The burner blower that was not making pressure - and the ammeter that said otherwise

At-a-Glance

The Setup

A 20 HP centrifugal blower supplying combustion air to a multi-burner oven built by an OEM in Maharashtra. Duty as supplied: 8,600 CMH at 305 mmWC, 20 HP, design air temperature 20 °C, tested to IS 4894 / ISO 5801 / AMCA 210 method, driven through a variable-frequency drive.

On a combustion-air line, pressure is not a comfort item: the burner needs a defined inlet pressure to hold its air-fuel ratio. If header pressure sags, the burner drifts, the oven's temperature profile drifts with it, and product goes off-spec long before anything mechanical fails. A production complaint, not a maintenance one.

The Complication

The site's argument was clean, and it is the one we hear most often: "The same fan and the same duct arrangement work at our other location." If an identical unit works elsewhere, the difference must be the fan. They asked for a new impeller design and an engineer on site.

That is the trap. "Identical arrangement" is a statement about drawings, and a fan does not respond to drawings. It responds to the system curve it is actually connected to, at the speed it is actually turning, in the air it is actually moving. And nothing mechanical was reported — no vibration, no bearing noise, no imbalance. The fan ran. It just was not making the datasheet number.

The Diagnosis & Fix

We took the readings jointly with the plant, so the numbers could not be contested later, and ruled out in the order that costs least:

1. Mechanical failure? No vibration, no bearing distress, no imbalance reported. Rule out.
2. Motor overloaded — is the fan straining? The opposite. Design current 15.25 A; measured ~11 A. A fan fighting its own defect does not run under rated load. An underloaded motor means it is moving less air than design, because the system will not accept more.
3. At design speed? No — the VFD was at 48 Hz, not 50 Hz when the readings were taken. Below design speed a fan cannot make design head.
4. In the air we sized for? No — 40 °C against a design 20 °C. Less dense air makes less pressure: roughly 10% of head on that gap alone.
5. What is downstream? About 50 mmWC of oven back-pressure, plus a duct splitting into three burner branches — resistance the original operating point did not carry.

Two routes went to the plant. The cheap one: a low-turbulence three-branch Y-type duct to split flow into the three burners smoothly and pull resistance down. The definitive one: if the system stays as built, re-rate the machine to the system — the same 8,600 CMH, but at 450 mmWC and 30 HP at 40 °C, with the caution that the new casing stands 83 mm taller, so the foundation needs modifying.

The Result

• Not a mechanical failure. A motor pulling 11 A against 15.25 A is not a machine straining against its own defect.
• The measurement was taken off-design. 48 Hz instead of 50 Hz, 40 °C instead of 20 °C — correct for those before concluding anything about the fan.
• The system was quantified. ~50 mmWC of oven back-pressure and a three-branch split, against a fan sized for a different resistance.
• We changed what we ship. Combustion and oven blowers are now sized on the worst-case system curve including back-pressure and the true operating air temperature, with VFD frequency and operating conditions recorded at commissioning.
• What the record does not confirm: no post-fix pressure reading, no confirmation that the Y-duct or the re-rated fan was installed, no customer acceptance. The root cause stayed our engineering position, not a jointly signed-off one, and we will not tell you it ended in a handshake because the file does not say so.
• Which is the last lesson: an RCA the customer has not agreed to is not a closed complaint. When a plant says "the identical unit works elsewhere," only comparative measurement at both sites settles it — not argument.

The Takeaway + Call to Action

If a fan is "not making pressure," take three readings before you touch the impeller: motor current against nameplate, actual speed (or VFD frequency), and air temperature. Under-current at reduced speed in hot air is a fan doing exactly what physics allows; a redesigned impeller buys you a bigger motor and the same complaint.

We service, diagnose and re-rate industrial fans and blowers of any make. If a fan on your plant is under-performing and nobody can agree why, we will come and take the readings with you.

— Jitamitra Electro Engineering · Technical Services

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