Low flow on site from a furnace blower — the test record, the duct gauge, and a motor that is not meant to reach its rated current

At-a-Glance

The Setup

A heat-treatment furnace leans on a high-pressure blower for its purge and cooling air. This one was specified for 8,300 CMH at 900 mmWC and tested at our works.

The Complication

Three readings came back from site: 835 mmWC available with all valves open, falling once the cooling circuit was connected; a motor current of 53 to 58 A; and a measured flow of 7,160 CMH. Two conclusions were drawn from them — that the blower was short of flow, and that, because the motor was not drawing its full 63 A rating during purging with every valve open, the blower was not doing its work. The end user asked whether the impeller could be made larger or wider.

The Diagnosis

We took the questions one at a time.

1. Is the blower on its duty? The works test, run to the IS 4894 procedure and cross-checked with AMCA 210-16, recorded 8,400 CMH at 925 mmWC against the 8,300 CMH at 900 mmWC specified.

2. What does the site gauge tell us? A pressure gauge on the duct is a reference reading. It does not give the fan's exact operating point, and it can read lower than the pressure actually available. The site method that does: at the fan inlet, close the inlet fully and read the shut-off pressure; open it until the gauge reads 900 mmWC; measure the air velocity there; and work out the flow from the open area. We offered to run that test on site.

3. Why does the motor not reach 63 A? The test record puts the duty at 29.5 kW and 51 A at 32 °C, and 54.7 A with the system fully open. The motor is chosen to cover the fully open condition, and the next standard motor that does is 37.3 kW, rated 63 A. A rating is the most a motor can carry, not a figure the fan is meant to draw — with every valve open, this blower is not expected to reach it.

4. Where did the flow go? The duct layout drawing and photographs showed sharp bends, T-joints and reductions in duct diameter. Each adds resistance; together they move the system's operating point to a lower flow than the one specified. A blower holds its curve; the system decides where on that curve it runs.

5. Why not a larger impeller? A larger or wider wheel draws more power. On this machine it would push the current too high. A resistance problem is solved in the ductwork.

The Result

• Works test record: 8,400 CMH at 925 mmWC, against 8,300 CMH at 900 mmWC specified.
• Site readings explained: the duct gauge is a reference; the motor's 63 A is a rating, not a target.
• Cause, from the layout: sharp bends, T-joints and duct reductions adding resistance.
• Impeller enlargement: not advised — it would raise the current too far.
• Offered: an on-site inlet test to read the blower's own operating point.
• The record closes at the explanation and the offer of an on-site inlet test; the customer's confirmation is not on file.

The Takeaway

Before a blower is condemned for low flow, three things settle most of it: the works test point, a measurement at the fan inlet on site, and the duct layout. And a motor drawing less than its rating is not, on its own, a weak fan — motors are sized to the next standard rating above what the fan needs.

We service centrifugal fans and blowers of any make — root-cause diagnosis, on-site testing and correction, and a written maintenance regime you can run.

— Jitamitra Electro Engineering · Technical Services

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