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Lab Hoods, Biosafety Cabinets, Room Pressure Differentials, and Air Flow Patterns
Lou DiBerardinisDirector, EHS Office
Massachusetts Institute of Technology
Enclosures
Negative Pressure - relatively small airflowAdvantages: Containment, low airflow
Disadvantages: Restricted Access
Examples:Enclosure: all sides closed, glove box, toxic gas cabinetBooths: 1 side completely open, Lab Hood, Paint BoothTunnels: 2 side open, Foundry, Foam Booth
Exterior Hoods
Containment captured by moving air past the source of generation into the hood.
Examples: Slot, Welding, Plating
Disadvantages - containment released in general environment 1stAdvantage - Easy Access to Work
Receiving Hoods
Exterior Hoods - designed to use some aspects of the generation to help
Examples:Grinding – particle momentumOver exhaust – thermal updraftDown draft table - gravity
Design of Enclosing Hoods
Containment given off inside hoodReduce openings - provide adequate inflow through any openingUse baffles or hood depth to get even air distribution
V = control informationQ = system design information
Design of Exterior Hoods
Determination of Capture VelocityFactors
Location of work vs. location of hoodNature of Material (gas, vapor, particulate)Generation of MaterialGeometry of hoodCross draftsWork Activities
Capture Velocity - Table 4-1(account for effect of cross drafts)
Picking capture Velocity - wide ranges are given
1. Use Vent Manual - Specific design data sheets2. Be conservative
low guess - no capturehigh guess - waste energy - eddying, etc.
Types of HoodsChemical
By-passVariable air volumeAuxiliary air supplyPerchloric acidWalk-in (floor mounted)
EnclosuresGlove boxWeigh stationDissecting hoodsSputum cytology
Biological Safety Cabinets
Special Local Exhaust
Ductless Hoods
Biosafety Cabinets
Provide:Personnel safetyEnvironmental safetyWork protectionClass II and Class III
Class I Biosafety Cabinet
Partial containment (open face) cabinetPersonnel safety (inflow velocity)Environmental safety (HEPA filtered exhaust air)
Class III Biosafety Cabinet
Total containment cabinet; 0.5 in. water negative pressureAccess via gloves Personnel, environmental, and work protectionInflow through HEPA filters, outflow through double HEPA filters
Class II Biosafety Cabinet
Partial containment cabinets (open face)Personnel, environment, and work protectionInflow velocity 75-100 fpmDownflow inside cabinet 40-60 fpmType A cabinets exhaust back into labType B cabinets exhaust to the atmosphere Via a roof fan
Class II, Type B Biosafety Cabinet
May use some amount (small) of volatile solvents. Air exhausted to atmosphere largely from rear work surface grill. All contaminated ducts and plenums under negative pressure or surrounded by negative pressure ductsMinimum f ace velocity: 100 fpm
Cabinet Certification
Design certification by NSF InternationalIn-place certification
After installationAnnuallyFollowing a moveFollowing internal inspection or repairs
Provisions For Decontamination
Spill kit storageFormaldehyde or hydrogen peroxide decon of cabinet and labUVGI
Room Ventilation
Air Temperature & Pressure Differentials and Air Airflow Patterns
Things Everyone Knows About Air Flow
1. The volume rate of air discharged from a room must be exactly equal to the volume rate of air induced or introduced.
2. Air flows from a zone of higher pressure to zones at lesser pressure and the pressure differential influences flow rate.
3. Air that is warmer than the surrounding air tends to rise; cooler air, to settle.
Room Pressure Differentials
Recommendations:CDC TB isolation rooms:>0.01 in. /2.5PaExperience: < 0.05 in. w/ 12.5 Pa
Is more better?
Loads on Doors and Partitions
For a 7.5 ft x 3.5 ft door: 26 sq ft/2.4 sq m Press. Diff. Load0.05 in. w 7 lb0.1 “ 14 “0. 2 “ 28 “
Plus Venturi effect
Light partition: 8 ft x 15 ft: 120 sq ft
0.1 in. w 70 lb0.2 “ 140 “
Mechanical Room Ventilation Systems
Ceiling supply and return air grillesVentilation rate:
cubic ft per min. (CFM)air changes per hour (ACH)(this is dilution ventilation; 67% of the air in a well mixed room, with its contaminants, is displaced during one ACH)
Displacement ventilation: “plug flow.”(one ACH removes 100% of room air.)
Measuring Airflow Direction, Velocity, & Pressure Differential
Direction: Smoke tube & Vanometer
Velocity: Anemometer, Vanometer
Pressure differential: 1/10 manometer and Vanometer
Equations to Calculate Pressure Differential from Velocity at Doorway
Velocity (V) in feet per minute and pressure differential (ΔP) in inches of water
ΔP = 3.2x10-7*V2
V in meters per second and ΔP in inches of water
ΔP = 0.12*V2
Entries & Air Mixing
Single door entries = air mixing (fan effect)
Double door entries (air lock) Pressurized double door entries
Exclusion & Containment Spaces
Exclusion spaces are at positive pressure to their surroundings to exclude entry of contaminating air.Containment spaces are at negative pressure to their surroundings to prevent escape of potentially contaminated air.Isolation rooms; sputum collection rooms: surgical operating rooms; laboratories; autopsy labs; morgues; infectious disease wards; clinics.
Simple Ventilation Systems
Natural ventilationopen windows & open wallswind effects on building pressure zones
Window & ceiling fansStack effect
Stack Effect
⎟⎟⎠
⎞⎜⎜⎝
⎛−=
iTTHkDraft 11*
0
• Draft in inches of water• H: Stack height• T0: Temperature in environment• Ti: Temperature inside stack
Roof Fans & Stacks for Safety Exhaust Air Devices