Air Quality Controls

• Engineering Controls

• Administrative Controls

• Personal Protective Equipment

Engineering Controls (Air)

• Periodic maintenance of plumbing, valves, ducting, air-handlers, filters. &c

• Remote controls for chemical operations

• Redesign of process to eliminate or reduce exposure-intensive steps

• Substitution of less hazardous chemicals

• Installation of effective ventilation system

Ventilation Terms

• Air Pressure: force of colliding air molecules

• Static Pressure: under influence of fan

• Velocity Pressure: inertia of molecules

• Capture Velocity: entrain mol. outside of duct

• Transport Velocity: entrain inside of duct

• Flow rate: volume/time

General Exhaust Ventilation

• Exchange air in work room(s) with outside “make-up” air– Capacity described in room changes per hour:

E=Q/V

Where Q is the volumetric flow rate, and

V is the volume of the room

• Intended to prevent contaminant concentration inside from rising to hazardous levels

• Presumes outside air is “cleaner” than inside

Effect of GEV during generation• Change in mass as f(time,conc):

M = G t - QC twhere G is generation rate (mg/min), C is concentration in

exhaust air (mg/m3), and Q is flow rate

• Divide by Volume to get C:C = G t/V - QC t/V = GenRate - RemRate

€

dC

dt=

G

V-

QC

V

• Burgess’ equation for conc as f(time):C = (G/Q)(1 - e-Qt/V)Notice that for large t, Cmax G/Q

Example

• A 300 m3 room through which 150 m3/hr of air is entering via infiltration (and exiting via exfiltration) is experiencing 0.5 ACH– So Q = V*E = 150 m3/hr

• Suppose the people in the room produce CO2

at the rate of 180 g/hr.

• At steady state, the CO2 concentration will be

Cmax G/Q = (180 g/hr)/(150 m3/hr) = 1.2 g/m3

Assuming what?

Hint: A = I + G – C - O

Effect of GEV after cut-off

• Can be calculated as a decay process:Ct = C0e-(Q/V)t

• Setting Ct = C0/2 we can calculate the half-life of the contaminant in the room:

1/2 = e-(Q/V)t

ln(1/2) = -(Q/V)t

t = ln(1/2)/ (-Q/V) = ln(1/2)(-V/Q)

t = 0.693(V/Q)

Example• Suppose there’s a benzene spill in the lab,

where the exchange rate E = 0.75/hr• After evaporation, the resulting

concentration is 50 ppm.• How long before it’s safe to go in?

– i.e. less than the 5 ppm action level

Ct = C0e-(Q/V)t

t = -ln(Ct/C0)/(Q/V)

t = -ln(Ct/C0)/(E)t = -ln(5 ppm/50 ppm)/(0.75/hr) 3 hrs

Issues with GEV

• Previous calculations assumed perfect mixing ( ideal transfer from room)

• One “room change” all air exchanged

• Exhaust system can bring contaminant into contact with more workers

• Seasonal changes (e.g. heating/cooling) can alter performance of system

Local Exhaust Ventilation

• Remove contaminant at its source

• Assumes “point sources”

• Lowers number of workers potentially exposed

• But usually more susceptible to over-ride and undetected failure

Elements of LEV

• Hood

• Ducts

• Treatment

• Fan

Hoods

• Aperture through which airborne contaminant is drawn into ventilation ducts

• Capture Velocity is that velocity of airflow required to draw contaminant into hood

• Velocity at distance x from hood:

v = kQ/(x2 + kA)

where k depends on opening shape

and Q = vhA

Types of Hoods

• Capture– Canopy– Lateral– Push-pull

• Enclosure

• Receiving

Ducts• Duct performance is governed by resistance

• Round ducts are less resistant than square– Why?

– As = (p/4)2 and Ac = c2/(4)

– Setting As = Ac, p = 2c/ ()1/2

– p = 1.128c– So for equal capacity, square has more surface

• Resistance is proportional to velocity

Fan Issues

• Noise

• Maintenance

Treatment

• Particulates– Settling Chambers– Baffles– Cyclones– Filters– Electrostatic Precipitators

Treatment

• Vapor and Gas– Scrubbers– Adsorbents– Combustors

Administrative Controls

• Reduced shifts in hazard area

• Allergy and respiratory ailment screening

• Employee health tracking

PPE: Respirators

• Air-purifying respirators– Filter mask (e.g. for dusts)– Adsorbent mask (e.g. for vapors)– Negative pressure

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are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

PPE: Respirators• Atmosphere-supplying respirators

– Self-Contained Breathing Apparatus (SCBA)– Supplied-Air Respirator (SAR)– Positive pressure

QuickTime™ and a decompressor

are needed to see this picture.

Respirator Issues

• Masks must fit properly– Qualitative fit testing: expose wearer to banana

oil or saccharin mist and ask if they detect– Quantitative fit testing: in chamber of known

concentration, measure concentration inside

• Workers must be trained (not all respirators are effective for all contaminants)

• Workers must wear them to be protected