Axial Fans for Chemical and Petrochemical Plants: Corrosion, Fumes and Hazardous Areas

Chemical and petrochemical plants ask more of a ventilation fan than almost any other industry: aggressive fumes, humid or salty air, and, in many areas, the possibility of explosive gas. A fan that moves plenty of air but corrodes in two years, or sits in a hazardous zone without the right construction, is a liability, not a solution. Specifying axial fans here starts from the hazard and the chemistry, and only then from the airflow.

Where Axial Fans Fit in a Chemical Plant

Area	Typical duty	Notes
Pump houses, compressor houses	General and dilution ventilation	Hazardous-area classification often applies
Process buildings and enclosures	Heat removal, fume dilution	Corrosive atmosphere likely
Laboratory and workshop buildings	General ventilation	Usually non-hazardous
Cooling towers and air-cooled heat exchanger bays	Forced or induced draught	Wet or corrosive duty
Battery and switchgear rooms	Ventilation, sometimes with gas hazards	Check hazardous classification
Process gas with high resistance	Scrubbers, stacks, and ducted gas	Centrifugal fans usually

Start With the Hazardous-Area Classification

Explosive atmospheres are classified into zones by the plant operator: zone 0, 1, or 2 for gas, and 20, 21, or 22 for dust. Zoning is the end user’s responsibility, and the fan’s construction must suit the zone inside the fan and outside it. Equipment for Group II surface industry is graded in categories 1, 2, and 3, and marked for gas group and temperature class.

Two mistakes are common. The first is treating spark-resistant construction, described in AMCA 99, as the same thing as ATEX. It is not: AMCA 99 classifies construction in levels A, B, and C, and does not guarantee safety or replace ATEX assessment. The second is ignoring the motor. An electric motor in the airstream, or in the zone outside the fan, must be suitable for that zone. Read our guide to ATEX and spark-resistant axial fans before writing the specification.

Ventilating for Flammable Vapour: A Simple Dilution Calculation

Ventilation can dilute a small release of vapour below a safe concentration. The basic mass balance is:

Airflow = release rate ÷ allowed concentration

Illustration. If 2 g/s of vapour can be released and the concentration must be kept at or below 0.5 g/m³, the airflow needed is 2 ÷ 0.5 = 4 m³/s. Real designs use the hazardous-area classification, the release characteristics, and applicable codes to set the allowed concentration, and they also consider airflow patterns, since dilution only works where the air actually moves. This calculation shows the principle, and it should not replace a proper hazardous-area design.

Corrosive Atmospheres

Chemical plants often combine acid or alkali fumes with humid air, and coastal plants add salt. Material choice depends on the chemical, concentration, and temperature.

• Galvanised or coated steel suits milder conditions.
• Stainless steel suits many conditions, with the right grade for chlorides.
• Fibre-reinforced plastic suits many aggressive fumes, with attention to temperature limits and static charge.
• Fasteners, motors, and bearings need the same attention as the casing.

Consider a motor outside the airstream where fumes pass through the fan. See corrosion-resistant axial fans.

Cooling Duties

Chemical processes reject large amounts of heat, and small cooling towers, closed-circuit coolers, and air-cooled heat exchanger bays use axial fans. Wet, corrosive, and sometimes hazardous conditions apply. See cooling towers, air-cooled condensers and heat exchangers.

Temperature Class and Hot Process Air

Where ventilation air can be hot, or where the surface temperature of the fan must stay below a gas’s ignition temperature, the temperature class matters as much as the zone. A fan that suits IIB gas at T4 does not automatically suit a hotter or more sensitive atmosphere. Give the gas group and temperature class in the specification.

Pump Houses, Compressor Houses and Enclosures

Pump and compressor houses release heat from motors and machinery, and sometimes small leaks of process fluid. Ventilation here serves two purposes: keeping temperatures acceptable for equipment and staff, and diluting any vapour that escapes. Air should enter at low level or on the side away from the extraction, sweep across the equipment, and leave through high-level fans, so that heavier-than-air vapours are not trapped at floor level. Where the classification requires it, ventilation failure should be alarmed, and the arrangement should keep operating in the way the safety case assumes. Plan for redundancy, since losing ventilation in a classified building can force a process shutdown.

Noise and Nuisance

Plants near communities often have boundary noise limits. Larger fans at lower speed reduce noise, and silencers add resistance that must be included. See axial fan noise.

Where Our Range Fits

NextAir Systems manufactures axial fans with impeller diameters up to 1,600 mm, static pressure up to 2,400 Pa, and air volume up to 185,000 m³/h. Ventilation, heat removal, and cooling duties in non-classified areas of chemical plants sit well within that range, and corrosion-resistant designs are confirmed case by case. NextAir Systems does not currently manufacture ATEX-certified or AMCA 99 spark-resistant fans. A standard fan is only suitable where the fan and its motor sit outside any classified zone and the air being handled is not explosive. For classified areas, or where the fan handles a flammable atmosphere, specify the fan from a specialist ATEX supplier. Tell us the chemicals and where the fan sits relative to your zone classification, and we will tell you plainly which case applies.

What to Specify

1. Hazardous-area zone inside and outside the fan, gas or dust group, and temperature class.
2. Chemicals present, concentration, temperature, and moisture.
3. Required airflow and how it was derived.
4. Whether the requirement is ATEX equipment, AMCA 99 spark-resistant construction, or both.
5. Material, coating, and motor arrangement.
6. Noise limits at the boundary or at work positions.
7. Documentation required.

Tell Us the Zone and the Chemicals

Send your chemicals, duty, and where the fan sits relative to any classified zone, and we’ll tell you plainly whether a standard fan suits it or you need a specialist supplier. Contact us for a free quote.

Frequently Asked Questions