Fume Extraction System Design: Laboratory Exhaust Engineering Guide

Fume Extraction System Design: Laboratory Exhaust Engineering

Fume extraction system design for laboratories requires integrating four components — fume hood, ductwork, exhaust fan, and discharge stack — into a single balanced system that captures chemical fumes and discharges them safely above the roof. The design must accommodate mixed acid exhaust, meet ANSI Z9.5 and NFPA 45 requirements, and provide consistent hood face velocity regardless of sash position. This fume extraction system design guide covers hood types and materials, hood performance testing per ASHRAE 110, manifold versus individual exhaust systems, VAV control sequences, stack dispersion design, emergency exhaust per NFPA 45, and duct material selection for specific chemicals. A complete worked example demonstrates a 30-hood system. A well-designed fume extraction system design integrates all four components from the start — specifying them separately results in mismatched CFM, pressure, and materials. For PP ductwork specifications, see our PP ductwork ventilation system design guide.

Key Takeaways

• Fume extraction system design integrates four components: hood, duct, fan, and stack — the hood face velocity (80-100 ft/min per ANSI Z9.5) determines the system CFM, which determines the duct size, fan size, and stack diameter. Every component depends on the hood selection.
• VAV fume hoods with modulating dampers reduce average exhaust CFM by 30 to 50 percent compared to constant-volume hoods — the fan VFD reduces speed proportionally, saving 20 to 35 percent in annual fan energy. The VAV control sequence maintains constant duct static pressure measured by a sensor 2/3 of the way down the main duct.
• PPs flame retardant duct is the standard material for fume hood exhaust — it resists mixed acids (HCl, HNO₃, H₂SO₄) and meets the IBC flame spread index of 25 or less. For HF exhaust, specify PP+FRP hybrid duct or PVC duct. For exhaust above 70°C, specify FRP.
• Manifold systems (one fan serving 8 to 40 hoods) cost 40 to 60 percent less than individual fans per hood. A single 25,000 CFM fan serving 30 hoods costs $14,000 versus 30 individual 800 CFM fans at $3,200 each ($96,000). The manifold duct must be sized for the peak CFM.
• The stack must discharge at 2,500 to 3,500 ft/min at least 10 ft above the roof per ANSI Z9.5 — this velocity provides adequate plume rise for atmospheric dispersion. A stack below 1,500 ft/min discharge velocity can cause re-entrainment of exhausted fumes into the building ventilation system.

Fume Hood Construction, Materials, and Performance Testing

Fume hoods are classified by construction material and airflow configuration. The interior liner material must match the chemical resistance requirements of the laboratory — PP-lined hoods are standard for mixed acid service (HCl, HNO₃, H₂SO₄), FRP-lined hoods for service up to 120°C, and SS 316L-lined hoods for perchloric acid and high-temperature service above 120°C. The hood exterior is typically painted steel — the exterior does not contact chemicals and does not require corrosion-resistant material. Standard hood sizes range from 4 ft to 8 ft width for chemical laboratories, 5 ft to 6 ft being the most common single-hood size. The hood depth is 30 to 36 inches, and the interior height is 36 to 48 inches. The hood must include a raised sill at the front edge (typically 4 inches high per ANSI Z9.5) to contain chemical spills within the hood — a spill on the sill surface flows back into the hood rather than onto the lab floor. The hood sash is tempered safety glass with a counterweight mechanism that holds the sash at any position. Modern VAV hoods include a sash position sensor — an ultrasonic or infrared sensor mounted at the top of the hood that measures the sash opening height and transmits the position to the VAV damper controller. The fume extraction system design must begin with the hood selection because the hood CFM and static pressure determine the duct size, fan size, and stack size. A common error is to select the duct size and fan size before the hood model is confirmed, resulting in a system that cannot achieve the hood’s rated CFM at the required static pressure.

Hood performance is certified per ASHRAE Standard 110 — a tracer gas test that measures the hood’s ability to contain a challenge gas (SF₆) at the hood face. The test is performed with the sash at 10-inch opening (the typical operating position) and the hood exhaust at the design CFM. The challenge gas is released inside the hood at 4 L/min, and sensors at the hood face measure any gas leakage into the room. The hood passes if the measured SF₆ concentration outside the hood is below 0.05 ppm (the ASHRAE 110 acceptance criterion for an “AS‑1” rated hood, the highest rating). The test is performed by a certified third-party testing agency and repeated after the hood is installed and connected to the exhaust ductwork. The ASHRAE 110 test verifies that the hood, duct connection, and exhaust fan together provide adequate containment at the design CFM. A hood that passes the factory ASHRAE 110 test but fails the field test has an installation issue — typically a duct connection that is too small or a fan that does not deliver the design CFM. The duct designer must verify that the fan delivers the hood’s rated CFM at the system static pressure, not just the hood CFM at the fan’s free-air rating. The hood manufacturer’s published CFM and static pressure drop data must be included in the system pressure loss calculation — omitting the hood entry loss underestimates the total system pressure by 0.5 to 1.5 in. W.G., resulting in a fan that delivers 15 to 25 percent less CFM than the hoods require. The ASHRAE 110 test should also be performed after any significant modification to the hood or duct system — changing a hood manifold connection or modifying the exhaust fan speed can change the hood containment characteristics even if the hood itself has not been altered.

The hood selection determines the operating cost of the entire fume extraction system over its 15 to 20 year life. A standard bypass hood at 100 ft/min face velocity requires 700 CFM for a 7 ft² opening, while a low-flow hood at 60 ft/min requires only 420 CFM — a 40 percent reduction. At $0.12/kWh and 6,000 operating hours per year, the 700 CFM hood costs $320 per year in fan energy at 0.8 in. W.G. system pressure, while the 420 CFM hood costs $195 per year — a savings of $125 per hood per year. For 30 hoods, the annual saving is $3,750. The low-flow hood premium of $2,000 to $3,000 per hood pays back in 16 to 24 months through fan energy savings. The fume extraction system design for new laboratory buildings should specify low-flow or VAV hoods as the default, with standard bypass hoods only for budget-constrained projects where first cost is the primary driver. For laboratory buildings with 20 or more hoods, the lifecycle cost analysis consistently favors low-flow or VAV hoods because the cumulative fan energy savings over 15 years exceed the initial hood cost premium by 3 to 5 times.

Duct Material Selection by Chemical Type

The duct material selection table above covers the most common chemical categories in laboratory fume extraction system design. PPs flame retardant duct (UL 94 V-0) is the standard for 80 percent of laboratory installations — it provides the chemical resistance for mixed acids and meets the fire rating required by IBC and NFPA 45. The PPs duct cost of $55 to $85 per foot installed for 24-inch diameter is the baseline. For HF exhaust, PP (either PP-H or PPs) is acceptable — solid FRP is not because HF attacks the glass fibers. For perchloric acid, SS 316L duct with a water-wash system is mandatory by NFPA 45 — perchloric acid vapors condense in the duct and form explosive perchlorate crystals. The water-wash system sprays water into the duct at regular intervals, typically every 8 hours, to flush away the crystals. The SS 316L duct for perchloric acid costs $140 to $220 per foot — 2 to 3 times PPs — but is required by code. The comprehensive fume extraction system design must include a material compatibility review for every chemical used in the laboratory — not just the most common ones — because an incompatible duct material can fail within 6 to 12 months of exposure to a chemical that the designer did not account for.

The duct material must also be selected for the exhaust temperature at each section. A laboratory with standard hoods at 25°C uses PPs throughout. A laboratory with walk-in hoods handling hot processes at 90°C requires FRP for the first 20 to 30 ft downstream of the hot hoods, transitioning to PPs where the gas has cooled below 70°C. The transition between FRP and PPs includes a flanged connection with a Viton gasket and independent support for both sections — the two materials have different thermal expansion rates. The cost of an FRP-to-PPs transition is $250 to $500 including flanges and gasket — negligible compared to replacing PPs that fails from thermal degradation. In fume extraction system design, the material selection must be verified for every hood with different exhaust chemistry or temperature — not assumed to be the same for all hoods.

Manifold Systems, VAV Control, and Emergency Exhaust

Manifold systems — where multiple fume hoods share a common exhaust duct and fan — are the standard approach for installations with 8 or more hoods. The manifold duct must be sized for the peak simultaneous CFM — typically 80 to 100 percent of the total hood CFM depending on the diversity factor. For a system with 30 hoods at 800 CFM each (24,000 CFM total), the manifold duct is sized for 24,000 CFM at 2,000 ft/min = 12.0 ft² = 44-inch diameter PPs duct at the fan inlet. The main duct diameter reduces progressively as branches are taken off — at the farthest hood, the main duct is sized for that single branch CFM; at the fan inlet, it is sized for the total CFM. The manifold system cost for 30 hoods is $240,000 including hoods, PPs ductwork, fan, controls, and installation — versus $380,000 for 30 individual hoods with dedicated fans and stacks (60 percent higher). The manifold system also provides simpler maintenance (one fan to inspect instead of 30) and better energy efficiency (the single fan operates at 50 to 70 percent load on average versus 30 individual fans operating at 30 to 50 percent load).

VAV control sequences for fume extraction systems: each fume hood has a modulating exhaust damper controlled by the sash position sensor. The damper modulates from 100 percent open at full sash to 30 to 40 percent open at fully closed sash. The exhaust fan has a VFD that maintains a static pressure setpoint measured by a pressure transmitter located 2/3 of the distance down the main duct from the fan (the ASHRAE-recommended sensor location for stable VAV control). The static pressure setpoint is 1.0 to 2.0 in. W.G. depending on the system design — the setpoint must be high enough that the farthest hood receives its design CFM when its damper is fully open. When multiple hoods close, the duct static pressure rises, the VFD slows the fan, and the duct static pressure returns to the setpoint. The VFD speed range is 30 to 100 percent — below 30 percent speed, the fan may stall because the fan curve at low speed does not intersect the system curve. Emergency exhaust override: when the fire alarm system or a manual emergency exhaust station is activated, all VAV dampers go to 100 percent open, and the fan VFD goes to 100 percent speed regardless of the static pressure setpoint. The fan motor must be sized for 100 percent speed continuous operation — the emergency exhaust condition may require 3 to 5 times the normal operating power. A VAV system that normally operates at 15 HP at 60 percent speed requires 15 × (1.0/0.60)³ = 69 HP at full speed during emergency exhaust. The motor must be sized for this emergency condition. NFPA 45 requires the emergency exhaust system to exhaust the entire building volume within 15 minutes — for a 30,000 ft² lab building with 12 ft ceilings, the emergency exhaust CFM must be at least 30,000 × 12 / 15 = 24,000 CFM.

Stack Dispersion Design

The exhaust stack is the final element of a fume extraction system design. The stack must discharge the exhausted fumes at sufficient height and velocity to prevent re-entrainment into the building’s air intakes or open windows. The stack diameter is sized for a discharge velocity of 2,500 to 3,500 ft/min at the design CFM — this velocity provides adequate plume rise for the exhaust gases to dilute in the atmosphere before reaching ground level. A 24,000 CFM system at 3,000 ft/min requires a stack area of 24,000 / 3,000 = 8.0 ft², requiring a 36-inch diameter stack. The stack height must be at least 10 ft above the roof surface per ANSI Z9.5, and at least 10 ft above any air intake within 50 ft of the stack. For laboratory buildings with rooftop penthouses, the stack must extend at least 7 ft above the penthouse roof if the penthouse is within 10 ft of the stack. The stack discharge velocity must be verified at both peak and partial flow conditions — a VAV system that operates at 50 percent average CFM must still maintain at least 1,500 ft/min discharge velocity at the minimum expected CFM to provide adequate plume rise at low-flow conditions.

The stack design should also consider the prevailing wind direction and nearby building obstructions. A wind tunnel study or computational fluid dynamics (CFD) analysis is recommended for buildings with complex roof geometries, penthouses, or nearby taller buildings that can create downwash of the exhaust plume. The stack must be fabricated from the same material as the duct — PPs or FRP depending on the exhaust temperature — and must include a rain cap or weatherhead that prevents rain entry while allowing free exhaust discharge. A gooseneck or 45-degree elbow with downward-facing opening is standard. The stack must have a condensate drain at the lowest point with a chemical-resistant trap and drain line to the building drainage system — without the drain, condensate accumulated in the stack bottom causes material degradation at the water line within 3 to 5 years. The stack must be independently supported from the roof structure — the weight of the stack must not be carried by the fan discharge flange. The stack support includes SS 304 or FRP brackets braced to the roof curb or structural steel, with a flexible connector at the fan-to-stack connection to isolate fan vibration from the stack.

The stack-to-fan connection is a common weak point in fume extraction system design. The flexible connector between the fan discharge and the stack must be fabricated from corrosion-resistant material (Viton-coated fabric or PTFE) and must be long enough to accommodate 0.5 to 1.0 inches of thermal expansion movement without stressing the fan or stack flanges. A flexible connector that is too short — less than 4 inches long for stack diameters above 24 inches — transmits fan vibration directly to the stack, which radiates the vibration as audible noise at the blade passage frequency. A connector that is too tight (installed in tension rather than with 10 to 15 percent slack) tears at the clamp connection within 6 to 12 months. For stacks that extend more than 20 ft above the roof, consider installing a flexible connector at both the fan discharge and at a mid-height stack joint to accommodate the differential thermal expansion between the PP or FRP stack and the steel roof curb. The total installed stack cost for a 36-inch diameter PPs stack at 15 ft height, including the flexible connector, support brackets, rain cap, and condensate drain, is $2,500 to $4,000. For the complete fan and stack design methodology, see our FRP blower selection guide.

Worked Example: 30-Hood Research Laboratory

A research laboratory building requires a fume extraction system for 30 fume hoods: 24 standard VAV hoods at 800 CFM each (100 ft/min, 8 ft² opening) and 6 walk-in hoods at 3,000 CFM each. Total peak CFM: 24 × 800 + 6 × 3,000 = 19,200 + 18,000 = 37,200 CFM. Average operating CFM at 50 percent sash position for VAV hoods: 24 × 400 + 6 × 3,000 = 9,600 + 18,000 = 27,600 CFM. Hood entry loss: 0.8 in. W.G. for standard VAV hoods, 1.2 in. W.G. for walk-in hoods. Duct material: PPs flame retardant duct with V-0 rating throughout (exhaust is mixed acids at 35°C) except the first 20 ft downstream of each walk-in hood, which is FRP (the walk-in hoods occasionally handle hot processes up to 90°C). Main duct sizing at 2,000 ft/min: 37,200 CFM requires 18.6 ft² = 48-inch diameter PPs duct at the fan inlet. Branch ducts for VAV hoods at 800 CFM: 8-inch diameter. Walk-in hood branches at 3,000 CFM: 16-inch diameter FRP for the first 20 ft, transitioning to PPs for the remainder. The FRP-to-PPs transitions use flanged connections with Viton gaskets and independent supports on both sides. The 120 ft of FRP duct costs $11,400 versus $9,600 for PPs — the $1,800 premium is justified because PPs at 90°C would degrade within 12 months.

Fan selection: total system pressure loss — straight duct (250 ft at 0.12 in. W.G./100 ft for 48-inch duct = 0.30 in. W.G.), fittings (10 long-radius elbows at 50 ft eq. + 8 tees at 80 ft eq = 500 + 640 = 1,140 ft eq., 11.4 × 0.12 = 1.37 in. W.G.), hood entry (average 0.9 in. W.G.), stack loss (0.30 in. W.G.) = 2.87 in. W.G. plus 15 percent margin = 3.30 in. W.G. Fan: FRP centrifugal, 630 mm backward-curved impeller at 1,200 RPM, 50 HP motor, belt drive, Arrangement 9, VFD for VAV speed control. The motor is sized for emergency exhaust at 100 percent speed: 50 HP × (1.0/0.65)³ = 182 HP — select 200 HP motor per NFPA 45 emergency exhaust requirement. Stack: 48-inch diameter PPs, 15 ft above roof, discharge velocity 37,200 CFM at 3,000 ft/min. Stack includes rain cap, condensate drain, and independent roof curb support.

System cost breakdown: 30 fume hoods at $6,500 average = $195,000, PPs ductwork 800 ft at $80/ft avg = $64,000, FRP duct 120 ft at $95/ft = $11,400, fan with 200 HP motor and VFD = $28,500, controls including VAV damper controllers and static pressure sensors = $32,000, installation labor at 35 percent of material = $79,600, commissioning and ASHRAE 110 testing at $1,200 per hood = $36,000 — total $446,500. Annual fan energy cost at $0.12/kWh: peak operation at 37,200 CFM uses 50 HP × 0.746 × 2,000 h = $8,952. Average operation at 27,600 CFM uses 50 HP × (27,600/37,200)³ = 50 × 0.41 = 20.5 HP × 0.746 × 4,000 h = $6,116. Total annual energy = $15,068. The constant-volume alternative costs 50 HP × 0.746 × 6,000 h = $22,380 — VAV saves $7,312 per year. The VAV control system premium of $32,000 pays back in 32,000 / 7,312 = 4.4 years from energy savings. Over a 15-year system life, the VAV system saves $109,680 in energy versus constant-volume operation — 3.4 times the VAV control investment.

Fume Extraction System Design FAQ

What components are included in fume extraction system design?
Four components: fume hood (captures fumes), ductwork (PPs or FRP, transports fumes), exhaust fan (roof-mounted FRP centrifugal, provides airflow), and stack (discharges fumes above roof). All four must be designed as an integrated system — the hood CFM determines the duct size, fan size, and stack diameter.

What hood face velocity is required per ANSI Z9.5?
80 to 100 ft/min at the hood opening for standard applications. The hood CFM is the face velocity times the hood opening area — a 7 ft² hood at 100 ft/min requires 700 CFM. VAV hoods vary the CFM with sash position, reducing average CFM by 30 to 50 percent compared to constant-volume hoods.

What duct material is used for perchloric acid exhaust?
SS 316L duct with a water-wash system — mandatory per NFPA 45. Perchloric acid vapors condense in the duct and form explosive crystals that can detonate on impact. The water-wash system sprays water into the duct at 8-hour intervals to dissolve and flush away the crystals.

How are multiple fume hoods connected to a common fan?
A manifold duct system where branches from each hood join a main duct sized for the total CFM at 2,000 ft/min. Balancing dampers at each branch are set during commissioning. Manifold systems cost 40 to 60 percent less than individual fans per hood for installations with 8 or more hoods.

How is the VAV fan controlled for a multi-hood system?
A static pressure sensor located 2/3 of the way down the main duct sends a signal to the fan VFD. The VFD maintains the static pressure setpoint by varying fan speed as hoods open and close. The motor must be sized for 100 percent speed emergency exhaust per NFPA 45 — the emergency power requirement is 3 to 5 times the normal operating power.

What stack design prevents re-entrainment of lab exhaust?
Stack discharge velocity of 2,500 to 3,500 ft/min at a height at least 10 ft above the roof and 10 ft above any air intake within 50 ft per ANSI Z9.5. The stack must include a rain cap, flexible connector at the fan connection, and a condensate drain at the lowest point. A flexible connector shorter than 4 inches transmits fan vibration to the stack and causes noise.

What is the payback period for VAV fume hood controls?
The VAV control system premium typically pays back in 3 to 5 years through fan energy savings. For the 30-hood system example, the $32,000 VAV control premium pays back in 4.4 years at $7,312 annual energy savings. Over 15 years, VAV saves $109,680 — 3.4 times the control investment.

How does the ASHRAE 110 hood test work?
SF₆ tracer gas is released inside the hood at 4 L/min with the sash at 10-inch opening. Sensors outside the hood measure any gas leakage. A passing result is SF₆ concentration below 0.05 ppm (AS‑1 rating). The test is performed at the factory and repeated in the field after installation.

Fume extraction system design integrates hoods, ductwork, fans, and stacks into a single system. The hood type and material determine the CFM and chemical resistance. PPs flame retardant duct is standard for mixed acid exhaust below 70°C. Manifold systems with one fan serving multiple hoods cost 40 to 60 percent less than individual fans. VAV controls with modulating dampers and VFD fan speed control save 30 to 50 percent in fan energy. The fan motor must be sized for emergency exhaust at 100 percent speed per NFPA 45 — the emergency power requirement is 3 to 5 times normal operating power. The stack must discharge at 2,500 to 3,500 ft/min at least 10 ft above the roof per ANSI Z9.5 with a flexible connector and condensate drain. The VAV control payback period is 3 to 5 years through energy savings. Contact XICHENG EP LTD for fume extraction system design and equipment supply for your laboratory project.

For PP duct and fitting specifications, see our PP ductwork ventilation system design guide. For fan selection, see our FRP blower selection guide.