Technical note

Stating a duty point properly

Two quotations for the same plant often differ by a frame size and a motor rating, and the reason is usually the specification rather than the supplier. Nine items make the comparison honest. Here is each one and where it comes from.

The core

Two numbers and a basis

A duty point is a volume flow and a pressure, plus the gas state that makes the pressure mean something. Without the third item the first two are decorative.

Volume flow
Actual cubic metres per hour at the fan inlet, not normal or standard metres cubed
Pressure
Static or total, stated which, in mmWC or kPa, at the same density basis
Gas state
Temperature, altitude or absolute pressure, and composition where it is not air

The list

The nine items

Send these and the quotation is real

  • – Volume airflow, actual m3/hr, with the basis stated
  • Pressure, marked static or total, and the density it was derived at
  • Gas temperature and, where it matters, composition and dust content
  • Altitude or site location, so density is not assumed
  • Continuous or intermittent duty, and how it is controlled
  • Arrangement preference or space constraint, with a sketch if you have one
  • Material constraints: corrosion, food contact, spark risk, wash-down
  • The electrical supply at the point of connection
  • Noise limit or distance to the nearest receptor

Items one to three decide the machine. Items four to nine decide whether the machine that arrives can be installed, run and maintained. A quotation built on three items is a price for a wheel; a quotation built on nine is a price for a working plant.

Sourcing

Where each number comes from

Airflow

From a design calculation on the hood or filter face velocity, or from a measured traverse with a hot wire or pitot. If it is measured, take three readings across the duct and state the duct size. An airflow quoted from a fan nameplate is not a measurement of anything.

Pressure

Sum the system losses – filter, duct, bend, inlet cone, discharge velocity – or measure the difference across the machine with a manometer. Measured pressure at a blinded filter is a true number about the plant, and a false number about the duty.

Temperature and gas

Read it at the fan, not in the process. A fan on the clean side of a bag filter moves ambient air; a fan on the dirty side of a kiln moves something else entirely, and the two duty points are not interchangeable.

Margins

Margins without overbuying

A margin on both flow and pressure is standard practice, and a works will usually apply something of the order of 10 per cent on flow and 15 per cent on pressure before it selects. That is a design allowance, not a licence to buy a bigger frame. Two rules we hold to:

Selected point
On the curve, at or left of peak efficiency, never out at the free-delivery end
Margin expressed as
A second duty point on the same curve, not a bigger machine
Never a margin on
Gas temperature, corrosivity or dust loading. Those are not quantities to be multiplied

Arrangement

Arrangement is part of the duty

Airflow and pressure describe the process. Arrangement describes how the machine reaches it, and it changes cost, footprint, maintenance access, spares holding and noise more than most buyers expect.

Dust Collection Fan Duty

Selection and wear notes for fans on bag filters, cyclones and workshop extraction, dirty side and clean side.

Duty notes

Pneumatic Conveying Blower Duty

Dilute phase conveying duty: line velocity, material rate, travel distance, pressure and wheel choice.

Duty notes

Aeration and Water Treatment Blower Duty

Aeration, backwash and effluent duty: head from diffuser depth, density corrections, turndown and corrosion.

Duty notes

Boiler and Kiln Draft Fan Duty

Forced draft and induced draft duty on boilers, kilns and dryers, with fly ash, temperature tiers and materials.

Duty notes

Paint Booth and Spray Exhaust Duty

Overspray extraction duty: sticky loads, buildup, cleaning access, wheel choice and explosion-risk awareness.

Duty notes

Tunnel and Enclosed Space Ventilation Duty

Tunnel, car park and shaft ventilation: reversible and emergency duty, corrosion protection and the axial decision.

Duty notes

Questions

Duty point questions

I only know the fan size and the motor. Can you select from that?

Sometimes. If the wheel diameter and speed can be read from the nameplate, a works curve can be reconstructed for standard air and the duty estimated. It is an inference rather than a measurement, and we mark it as such on the schedule.

Should I quote pressure drop of my equipment or pressure at the fan?

Pressure at the fan, which is the equipment loss plus every duct loss plus the discharge velocity term. Quoting the filter alone is the most common way to end up with a machine 20 per cent short.

What if I have two possible duties?

Send both. A machine that covers two points on one curve is often the same machine that covers one, and it is far cheaper to decide now than to change a wheel later.

Next

Put the numbers in the worksheet

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