Specifying a High-Static Pharma Process Blower

The line on a pharma process-blower RFQ that actually sizes the machine is not the airflow — it is the static pressure, and only if it is stated at the fan outlet and labelled as static. We recently built a direct-driven process blower for a pharma-equipment customer at roughly 11,050 m³/hr and 610 mmWG fan static pressure. The flow is unremarkable. The 610 mmWG is what decided the wheel, the speed and the motor. Here is how that selection reads, anonymised from the real summary report and data sheet.

Why the static line, not the flow, sizes the fan

A centrifugal fan develops pressure from tip speed — and fan pressure rises with roughly the square of that tip speed. So the static duty you write down sets the tip speed the wheel must reach; tip speed then sets the combination of diameter and rpm; and that combination sets absorbed power and therefore motor rating. At this duty the wheel runs at about 101 m/s tip speed to make 610 mmWG, drawing roughly 24.8 kW at the duty point behind a 40 HP motor.

Flow does matter — it sets the inlet area and wheel width — but two fans at the same 11,050 m³/hr can be entirely different machines once one is asked for 150 mmWG and the other for 610. That is why a quotation only means something when it says which pressure it states. Fan static pressure (FSP) excludes the velocity pressure at the outlet; fan total pressure (FTP) includes it, and at real outlet velocities the gap is not rounding error. If your consultant's schedule gives total pressure, pass it on as total — do not relabel it. We convert it and show the conversion.

Single inlet, not double — because it is high-static

The instinct on anything near a cleanroom or pharma AHU is a double-inlet (DWDI) wheel — and for AHU supply and return duty that instinct is right: DWDI moves high volume at lower pressure and packs into a shorter plenum. But a 610 mmWG scrubber or process blower is a different animal. High static at modest flow is single-inlet (SWSI) territory: one impeller, one clean inlet approach, bearings out of the airstream. We selected a single-width backward-inclined wheel of about 665 mm here for exactly that reason. Get the width question answered by the duty, not by habit — see our backward-curved fan range.

The duty, anonymised

Parameter	Value (served)
Volume flow	~11,050 m³/hr
Fan static pressure (at outlet)	610 mmWG
Operating temperature	30 °C
Gas density	1.157 kg/m³
Wheel	single-inlet, backward-inclined, ~665 mm
Speed · tip speed	2,890 rpm · ~101 m/s
Absorbed power at duty	~24.8 kW
Motor · drive	40 HP · direct-driven

Every figure traces to the selection report and data sheet for this job; all of it sits well inside our served envelope. Temperature rise across the fan works out to under 7 °C, which matters when the discharge feeds back toward conditioned space.

Direct-driven and hygienic on purpose

Two construction decisions follow the pharma setting. First, direct drive: no belts means no belt-dust shedding and one fewer wear item to service near a clean process — but it fixes the fan close to motor speed (about 2,890 rpm on a 2-pole), so the wheel diameter has to do all the work of hitting 610 mmWG. Lose the belt-ratio freedom and diameter becomes the tuning knob. Second, wetted parts in stainless: the impeller and hub are SS304 for a cleanable, corrosion-resistant gas path, on an epoxy-coated 4 mm mild-steel casing. The build also carried a silicone inlet flexible connector to isolate vibration, an outlet silencer, a drain plug, an aluminium spark-proof ring at the inlet, and anti-vibration mounts. One honest limit worth stating up front: a gland/plate shaft seal reduces but does not achieve 100% sealing.

The rotor was balanced to the ISO 1940 G2.5 grade and the fan tested to the IS 4894 method, with the test record naming its setup — no fan carries an AMCA product stamp, so the honest claim is always the method, not a certificate. Pushing rpm to chase static is expensive because power climbs with the cube of speed (the cube law), which is one more reason to fix the static number precisely before selecting.

What to put on your RFQ

• State the required static pressure at the fan outlet — and say it is static.
• Give gas, temperature and altitude; density changes every pressure number.
• Say whether the duty is clean high-static process/scrubber air (single-inlet) or high-volume AHU supply and return (double-inlet).
• Name the wetted-part material and drive type your process needs, and ask each bidder to confirm the pressure basis (FSP or FTP) in the quotation itself.

Do that and the machine gets sized by engineering. See also seven mistakes when buying an industrial fan.

Talk to us about pharma process blowers →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.