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Published byAmbrose Pope Modified over 9 years ago
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Air Quality Controls Engineering Controls Administrative Controls Personal Protective Equipment
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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Elements of LEV Hood Ducts Treatment Fan
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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
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Types of Hoods Capture –Canopy –Lateral –Push-pull Enclosure Receiving
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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
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Fan Issues Noise Maintenance
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Treatment Particulates –Settling Chambers –Baffles –Cyclones –Filters –Electrostatic Precipitators
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Treatment Vapor and Gas –Scrubbers –Adsorbents –Combustors
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Administrative Controls Reduced shifts in hazard area Allergy and respiratory ailment screening Employee health tracking
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PPE: Respirators Air-purifying respirators –Filter mask (e.g. for dusts) –Adsorbent mask (e.g. for vapors) –Negative pressure
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PPE: Respirators Atmosphere-supplying respirators –Self-Contained Breathing Apparatus (SCBA) –Supplied-Air Respirator (SAR) –Positive pressure
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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
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