Exhaust Snorkel Sizing Guide: Arm Diameter, CFM and Fan Capacity
Choosing an extraction arm comes down to three numbers: the hood size the work needs, the airflow that hood requires, and whether the fan you already have can deliver it. This guide walks through all three, with the airflow ranges for 2 inch, 3 inch, 4 inch, 100 mm and 6 inch arms.
Start With the Work, Not the Arm
Almost every sizing mistake we see starts at the wrong end: someone picks an arm diameter first, then tries to make the work fit it. Source capture works the other way around. The hood has to be able to sit close to the emission point, the arm has to reach that position from wherever it is mounted, and the airflow has to be enough to pull the plume into the hood before room air currents carry it away.
So begin by describing the work in four terms: what is released (solder smoke, solvent vapor, acid mist, nuisance odor, dust), how big the source is, where on the bench it happens, and whether that location moves. A soldering iron tip is a point source that stays in one place. Open beakers on a chemistry bench move around the bench. A vapor degreasing tray is a large open surface that does not move at all. Those three cases need different hoods and different arm diameters even though all three are described as bench work.
The second question is whether the contaminant is buoyant. Heat makes a plume rise, which means a dome or bell hood positioned above the work does most of the job for you. Cool, heavy vapors do not rise, so the hood has to be beside the source rather than above it, and the airflow matters more.
Choosing Arm Diameter
Arm diameter is the practical shorthand for capacity. Larger arms move more air, reach further without losing capture, and carry heavier or hotter contaminants; they also cost more, weigh more, need more structure to mount, and take more fan capacity. These are the ranges that cover almost all laboratory and light industrial work.
| Arm diameter | Typical hood | Suits | Notes |
|---|---|---|---|
| 2 inch | Small round or slot hood, 3 to 5 inch | Soldering, single point sources, small analytical work | Least intrusive on a crowded bench; capture range is very short, so the hood has to be right at the work |
| 3 inch | 6 to 8 inch dome or bell hood | General chemistry benches, small open vessels, sample prep, dental and lab odor control | The most common laboratory size; a good balance of capture and fan load |
| 4 inch / 100 mm | 8 to 12 inch dome, bell or metal hood | Larger open vessels, solvent work, adhesives, heat, ESD safe assembly | Standard where the source is larger or hotter, or where the arm has to reach further |
| 6 inch | 12 inch and larger metal hood or canopy | Welding, grinding, large open tanks, heavy industrial work | Needs real fan capacity and proper structural mounting; rarely required for bench chemistry |
Reach is the other half of the decision. A longer arm is only useful if the hood still captures at full extension, so it is normal to step up one diameter when the arm has to cover a wide bench or a deep workstation from a single mounting point.
Airflow (CFM) by Arm Size
Airflow is not a mystery number; it is duct area multiplied by transport velocity. The transport velocity range used for fume, smoke and light vapor is 2,000 to 3,000 feet per minute, which is what keeps particulate moving in the duct rather than settling in it. Applying that range to each duct size gives the figures below.
| Arm diameter | Duct area | At 2,000 fpm | At 3,000 fpm |
|---|---|---|---|
| 2 inch | 0.022 sq ft | about 45 CFM | about 65 CFM |
| 3 inch | 0.049 sq ft | about 100 CFM | about 145 CFM |
| 4 inch / 100 mm | 0.087 sq ft | about 175 CFM | about 260 CFM |
| 6 inch | 0.196 sq ft | about 390 CFM | about 590 CFM |
Use your engineer’s number when there is one
Where a mechanical engineer, industrial hygienist or a written ventilation specification gives a design airflow for the station, that figure governs. The ranges above are for scoping an arm and checking a fan, not for replacing a design. The American Conference of Governmental Industrial Hygienists publishes the standard reference on industrial ventilation design if you need the underlying method.
Capture velocity at the hood is the figure that decides whether the arm actually works. For bench contaminants released with little velocity into reasonably still air, a design capture velocity in the range of 100 to 200 feet per minute at the source is normal. The catch is distance: capture velocity falls away very quickly as the hood moves back from the source, which is why the same arm can perform well at four inches from the work and do almost nothing at eighteen inches.
The manual damper on the arm matters more than it looks. When two or more arms share a fan, the damper is how you give the station in use the airflow it needs instead of splitting the fan’s capacity evenly between one arm that is working and one that is idle. It is also the cheapest way to commission a system: set the dampers with a velometer at the hood face rather than assuming the balance is right.
Will Your Existing Fan Handle It
This is the check that gets skipped, and it is the one that causes disappointment after installation. An extraction arm is not a self contained appliance; unless it is a portable filtered unit, it is a fitting on a fan system, and the fan has to have real spare capacity at the static pressure the new run adds.
Add the simultaneous airflow
Total the design CFM of every arm that can realistically be open at the same time, not the total number of arms installed.
Add the system losses
Duct runs, elbows, the arm itself, any filter, and the discharge all add static pressure. Long runs and multiple elbows add more than people expect.
Compare against the fan curve
Check the fan can deliver that airflow at that static pressure, not just at free air. A fan rated for 800 CFM at zero static may deliver far less on a real duct.
Verify at the hood
After installation, measure at the hood face. Design intent and installed reality differ often enough that commissioning measurements are worth the hour they take.
If the existing fan does not have the capacity, the options are a larger fan, a dedicated fan for the new arms, or a portable filtered unit that needs no ductwork at all. Portable units are genuinely useful where there is no exhaust path, where the work moves between rooms, or where a landlord will not allow a roof penetration. Their limitation is filtration: a filter that suits solvent vapor is not the same as one that suits acid mist, and filters are a consumable with a service interval.
Portable extraction is the right answer more often than people assume, particularly in leased buildings and renovated spaces where the roof and the mechanical shafts are out of scope. The tradeoff is that the filter, not the roof fan, becomes the thing you have to plan around: match the filter media to the chemistry, keep a spare set on the shelf, and log the change intervals.
Capture Hood Styles and Materials
The hood is the part that does the capturing, so it deserves as much attention as the arm. Four distinctions matter in practice.
A conductive inlet hood on a four inch arm, used where static discharge is a risk to the product being assembled. The arm, the tube segments and the hood all have to be specified as conductive together; a conductive hood on a standard arm does not give you a grounded path.
Material choice is also a chemical question. Polypropylene arms and hoods suit acids and corrosives that would attack painted steel; steel suits heat; conductive assemblies suit electronics and aerospace assembly work. Where flammable atmospheres are involved, the specification moves beyond the arm to explosion proof or ATEX rated fans and grounding, which is a different conversation and needs to be flagged early.
Mounting and Reach
Mounting decides reach, and reach decides whether the hood can get to the work. The short version is that wall brackets suit benches against a wall, ceiling drops suit islands and free standing benches, and bench or tripod mounts suit stations that move. Ceiling height is the variable that catches people out, because a high bay ceiling needs a drop tube before the arm even starts.
That subject has its own page, with drop length, clearance and structural backing: Exhaust snorkel mounting and clearance guide.
A ceiling mounted arm over a teaching bench. Ceiling mounted installations free the bench and the wall completely, but the mounting height has to be worked out before the arm is quoted: the drop tube, not the arm, sets whether the hood can reach the work surface.
Seven Sizing Mistakes Worth Avoiding
- Sizing the arm before describing the work, then discovering the hood cannot sit close enough to capture anything.
- Adding arms to an exhaust system that is already at its airflow limit, which quietly degrades every other station on the same duct.
- Forgetting the drop tube on a high ceiling, so the arm is quoted correctly and still cannot reach the bench.
- Ordering without the wall or ceiling bracket, which is the single most common reason a snorkel order needs a second quote.
- Specifying a clear polycarbonate dome over heat or hot tooling, where a metal hood belongs.
- Leaving out the manual damper on a shared system, then having no way to balance it during commissioning.
- Treating 100 mm and 4 inch as a problem rather than a transition detail at the duct connection.
None of these are exotic. They are the questions our own sales conversations come back to week after week, which is why this page exists: if the answers are here, the quote can be right the first time.
A ten inch metal capture hood on a telescopic arm. Hood size, arm diameter and reach are one decision, not three: the hood has to cover the source, the arm has to carry the airflow that hood needs, and the mounting has to put the hood where the work actually happens.
Frequently Asked Questions
What size exhaust snorkel do I need?
Start from the work, not the arm. Match the capture hood to the size of the source so the hood can sit within about one hood diameter of it, then size the arm to the airflow that hood needs. In practice most laboratory bench work is handled by a 3 inch or 4 inch arm; 2 inch arms suit soldering and very small point sources, and 6 inch arms suit large open vessels, welding and heavier industrial work.
How many CFM does an extraction arm need?
Airflow follows from duct size and transport velocity. Using the standard 2,000 to 3,000 feet per minute transport velocity range for fume and smoke, a 2 inch arm moves roughly 45 to 65 CFM, a 3 inch arm about 100 to 145 CFM, a 4 inch (or 100 mm) arm about 175 to 260 CFM, and a 6 inch arm about 390 to 590 CFM. Use the design figure your ventilation engineer specifies where one exists.
How many arms can one fan run?
Add the design airflow of every arm that can be open at the same time, then add duct and filter losses. A fan sized for 400 CFM will run two 3 inch arms comfortably and two 4 inch arms only if both are rarely open together. Manual dampers at each arm let you balance a shared system so the station in use gets the airflow instead of splitting it with idle stations.
Is 100 mm the same as a 4 inch arm?
Close enough to specify interchangeably in most cases. 100 mm is 3.94 inches, so European 100 mm arms and North American 4 inch arms carry very similar airflow. Confirm the connection detail at the duct, since the transition, not the arm, is where the mismatch shows up.
Can I add an extraction arm to an existing exhaust system?
Often yes, and it is the first thing worth checking. What matters is whether the existing fan has spare capacity at the static pressure the new run adds. If the system is already at its limit, adding an arm quietly reduces airflow everywhere else on the same duct, which is the most common reason a new arm disappoints after installation.
Does the hood style change the airflow?
Yes. A dome or bell hood captures more effectively per CFM than a flat inlet because it collects the rising plume rather than pulling across it. Metal hoods suit heat and hot tooling; clear polycarbonate domes let an operator see the work; conductive and ESD safe hoods are used where static discharge matters.
How close does the hood have to be to the source?
Closer than most people expect. Capture velocity falls off sharply with distance, so a hood placed one duct diameter away from the source sees only a small fraction of its face velocity. Position the hood within about one hood diameter of the emission point and reposition it when the work moves, which is what an articulated or telescopic arm is for.
Related Pages
Tell Us the Work and We Will Size It
Send the bench layout, ceiling height and what you are handling. We will come back with arm diameter, hood style, mounting and the airflow it needs, and tell you plainly whether your existing fan can carry it.