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