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  t where G is generation rate (mg/min), C is concentration in exhaust air (mg/m 3 ), and Q is flow rate Divide by Volume to get  C:  C = G  t/V - QC  t/V = GenRate - RemRate Burgess’ equation for conc as f(time): C = (G/Q)(1 - e -Qt/V ) Notice that for large t, C max  G/Q

Example A 300 m 3 room through which 150 m 3 /hr of air is entering via infiltration (and exiting via exfiltration) is experiencing 0.5 ACH –So Q = V*E = 150 m 3 /hr Suppose the people in the room produce CO 2 at the rate of 180 g/hr. At steady state, the CO 2 concentration will be C max  G/Q = (180 g/hr)/(150 m 3 /hr) = 1.2 g/m 3 Assuming what? Hint: A = I + G – C - O

Effect of GEV after cut-off Can be calculated as a decay process: C t = C 0 e -(Q/V)t Setting C t = C 0 /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 C t = C 0 e -(Q/V)t t = -ln(C t /C 0 )/(Q/V) t = -ln(C t /C 0 )/(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/(x 2 + kA) where k depends on opening shape and Q = v h A

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? –A s = (p/4) 2 and A c = c 2 /(4  ) –Setting A s = A c, 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

PPE: Respirators Atmosphere-supplying respirators –Self-Contained Breathing Apparatus (SCBA) –Supplied-Air Respirator (SAR) –Positive pressure

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