What is the difference between a Primary Air fan and a Forced Draft fan?
Both push air on the clean side of combustion, but a PA fan does more. An FD fan delivers combustion air to the windbox at moderate static. A PA fan drives primary air at higher static through the pulveriser to dry and transport the fuel, then carries the fuel-air mixture down the pipe to the burner. That means higher pressure, higher tip speed, and a curve that must stay stable as the mill feeder rate swings. Cold-PA fans handle clean air; hot-PA and combined-PA fans carry light coal dust off the mill, so unlike an FD fan they often need leading-edge wear protection. We engineer the PA fan to the mill and burner resistance, not to a generic FD rating.
What is the difference between cold PA and hot PA, and does it change the fan?
Yes, it changes the whole build. A cold-PA fan sits upstream of the air pre-heater and mill and handles clean ambient air at high static, so the focus is tip speed, curve stability and efficiency. A hot-PA fan sits downstream of the APH where the primary air runs 200 to 350 °C and, on combined-PA arrangements, carries light coal dust off the mill, so it also gets upgraded casing metallurgy, shaft thermal sizing and leading-edge wear protection. Tell us where the fan sits in the mill circuit and we build to that position rather than a single default.
How do you keep a PA fan stable as mill load swings?
Primary-air flow tracks the feeder rate, so the duty point moves continuously with mill load, and a fan sized onto the flat or rising part of its curve can stall under back-pressure swings and oscillate the fuel-air supply. We engineer the duty point onto the falling portion of the pressure-flow curve, typically 5 to 15 percent to the right of the peak, so the fan is inherently stable across the mill-load range. We then verify the curve on the 200 HP VFD test rig before dispatch.
PA fans run at high static. How do you manage the tip speed and wheel stress?
The static a PA fan makes comes from tip speed, and higher tip speed drives up wheel stress and blade-root fatigue on a fan that runs continuously. We engineer the wheel to the pressure the mill and burner actually demand, check blade-root stress against continuous duty rather than a catalogue rating, and balance the rotor to ISO 21940 G6.3 as standard, with G2.5 or G1.0 on application, so the high-speed wheel runs true. On hot-PA and combined-PA duty we also hard-face the leading edges so light coal dust does not erode the wheel unevenly and unbalance it.
Our PA fan carries pulverised coal dust. What wear protection do you fit?
Hot-PA and combined-PA fans carry light pulverised-coal dust off the mill, a far lighter load than the fly-ash on an ID fan but still enough to erode a high-tip-speed wheel. We fit chrome-carbide hard-facing on the blade leading edges and choose impeller geometry that sheds dust from the blade root, and where the loading warrants it we bolt in AR400 wear plates at the volute throat and outlet with access doors for in-place replacement. Cold-PA fans on clean air upstream of the mill need no wear scope at all. We size the wear package to your stated coal-dust condition.
Should I specify VFD or an inlet vane damper for control?
VFD is our default. Primary-air demand tracks mill load, so a PA fan turns down meaningfully across the firing range, and VFD speed control is more efficient than an inlet vane damper across the operating range because it avoids the throttling loss at part-load. Speed control also holds the fuel-air ratio steady as firing swings, which matters for stable flame. Inlet vane dampers remain available for legacy retrofit where the existing motor and starter cannot accommodate a drive. We quote whichever your installation calls for.
What is the lead time for a standard PA fan?
A standard engineered PA fan runs roughly 9 to 14 weeks order-to-dispatch: offer in 3 to 5 working days, GA drawing 2 to 3 weeks from PO, manufacture, balance and paint 6 to 10 weeks, and performance test plus FAT 1 week. A hot-PA or combined-PA build with wear protection and thermal scope sits at the longer end. API 673 refinery-adjacent duty runs longer, roughly 13 to 19 weeks, with a 7 to 10 working-day offer turnaround.
Do you build PA fans to API 673, CE and ATEX requirements, and are your fans certified?
Yes to the build; let us be precise about the claims. We design and build to API 673 for oil-and-gas and fired-heater-adjacent service as project-specific scope (allow 7 to 10 working days for the offer). CE is self-declared per 2006/42/EC and 2014/35/EU, and ATEX Zone 2/22 is self-declared per 2014/34/EU, Category 3, where the area classification calls for it. Those are self-declarations of conformity, not third-party certifications. Performance is tested in-house to the AMCA 210 / ISO 5801 method on our 200 HP VFD test rig; that is testing to the AMCA method, not an AMCA certification, and we are not an AMCA member. Our only third-party certification is ISO 9001:2015. Balance is to ISO 21940 G6.3 as standard, and bearing life is a design target of L10h at least 40,000 hours continuous.
