NC Industrial Ventilation Conference

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1 Have you ever heard this from plant operations? Monitoring & Maintenance of Ventilation Systems, Part I (MM-1-1) 1) NC Industrial Ventilation Raleigh, NC April 2010 The new local exhaust ventilation system we started up a month ago doesn t work! Engineering didn t design it right! MM MM Tips to Prevent this Problem at Your Site Where do you start? Why IVS fail Common failure modes for Hoods Ducts Fans Collectors Tuesday Ventilation System Measurements MM-1-1 Where Do You Start? Broadly, are system problems due to Original design? System modified without redesign? Poor operating practice? (ie ie, adjusting blast gates) Little or no system monitoring & maintenance? Getting started Define the problem areas & symptoms Gather visual and measurement data on system MM Defining the Problem Data Gathering - Qualitative Observable Symptoms? Qualitative (visible, olfactory, etc.) Quantitative symptoms (air samples) Operator complaints Specific locations affected? Reasons, frequency of equipment entry? One hood/enclosure? Multiple hoods/enclosures? Nowhere near a hood/enclosure? Original system design work as intended? Design basis for each hood/enclosure System layout and schematic System Baseline airflows and static pressures System IH Evaluation at startup Problem area data today? MM MM Module MM-1-1 1

Data Gathering Qualitative, 2 System modification without redesign? Site have a Change Management System with IVS knowledgeable people part of the process?

Designated system owner Operators (line and IVS) trained Blast gates locked or adjusted whenever anyone wants Monitoring & Maintenance?

2 Data Gathering Qualitative, 2 System modification without redesign? Site have a Change Management System with IVS knowledgeable people part of the process? Compare original schematic to as installed Visual clues if no documentation Problem area data today MM Data Gathering Qualitative, Operating practices? Designated system owner Operators (line and IVS) trained Blast gates locked or adjusted whenever anyone wants Monitoring & Maintenance? System Baseline Documentation available as reference for M&M Installed monitoring devices or routine system wide data gathering Breakdown or predictive maintenance Recent maintenance on system MM Data Gathering - Quantitative Data Gathering Quantitative, 2 Hoods/enclosures Measure area of opening Measure average face velocity across opening Process at hood cause interference? External air currents? Ducts & Fans Duct layout diameters, lengths, types of transitions at junctions Duct conveying velocity and static ti pressures Fan shaft speed, motor amps Air cleaning device Differential pressure Operating at base condition? MM MM Causes of IVS Failures Sudden Failure Modes major airflow reduction Broken fan belt Water in baghouse cleaning compressed air Bag sucked into duct Baghouse dust removal system backs up into baghouse Explosion vent opens Causes of IVS Failures Gradual Failure Modes Dust buildup in elbows Dust erodes holes in ducts Increasing filter differential pressure Bag cleaning system Moisture sensitive dust Slipping fan belts Dust visible in exhaust Duct gaskets or access door seals leaking audibly MM MM Module MM-1-1 2

Causes of IVS Failures Gradual Failure Collector High Differential Pressure Causes of IVS Failures where Branch versus System failure? where in the system?

Poor suction some hoods, harder suction at other hoods Filter and fan not obviously affected System-wide No or low airflow Similar effects seen at all branches MM-1-1 14 Causes of IVS Failures Change

~ 11 2 or 121 11 2 +8 2 = 185 ~ 14 2 or 196, ~ 1 2 or 169 C ID D, inches Q, CFM V, fpm VP/ 100 A 8 1500 4200.

3 Causes of IVS Failures Gradual Failure Collector High Differential Pressure Causes of IVS Failures where Branch versus System failure? where in the system? High bag differential pressure due to improper bag conditioning (seeding) at startup of new bags Scrubber venturi plugging another example MM Branches - Localized impact Dusting at some hoods Poor suction some hoods, harder suction at other hoods Filter and fan not obviously affected System-wide No or low airflow Similar effects seen at all branches MM Causes of IVS Failures Change - Add a duct branch/hood Causes of IVS Failures Change Remove Remove a duct branch/hood A B Rough Rule of Thumb: Downstream duct diameter 2 ~ sum of squares of upstream duct diameters = 128 ~ 11 2 or = 185 ~ 14 2 or 196, ~ 1 2 or 169 C ID D, inches Q, CFM V, fpm VP/ 100 A B 8 C Impossible conveying velocity requirements that fans cannot deliver. Only option with this duct is branch ON/OFF procedure good luck! MM Don t blank a branch like these. It starves airflow and velocity downstream causing dust dropout. Unless duct changed, bleed air equivalent on removed branch. MM Baseline Performance Criteria: Airflow: + 10 % design Pressure: + 20% baseline All duct branches Turnover document - engineering to operations along with air monitoring results ISOMETRIC VIEW - OPERATOR FRIENDLY No Documented Proof of Performance MM Causes of IVS Failures - Changes to Calculated System Resistance : Dust plugs ducts, elbows first - more resistance Baghouse filter media blinds - poor startup seeding Bags bridge in filter - dust removal failure or filter design incorrect for particulate collected Access door left open bypass duct network Fan belts wear and slip Balancing orifices or blast gates removed/changed Unauthorized changes to system Design not robust enough to minimize duct plugging Proof of performance not measured or documented MM Module MM-1-1

Causes of IVS Failures Lack of Management Support Lack of management ownership and support No one accountable for system operation No trained personnel on site

Working Harder, Not Smarter IVS Operating Skills Needed 5 functional levels of increasing skill General Awareness User Operator Troubleshooter Change Reviewer Site

performing without maintenance due to Physical reasons Management reasons Predictive M&M is Before the Fact control of exposures Staff for success, but keep

person and hood Protection is general air movement into hood Low air velocity must pull dust into hood Airflow requirement increases with distance Welding hoods,

4 Causes of IVS Failures Lack of Management Support Lack of management ownership and support No one accountable for system operation No trained personnel on site Cannot get downtime to clean/maintain i system No system in place to ensure changes done by competent resources Breakdown rather than predictive maintenance Working Harder, Not Smarter IVS Operating Skills Needed 5 functional levels of increasing skill General Awareness User Operator Troubleshooter Change Reviewer Site complexity and risk assessment determines job description and number of people trained at each level MM MM Summary IV Systems degrade and stop performing without maintenance due to Physical reasons Management reasons Predictive M&M is Before the Fact control of exposures Staff for success, but keep looking for the best value Best to work with Baselined IVS Hood/Enclosure Failure Modes MM MM Open Faced Hoods OBJECTIVE: CAPTURE Dust source between person and hood Protection is general air movement into hood Low air velocity must pull dust into hood Airflow requirement increases with distance Welding hoods, simple elephant trunk hoods Key Principle for Open Hoods Flow Rate as Distance from Hood Xti times 2 Qti times 4 (move hood away and performance drops) MM MM Module MM-1-1 4

Capture Velocities Capture Velocities factors to decide upper or lower end of range Dispersion Conditions Release with practically no velocity into quiet air Example Evaporation from tanks;

Capture Velocity, ft/min 50-100 Released at low velocity Spray booths; intermittent 100-200 into moderately still air container filling; welding; plating; pickling Active generation into zone of

500-2000 Lower End of Range Room air currents minimal or favorable to capture Contaminants of low toxicity or of nuisance value only Intermittent, low production Large hood-large air mass in motion

5 Capture Velocities Capture Velocities factors to decide upper or lower end of range Dispersion Conditions Release with practically no velocity into quiet air Example Evaporation from tanks; degreasing, etc. Capture Velocity, ft/min Released at low velocity Spray booths; intermittent into moderately still air container filling; welding; plating; pickling Active generation into zone of rapid air motion Released at high initial velocity into zone of very rapid air motion Spray painting in shallow booths; barrel filling; conveyor loading Grinding; abrasive blasting; tumbling Lower End of Range Room air currents minimal or favorable to capture Contaminants of low toxicity or of nuisance value only Intermittent, low production Large hood-large air mass in motion Upper End of Range Disturbing room air currents Contaminants of high toxicity High production, heavy use Small hood-local control only Table 6-1: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice for Design, 26th Edition. Copyright Reprinted with permission. MM Table 6-1: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice for Design, 26th Edition. Copyright Reprinted with permission. MM Hood Shape and Distance Determine Required Airflow Hood Calculations - Math Review Continuity Equation: Q = VxA = V 1 xa 1 = V 2 xa 2 Q-ft /min, V-ft/min, A-ft 2 Duct Area: A = pi x radius 2 = pi x (diameter/2) 2 A=.14x(D/2) 2 x(1 ft/12in) 2 = pi x D 2 /576 Fig. 6-11: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice, 26th Edition. Copyright Reprinted with permission." MM Velocity-Velocity Pressure: V = 1096x(VP/df) 1/2 = 4005x(VP) 1/2 (sea level) or VP = (V/4005) 2 (VP-inches water column) MM Calculate Hood Face Velocity Hood Face Velocity Solution Find V duct V d = 4005x(1.1) 1/2 = 4200 ft/min Find A duct A d = pi x (4) 2 /576 = sq.ft. Find A hood A h = 4 x10 x(1 ft/12 ) 2 = sq.ft. Calculate V hood V h = V d x (A d /A h ) = 4200 x (0.087/0.278) = 115 ft/min MM MM Module MM-1-1 5

Calculate Hood SP Calculate Hood SP 1. Find duct VP 2.

Find acceleration loss Included angle = 90 o 4.

Industrial Ventilation: A Manual of Recommended Practice for Design, 26th

Copyright 2007. Reprinted with permission.

1. Duct VP = 1.1 w.c. (diagram) 2. Entry loss = 0.25 x VP.

Hood SP = acceleration loss + entry loss = 1VP+0.25VP=VP(1+0.25) =1.1x1.

Every hood has a specific SP h based on its shape and the airflow through it.

6 Calculate Hood SP Calculate Hood SP 1. Find duct VP 2. Find entry loss coefficient for shape. Find acceleration loss Included angle = 90 o 4. Calculate Hood SP MM MM Hood Entry Losses Shapes Hood Entry Losses - Transitions Fig. 9-a: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice for Design, 26th Edition. Copyright Reprinted with permission. MM-1-1 Fig. 9-a: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice for Design, 26th Edition. Copyright Reprinted with permission. MM Calculate Hood SP Using Hood Static Pressure (SP h ) for Monitoring 1. Duct VP = 1.1 w.c. (diagram) 2. Entry loss = 0.25 x VP. Acceleration loss = 1 x VP 4. Hood SP = acceleration loss + entry loss = 1VP+0.25VP=VP(1+0.25) =1.1x1.25 = 1.4 w.c. Every hood has a specific SP h based on its shape and the airflow through it. SP h values can be found in Industrial Ventilation, A Recommended Practice SP h values can be measured in the field at startup Local SP h indication (Magnehelic or manometer) provides operator warning of IVS problems MM MM Module MM-1-1 6

movement Avoid giving particle escape velocity from process or manual handling procedure Bag

from top unequal air paths => Baffle moves air inlet to back wall equal poor face velocity

Contaminant totally surrounded by enclosure Protection: negative pressure and inward air

Examples: belt conveyor housings, bins, Loss In Weight bins Need air bleed to maintain duct

Velocity with Sealed Enclosures Biggest Hood & Enclosure Failure Cause: Room Air Currents Good

7 Open Enclosures Uniform Velocity Profile Techniques to Get Equal Air Path Lengths OBJECTIVE: CONTAIN Person in front of cabinet, dust inside cabinet Protection: low velocity inward air movement Avoid giving particle escape velocity from process or manual handling procedure Bag dumps, super sack dumps, dump cabinets Bag dump open face Internal enclosure baffle Air inlet from top unequal air paths => Baffle moves air inlet to back wall equal poor face velocity profile (arrow length) air path lengths & velocities Air inlet from back equal air path lengths also possible decide based on layout MM MM Uniform Velocity Profile Slots Hood opening: 60 x 60 slots, 2 x 6 Duct diameter 12 Rectangular to round duct transition 120 degrees Contaminant totally surrounded by enclosure Protection: negative pressure and inward air movement Low air velocity at access openings Avoid enclosure positive i pressure - dusting Examples: belt conveyor housings, bins, Loss In Weight bins Need air bleed to maintain duct conveying velocity Sealed Enclosures MM MM Air Bleeds Maintain Duct Conveying Velocity with Sealed Enclosures Biggest Hood & Enclosure Failure Cause: Room Air Currents Good Air Bleed Design High velocity HVAC diffusers How does air get into the duct? Open windows Pedestal fans MM MM Module MM-1-1 7

Face Velocity Impact: Unauthorized Changes Other Hood/Enclosure Failure Modes Open hood moved out of position Excessive particle momentum Face velocity too low to stop

Visual Checks React to visible emissions to room Check for hood modifications First line of defense monitor air flow Install Hood Static Pressure gauges with action

Bypassing thru open access doors Hood face modifications MM-1-1 45 MM-1-1 46 Duct & Fan Failure Modes Key Duct Design Principles Round ducts preferred Smooth duct

8 Face Velocity Impact: Unauthorized Changes Other Hood/Enclosure Failure Modes Open hood moved out of position Excessive particle momentum Face velocity too low to stop high energy particles Process design steps to slow particles Poor face velocity profile Unequal air paths External air currents Changing hood airflow Physical changes to hood (ie, dust buildup inside, etc.) Connected IVS degrades No air bleed on sealed enclosure HVAC Supply insufficient MM MM Hood Maintenance Hood Maintenance, cont. Visual Checks React to visible emissions to room Check for hood modifications First line of defense monitor air flow Install Hood Static Pressure gauges with action limits (+ 20% Baseline) OR take routine Face Velocity measurements Things that can change airflow: Dirty screens Deposited contaminants in plenum behind hood opening Bypassing thru open access doors Hood face modifications MM MM Duct & Fan Failure Modes Key Duct Design Principles Round ducts preferred Smooth duct interior, not spiral wound Adequate conveying velocity - ALL BRANCHES target + 10% design Elbow radius 2.5 R/D mitered or 2.0 R/D smooth is best to minimize duct fouling Merge two dusty streams at angle appropriate for dust properties Balance and Baseline systems to prove you got what you paid for MM MM Module MM-1-1 8

Design Principles - Duct Sizing Size Ducts for Conveying Velocity Range Dust Control Systems: 500 to 4500 fpm. Aerosol or other small particle Control Systems: 2500 to 500 fpm.

Vapors, gases, smoke Range of Minimum Duct Design Velocities Examples All vapors, gases, smoke Minimum Design Velocity, fpm Any desired velocity (economic optimum velocity usually) 1000-2000 Fumes

Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice, 25th Edition. Copyright 2004. Reprinted with permission.

dust, granite dust, general material handling, brick dust, clay dust, foundry (general,) limestone dust Sawdust (heavy & wet,) metal turnings, foundry tumbling barrels and shake-out, sand blast dust,

9 Design Principles - Duct Sizing Size Ducts for Conveying Velocity Range Dust Control Systems: 500 to 4500 fpm. Aerosol or other small particle Control Systems: 2500 to 500 fpm. 2 2 Q A ( D / 4)(1 /144 ) D / 576 where : V 2 A cross - sectional area of duct (ft ) D duct diameter (inches) Q airflow (ft / min) V air veloci ty in duct (ft/min) MM Nature of Contaminant Vapors, gases, smoke Range of Minimum Duct Design Velocities Examples All vapors, gases, smoke Minimum Design Velocity, fpm Any desired velocity (economic optimum velocity usually) Fumes Welding Very fine Fine rubber dust, Bakelite molding powder dust, jute lint, cotton dust, light dust shavings (light,) soap dust, leather shavings Table -1: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice, 25th Edition. Copyright Reprinted with permission. MM Nature of contaminant Average industrial dust Heavy dusts Heavy or moist Range of Minimum Duct Design Velocities Examples Grinding dust, buffing lint (dry), wool jute, coffee beans, shoe dust, granite dust, general material handling, brick dust, clay dust, foundry (general,) limestone dust Sawdust (heavy & wet,) metal turnings, foundry tumbling barrels and shake-out, sand blast dust, wood blocks, hog waste, brass turnings, cast iron boring dust, lead dust Lead dusts with small chips, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky,) quick lime dust Minimum transport velocity, fpm > 4500 Table -1: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice, 25th Edition. Copyright Reprinted with permission. MM Duct Conveying Velocity Changes with Diameter Find V & VP Q=VA=V A A A =V B A B, V B =V A (A A /A B ) Remember duct area A=Pi D 2 /4 V B =V A (D A /D B ) 2 = 1000(8/4) 2 = 4000 fpm V = 4005(VP) 1/2, VP=(V/4005) 2 VP A = (1000/4005) 2 = w.c. VP B = (4000/4005) 2 = w.c. MM Pressure Loss Overcoming Straight Duct Friction Maintenance Saver Long Radius Elbows Smaller ducts have greater resistance per length. They create high system pressure drop on the governing leg of the system. MM MM Module MM-1-1 9

Branch Entries: Merge angle based on contaminant characteristics Branch Entries: Do not use T-Connection 15 degree 0 degree

for Design, 26th Edition. Copyright 2007. Reprinted with permission. MM-1-1 57 Note: 11 S.P.

Change in one branch affects entire system balance MM-1-1 58 Which Method to Balance Systems Is Used at Your Site?

Balance by Design Size ducts to restrict flow or add flow Adds 10-20% more airflow to system Balance by Blast Gate

10 Branch Entries: Merge angle based on contaminant characteristics Branch Entries: Do not use T-Connection 15 degree 0 degree MM MM Pressure Loss Branch Entries Balancing Airflows Between Branches Very sticky Mildly sticky Dry, free flow NO, don t do this!!!! Fig. 9-f: From American of Governmental Industrial Hygienists (ACGIH ), Industrial Ventilation: A Manual of Recommended Practice for Design, 26th Edition. Copyright Reprinted with permission. MM Note: 11 S.P. to fan, 7 just to get to bag house inlet 1. w/o balancing devices, airflow takes path of least resistance 2. Change in one branch affects entire system balance MM Which Method to Balance Systems Is Used at Your Site? At a glance, duct design principles met? Balance by Design Size ducts to restrict flow or add flow Adds 10-20% more airflow to system Balance by Blast Gate Adjustable (both advantage and disadvantage) Balance by Fixed Orifice Pre-calculate orifice size based on actual duct construction most accurate balance KEY PRINCIPLES 1. Size duct for conveying velocity in all branches 2. Maintain i conveying velocity through merge of two dusty air streams Good, Bad, & Ugly all in one photo! MM MM Module MM

Visual Damage-dents, holes? Open duct access doors?

Duct Maintenance Maintenance Access Fittings & elbows should be flanged (or have quick disconnect couplings) at both ends.

Provide quick opening access door in large diameter ductwork (18 dia. and above).

MM-1-1 61 MM-1-1 62 Remote sensing of duct static pressure saves climbing ladders Use single piece of tubing Need to wait for reading to equalize Check for plugging

11 Visual Damage-dents, holes? Open duct access doors? Local gauges with action limits Routine monitoring i Duct network static pressures Strategic airflows Take Troubleshooting action > + 20% Baseline static pressure Duct Maintenance Maintenance Access Fittings & elbows should be flanged (or have quick disconnect couplings) at both ends. For frequently removed section or complex sections - match mark adjoining duct sections for efficient re- assembly. Provide quick opening access door in large diameter ductwork (18 dia. and above). Consider permanent platforms for hard to reach duct section requiring frequent cleanout. MM MM Remote sensing of duct static pressure saves climbing ladders Use single piece of tubing Need to wait for reading to equalize Check for plugging if reading doesn t change Monitoring - Test Ports Fan Failure Modes - Fan Drive System Belts lose or worn? Pulleys aligned? Bearings lubed? Vibration? Courtesy Dwyer Co. MM MM Fan Design Principles Design Operating Point Match the fan performance curve with the system Oversizing for possible future need may be inefficient operating point for current need Consequence major energy costs For long term reliability Heavy duty industrial exhausters Optimal shaft alignment and balance Don t exceed fan mfr s maximum shaft speed recco. Avoid fan performance degradation (system effects) due to poor fan inlet and discharge duct design Pressure Fan System Operating Point System balance calculations determine fan requirement Fan Performance Tables locate the vendor s fan that can deliver requirement Flow MM MM Module MM

First Law: Airflow Q Second Law: Static Pressure Third Law: Power rough approximation: Fan Laws N Q N 2 2 N SP SP2 N N PWR PWR2 N 2 N 2 m airflow SP Pa h

Increase flow at the system hoods to 14,000 CFM without changing any of the system duct.

Calculate new fan shaft speed 2. Calculate new static pressure required.

(14000/12546) 2 2 /Q 1 ) = 9.95 Brake horsepower change HP 2 =HP 1 (Q 2 /Q 1 ) =2.2(14000/12546) =2.

12 First Law: Airflow Q Second Law: Static Pressure Third Law: Power rough approximation: Fan Laws N Q N 2 2 N SP SP2 N N PWR PWR2 N 2 N 2 m airflow SP Pa h PWR( kw ) Calculation Fan Capable of Change? Performance of existing fan 12,546 8 FSP Fan shaft speed 875 RPM and 2.2 BHP. Increase flow at the system hoods to 14,000 CFM without changing any of the system duct. The Determine new operating volume, pressure and horsepower if the change is made by only a fan speed increase. MM MM Fan Laws Calculation 1. Calculate new fan shaft speed 2. Calculate new static pressure required. Calculate new brake horsepower Speed change Fan Laws Calculation N 2 =N 1 (Q 2 /Q 1 )=875(14000/12546)=976 RPM Static pressure change P 2=P 1 1(Q 2 =8 (14000/12546) 2 2 /Q 1 ) = 9.95 Brake horsepower change HP 2 =HP 1 (Q 2 /Q 1 ) =2.2(14000/12546) =2.2 BHP (versus 25 HP motor) fan motor upgrade to next size, 40 HP MCC & wiring upgrade size 2 size MM MM Effect of Fan Speed Change No Fan System Effects (1.1) 2 =1.21 (1.1) =1. No loss stack 4 x (D+1 ) Inlet duct - 5D or greater System effects can add several inches Fan Static Pressure to overcome poor inlet conditions (1.1) MM MM Module MM

Stack Weather Head Critical to Dispersion Fan Maintenance Recommended No Loss Stackhead bird s eye view! DON T USE THESE! MM-1-1 7 Visual Overheated bearings?

Routine Maintenance Lubricate shaft bearings fan, motor Check mechanical components (belts, bearings, fan impellor & housing clean) Check vibration isolators Vibration analysis Motor condition &

13 Stack Weather Head Critical to Dispersion Fan Maintenance Recommended No Loss Stackhead bird s eye view! DON T USE THESE! MM Visual Overheated bearings? Unusual noises or vibrations (belts aligned?, impeller out of balance-erosion/buildup?) Air leaks on flexible connections? Routine Maintenance Lubricate shaft bearings fan, motor Check mechanical components (belts, bearings, fan impellor & housing clean) Check vibration isolators Vibration analysis Motor condition & electrical current draw MM AMCA Useful Fan References Publication 201 Fans & Systems Publication 202 Troubleshooting Fans ACGIH Industrial Ventilation, A Manual of Recommended Practice for Design, 26 th edition Industrial Ventilation, A Manual of Recommended Practice for Operation and Maintenance, 1 st edition Your Fan Manufacturer s literature Key Issues for Any Collector Meeting environmental emission permit requirements contaminant collection efficiency collected contaminant recycle or disposal Operating the collector within its design differential pressure (DP) range DP high acts like a damper & reduces airflow in rest of system DP low bypassing of collector and DC System? cause higher than desired airflow in DC System? MM MM Types of Collectors SUMMARY RELIABLE IVS PERFORMANCE Particulate Collectors (dusts/mists/fumes) Fabric Filters or Baghouses Cyclones Particle Scrubbers Electrostatic Precipitators Mist Eliminators Vapor/Gas Collectors Absorbers Gas Scrubbers Thermal & Catalytic Oxidizers Bio Filters Reliable Hood/Enclosure Exhaust Airflows Interference by room air currents Modifications that degrade performance Keep duct conveying velocities within range Air flow balanced between all branches Design systems with maintenance in mind Fan inlets and exhausts avoid fan system effects Keep air cleaning device differential pressures within Base Condition range MM MM Module MM-1-1 1

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