Low pressure on a combustion-air blower, or a reading taken on the wrong basis?

At-a-Glance

(Corrective action delivered; customer sign-off not evidenced in the internal record.)

The Setup

A combustion-systems OEM builds burner and furnace packages and buys its combustion-air blowers in. The blowers here were built to a design static pressure of 305 mmWC, supplied as a small cluster in the 10–20 HP class, and installed on a furnace at an end-user plant in Tamil Nadu.

Combustion air is not a comfort duty: the blower sets the air side of the air-to-fuel ratio. Under-deliver pressure and the burner runs rich, the flame profile shifts, process temperature wanders — on a heat-treatment line, a quality problem, not just an energy one. And the blower was a bought-out item on the OEM's package, so the end-user was looking at them.

The Complication

The site reported low delivered pressure, measured on their pressure gauge. The customer also highlighted a static-pressure figure on the works test report we had issued, and asked us to explain it.

The obvious suspect is the fan. It is always the fan — and the reflex supplier response is to send an engineer and open the casing. We did not, for one reason: nothing else in the complaint behaved like a fan fault. No vibration. No noise. No bearing temperature. No motor current anomaly. A fan genuinely failing to make pressure — reversed impeller, slipping drive, a stalled aerodynamic condition — almost never fails silently and cleanly, with every other parameter normal. A number that is wrong all by itself is very often a number that was taken wrong.

So the first thing we put on trial was not the fan. It was the measurement.

The Diagnosis & Fix

Ruled out first — the mechanical failure modes. No vibration, noise, bearing or motor evidence anywhere in the complaint. Nothing to support a hardware hypothesis.

Tested second — the measurement point. A fan's static pressure is defined at a specific plane. Read it elsewhere on the ductwork and you get a number that is real, but is not the number on the datasheet. We asked the site to re-measure at the transition piece with a calibrated digital manometer, and issued a tapping-location diagram with guidelines.

Tested third — the acceptance basis. The substantive finding. Our works test report carries two sets of figures: at test conditions and at design conditions. They differ, and are meant to: the air temperature, density and RPM at the moment of testing are not those the fan was designed around, and density scales pressure directly. The figure the customer had highlighted was the test-condition value. The acceptance value is the design-condition value.

At design conditions, the fan makes 312 mmWC against a required 305 mmWC. It meets duty.

Three preventive changes went back into our own process: name the tapping point and instrument on the commissioning sheet; label the design-condition figure on every test report explicitly as the acceptance figure; and agree the acceptance basis in writing at order stage.

The Result

• What we proved: The works test record shows the fan achieving 312 mmWC against a required 305 mmWC at design conditions — it meets its specified duty.
• What we ruled out: Any mechanical failure mode. No hardware defect was found, and none is implied by the evidence.
• What we delivered: A corrected field-measurement protocol (transition-piece tapping, calibrated digital manometer, guidelines, diagram), plus a written clarification of the test- versus design-condition figures. A site visit was held in reserve, should a correctly-taken reading still show a shortfall.
• What we changed on our side: The commissioning sheet, the test-report format and the order-stage acceptance wording.
• What the record does not confirm: We hold no entry recording the customer's re-measurement, their agreement, or a formal sign-off. We are not claiming a closure we cannot evidence. No spares were issued and no rework was performed.

The Takeaway + Call to Action

If a fan "isn't making pressure" but is quiet, cool, smooth and drawing normal current, be suspicious of the number before you are suspicious of the machine. Pressure means nothing without three things attached: where it was tapped, what it was tapped with, and under what air conditions it was read. Change any of the three and the value changes legitimately, with nothing wrong anywhere. Many "performance shortfall" disputes are two parties correctly measuring different things.

We service, diagnose and re-test centrifugal fans and blowers of any make, not only our own: performance verification, balancing to ISO 21940, testing to IS 4894 / ISO 5801 / AMCA 210 method, ISO 14694 vibration assessment. If a fan on your plant is under dispute, we would rather help you measure it properly than replace it unnecessarily.

— Jitamitra Electro Engineering · Technical Services

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