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11.5 Ventilation System Troubleshooting and Testing

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CIH Certified Industrial Hygienist Exam

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1 Chapter 1: The CIH Exam and How to Use This Guide

1.1 CIH Exam Format, Scoring, and Test-Day Logistics 1.2 Eligibility, Application Windows, Fees, and Certification Maintenance 1.3 The BGC Blueprint, Subject Areas, and a Calculation-First Study Plan

2 Chapter 2: Basic Science & Calculations

2.1 Gas Laws and Vapor Calculations 2.2 Unit Conversions: ppm, mg/m³, and Standard Temperature/Pressure 2.3 Fluid Mechanics and Airflow Fundamentals

3 Chapter 3: Air Sampling & Instrumentation

3.1 Active and Passive Air Sampling Strategies 3.2 Particulate and Aerosol Sampling 3.3 Direct-Reading Instruments and Calibration

4 Chapter 4: Analytical Chemistry & Lab Methods

4.1 Gas Chromatography and Mass Spectrometry 4.2 High-Performance Liquid Chromatography and Ion Chromatography 4.3 Spectroscopy and Elemental Analysis 4.4 Gravimetric, Microscopy, and Wet Chemical Methods 4.5 Laboratory Quality Assurance, Chain of Custody, and Desorption Efficiency

5 Chapter 5: Biostatistics & Epidemiology

5.1 Lognormal Distributions and Exposure Data Statistics 5.2 Upper Confidence Limits (UCL95) and Compliance Decision Rules 5.3 Epidemiological Study Designs and Measures of Association 5.4 Sampling Strategy: Sample Size, Power, and Worker Selection

6 Chapter 6: Toxicology & Biological Monitoring

6.1 Dose-Response Relationships and Points of Departure 6.2 Toxicokinetics: Absorption, Distribution, Metabolism, and Elimination 6.3 Aerosol Deposition, Clearance, and Respiratory Tract Dosimetry 6.4 Hepatic, Renal, Neurologic, and Hematologic Target Organ Toxicity 6.5 Pulmonary Toxicology, Carcinogenicity, and Reproductive Hazards 6.6 OEL Frameworks, Classifications, and Excursion Limits 6.7 Chemical Mixtures and Extended Work-Shift OEL Adjustments 6.8 Biological Exposure Indices (BEIs) and Biomonitoring

7 Chapter 7: Biohazards & Indoor Environmental Quality

7.1 Mold, Fungi, and Microbial Contaminants 7.2 Infectious Agents, Bloodborne Pathogens, and Biosafety Levels 7.3 Indoor Environmental Quality (IEQ) Investigations 7.4 Sanitation, Potable Water, and Personal Hygiene Programs

8 Chapter 8: Community Exposure & Environmental Health

8.1 Clean Air Act Framework, NAAQS, and New Source Review 8.2 Hazardous Air Pollutants, CEMS, and EPA Reference Methods 8.3 Atmospheric Stability, Lapse Rates, and Plume Morphology 8.4 Gaussian Plume Modeling, Plume Rise, and Environmental Fate 8.5 HAZWOPER Scope, Mandates, and Training Tiers 8.6 Protection Levels, Site Zoning, Incident Command, and Decontamination 8.7 Water Pollution, Hazardous Waste, and Community Right-to-Know

9 Chapter 9: Health Risk Analysis & Hazard Communication

9.1 GHS Hazard Classification: Health and Physical Hazard Criteria 9.2 GHS Labels, Safety Data Sheets, and Trade Secret Rules 9.3 The AIHA Exposure Assessment Strategy and Similar Exposure Groups 9.4 Qualitative Exposure Profiling and the AIHA Risk Matrix 9.5 Quantitative Health Risk Characterization: HQ, HI, and ELCR 9.6 NIOSH Occupational Exposure Banding 9.7 Risk Communication and Reporting to Affected Parties

10 Chapter 10: Industrial Ventilation Principles & LEV Design

10.1 Elimination, Substitution, Isolation, Containment, and Process Change 10.2 LEV Hood Taxonomy and Capture Velocity Selection 10.3 Hood Airflow Equations, Entry Losses, and Hood Static Pressure 10.4 Particulate Transport Aerodynamics and Duct Sizing 10.5 Duct Friction, Fitting Losses, and System Balancing 10.6 Fan Categories and Pressure Dynamics Across the Fan 10.7 System Curves, the Fan Laws, and Density Corrections

11 Chapter 11: General Ventilation, Air Cleaning & Diagnostics

11.1 Dilution Ventilation Principles and Steady-State Modeling 11.2 Transient Purging and Air Distribution Geometry 11.3 Particulate Air Cleaning: Cyclones, Baghouses, Scrubbers, and ESPs 11.4 Gaseous Emission Control and Air Cleaner Selection 11.5 Ventilation System Troubleshooting and Testing

12 Chapter 12: Noise Measurement & Hearing Conservation

12.1 Acoustic Physics and Decibel Mathematics 12.2 Sound Propagation, Octave Bands, and Frequency Weighting 12.3 Noise Dose Metrics and OSHA versus ACGIH Criteria 12.4 Exchange Rates, Extended Shifts, and Dosimetry Field Protocols 12.5 Engineering Noise Controls and Hearing Protector Attenuation 12.6 Hearing Conservation Programs and Standard Threshold Shifts 12.7 Hand-Arm and Whole-Body Vibration

13 Chapter 13: Thermal Stress Assessment & Control

13.1 Wet Bulb Globe Temperature (WBGT) and Heat Stress Indices 13.2 Heat-Related Disorders, Acclimatization, and Work/Rest Regimens 13.3 Cold Stress Evaluation and Wind Chill

14 Chapter 14: Ionizing Radiation Protection

14.1 Ionizing Radiation Physics and Radioactive Decay Kinetics 14.2 Radiation Units, Radiobiology, and Health Effects 14.3 ALARA, Stay-Time, and the Inverse Square Law 14.4 Shielding Physics, Material Selection, and Gamma Constants 14.5 Radiation Detection Instruments and Spectrometry 14.6 Personnel Dosimetry, Dose Limits, and Contamination Surveys

15 Chapter 15: Non-Ionizing Radiation & Laser Safety

15.1 The Non-Ionizing Spectrum and Static/ELF Field Exposures 15.2 Radiofrequency and Microwave Exposure: SAR, Standards, and Surveys 15.3 Ultraviolet, Visible, and Infrared Radiation Hazards 15.4 Laser Safety Classifications and Optical Density Calculations 15.5 Illumination, Lighting Design, and Visual Performance

16 Chapter 16: Non-Engineering Controls & Personal Protective Equipment

16.1 Respiratory Protection Programs and Assigned Protection Factors 16.2 Maximum Use Concentration, Oxygen Deficiency, and Breathing Air 16.3 Particulate Filters, Sorbent Cartridges, and Change Schedules 16.4 Respirator Fit Testing Protocols 16.5 Chemical Protective Clothing and Dermal Exposure Control

17 Chapter 17: Ergonomics & Workstation Design

17.1 Soft-Tissue Biomechanics and WMSD Pathophysiology 17.2 Ergonomic Risk Factors and Their Interaction Effects 17.3 The Revised NIOSH Lifting Equation 17.4 RULA, REBA, the Strain Index, and the ACGIH HAL TLV 17.5 Anthropometry and Workstation Geometry 17.6 VDT Workstations, Hand Tool Design, and Administrative Ergonomics

18 Chapter 18: Work Environments & Industrial Processes

18.1 Welding Fume Emissions, Metal Fume Fever, and Cr(VI)/Manganese Toxicology 18.2 Foundry Operations and Hot Work Fire Prevention 18.3 Vapor Degreasing, Pickling, Electroplating, and Cyanide Baths 18.4 Spray Coatings, Isocyanates, and Semiconductor Manufacturing 18.5 Confined Spaces and High-Hazard Industrial Operations 18.6 Asbestos, Lead, and Silica: Regulated Substance Programs 18.7 Abrasive Blasting and Surface Preparation

19 Chapter 19: IH Program Management & Professional Ethics

19.1 The BGC Code of Ethics: Canons and Enforcement 19.2 Professional Liability, Standard of Care, and Expert Testimony 19.3 Occupational Health & Safety Management Systems (ISO 45001, ANSI Z10) 19.4 Exposure Records Access and Medical Surveillance 19.5 OSHA Injury and Illness Recordkeeping and Enforcement

11.5 Ventilation System Troubleshooting and Testing

Key Takeaways Explain This Topic AI

Standard Pitot-static tubes measure velocity pressure (VP = TP - SP) and static pressure; duct velocity is calculated via V = 4005 × √(VP / d_f), where d_f is the air density correction factor (d_f = [530 / (T_act + 460)] × [P_act / 29.92]).

Duct velocity traverses must use the Log-Tchebycheff method (preferred for round and rectangular ducts) or Equal Area method, requiring 6 to 10 traverse points per axis across two perpendicular planes positioned ≥ 7.5 duct diameters downstream and ≥ 1.5 diameters upstream of aerodynamic disturbances.

Average duct velocity must be calculated by averaging the SQUARE ROOTS of individual velocity pressure measurements (V_avg = [4005 / n] × Σ√VP_i), NEVER by averaging VP values directly.

Thermal (hot-wire) anemometers are required for low face velocities (0–500 fpm) at chemical fume hoods and paint booths; rotating vane anemometers are suited for large supply diffusers, while smoke tubes provide non-quantitative flow visualization and containment verification.

Static pressure profiling systematically diagnoses ventilation faults: decreased |SP_h| indicates upstream blockage or duct collapse; elevated negative static pressure indicates downstream clogging; backward centrifugal fan rotation moves air in the forward direction but at only 40–60% of design capacity.

Last updated: August 2026

Ventilation System Troubleshooting and Testing

Industrial ventilation systems require systematic aerodynamic commissioning, regular quantitative performance verification, and methodical diagnostic troubleshooting. Changes in system aerodynamics—such as duct sedimentation, filter blinding, blast gate tampering, fan belt slippage, or physical duct collapse—compromise contaminant capture and expose workers to chemical and physical hazards. Industrial hygienists must utilize precision aerodynamic instrumentation, rigorous traverse protocols, and static pressure profiling to verify system compliance with ACGIH, ANSI/ASSP Z9, and OSHA standards.

1. Aerodynamic Instrumentation and Measurement Principles

Accurate quantification of ventilation system performance requires measuring three interrelated fluid pressures: Total Pressure (TP) , Static Pressure (SP) , and Velocity Pressure (VP) , governed by Bernoulli's energy relationship: T P = S P + V P ⟺ V P = T P − S P TP = SP + VP \quad \Longleftrightarrow \quad VP = TP - SP TP = SP + V P ⟺ V P = TP − SP +-------------------------------------------------------------------------+

  • | STANDARD PITOT-STATIC TUBE GEOMETRY |
  • +-------------------------------------------------------------------------+
  • | |
  • | Impact Opening (FACES FLOW) Static Pressure Holes |
  • | Senses Total Pressure (TP) (Perpendicular to flow) |
  • | | Senses Static Pressure (SP) |
  • | v | |
  • | ====\ v |
  • | Flow ====>)===================================+========+----+ |
  • | ====>====/ Inner Tube (TP) | | | |
  • | =========================================+========+ | |
  • | Outer Jacket (SP) | |
  • | | |
  • | | |
  • | SP Port | TP Port |
  • | (-) | (+) |
  • | | | | |
  • | v v v |
  • | [ Manometer ] |
  • | Displays: VP! |
  • +-------------------------------------------------------------------------+
  • The Standard Pitot-Static Tube

The standard Pitot-static tube consists of two concentric tubes:

Total Pressure Port (Inner Tube): Points directly upstream into the airflow. Air comes to an aerodynamic stagnation point at the tip, converting all fluid kinetic energy into impact pressure (TP).

Static Pressure Ports (Outer Tube): A ring of small, smooth radial holes drilled perpendicular to the airflow along the tube stem, sensing only potential pressure (SP).

Measuring Velocity Pressure (VP): Connecting the TP tap to the positive (+) port of a differential manometer and the SP tap to the negative (-) port yields VP directly: VP = TP - SP.

Directional Sensitivity: A standard Pitot tube must be aligned within ±15° of the duct airflow centerline to avoid measurement errors.

Standard vs. Actual Air Density Corrections

Velocity is calculated from velocity pressure using the standard air relationship (0.075 lb/ft³ density at 70°F and 29.92 in. Hg): V = 4005 V P V = 4005 \sqrt{VP} V = 4005 V P ​

When gas temperature, barometric pressure, or altitude deviate from standard conditions, the industrial hygienist must apply the air density correction factor (df) : d f = ( 530 T act ( ∘ F ) + 460 ) × ( P act ( in. Hg ) 29.92 ) d_f = \left(\frac{530}{T_{\text{act}}(^\circ\text{F}) + 460}\right) \times \left(\frac{P_{\text{act}}(\text{in. Hg})}{29.92}\right) d f ​ = ( T act ​ ( ∘ F ) + 460 530 ​ ) × ( 29.92 P act ​ ( in. Hg ) ​ ) V = 4005 V P d f \mathbf{V = 4005 \sqrt{\frac{VP}{d_f}}} V = 4005 d f ​ VP ​ ​

(where Tact is actual gas temperature in °F, and Pact is actual absolute barometric pressure in in. Hg) .

2. Duct Velocity Traverse Protocols: Log-Tchebycheff vs. Equal Area

Because fluid friction against duct walls creates a parabolic boundary layer velocity profile (velocity is zero at the walls and maximum at the centerline), a single centerline measurement cannot represent true average volumetric flow. +-------------------------------------------------------------------------+

  • | BOUNDARY LAYER VELOCITY PROFILE |
  • +-------------------------------------------------------------------------+
  • | |
  • | Duct Wall -----------------------------------------------+ |
  • | ---=== | (v ~ 0) |
  • | ----------====== | |
  • | ------------------========= (V_centerline) | Max Vel |
  • | ----------====== | |
  • | ---=== | (v ~ 0) |
  • | Duct Wall -----------------------------------------------+ |
  • | |
  • | * Centerline velocity is typically 10% to 20% HIGHER than true average!|
  • | * Centerline factor: V_avg = (0.80 to 0.90) × V_centerline (Estimate) |
  • | * Accurate Flow Measurement MANDATES a Full Multipoint Traverse! |
  • +-------------------------------------------------------------------------+
  • Traverse Placement Standards

To establish stable, fully developed laminar-turbulent flow free of swirling eddies, traverse test holes must be positioned:

≥ 7.5 to 8.5 duct diameters downstream of any elbow, fan, expansion, or disturbance.

≥ 1.5 to 2.5 duct diameters upstream of any disturbance.

1. The Log-Tchebycheff Traverse Method (ACGIH & ASHRAE Standard)

The Log-Tchebycheff (Log-T) method is the internationally recognized gold standard for round and rectangular ducts. Unlike Equal Area methods, Log-T mathematically accounts for wall shear boundary layer friction, placing traverse points at precise fractional radius locations to eliminate boundary layer overestimation. +-------------------------------------------------------------------------+

  • | 10-POINT LOG-TCHEBYCHEFF TRAVERSE LOCATIONS |
  • +-------------------------------------------------------------------------+
  • | |
  • | Round Duct: Traverse along TWO perpendicular axes (90° apart). |
  • | For Duct Diameter D, measure at distances from inside wall: |
  • | |
  • | Point 1: 0.019 × D | Point 6: 0.644 × D |
  • | Point 2: 0.077 × D | Point 7: 0.806 × D |
  • | Point 3: 0.153 × D | Point 8: 0.888 × D |
  • | Point 4: 0.217 × D | Point 9: 0.945 × D |
  • | Point 5: 0.361 × D | Point 10: 0.981 × D |
  • | |
  • | Axis 1 |
  • | ^ |
  • | ( 1 | 10 ) |
  • | ( 2 | 9 ) |
  • | ( 3 | 8 ) |
  • | ( 4 | 7 ) |
  • | <------(----5----+----6----)------> Axis 2 |
  • | ( 7 | 4 ) |
  • | ( 8 | 3 ) |
  • | ( 9 | 2 ) |
  • | ( 10| 1 ) |
  • | v |
  • +-------------------------------------------------------------------------+

Number of Points for Round Ducts:

Duct Diameter < 6 inches: Not recommended for Pitot traverse (use orifice/venturi meter).

6 in ≤ D ≤ 10 in: 6 to 8 points per axis (12 to 16 total points across 2 axes).

Duct Diameter > 10 inches: 10 points per axis (20 total points across 2 axes).

Rectangular Ducts: The duct cross-section is divided into equal rectangular sub-zones, with a minimum of 16 to 64 traverse points (measuring at zone geometric centers).

CRITICAL RULE: Averaging Traverse Velocities

Because velocity is proportional to the square root of velocity pressure (V ∝ √(VP)), YOU MUST NEVER AVERAGE THE VELOCITY PRESSURES DIRECTLY! Averaging VP values introduces severe positive mathematical error. V avg = 4005 n ∑ i = 1 n V P i = 4005 × ( V P 1 + V P 2 + ⋯ + V P n n ) \mathbf{V_{\text{avg}} = \frac{4005}{n} \sum_{i=1}^{n} \sqrt{VP_i} = 4005 \times \left(\frac{\sqrt{VP_1} + \sqrt{VP_2} + \dots + \sqrt{VP_n}}{n}\right)} V avg ​ = n 4005 ​ i = 1 ∑ n ​ V P i ​ ​ = 4005 × ( n V P 1 ​ ​ + V P 2 ​ ​ + ⋯ + V P n ​ ​ ​ ) Q = V avg × A duct \mathbf{Q = V_{\text{avg}} \times A_{\text{duct}}} Q = V avg ​ × A duct ​

3. Specialized Anemometry and Flow Visualization

Instrument Operating Principle Velocity Range Primary IH Applications Key Limitations / Caveats

Pitot-Static Tube & Micromanometer Differential pressure (TP - SP = VP) 600 to 10,000+ fpm (3 to 50 m/s) Duct traverses, fan testing, high-velocity exhaust stacks Inaccurate below 600 fpm (VP < 0.02 in. w.g.); clogs in heavy dust.

Thermal (Hot-Wire) Anemometer Convective heat loss from an electrically heated microscopic filament 0 to 500 fpm (0 to 2.5 m/s) Fume hood face velocities, cleanroom laminar flow, room cross-drafts Fragile sensor wire; contaminated by dust/grease; flammable atmosphere hazard (non-IS).

Rotating Vane Anemometer Airflow rotates mechanical miniature turbine propeller 100 to 3,000 fpm (0.5 to 15 m/s) Large supply diffusers, exhaust grilles, paint spray booths Mechanical bearing inertia; requires cross-sectional sweeping technique and area correction (Kd).

Deflecting Vane (Velometer) Air entering probe pushes a pivoted spring-loaded vane 50 to 5,000 fpm Rough field screening, spray booth face checks Lower precision (±5–10%); requires zero leveling.

Chemical Smoke Tubes Titanium tetrachloride (TiCl4) or stannic chloride reacting with ambient humidity Qualitative visual Chemical fume hood boundary containment, room draft tracing Corrosive acid smoke (HCl); irritant; prohibited in cleanrooms and around optics.

Theatrical / Glycol Foggers Heated glycol/glycerol condensation aerosol Qualitative visual Large-scale capture visualization, room air distribution mapping Leaves slight glycol residue; dense fog can trip optical smoke detectors.

4. Diagnostic Static Pressure Profiling

Static pressure (SP) represents the mechanical suction energy in an exhaust system. Because static pressure losses along a duct run are strictly governed by aerodynamic resistance, tracing the static pressure profile along the entire LEV system from hood to stack is the most powerful diagnostic tool for identifying system malfunctions . +-------------------------------------------------------------------------+

  • | NORMAL SYSTEM STATIC PRESSURE PROFILE |
  • +-------------------------------------------------------------------------+
  • | |
  • | Atmospheric Pressure (0.0 in. w.g.) ---------------------------------- |
  • | \ |
  • | Hood SP: -1.2 in. w.g. ---------+ \ Duct Friction Loss |
  • | \ \ |
  • | \ v |
  • | +-------------> Fan Inlet SP: -6.5" |
  • | | |
  • | [ EXHAUST FAN ] |
  • | | |
  • | Fan Outlet SP: +2.0 in. w.g. --------------------------+ |
  • | \ |
  • | \ Stack Friction & Loss |
  • | v |
  • | Discharge Stack Tip: 0.0 in. w.g. ------------------------------------ |
  • +-------------------------------------------------------------------------+
  • Systematic Diagnostic Malfunction Matrix

Diagnostic Observation / Deviation Probable Physical Root Cause Aerodynamic & Mechanical Explanation

Hood static pressure magnitude (|SPh|) and airflow (Q) both decreased; Fan inlet |SPi| also decreased Upstream duct collapse, clogged hood mouth, or blocked branch Obstruction upstream reduces airflow through the entire system. Reduced flow lowers friction losses, causing static pressure magnitudes everywhere to drop toward zero.

Hood airflow (Q) decreased, but negative static pressure at an intermediate tap is abnormally HIGH (more negative) Duct plugging / settled dust bed immediately downstream of test tap The blockage restricts flow, creating a high throttling resistance (severe orifice loss) immediately upstream of the plug.

System airflow (Q) decreased by 40--50%; Static pressures throughout system reduced; Fan motor amperage abnormally LOW Centrifugal fan rotating BACKWARD (reverse electrical phase wiring) Backward rotation of a backward-curved or radial centrifugal fan still moves air in the forward direction, but at only 40% to 60% of rated capacity , generating low pressure and low motor load!

Airflow (Q) and all static pressures reduced; Fan motor amperage normal or low; Fan RPM lower than design Fan V-belt slippage or loose belt tension Belt wear or loose motor mounts allow drive belts to slip on the sheaves, reducing fan rotational speed (RPM). Airflow drops directly with RPM (Q2 = Q1 [RPM2 / RPM1]).

Static pressure drop across baghouse (Δ P) abnormally high (> 8--10 in. w.g.); Hood flow starved Filter bag blinding (over-pressurization) or failure of pulse-jet cleaning system Dust cake becomes compacted or compressed air pulse header pressure drops (< 60 psig), freezing the cleaning cycle and choking system flow.

Fan motor tripping overload breakers; Total airflow (Q) at fan abnormally high, but hood capture velocities starved Duct rupture, detached branch, or open cleanout inspection door near fan inlet A massive leak near the fan allows clean room air to enter under low resistance. System operating point shifts to maximum flow on the fan curve, overloading the motor while starving upstream hoods.

  • 5. Worked Step-by-Step Calculation Examples
  • Worked Example 10.3.1: 10-Point Log-Tchebycheff Duct Traverse Calculation

Problem: An industrial hygienist conducts a 10-point Log-Tchebycheff traverse across two axes (20 total points) in a 12-inch diameter round exhaust duct (D = 12 in = 1.0 ft, A = (π · 1²)/4 = 0.7854 ft²). Standard air conditions apply. The measured velocity pressures (VP, in in. w.g.) across the 10 points on Axis 1 and Axis 2 are recorded as follows:

  • Axis 1 VP measurements: 0.36, 0.49, 0.64, 0.81, 0.81, 0.81, 0.64, 0.49, 0.36, 0.25
  • Axis 2 VP measurements: 0.36, 0.49, 0.64, 0.81, 0.81, 0.81, 0.64, 0.49, 0.36, 0.25

Calculate the square roots of the velocity pressures for each measurement point.

Determine the true average duct velocity (Vavg) in fpm.

Calculate the total volumetric airflow rate (Q) in cfm.

Show the error that would result from incorrectly averaging VP directly before taking the square root.

Solution Steps:

Compute √(VP) for each point:

  • Points with VP = 0.81 → √(0.81) = 0.90 (6 points total)
  • Points with VP = 0.64 → √(0.64) = 0.80 (4 points total)
  • Points with VP = 0.49 → √(0.49) = 0.70 (4 points total)
  • Points with VP = 0.36 → √(0.36) = 0.60 (4 points total)
  • Points with VP = 0.25 → √(0.25) = 0.50 (2 points total)

Calculate average of the square roots ((√(VP))avg): ∑ V P = ( 6 × 0.90 ) + ( 4 × 0.80 ) + ( 4 × 0.70 ) + ( 4 × 0.60 ) + ( 2 × 0.50 ) \sum \sqrt{VP} = (6 \times 0.90) + (4 \times 0.80) + (4 \times 0.70) + (4 \times 0.60) + (2 \times 0.50) ∑ V P ​ = ( 6 × 0.90 ) + ( 4 × 0.80 ) + ( 4 × 0.70 ) + ( 4 × 0.60 ) + ( 2 × 0.50 ) ∑ V P = 5.40 + 3.20 + 2.80 + 2.40 + 1.00 = 14.80 \sum \sqrt{VP} = 5.40 + 3.20 + 2.80 + 2.40 + 1.00 = 14.80 ∑ V P ​ = 5.40 + 3.20 + 2.80 + 2.40 + 1.00 = 14.80 ( V P ) avg = 14.80 20 = 0.740 (\sqrt{VP})_{\text{avg}} = \frac{14.80}{20} = 0.740 ( V P ​ ) avg ​ = 20 14.80 ​ = 0.740

Calculate true average velocity and volumetric flow rate: V avg = 4005 × ( V P ) avg = 4005 × 0.740 = 2963.7 fpm ≈ 2964 fpm V_{\text{avg}} = 4005 \times (\sqrt{VP})_{\text{avg}} = 4005 \times 0.740 = 2963.7\text{ fpm} \approx 2964\text{ fpm} V avg ​ = 4005 × ( V P ​ ) avg ​ = 4005 × 0.740 = 2963.7 fpm ≈ 2964 fpm Q = V avg × A duct = 2963.7 fpm × 0.7854 ft 2 = 2327.7 cfm ≈ 2328 cfm Q = V_{\text{avg}} \times A_{\text{duct}} = 2963.7\text{ fpm} \times 0.7854\text{ ft}^2 = 2327.7\text{ cfm} \approx 2328\text{ cfm} Q = V avg ​ × A duct ​ = 2963.7 fpm × 0.7854 ft 2 = 2327.7 cfm ≈ 2328 cfm

Demonstrate error of averaging VP directly: ∑ V P = ( 6 × 0.81 ) + ( 4 × 0.64 ) + ( 4 × 0.49 ) + ( 4 × 0.36 ) + ( 2 × 0.25 ) = 4.86 + 2.56 + 1.96 + 1.44 + 0.50 = 11.32 \sum VP = (6 \times 0.81) + (4 \times 0.64) + (4 \times 0.49) + (4 \times 0.36) + (2 \times 0.25) = 4.86 + 2.56 + 1.96 + 1.44 + 0.50 = 11.32 ∑ V P = ( 6 × 0.81 ) + ( 4 × 0.64 ) + ( 4 × 0.49 ) + ( 4 × 0.36 ) + ( 2 × 0.25 ) = 4.86 + 2.56 + 1.96 + 1.44 + 0.50 = 11.32 V P arithmetic avg = 11.32 20 = 0.566 in. w.g. VP_{\text{arithmetic avg}} = \frac{11.32}{20} = 0.566\text{ in. w.g.} V P arithmetic avg ​ = 20 11.32 ​ = 0.566 in. w.g. V erroneous = 4005 × 0.566 = 4005 × 0.7523 = 3013.1 fpm V_{\text{erroneous}} = 4005 \times \sqrt{0.566} = 4005 \times 0.7523 = 3013.1\text{ fpm} V erroneous ​ = 4005 × 0.566 ​ = 4005 × 0.7523 = 3013.1 fpm Error = 3013.1 − 2963.7 2963.7 × 100 = + 1.67 % overestimation \text{Error} = \frac{3013.1 - 2963.7}{2963.7} \times 100 = +1.67\% \text{ overestimation} Error = 2963.7 3013.1 − 2963.7 ​ × 100 = + 1.67% overestimation

Result: The true average duct velocity is 2964 fpm and the volumetric flow rate is 2328 cfm .

Worked Example 10.3.2: Air Density Correction for High-Temperature Exhaust

Problem: A hot process exhaust duct carries flue gas at a temperature Tact = 250°F. The facility is located at an elevation of 5,000 ft, where actual barometric pressure is Pact = 24.90 in. Hg. A Pitot-static tube traverse yields an average square root of velocity pressure (√(VP))avg = 0.850 (in. w.g.)(1/2). The round duct diameter is 16 inches (A = 1.3963 ft²).

Calculate the air density correction factor (df).

Calculate the actual duct velocity (Vact) in fpm.

Determine the actual volumetric airflow rate (Qact) in ACFM.

Solution Steps:

Calculate density correction factor (df): d f = ( 530 T act + 460 ) × ( P act 29.92 ) = ( 530 250 + 460 ) × ( 24.90 29.92 ) d_f = \left(\frac{530}{T_{\text{act}} + 460}\right) \times \left(\frac{P_{\text{act}}}{29.92}\right) = \left(\frac{530}{250 + 460}\right) \times \left(\frac{24.90}{29.92}\right) d f ​ = ( T act ​ + 460 530 ​ ) × ( 29.92 P act ​ ​ ) = ( 250 + 460 530 ​ ) × ( 29.92 24.90 ​ ) d f = ( 530 710 ) × ( 0.8322 ) = 0.74648 × 0.8322 = 0.6212 d_f = \left(\frac{530}{710}\right) \times (0.8322) = 0.74648 \times 0.8322 = 0.6212 d f ​ = ( 710 530 ​ ) × ( 0.8322 ) = 0.74648 × 0.8322 = 0.6212

Calculate actual duct velocity (Vact): V act = 4005 × ( V P ) avg d f = 4005 × 0.850 0.6212 = 4005 × 0.850 0.7882 = 4005 × 1.0784 = 4319.1 fpm ≈ 4319 fpm V_{\text{act}} = 4005 \times \frac{(\sqrt{VP})_{\text{avg}}}{\sqrt{d_f}} = 4005 \times \frac{0.850}{\sqrt{0.6212}} = 4005 \times \frac{0.850}{0.7882} = 4005 \times 1.0784 = 4319.1\text{ fpm} \approx 4319\text{ fpm} V act ​ = 4005 × d f ​ ​ ( V P ​ ) avg ​ ​ = 4005 × 0.6212 ​ 0.850 ​ = 4005 × 0.7882 0.850 ​ = 4005 × 1.0784 = 4319.1 fpm ≈ 4319 fpm

(Without density correction, the uncorrected velocity would be 4005 × 0.850 = 3404 fpm, a 21.2% underestimation!)

Calculate actual volumetric flow rate (Qact): Q act = V act × A duct = 4319.1 fpm × 1.3963 ft 2 = 6030.8 ACFM ≈ 6031 ACFM Q_{\text{act}} = V_{\text{act}} \times A_{\text{duct}} = 4319.1\text{ fpm} \times 1.3963\text{ ft}^2 = 6030.8\text{ ACFM} \approx 6031\text{ ACFM} Q act ​ = V act ​ × A duct ​ = 4319.1 fpm × 1.3963 ft 2 = 6030.8 ACFM ≈ 6031 ACFM

Result: The air density correction factor is 0.621 , actual duct velocity is 4319 fpm , and actual airflow is 6031 ACFM .

Worked Example 10.3.3: Diagnostic Static Pressure Profile Evaluation

Problem: Baseline commissioning data for a local exhaust branch showed a hood static pressure SPh = -1.50 in. w.g. and airflow Q = 1,200 cfm. During an annual audit, the industrial hygienist measures SPh = -0.60 in. w.g.

Calculate the current actual airflow (Q2) passing through the hood.

If the fan inlet static pressure also dropped from -8.0 in. w.g. down to -3.2 in. w.g., determine whether the defect is located upstream or downstream of the fan.

Solution Steps:

Calculate current airflow (Q ∝ √(|SPh|)): Q 2 Q 1 = ∣ S P h , 2 ∣ ∣ S P h , 1 ∣ = 0.60 1.50 = 0.40 = 0.6325 \frac{Q_2}{Q_1} = \sqrt{\frac{|SP_{h,2}|}{|SP_{h,1}|}} = \sqrt{\frac{0.60}{1.50}} = \sqrt{0.40} = 0.6325 Q 1 ​ Q 2 ​ ​ = ∣ S P h , 1 ​ ∣ ∣ S P h , 2 ​ ∣ ​ ​ = 1.50 0.60 ​ ​ = 0.40 ​ = 0.6325 Q 2 = 1200 cfm × 0.6325 = 758.9 cfm ≈ 759 cfm Q_2 = 1200\text{ cfm} \times 0.6325 = 758.9\text{ cfm} \approx 759\text{ cfm} Q 2 ​ = 1200 cfm × 0.6325 = 758.9 cfm ≈ 759 cfm

Diagnostic Evaluation:

Airflow has decreased by 36.8%.

Both hood static pressure magnitude and fan inlet suction magnitude decreased by the exact same ratio (0.60/1.50 = 0.40 and 3.2/8.0 = 0.40).

This systemic, uniform reduction in both airflow and static pressures across all points confirms that total system airflow is reduced due to an upstream obstruction, severe fan belt slippage, or backward fan rotation (not a downstream plug, which would have elevated negative static pressures upstream of the blockage).

Result: Current branch airflow is 759 cfm , confirming significant loss of capture efficiency.