A plating line vents chrome and nickel fumes into a duct. A grinding operation ahead of a baghouse pulls fine metal dust through a wheel turning at a few thousand RPM. A digester pulls off methane. In each case the airstream carries something that will ignite given an ignition source, and at the right concentration will do more than ignite. And in each, the fan is the one component in the system with parts moving fast enough to supply the source.
Somebody has to decide how that fan gets built, and the decision happens at the specification stage, often months before anyone asks for a quote. ANSI/AMCA Standard 99-25 lays out the options in Section 8. Working out which option your airstream calls for is the part left to you.
The classification, and which edition to work from
The Air Movement and Control Association classifies spark-resistant construction in ANSI/AMCA Standard 99, the Standards Handbook. The current edition is 99-25, listed on AMCA's publications page.
Check the edition printed on whatever reference document you are working from. Plenty of the material circulating in this industry, including engineering letters and technical bulletins that surface first in a search, cites editions going back to 2016 and in a few cases to the 1980s. The general shape of the classification has held steady across those revisions. The details are where specification language gets written, so work from the current text.
The three types, briefly
Type A is the most restrictive. All parts of the fan in contact with the airstream are nonferrous.
Type B allows ferrous hubs, shafts and hardware alongside a nonferrous impeller and a nonferrous ring around the shaft opening, provided the construction is such that a shift of the impeller or shaft cannot let two ferrous parts rub or strike.
Type C is the entry level. The fan is built so that a shift of the wheel cannot bring two ferrous parts into contact.
The standard puts the definition of nonferrous in its notes, separate from the classification: any material containing less than 5 percent iron, or any other material with a demonstrated ability to be spark resistant. Stainless steel sits on the wrong side of that line, which catches people who reach for it as the corrosion answer and assume it settled the spark question at the same time.
What the classification does not cover
Read the standard's own limits before leaning on it. The standard states that its use implies no guarantee of safety for any level of spark resistance, and that spark-resistant construction does not protect against ignition caused by catastrophic failure or by airstream material present in the system. AMCA publishes formal interpretations on this point, including one addressing the status of that note, on its publications page.
Two further items sit outside the classification and inside your scope. Bearings, drive components and electrical devices are not to be placed in the air or gas stream unless they are constructed or enclosed so that a failure of the component cannot ignite the surrounding stream. And grounding the fan is the user's responsibility, which matters more than it sounds like it does. More on that below.
Both of those, and the definition of nonferrous above, live in the standard's numbered notes, separate from the classification table. The table is what gets copied into specifications and technical bulletins. The notes are where the conditions are, and they are the part that goes missing in transit.
The classification describes how the fan is built. Treating it as a substitute for the hazard analysis the process requires is where people get into trouble.
Spark resistant and explosion proof are two different specifications
These two get treated as one requirement, and they are two.
Explosion proof is an enclosure term. It comes out of the electrical codes and it describes equipment built so that an internal ignition stays sealed inside the enclosure rather than reaching the atmosphere around it. Spark-resistant construction is an airstream term. It comes out of AMCA 99 and it describes how the fan is built so it is less likely to make a spark inside the air it is moving.
A fan cannot be explosion proof in that enclosure sense, and the reason is structural rather than a matter of build quality. Explosion-proof protection depends on isolating equipment from the explosive atmosphere, and a fan exists to pull that atmosphere through itself. Every fan is, by design, full of the thing you are worried about.
So, the two requirements answer two different questions, and both can apply on the same job. If the hazard is inside the airstream, the answer is spark-resistant construction on the fan. If the hazard is the atmosphere in the room around the equipment, the answer is a motor and electrical package classified for that location. Write both into the specification separately, each stated once, and the vendor has a clear target on each.
Working from the airstream to a construction type
Begin with what is in the airstream, then go to the catalog.
Combustible dust and flammable vapor behave differently and can land on different types. Dust settles, accumulates on surfaces and gets disturbed later, and it turns from a fire problem into an explosion problem once it is suspended in air at the right concentration, which is precisely what a fan does to it. Vapor carries a flame only between a lower and an upper concentration limit, and where it sits between them moves with temperature and ventilation rate. A dust collection fan handling fine metal or organic dust is a different specification problem from a solvent exhaust fan on a finishing line, even when both end up wanting spark-resistant construction.
Then ask what else the airstream is doing to the fan, because corrosion and spark resistance frequently arrive together and pull in opposite directions. Aluminum is the common nonferrous answer for Type A construction. Aluminum in the presence of rusted steel deserves specific attention, since work by the U.S. Bureau of Mines and others found that aluminum impellers rubbing on rusty steel can produce high-intensity sparking. If the airstream is also chemically aggressive, the material question and the spark question have to be answered together, in one pass. Our guide to corrosion-resistant fan selection covers the material side of that.
Then widen the question to the air around the fan, beyond what moves through it. On coastal sites the ambient air carries salt. Along stretches of the Gulf chemical corridor the air surrounding the plant is acidic on its own, before anything enters a duct. Those conditions decide the material, and the material decides which spark-resistant construction is available to you.
What changes when the fan is fiberglass
The classification reasons in ferrous and nonferrous terms, which does real work on a steel or aluminum fan. An FRP fan has no ferrous parts in the airstream to strike in the first place, so the three types answer a question that fan never raises.
The standard does speak to it, again in the notes. All structural components within the airstream, including non-metallic materials, must be suitable for conducting static charge safely to ground so that electrical potential cannot build. That is a clear requirement. What comes with it is a classification built around metal, so you are left holding a requirement and no construction type that satisfies it.
The requirement exists because fiberglass is an insulator. Static accumulates on the surfaces the airstream touches, and a discharge inside a combustible airstream is precisely the ignition source the whole exercise exists to prevent.
The construction comes from ASTM D4167, Standard Specification for Fiber-Reinforced Plastic Fans and Blowers, maintained by ASTM Committee D20. Per that specification, rendering the fan wheel and housing electrostatically conductive results in a spark-resistant fan. That decision turns on conductivity, where the AMCA types turn on alloys, which is why an FRP fan wants the ASTM reference written in alongside whatever AMCA type appears in the specification. The two documents answer different halves of the same question.
Hartzell's fiberglass catalog carries this as a construction option. For fiberglass equipment handling fumes that are both corrosive and potentially explosive, interior airstream surfaces are coated with a carbon-rich resin coat, and grounding straps run from the side of the housing to the fan's steel base. Specifying ASTM D4167 construction adds a synthetic surfacing veil, an electrostatically conductive coating applied to the housing and impeller airstream surfaces, special nameplates, and a special final dynamic balance. Construction options are listed on the fiberglass backward curved centrifugal fan page, and the scrubber applications where this comes up most often are covered separately.
One link in that chain belongs to the field. The straps carry charge from the housing to the fan base. Grounding the base at installation is what completes the path, and a conductive coating tied to an ungrounded base moves charge nowhere at all. Name the responsible party for that step in the scope of work.
The provision nobody asked for
There is a gap between what a specification asks for and what an airstream requires, and on any given job that gap gets filled or it gets left open.
Our fiberglass application engineers describe the industry practice this way: others will say they meet the standard, but where a specification does not ask for the unit to be spark resistant, they will not apply the spark resistance part of it. Hartzell Air Movement applies the provisions when the airstream warrants them, whether or not the specification remembered to.
That position costs something, and saying so is fairer than pretending otherwise. Construction the customer did not specify is construction the customer did not budget, and Hartzell is not the lowest-price fan in most quotes it appears in. The argument for doing it anyway is that the person writing the specification is not always the person who knows what the airstream is carrying, and the fan does not care whose oversight it was.
What to put in the specification
Five items, in this order:
1. The construction type, named by its ANSI/AMCA 99 classification. For an FRP fan, reference ASTM D4167 construction and call out electrostatic conductivity of the wheel and housing explicitly.
2. What is in the airstream: the material, whether it is combustible, potentially explosive or both, its concentration where you know it, the temperature, and whether moisture is present.
3. Whether ambient conditions at the installation add a corrosion requirement on top of the spark requirement.
4. Who grounds the fan base, named explicitly in the scope of work.
5. The electrical classification requirement for the location, stated separately from the fan construction so the two are not read as one item.
If you are working a specification where the airstream is doing more than one of these things at once, that is a call worth making at selection, while you still have options. Send the airstream data and the arrangement drawing and our application engineers will work the construction question through with you: 1-800-336-3267 or info@hartzell.com.
Frequently Asked Questions
What's the difference between spark-resistant and explosion-proof?
Spark-resistant construction is an airstream requirement. It governs how the fan itself is built so it's less likely to generate a spark inside the air it's moving. Explosion-proof is an enclosure requirement that comes from electrical code and applies to equipment near the fan, not the fan's airstream construction. A fan can't be built "explosion-proof" in that sense, since its job is to pull the surrounding atmosphere through itself. Both can apply on the same job, but they answer different questions and belong in the specification separately.
What are the three AMCA spark-resistant construction types?
Type A is the most restrictive: all fan parts in contact with the airstream are nonferrous. Type B allows ferrous hubs, shafts, and hardware alongside a nonferrous impeller and a nonferrous ring at the shaft opening. Type C is the entry level, built so a shift of the wheel can't bring two ferrous parts into contact. All three come from Section 8 of ANSI/AMCA Standard 99-25.
What counts as "nonferrous" under the standard?
Any material with less than 5 percent iron, or any other material with demonstrated spark-resistant ability. Stainless steel doesn't automatically qualify just because it resists corrosion; the corrosion answer and the spark answer are separate questions.
Does an FRP fan need an AMCA spark-resistant type?
The AMCA classification reasons in ferrous and nonferrous terms, which doesn't map cleanly onto a fiberglass fan with no ferrous parts in the airstream. What an FRP fan does need is a way to safely conduct static charge to ground, which is where ASTM D4167 construction comes in: rendering the wheel and housing electrostatically conductive. Specify both references, AMCA for the metal logic where it applies and ASTM D4167 for the fiberglass answer, rather than assuming one covers the other.
Who's responsible for grounding the fan?
The user, at installation. This applies regardless of construction type or material. A conductive coating or grounding strap that terminates at an ungrounded base doesn't complete the path, so name the responsible party for this step explicitly in the scope of work.