Presentation on theme: "General and Local Exhaust Ventilation"— Presentation transcript:
1General and Local Exhaust Ventilation SAND No C Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
2Overview of Presentation DefinitionsCommon terminologyPurposeHazard assessmentGeneral ventilationLocal exhaust ventilationVentilation evaluationTroubleshootingExercisesLecture from the viewpoint of occupational health and safety, not emergency releases or incidents.American Conference of Governmental Industrial Hygienists(ACGIH) Ventilation Manual 27th Edition
3DefinitionsHeating, ventilating and air conditioning (HVAC): refers to the distribution system for heating, ventilating, cooling, dehumidifying and cleansing air.Replacement/Supply air: refers to replacement air for HVAC and local exhaust ventilation.General ventilation: refers to ventilation that controls the air environment by removing and replacing contaminated air before chemical concentrations reach unacceptable levels.Local exhaust ventilation (LEV): refers to systems designed to enclose, or capture and remove contaminated air at the source.
4Common Terms Q = volume of air in cubic meters V = velocity of air in meters per secondDuct velocity-velocity required to transport the contaminantFace velocity-velocity on the front of an enclosing hoodCapture velocity-velocity required to capture contaminant at point of generationA = cross sectional area of hood opening in square metersX = distance of ventilation from the source in meters
5Purposes of Ventilation Protect workers from health hazardsDilute, capture, or contain contaminantsProtect workers from hot processesOvens, foundriesProtect the productSemiconductorElectronicsPharmaceuticalsSlot HoodCanopy Hood
6Purposes of Ventilation Emergency ventilationStandalone fansDetectors connected to ventilation or scrubber systemsSafe roomPositive pressureEnclosed vented rooms or cabinetsGas cabinetsComply with health and safety regulationsPhoto credit: Emergency Responder ProductsRoom goes negative when there is a release or positive pressure in the control or safe room to keep toxic gases out.Photo credit: Advanced Specialty Gas Equipment
7Hazard Assessment What are the airborne contaminants? ParticlesSolvent vaporsAcid mistsMetal fumesHow do to the workers interact with the source contaminant?Are workers exposed to air contaminants in concentrations over an exposure limit?*Requires air monitoring of the taskDilution or local exhaust ventilation?Picture Credit : International Labor Organization
8General Ventilation Natural Ventilation: Useful for hot processesChimney effectWindows and doors kept openExample: a warehouse opens the windows to create natural ventilationQ = 0.2 AVA = square meters (area of open doors)V = wind speed in kilometers/hourQ = estimates the volumetric flow rate through the building (m3/s)Q in cubic meters/secondEstimates flow rate from the wind blowing through the windows with average wind speed in the area. Compare to ASHRAE Standard
9General Ventilation Dilution Ventilation Principles -Heat control -Dilution of odors, flammables-Not for control of toxicsPrinciples-Contaminant emissions must be widely dispersed-Exhaust openings must be near contaminant source-The worker must not be downstream of contaminant-Air flow over worker should not exceed meters/sec
11Local Exhaust Ventilation (LEV) Use when contaminant concentration cannot be controlled by dilution ventilation or other controlsSelect the type of LEV from hazard assessmentWhich type is best to capture the contaminant?Enclosed or capture hood?Consider worker’s needsWhat duct transport velocity is required to carry the contaminant? Heavy particles?What face or capture velocity is required?Select duct material for the contaminantEnsure enough replacement air/adequate fan size
13Local Exhaust Ventilation (LEV) Volumetric Flow Rate, Q = VA [Circular Opening]Q = V1A1Q = V2A2Q = Volumetric flow rate, in cubic meters/secondV = Average velocity, in meters/secondA = Cross-sectional area in square meters
14Local Exhaust Ventilation (LEV) Duct diameter = 1 meterV = 600 meters/secondWhat is Q?Duct diameter = 0.5 meterWhat is the duct velocity (V)?For circular ducts A = d2/4Q = VAQ = (600 m/s)([1m]2/4)Q = 471 meters3/secondQ = VA 471 meters3/s = V ([0.5m]2/4) V = 2400 meters/secondThese equations show how when the duct diameter is less, the velocity increases.
15Capture of contaminant is only effective within one (1) duct diameter Local Exhaust Ventilation (LEV)JETAirDuct30 Duct DiametersTop picture is exitCapture of contaminant is only effective within one (1) duct diametervfaceD = Duct diameter
16ACGIH Ventilation Manual These diagrams show different kinds of hoods, and how their air flows differ. The air flow formulas are given more legibly in the next slide.ACGIH Ventilation Manual
17Local Exhaust Ventilation (LEV) Capture Velocity (Vc) : [Plain Opening]xvcaptureQ = vfaceUpper photo illustrates capture of something a distance away from the face of the hood. Determine the capture velocity needed to capture the contaminant, then you know what Q you will need—fan size, etc.Q = Vc (10x2 + A)X = distance of source from hood face
18Recommended Capture Velocities CONDITIONEXAMPLESCAPTURE VELOCITYRange in meters/secondNo velocity,Quiet airEvaporation from tanks, degreasers0.25 – 0.5Low velocity, moderately still airSpray booths, container filling, welding, plating0.5 – 1.0Active generation into rapid air motionSpray painting (shallow booths), crushers1.0 – 2.5High initial velocity into very rapid air motionGrinding, abrasive blasting, tumbling2.5 – 10.1ACGIH Ventilation Manual
19Recommended Duct Velocities CONTAMINANTEXAMPLESDUCT VELOCITYMeters/secondVapors, gases, smoke5.0 – 10.1FumesWelding10.1 – 12.7Very fine dustCotton lint12.7 – 15.2Dry dusts & powdersCotton dust15.2 – 20.3Industrial dustGrinding dust, limestone dust17.8 – 20.3Heavy dustSawdust, metal turnings20.3 – 22.9Heavy/moist dustsLead dusts, cement dust> 22.9Add m/s in third columnACGIH Ventilation Manual
20Local Exhaust Ventilation (LEV) Canopy hood:Best for controlling hot processesNot good for capturing dusts, or vaporsNot good where cross- drafts existWorker must not put head under canopy20
21Local Exhaust Ventilation (LEV) “Elephant trunk”:Good for welding fumes, small process tasks, machining, disconnecting process linesPlace close to contaminantEnsure adequate capture velocity at distance from contaminantFlanged opening captures contaminant better
22Local Exhaust Ventilation (LEV) Downdraft hood:Vapors pulled down through grillCapture velocity depends on source distance from grillNot for hot operations
23Local Exhaust Ventilation (LEV) Slot ventilation:Best for liquid open surface tanksAcid bathsPlating tanksPulls air across the tank away from workerSide enclosures prevent cross draftsPush-Pull design is optional (push jet)
24Local Exhaust Ventilation (LEV) Fume hood:Laboratory useBest for small amounts of chemicalsSash must be kept at set levelNO storage of equipment in the hood!
25Local Exhaust Ventilation (LEV) Enclosures:Example:Paint boothsControl of exposure to liquid aerosols and vaporsFlammability hazardMust have scheduled filter changeoutOperator must be upstream
26Local Exhaust Ventilation (LEV) Other vented enclosuresGlove boxesFurnaces/ovensAbrasive blastingPhoto credit: U. S. Department of Labor. OSHAPhoto credit: Borel Furnaces and Ovens
27Local Exhaust Ventilation (LEV) Exhaust Systems:Do not place exhaust stack near air intakesRe-entrains contaminants into the buildingDo not use rain capsStack height depends on:Contaminant temperatureBuilding heightAtmospheric conditionsDischarge velocityIdeal discharge velocity is 15 meters per second
28Ventilation Systems Evaluation Evaluate capture velocityQuantitatively-anemometers, velometersQualitatively-smoke tubes,Visualizes air movementUse water vapor for clean roomsPhoto Credit: All Products Inc.Regarding fume hoods, the Z9.5 standard adopts the ASHRAE 110 Test, a performance test, and specifies the minimum acceptable level of containment for a fume hood when a specific flow rate of a detectable gas is released into an operating hood, and containment loss is measured in the breathing zone of a mannaquin placed in front of the hood.
29Ventilation Systems Evaluation Air velocity measurementsMeasure air velocities (meter/sec) at a number of pointsAverage the results and determine volumetric flow rate: Q = VAAll instruments must be calibrated periodicallyTypes:Swinging vane velometerHot-wire anemometer
30Troubleshooting Wrong hood for process Example: canopy hood for toxics Insufficient capture velocityInsufficient duct velocity~14 meters/second for vapors~18 meters/second for dustToo much air flow = turbulenceTraffic or competing air currentsInsufficient make up airNegative pressureCan’t open doors
31ExerciseWhat is the preferred ventilation system for the following situation?Dilute non-toxic odors in the warehouseA) General ventilationB) Local exhaust ventilation
32ExerciseWhat is the preferred ventilation system for the following situation?Acid processing bath with open surface areaA) Lab fume hoodB) Slot ventilationC) Elephant trunkD) Canopy hoodE) Paint booth
33ExerciseWhat is the preferred ventilation system for the following situation?Welding tableA) Lab fume hoodB) Slot ventilationC) Elephant trunkD) Canopy hoodE) Paint booth
34ExerciseWhat is the preferred ventilation system for the following situation?Chemical analysis of small samples for quality controlA) Lab fume hoodB) Slot ventilationC) Elephant trunkD) Canopy hoodE) Paint booth
35ExerciseWhat is the preferred ventilation system for the following situation?Spray painting a large piece of equipmentA) Lab fume hoodB) Slot ventilationC) Elephant trunkD) Canopy hoodE) Paint booth
36US Standards & Guidelines ACGIHAmerican Conference of Governmental Industrial HygienistsIndustrial Ventilation, A Manual of Recommended PracticeAIHAAmerican Industrial Hygiene AssociationStandard Z9.2, Fundamentals Governing the Design and Operation of Local Exhaust Ventilation SystemsASHRAEAmerican Society of Heating, Refrigeration and Air Conditioning EngineersStandard , Ventilation for Acceptable Indoor Air QualityOSHAOccupational Safety and Health AdministrationVentilation, 29 Code of Federal Regulations
37Summary of Presentation Provided ventilation definitions and terminologySummarized the purpose of ventilationDescribed general exhaust ventilationDescribed local exhaust ventilationDemonstrated volumetric flow rate and capture velocity calculationsDescribed how to evaluate a ventilation systemProvided examples of ventilation problems (troubleshooting)Listed ventilation standards and guidelines