Industrial Exhaust System Design: Hood to Stack
Industrial exhaust system design for chemical processes requires a five-step workflow that integrates hood capture, duct transport, air cleaning, fan movement, and stack discharge into a single balanced system. This guide covers the complete design sequence for corrosive exhaust systems using PP or FRP ductwork with FRP fans. The five steps are: capture velocity and hood design (Step 1), duct sizing by the velocity method (Step 2), system pressure loss calculation including duct friction, fitting losses, and equipment pressure drop (Step 3), fan selection from the system curve (Step 4), and stack discharge design for atmospheric dispersion (Step 5). A complete worked example at the end demonstrates all five steps for a 10,000 CFM HCl exhaust system. For component-level design details, see our PP ductwork design guide for duct sizing and our FRP blower selection guide for fan selection.
Key Takeaways
- Industrial exhaust system design follows five sequential steps: hood design, duct sizing, pressure loss calculation, fan selection, and stack design. Skipping any step results in an unbalanced system that cannot deliver the required capture or transport velocity.
- The hood capture velocity is the starting point for the entire design — it determines the CFM required at each hood. OSHA 29 CFR 1910.94 specifies minimum capture velocities for common processes ranging from 150 ft/min for plating tanks to 200 ft/min for chrome plating.
- Duct velocity for chemical exhaust is 1,500 to 2,000 ft/min for gas-phase fumes. For systems with particulate, the velocity must be 3,500 to 4,500 ft/min depending on particle size and density.
- System pressure loss must include duct friction, fitting equivalent lengths, and connected equipment (scrubber, mist eliminator, HEPA filter). A 10,000 CFM scrubber system typically has 5.0 to 8.0 in. W.G. total pressure loss — the fan must be selected for this value at the design CFM.
- The stack discharge must be at least 10 ft above the roof and 10 ft from any air intake within 50 ft per ANSI Z9.5 to prevent re-entrainment of contaminated exhaust into the building ventilation system.
Design Sequence: 5-Step Workflow
An industrial exhaust system design for chemical exhaust follows five sequential steps that must be completed in order. Step 1 — hood design: determine the capture velocity required for the process per OSHA 29 CFR 1910.94 or ACGIH Industrial Ventilation Manual, calculate the hood entry loss, and determine the CFM required at each hood. Step 2 — duct sizing: lay out the duct route from each hood to the fan inlet, select the duct diameter for each section based on the transport velocity required for the fume or particulate, and determine the duct material (PP, FRP, or SS) based on the gas chemistry and temperature. Step 3 — pressure loss calculation: calculate the friction loss for each straight duct section using the Darcy-Weisbach equation, add the equivalent length losses for each fitting (elbows, tees, reducers, dampers), and add the pressure drop of connected equipment (scrubber, mist eliminator, filters). Sum all losses to get the total system pressure loss at the fan inlet. Step 4 — fan selection: select a fan that delivers the design CFM at the total system pressure loss plus 10 to 15 percent safety margin, and verify that the fan operates within its stable performance range at the design operating point. Step 5 — stack design: size the discharge stack for the design CFM at a discharge velocity of 2,000 to 3,000 ft/min for plume rise, and verify that the stack discharge point is at least 10 ft above the roof surface and 10 ft from any air intake within 50 ft.
Each step produces an output that is the input for the next step. The hood CFM determines the duct diameter. The duct diameter and layout determine the system pressure loss. The system pressure loss and CFM determine the fan selection. The fan discharge CFM determines the stack diameter. The five-step sequence must be followed in order — designing the system backward (selecting the fan first, then designing the duct to match) results in a system that cannot maintain the required capture velocity at the hoods because the duct was sized for the fan’s available pressure rather than the transport velocity requirement. A complete industrial exhaust system design that follows these five steps in order produces a system that meets the OSHA capture velocity requirement, maintains transport velocity in all duct sections, and operates the fan within its stable performance range.
Step 1: Hood Design and Capture Velocity
The hood capture velocity is the air velocity at the point of contaminant generation required to pull the contaminant into the hood before it escapes into the worker’s breathing zone. OSHA 29 CFR 1910.94 Table G-4 specifies minimum capture velocities for common industrial processes. For open-surface tanks: 100 ft/min for non-hazardous gases, 150 ft/min for moderately hazardous gases (HCl, H₂SO₄ at room temperature), and 200 ft/min for highly hazardous gases (chrome plating, HF etching). For grinding and abrading: 1,500 to 2,000 ft/min at the grinding wheel surface. For material handling and transfer points: 200 to 400 ft/min at the dust generation point. The capture velocity is measured at the maximum distance from the hood face to the farthest point of contaminant generation — typically 6 to 18 inches from the hood face for tank exhaust hoods. The CFM required at the hood is calculated as the product of the hood face area (in ft²) times the capture velocity (in ft/min) times a correction factor for hood efficiency. A slotted hood exhausting a 4 ft × 3 ft plating tank at 150 ft/min capture velocity requires 4 × 3 × 150 = 1,800 CFM. The hood entry loss — the pressure drop across the hood itself — is 0.5 to 1.5 in. W.G. depending on the hood type and shape, and must be included in the system pressure loss calculation in Step 3.
The hood design must include a slot or opening that generates a uniform velocity profile across the entire hood face — a hood with uneven slot opening produces high velocity at the slot ends and low velocity at the center, allowing contaminant to escape from the center of the tank. The slot width for tank exhaust hoods is 1 to 2 inches, and the slot velocity (the velocity through the slot itself) must be 1,500 to 2,000 ft/min to maintain uniform flow distribution. A slot velocity below 1,500 ft/min does not generate enough pressure differential across the slot to distribute the flow evenly — the slot near the duct connection draws more air than the far end of the slot, and the capture velocity at the far end of the tank drops below the required minimum. For tanks longer than 6 ft, install slots on both sides of the tank with a separate duct connection at each slot end. The hood design is the starting point of any industrial exhaust system design — if the hood does not capture the contaminant, no amount of duct sizing or fan selection can compensate. Per OSHA 29 CFR 1910.94, the hood capture velocity must be verified by airflow measurement after system installation and documented in the system records.
Step 2: Duct Sizing
Duct sizing selects the duct diameter for each section of the system based on the CFM from Step 1 and the transport velocity required for the exhaust stream. For gas-phase chemical exhaust (no particulate), the minimum transport velocity is 1,500 to 2,000 ft/min. For exhaust with fine particulates (dust, powder), the required velocity is 2,500 to 3,500 ft/min. For heavy particulates (grinding dust, metal chips), the velocity must be 3,500 to 4,500 ft/min. The duct area is calculated from Q = A × V, and the diameter is selected from the nearest standard size. For a 1,800 CFM branch at 2,000 ft/min, the required area is 1,800 ÷ 2,000 = 0.9 ft². The diameter is √(4 × 0.9 ÷ π) × 12 = 12.9 inches — select 14-inch (355 mm) standard PP duct. The duct material is selected based on the gas chemistry and temperature: PP-H for chemical exhaust below 80°C, FRP for 80-120°C, SS 316L for above 120°C or abrasive service. For chemical exhaust systems, the duct material must be selected after the gas chemistry is known — a system handling HCl at 60°C uses PP-H duct, while the same system at 100°C uses FRP. The duct sizing step must be repeated for each section of the system — the main duct downstream of multiple branch junctions carries the combined CFM and requires a larger diameter than the individual branches.
The duct layout must include balancing dampers at each branch takeoff and a pressure measurement port at the fan inlet. The balancing dampers allow airflow adjustment during commissioning to achieve the design CFM at each hood despite variations in branch length and fitting count. The pressure measurement port at the fan inlet allows verification of the total system pressure loss against the design calculation — the measured pressure should be within ±15 percent of the calculated value. If the measured pressure exceeds the calculated value by more than 15 percent, the duct may be undersized, blocked, or may have more fittings than accounted for in the design. If the measured pressure is more than 15 percent below the calculated value, the duct may be oversized or the hoods may be blocked. For the duct sizing methodology with a complete worked example, see our PP ductwork ventilation system design guide. For fitting equivalent length data, see our PP duct fittings guide.
Step 3: System Pressure Loss Calculation
The total system pressure loss is the sum of four components: straight duct friction loss, fitting equivalent length losses, equipment pressure drop, and hood entry loss. Straight duct friction loss is calculated using the Darcy-Weisbach equation for each duct section. For smooth PP duct at 2,000 ft/min, the friction loss is 0.4 to 1.2 in. W.G. per 100 ft depending on duct diameter as shown in the pillar friction loss table. Fitting equivalent length losses are calculated by multiplying the equivalent length of each fitting (from the PP duct fittings guide) by the friction loss per 100 ft for the corresponding duct diameter. A 14-inch long-radius 90° elbow adds 13 ft of equivalent length — at 0.6 in. W.G./100 ft for 14-inch duct at 2,000 ft/min, the elbow adds 0.13 × 0.6 = 0.08 in. W.G. Equipment pressure drop includes the scrubber or air cleaner (3.0 to 8.0 in. W.G. for a packed bed scrubber at design flow), mist eliminator (0.5 to 1.5 in. W.G.), and HEPA filter (1.0 to 4.0 in. W.G. when dirty). The hood entry loss is 0.5 to 1.5 in. W.G. depending on hood type.
For a typical industrial exhaust system design with 150 ft of duct, 6 elbows, 4 tees, one scrubber at 5.0 in. W.G., and one hood at 0.8 in. W.G. entry loss, the total pressure loss is: straight duct (150 ft at 0.6 in. W.G./100 ft = 0.90 in. W.G.) plus fitting losses (6 elbows at 13 ft eq. = 78 ft, 0.78 × 0.6 = 0.47 in. W.G. plus 4 tees at 18 ft eq. = 72 ft, 0.72 × 0.6 = 0.43 in. W.G., total fitting loss = 0.90 in. W.G.) plus equipment (5.0 in. W.G.) plus hood entry (0.80 in. W.G.) = 7.60 in. W.G. total. This is the static pressure requirement at the fan inlet. Add 10 to 15 percent safety margin: 7.60 × 1.15 = 8.74 in. W.G. — the fan must be selected for 10,000 CFM at 9.0 in. W.G. The pressure loss calculation must include every component in the system — a single omitted component (such as a mist eliminator that the designer forgot to include) can add 1.0 to 2.0 in. W.G. of unaccounted pressure loss that leaves the fan unable to deliver the design CFM by 15 to 25 percent.
Step 4: Fan Selection from the System Curve
Fan selection in an industrial exhaust system design requires matching the fan performance curve to the system resistance curve at the design operating point. The fan manufacturer provides a performance curve for each fan model showing the CFM versus static pressure at various speeds (RPM). Per AMCA Standard 99, fan performance data must be certified and the operating point must be within the fan’s stable range. The system resistance curve is a parabola calculated from ΔP = k × Q², where k is a constant derived from the system pressure loss calculation at one flow rate. For the worked example above, the system pressure loss at 10,000 CFM is 8.74 in. W.G., so k = 8.74 ÷ (10,000/10,000)² = 8.74. At 8,000 CFM, the system pressure loss is 8.74 × (8,000/10,000)² = 5.59 in. W.G. The fan is selected at the intersection of the fan performance curve and the system curve at the design point. For a 10,000 CFM system at 9.0 in. W.G., a backward-curved FRP centrifugal fan with a 630 mm impeller at 1,400 RPM and a 30 HP motor is typical. The selected fan must operate within its stable performance range — the operating point must be to the right of the fan curve’s peak pressure point (the stall region). A fan operating in stall has fluctuating airflow and vibration that damages the fan bearings and motor within 6 to 12 months.
The fan type selection — centrifugal vs axial — is determined by the system pressure requirement. For the worked example at 9.0 in. W.G., a centrifugal fan is the only option because axial fans cannot generate static pressure above 500 Pa (2.0 in. W.G.). The fan drive type — belt drive vs direct drive — is selected based on the operating temperature and speed flexibility requirements. For exhaust temperatures below 80°C, direct drive with a VFD provides 97 to 99 percent transmission efficiency and allows speed adjustment for future system modifications. For exhaust temperatures above 80°C, belt drive keeps the motor outside the airstream and extends motor life. For the complete fan selection methodology covering all types, see our FRP blower selection guide. The fan must be selected after the system pressure loss is known — selecting the fan first and designing the duct to match the fan’s available pressure results in a system that either cannot meet the capture velocity (duct too large, velocity too low at the hoods) or has excessive pressure loss (duct too small, velocity too high, fan operating in stall).
Step 5: Stack Design and Discharge
The stack discharge is the final element of an industrial exhaust system design. The stack diameter is sized for a discharge velocity of 2,000 to 3,000 ft/min at the design CFM — sufficient velocity provides plume rise that carries the exhaust away from the building and prevents re-entrainment into the building’s air intakes. For a 10,000 CFM system, the stack area is 10,000 ÷ 2,500 = 4.0 ft², requiring a 24-inch diameter stack. The stack height must be at least 10 ft above the roof surface and 10 ft above any air intake within 50 ft per ANSI Z9.5. For laboratory buildings, the stack height must also meet the exhaust stack design criteria in the IMC — typically 10 ft above the roof for buildings up to 3 stories, with additional height for taller buildings. The stack material must match the duct material for the system — PP for exhaust below 80°C, FRP for 80-120°C. The stack must include a rain cap or weatherhead that prevents rain entry while allowing the exhaust to discharge freely — a gooseneck or 45-degree elbow with a downward-facing opening is standard. The stack discharge velocity is the critical parameter for atmospheric dispersion — higher velocity increases plume rise and reduces ground-level concentrations of the exhaust contaminants. A stack with discharge velocity below 1,500 ft/min produces minimal plume rise, and the exhaust may be drawn down into the building wake on the downwind side, causing re-entrainment through open windows and air intakes.
The stack must be supported independently from the fan — the weight of the stack is carried by the roof curb or structural steel, not by the fan discharge flange. The fan-to-stack connection includes a flexible connector to isolate fan vibration from the stack. The stack must be braced for wind loads per ASCE 7 — a 24-inch diameter PP stack extending 15 ft above the roof at a 110 mph wind zone experiences a wind load of 150 to 300 lb at the stack top, which must be resisted by the stack bracing and roof curb connection. For PP stacks, the bracing attachment to the stack must use a SS 304 or PP band clamp that grips the stack without crushing it — a U-bolt or pipe clamp attachment cold-flows into the PP over 2 to 4 years and causes the stack to lean at the bracing point. The stack design must also include a drain port at the lowest point to allow condensate to drain from the stack — without a drain, condensate accumulates in the stack bottom and causes PP degradation at the water line within 3 to 5 years in exhaust systems handling saturated gas streams.
Material Selection for the Exhaust System
The material for each component of an industrial exhaust system design — hood, duct, fan, stack — must be selected based on the gas chemistry and temperature at that component’s location in the system. For chemical exhaust systems, the material options are PP (PP-H or PPs), FRP (vinyl ester or epoxy novolac), and SS 316L. PP-H is the standard material for exhaust below 80°C containing HCl, H₂SO₄ up to 70 percent, HF, and NaOH — PP-H covers 80 to 90 percent of chemical exhaust applications. PPs is required when building codes mandate a UL 94 V-0 fire rating for duct passing through fire-rated barriers. FRP with vinyl ester resin handles service up to 120°C and is selected when the exhaust temperature exceeds 80°C or when PP is not chemically compatible (nitric acid above 10 percent). SS 316L is selected when the temperature exceeds 120°C or when abrasive particles are present in the gas stream. The fan material must match the system material — an FRP fan is selected for FRP duct systems, and a PP/FRP hybrid fan is selected for PP duct systems that require the FRP outer structural layer.
The material selection must consider the entire system temperature profile — the exhaust gas cools as it travels through the duct, and the temperature at the fan inlet may be 10 to 30°C lower than at the hood. A system that generates exhaust at 90°C (too hot for PP) may cool to 70°C (within PP range) by the time it reaches the duct sections 100 ft downstream. In this case, the first 50 ft of duct (from the hood to the point where the gas cools below 80°C) must be FRP, and the remaining duct can be PP. The transition between FRP and PP must include a flanged connection with a Viton gasket, and the PP duct section must be supported independently of the FRP section because the two materials have different coefficients of thermal expansion. Material transitions within the same duct run are common in industrial exhaust system design for high-temperature processes — the material cost saving from specifying PP for the downstream sections typically pays for the flanged transition hardware within the first 100 ft of the run. The complete material comparison including 10-year TCO for PP, FRP, and SS 316L is covered in our PP ductwork design guide.
Worked Example: 10,000 CFM HCl Exhaust System
A chemical plant requires an exhaust system for four HCl pickling tanks, each 4 ft × 3 ft. Step 1 — hood design: each tank requires 150 ft/min capture velocity per OSHA 1910.94 for moderately hazardous gas. Each slot hood exhausts 4 × 3 × 150 = 1,800 CFM. Four tanks require 4 × 1,800 = 7,200 CFM total hood exhaust. The hood entry loss per tank is 0.8 in. W.G. Step 2 — duct sizing: the gas is HCl at 60°C — select PP-H duct at 2,000 ft/min transport velocity. Each 1,800 CFM branch requires 1,800 ÷ 2,000 = 0.9 ft² area = 12.9-inch diameter — select 14-inch PP duct. The main duct downstream of all four branches carries 7,200 CFM at 2,000 ft/min = 3.6 ft² = 25.7-inch — select 24-inch PP duct (nearest standard size). Step 3 — pressure loss: total duct length 150 ft, 6 long-radius 90° elbows (R=1.5D), 4 tee 45° branches, one packed bed scrubber at 5.0 in. W.G., one mist eliminator at 1.0 in. W.G. Straight duct friction: 150 ft of 24-inch PP at 2,000 ft/min at 0.14 in. W.G./100 ft = 0.21 in. W.G. Fitting losses: 6 elbows at 23 ft eq. each = 138 ft, 1.38 × 0.14 = 0.19 in. W.G. 4 tees at 44 ft eq. each = 176 ft, 1.76 × 0.14 = 0.25 in. W.G. Total fitting loss = 0.44 in. W.G. Equipment: scrubber 5.0 plus mist eliminator 1.0 = 6.0 in. W.G. Hood entry: 0.8 in. W.G. per tank = 0.8 in. W.G. Total system loss = 0.21 + 0.44 + 6.0 + 0.8 = 7.45 in. W.G. Plus 15 percent margin = 8.57 in. W.G. Step 4 — fan selection: a backward-curved FRP centrifugal fan with 630 mm impeller at 1,350 RPM, belt drive, 30 HP motor, delivering 7,200 CFM at 9.0 in. W.G. (fan sized with margin for future addition of a fifth tank). Step 5 — stack: 24-inch diameter PP stack, 12 ft above roof, discharge velocity 7,200 ÷ (π × 12² ÷ 144) = 7,200 ÷ 3.14 = 2,292 ft/min. Stack height 12 ft (10 ft minimum plus 2 ft roof parapet clearance). The total installed system cost: $38,000 including PP duct, FRP fan, scrubber, and installation.
Industrial Exhaust System Design FAQ
What are the five steps of industrial exhaust system design?
Hood design (capture velocity and CFM), duct sizing (velocity method), pressure loss calculation (duct friction + fittings + equipment), fan selection (system curve matching), and stack design (discharge velocity and height).
What capture velocity is required for chemical exhaust hoods?
100 ft/min for non-hazardous gases, 150 ft/min for moderately hazardous gases (HCl, H₂SO₄), and 200 ft/min for highly hazardous gases (chrome, HF) per OSHA 29 CFR 1910.94. The hood CFM is the hood face area times the capture velocity.
What duct velocity is used for chemical exhaust systems?
1,500 to 2,000 ft/min for gas-phase chemical fumes. Systems with fine dust require 2,500 to 3,500 ft/min, and heavy particulates require 3,500 to 4,500 ft/min. The duct diameter is selected to maintain the required velocity at the design CFM.
How is system pressure loss calculated?
Sum of straight duct friction loss (Darcy-Weisbach), fitting equivalent length losses, equipment pressure drop (scrubber, mist eliminator, filters), and hood entry loss. Add 10 to 15 percent safety margin for the fan selection.
How is the fan selected for an industrial exhaust system?
The fan performance curve must intersect the system resistance curve at the design operating point. The system curve is ΔP = k × Q². Select a centrifugal fan for systems above 500 Pa (2.0 in. W.G.) — axial fans cannot operate at these pressures.
What stack height is required for chemical exhaust discharge?
Minimum 10 ft above the roof and 10 ft above any air intake within 50 ft per ANSI Z9.5. The stack diameter is sized for 2,000 to 3,000 ft/min discharge velocity to provide adequate plume rise. Contact XICHENG EP LTD for design assistance on your industrial exhaust system.
Industrial exhaust system design follows five sequential steps: hood design, duct sizing, pressure loss calculation, fan selection, and stack design. Each step produces the input for the next step, and the sequence must be followed in order. The hood capture velocity determines the CFM. The CFM and transport velocity determine the duct diameter. The duct diameter and layout determine the system pressure loss. The system pressure loss and CFM determine the fan selection. The fan CFM determines the stack diameter and height. For the component design details referenced in this guide, see our PP ductwork design guide for duct sizing and our FRP blower selection guide for fan selection. Contact XICHENG EP LTD for complete industrial exhaust system design services.
For component-level design details, see our PP ductwork design guide for duct sizing and our fan sizing guide.