A fan in a hazardous area has to do two jobs at once: move the air you need, and survive the atmosphere it is sitting in. Get either one wrong and the failure is not a maintenance nuisance. A fan that stalls in a solvent-laden atmosphere, or a motor whose surface runs hotter than the ignition temperature of the gas around it, stops being ventilation equipment and becomes the ignition source itself.
This guide covers what actually makes a fan explosion proof, which fan type suits which duty, how to size airflow and pressure properly rather than by rule of thumb, and the installation details that decide whether the installation still complies a year after commissioning.
What makes a fan “explosion proof”
The phrase is a description, not a certification. What makes a fan legal in a classified area is an Ex marking on the complete assembly, with a certificate covering the motor, the impeller and the housing together.
Three mechanical risks drive fan certification, and none of them are the same as the risks on a luminaire or a junction box:
- Rubbing contact. A fan has a rotating part with a small running clearance inside a stationary housing. If the impeller touches the casing, or a foreign object enters the airway, the contact can produce sparks or hot particles. Certified fans manage this through impeller material, minimum clearances, and a casing built to the same protection concept as the motor.
- Motor surface temperature. The motor is the heat source. Its surface temperature under fault conditions has to stay below the ignition temperature of the surrounding gas – that is the T class.
- Conveyed gas temperature. If the fan moves hot process air rather than ambient air, the air itself heats the housing. A fan certified T4 on ambient air may not be T4 on 80 °C process air. This is the single most commonly missed point in fan selection.
Protection concepts used on fans
Fans use a narrower set of protection concepts than lighting or control gear, because the rotating assembly limits what is practical. The table below maps the concept to the zone it permits.
| Marking | Concept | How it works on a fan | Typical zone use |
|---|---|---|---|
| Ex d | Flameproof enclosure | Motor and terminal chamber contain an internal explosion and quench the flame path; housing and impeller clearances controlled | Zone 1 and 2, gas |
| Ex e | Increased safety | No sparks or hot surfaces in normal operation; reinforced insulation, larger creepage and clearance, thermal protection | Zone 1 and 2, gas |
| Ex n | Non-sparking | No arcing parts in normal operation, no ignition-capable surfaces in normal operation | Zone 2 only, gas |
| Ex tb | Protection by enclosure (dust) | Dust-tight housing, surface temperature limited for a dust layer of stated thickness | Zone 21 and 22, dust |
The commercial difference between these lines is large. An Ex n fan for a Zone 2 position costs a fraction of an Ex d fan certified for Zone 1, and specifying Ex d where Ex n would do is one of the most common ways to overpay on a ventilation package. The reverse mistake – ordering an Ex n fan for a position that later turns out to be Zone 1 – means replacing equipment after installation.
Confirm the zone from the area classification drawing before the fan is quoted, not after. Our guide to hazardous area classification covers how zones and groups are derived, and Ex d vs Ex e vs Ex i explains where each concept is the right commercial answer.
Choosing the fan type
Fan type is decided by the resistance of the system, not by preference. Axial fans move a lot of air against little resistance; centrifugal fans move less air against much more.
| Type | Pressure capability | Best duty | Example from our range |
|---|---|---|---|
| Axial | Low | Whole-room ventilation, wall or roof mounting, short duct runs | CBF series, BT35 series |
| Oscillating / wall axial | Very low | Local air movement and spot cooling in workshops and tank farms | FB series |
| Axial with ducting | Low to medium | Tunnel and roadway ventilation, longer ducted runs | FBDY series |
| Contra-rotating axial | Medium to high | Mine main ventilation, high resistance with limited space | FBDCZ series |
| Centrifugal | High | Ducted extract with filters, scrubbers or long duct networks | Specified to duty |
If a fan is being asked to push air through a filter bank, over an activated-carbon bed, or along a duct run with several bends and dampers, the resistance is high and an axial fan will simply sit far back on its curve and deliver a fraction of its free-air figure. Size the system first, then pick the fan that owns that duty point.
Sizing: airflow and pressure together
A fan has one performance curve for one duty point at one air density. Quoting “10,000 m³/h” without a pressure figure is meaningless, and it is the most frequent error we see on enquiry sheets.
Three corrections decide whether a fan actually delivers:
- System resistance. Add up the pressure losses of the duct, bends, grilles, filters and weather louvres at the design airflow. The fan’s curve must cross the system curve at the flow you need. A fan selected at its free-air delivery will under-perform badly once ducted.
- Air density. Published fan curves assume a stated air density, usually around 1.2 kg/m³ at 20 °C and sea level. Air at 60 °C is roughly 0.15 of that density lower, so the same fan moves more volume but less mass. If the requirement is a mass flow or a cooling duty, correct for temperature and altitude before selecting.
- Site altitude. A fan at 2,000 m moves appreciably less mass flow than the same unit at sea level for the same shaft speed.
If the airflow is being calculated from a required ventilation rate rather than from a cooling load, the figure usually comes from a standard rather than from judgement. Battery rooms are the clearest example: charging lead-acid cells releases hydrogen, and the ventilation rate is set to hold the concentration below 25% of the lower explosive limit, which for hydrogen is 4% by volume – so the target is 1% hydrogen. The rate depends on the number of cells and the charging current, and it is a calculation, not a guess.
| Application | What is being diluted | Basis of the ventilation rate |
|---|---|---|
| Battery charging room | Hydrogen from gassing | Hold below 25% LEL, from cell count and charge current (IEC 62485-2) |
| Solvent storage or mixing room | Vapour from spills and open handling | Air changes per hour or a dispersion calculation – check the classification drawing |
| Paint or coating booth | Solvent vapour plus overspray | Face velocity at the opening, maintained by a dedicated extract fan |
| Pump or compressor house | Leakage from flanges and seals | Air changes per hour, extract at high level for lighter-than-air gases |
| Coal or grain handling | Combustible dust generation | Dust control and extraction, with Ex tb or Group I equipment as classified |
Two different worlds: petrochemical and coal mining
Group I (mining) and Group II (surface industry) fans are not interchangeable, and the difference is not only the certificate.
Surface industry. Petrochemical, chemical, pharmaceutical and paint plants need fans certified for Group IIA, IIB or IIC depending on the gas, with an EPL of Gb for Zone 1 or Gc for Zone 2. Impeller material and clearance control, motor thermal protection and a corrosion-appropriate housing are the deciding factors. For clean ambient-air duty, an axial fan with a flameproof motor is the standard answer.
Coal mining. Underground ventilation is a different problem. Mine fans are Group I, usually flameproof with EPL Mb, and they are sized for the resistance of a whole roadway or a face ventilation system rather than a room. They run continuously, they run under heavy dust loading, and they often need contra-rotating stages to develop the pressure without an impeller tip speed that would itself be a hazard. The FBDY and FBDCZ series are built for that duty; a surface-industry axial fan is simply not a substitute.
Group I equipment also carries an extra materials restriction: the standard limits the aluminium and magnesium content of external parts that could be struck, because a light-metal impact against rusty steel or rock can produce an incendive spark. That is a Group I requirement that does not apply in the same way to Group II equipment.
Installation details that decide compliance
Fans are certified as units, but they are installed as systems. Most non-conformities we are asked to resolve are in the installation, not in the equipment.
- Flexible duct connectors. A fabric or rubber anti-vibration coupling between the fan and the duct has to be antistatic and, where required, bonded to earth. An insulating coupling can accumulate a static charge in a duct carrying dust or a flammable vapour.
- Earthing and bonding. The housing, the motor frame, the ductwork and any flexible connection must be bonded. This matters most for ducting carrying dust, where static charge generation is continuous.
- Variable frequency drives. Fitting a VFD is not a neutral change. The motor must be certified for inverter duty – thermal protection, insulation class and, on longer cable runs, insulated bearings or a shaft-earthing ring to stop bearing currents. The drive itself must sit outside the hazardous area, or in a certified enclosure with the correct concept. A VFD running a certified motor outside its certified conditions voids the installation.
- Guards and airflow path. Inlet and outlet guards are not cosmetic. They keep foreign objects out of the running clearance. Removing a certified guard to improve airflow is a modification to certified equipment.
- Impeller clearance and spare parts. After any overhaul, the running clearance must be restored to the certified value. A replacement impeller from a third party is not a replacement for a certified impeller.
- Weather protection. A roof-mounted fan needs the correct IP rating for the position, and outdoor housings should be specified with the same care as an outdoor luminaire – the certificate states the IP rating, and it is the certificate figure that will be inspected. Our notes on IP ratings on Ex equipment apply to fan housings unchanged.
Seven selection mistakes that cost the most
- Selecting on airflow alone. No pressure figure, no valid selection. Always quote both, with the design air density.
- Ignoring the conveyed air temperature. The T class must survive the process air, not just the ambient air.
- Mixing an Ex motor with a standard fan. The assembly is what is certified.
- Over-specifying Zone 1 equipment in a Zone 2 position. Legal, but expensive. Check the classification drawing first.
- Fitting a VFD without a certified inverter-duty motor. Common on retrofits, and a frequent audit finding.
- Placing the extract at the wrong elevation for the gas density involved.
- Treating the motor as the only hazardous part. Impeller material, clearance and the fan casing are part of the certified assembly.
What to send us for an accurate fan quote
Five lines of information are enough for a firm selection rather than a guess:
- Zone, gas group and temperature class for the position, taken from the area classification drawing.
- Required airflow, and the total system static pressure at that airflow. If the ventilation rate is calculated rather than measured, tell us the basis.
- Air temperature and composition – ambient air, or a process stream, and whether there is dust or corrosive vapour in it.
- Mounting arrangement: wall, roof, inline ducted, or underground roadway, and whether ductwork already exists.
- Whether a VFD will be used, so the motor can be specified for inverter duty from the start.
If a certificate is needed for an acceptance inspection, say so on the enquiry. We issue fans with the Ex certificate stating the concept, gas group, temperature class, EPL and IP rating for the specific model, and we can match the cable entries and accessories to it. How to check those claims against the documentation is covered in how to verify an explosion-proof certificate.
Fan selection questions we get asked most
Can I use a standard industrial fan if the area is only Zone 2?
No. Zone 2 still requires certified equipment – normally a non-sparking Ex n design with an EPL of Gc. Zone 2 means the atmosphere is unlikely during normal operation, not that it is impossible. A standard fan has no certificate and no declared temperature class, so there is nothing to inspect against.
Is an Ex d motor enough to make a fan explosion proof?
No. The certificate must cover the fan assembly, including the impeller and housing, with the running clearances and materials as tested. A flameproof motor fitted to an uncertified fan leaves the assembly non-compliant.
Why does my fan deliver less air than the catalogue figure once it is ducted?
Because the catalogue figure is the free-air delivery, with no system resistance. Once duct, bends, filters and louvres are added, the fan moves along its curve to a lower airflow at higher pressure. Select at the duty point, not at free air.
Does the T class refer to the motor or to the air the fan is moving?
Both are relevant. The T class is the maximum surface temperature of the equipment under the conditions of certification. If the fan conveys hot air, that air raises the surface temperature of the housing and impeller, so the certification conditions and the process air temperature both have to be checked.
Can a certified fan be run from a variable frequency drive?
Only if the motor is certified for inverter duty and the drive is installed outside the hazardous area or in a suitable certified enclosure. Long motor cables introduce overvoltages and bearing currents that a standard motor is not certified to withstand, so this is a design decision, not an accessory.
Do I need a different fan for hydrogen than for methane?
Yes, in most cases. Hydrogen is Group IIC and methane is Group IIA. Hydrogen also has a very low minimum ignition energy and a wide flammable range, which puts it in the most demanding group. It also tends to accumulate at high level, which affects where the extract point is placed.
Send us the zone and the airflow, and we will select the fan. Tell us the gas group, temperature class, required airflow, system static pressure and whether the ductwork already exists – we will come back with the fan model, the motor duty and the Ex certificate that covers the assembly, including the driver arrangement.
