down; roof upblast with windband]
Fan: [propeller diameter; blade material and pitch; tube axial or vane axial section]
Drive: [arrangement; motor kW or hp, frame and enclosure; sheave type; belt guard]
Bearings: [sealed or greased; L10 life; open or sealed bearing enclosure]
Tests and documents: [AMCA 210/300 ratings; balance grade and vibration record; finish]

See manufacturer records Send an enquiry through CNFX