A certified and permitted extraction installation in OR, WA, CO, NV, and MD requires not only certified equipment, but an extraction booth that meets IBC, IFC, IEC, and NFPA-58. The design below illustrates one approach to meeting those requirements.
In this case it is constructed of 2X2 MS tubing, with a 16 ga inner and outer skin, but it could be constructed of metal studs and double drywall layers to meet 2 hour firewall requirements.
In addition to firewalls, the booth must meet NEMA 7, Class I, Div I or II electrical. Class I says the atmosphere is always explosive and Class II says that it normally isn’t but has the potential to be.
Butane has a Lower Explosive Limit (LEL) of about 1.86% and an Upper Explosive Limit of about 8.41%, which means that below 1.86% concentration in atmosphere containing 21% oxygen, the mixture is too lean to ignite, and above 8.41% it is too rich.
In practice, there is normally not an explosive atmosphere in the closed loop extraction booth, and because it is built to ASME Section VIII standards, most leaks with a closed loop system are minor leaks at damaged gaskets, worn seals, and loose fittings. That supports building a Class I, Div I room, but operating it as a Class II until LPG levels in the room reach 10% of LEL, as determined by a Hydrocarbon Detector, and then spooling up the exhaust system to Class I levels.
A good reason that you might want to do that, is HVAC costs, as well as operator comfort. Exhausted air must be replaced, bringing to question where that replacement air comes from and whether it is tempered for operator comfort and process control.
Ventilation Handbook, A Manual of Recommended Practices, by the American Conference of Governmental Industrial Hygienists recommends 100 to 200 surface feet air velocities to capture and convey flammable vapors released at medium velocity into moderately still air, such as a paint spray booth.
Some regulations are also written as 100 CFM per square foot of area, so how do those compare.
An Extraction booth 8′ deep X 7′ wide x 8′ tall would require 5600 CFM (8 X 7 X 100sf) for the first, and the latter would require 5600 CFM (7 X 8 X 100).
Perversely, where those numbers diverge is when the booth is proportionally longer or taller. For instance a booth 10′ deep, rather than 8′ would require 7000 cfm, even though the equipment is at the far end in front of the exhaust system.
By comparison, using the booth face area, instead of the area of its footprint, is based on moving a column of air the cross section of the extraction booth in a straight line horizontally at a capture velocity of 100 feet per minute, as recommended in the following tables copied from Ventilation Handbook, 17th Addition, page 4-5, Table 4-1.
Hood Design Data 4-5
Table 4-1
| Condition of Dispersion of Contaminant | Examples | Capture Velocity, fpm |
| Released with practically no velocity into quiet air. | Evaporation from tanks; degreasing, etc | 50-100 |
| Released at low velocity into moderately still air. | Spray booths; intermitten container filling; low speed conveyor transfers; welding; plating; pickling | 100-200 |
| Active generation into zone of rapid air motion | Spray painting in shallow booths; barrel filling; conveyor loading; crushers | 200-500 |
| Released at high initial velocity into zone of very rapid air motion | Grinding; abrasive blasting, tumbling | 500-2000 |
| In each category above, a range of cap0ture velocity is shown. The proper choice of values depends on several factors: Lower End of Range Upper End of Range 1. Room air currents minimal or favorable to capture 1. Disturbing room air currents2. Contaminants of low toxicity or of nuisance value 2. Contaminant of high toxicity Only.3. Intermittent, low production 3. High production, heavy use4. Large hood-large air mass in motion. 4. Small hood-local control only. |
Butane is about two times as heavy as air and Propane about 1.5 times, so both would like to sink and pool at floor level. One basic rule of ventilation, is where possible help the conveyed substance go where it wants, so instead of a hood, or a plenum with slots along its face, a slot at the bottom is the most effective way to remove it, with air makeup coming from the top.
The size of that slot is determined by how much air is being moved and how fast. Industrial Ventilation Table 4-2, gives 1000 to 1200 fpm duct velocity for all vapors, gases, and smoke, so in my illustration I’ve used 1000 surface velocity and therefore at 1000 sf, 5600 cfm would require a slot with an area of 5.6 feet, which at the width of the 84″ booth, would be .8 feet, or 9.6 inches tall.
Table 4-2 Range of Design Velocities
| Nature of Contaminant | Examples | Design Velocity |
| Vapors, gases, smoke | All vapors gases, and smokes | Any desired velocity(economic optimumVelocity usually 1000 to1200 fpm) |
| Fumes | Zinc and aluminum oxide fumes | 1400-2000 |
| Very fine light dust | Cotton, lint, wood flour, litho powder | 2000-2500 |
| Dry dusts and powders | Fine rubber dust, Bakelite molding powder, jute lint cotton dust, shavings (light), soap dust, leather shavings | 2500-3500 |
| Average industrial dust | Sawdust (heavy and wet), grinding dust, buffing lint (dry), wool jute dust (shaker waste), coffee beans, shoe dust, granite dust, silica flour, general material handling, brick cutting, clay dust, foundry (general), limestone dust, packaging and weighing asbestos dust in textile industries. | 3500-4000 |
| Heavy dust | Metal turnings, foundry tumbling barrels and shakeout, sandblast dust, wood blocks, hot waste, brass turnings, cast iron boring dust, lead dust | 4000-4500 |
| Heavy or moist dust | Lead dust with small chips, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky), quick-lime dust | 4500-up |
In my illustration below, air makeup is through a 66% open 1/4″ staggered on .385 centers, perforated A-36 MS metal ceiling, which slows it down to 154 surface feet so as to reduce turbulence in the room.
Simplified, the basic design is as follows:
Example of booth with ceiling intake and lower slot exhaust
Below is an example of a different approach exhausting directly with NEMA 7, Class I, Div I fans that was licensed and permitted in Washington:
Alternative design using explosion proof exhaust blowers