12.1 Extraction Booth and Ventilation System Design

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 currents

2.  Contaminants of low toxicity or of nuisance value   2.  Contaminant of high toxicity

     Only.

3.  Intermittent, low production                                        3.  High production, heavy use

4.  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 optimum

Velocity usually 1000 to

1200 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

 

 

 

 


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