Combustion-air blowers for burners: sizing and interfaces

What a combustion-air blower actually has to do

Furnaces, dryers, kilns, thermal oil heaters, incinerators and process burners all need a controlled supply of combustion air. The blower or fan is the machine that provides it, but the burner decides how much air is needed and at what pressure. Fuel and air must stay in the correct ratio at every firing rate, or the flame becomes unstable, burns rich and sooty, or wastes fuel heating surplus air.

That makes the blower one part of a chain: intake and filter, blower, silencer, duct, damper or air register, burner head and finally the combustion chamber, which may itself be under positive pressure or draught. Every element adds resistance, and several of them change with firing rate. A good interface design defines that chain clearly, puts measurement points at its boundaries and agrees who is responsible for each piece.

• The burner supplier owns the air-demand curve, the burner pressure requirement and the burner management (flame safeguard) logic.
• The combustion or process engineer owns the firing envelope, fuel specification and furnace pressure behaviour.
• The blower supplier owns the machine’s performance map, temperature limits and minimum stable flow.
• The integrator owns the duct, dampers, instruments, controls and electrical interfaces, as agreed in writing.

Why air mass flow matters more than a volume figure

Combustion depends on the mass of oxygen reaching the flame, so the true requirement is air mass flow in kg/h. Blowers, however, are rated in volume at their inlet, and the same mass of air occupies more volume when it is hot or at lower pressure. In Telangana and Andhra Pradesh, summer intake air at 40–45 °C is far less dense than air at a 20 °C rating condition, and plants on the Deccan plateau also sit at a lower barometric pressure than a coastal site.

For dry air treated as an ideal gas, density is ρ = P / (R × T), with absolute pressure in pascals, R = 287 J/(kg·K) and temperature in kelvin. Some typical values:

The hot, lower-pressure case needs roughly 12–16% more inlet volume for the same air mass. A blower sized on a reference volume without this correction can starve the burner at peak fire on a summer afternoon. Always state whether a flow is actual m³/h at the inlet, normal m³/h or standard m³/h, and give the reference conditions. Humidity lowers density further and reduces the oxygen share, so the burner supplier may ask for it to be included.

How to build the pressure requirement

The blower’s discharge pressure must overcome every loss between its inlet and the combustion chamber, while still leaving the burner its required pressure at the point where air is proved. Define the measurement planes first: blower suction flange, blower discharge flange, burner air inlet and combustion chamber reference. Then list each loss for each firing case.

Notice that the damper loss falls at peak fire because the damper opens, while the other losses rise roughly with the square of flow. That is why a single square-law multiplier applied to the whole system gives the wrong answer. Fixed elements follow Δp = K × ρ × v² / 2 reasonably well; moving dampers and changing furnace pressure do not. The figures above illustrate the method only; real values come from the actual burner, duct and process.

The ideal air power at the peak case is flow times pressure: (2,230 / 3,600) m³/s × 3,000 Pa, about 1.9 kW. Shaft power is higher once blower efficiency is included, and the motor must also cover the densest (coldest) inlet case, which absorbs the most power.

Which blower type suits combustion air?

“Blower” is used loosely on site. The right machine depends on pressure, flow, turndown and how the burner controls air.

A positive-displacement blower working against a closing burner damper does not reduce its flow the way a fan does; pressure and temperature rise instead. Such a system needs relief, bypass or speed control approved by the blower maker. A centrifugal machine, by contrast, can surge if the damper throttles it below its stable flow.

Turndown: matching low fire without unstable operation

Low fire is often the hardest case. Suppose the burner needs about 745 actual m³/h at low fire, but the chosen blower’s stable minimum at the required pressure is 1,000 m³/h. Speed reduction alone cannot close that gap. Options include:

1. Choosing a blower with a wider stable range, or a smaller unit for the duty.
2. Splitting the duty between two blowers, staged by firing rate.
3. Combining moderate speed reduction with burner air-register control, within both makers’ limits.
4. A recirculation or bypass arrangement approved by the blower maker and the burner supplier. Simple arithmetic (1,000 − 745 = 255 m³/h recirculated) does not prove a safe design; heating, check valves and the effect on air proving must be reviewed.

Variable-speed drives are popular here, and they save energy compared with throttling. But a reduced speed does not guarantee adequate motor cooling or stable operation, and fan affinity laws are a screening tool only. The final control curve linking burner demand, blower speed and damper position must be approved by the burner supplier.

Controls and safety: where the blower stops and burner safety begins

A blower running command or a drive speed signal is not proof that combustion air is reaching the burner. Air proving is a burner-management safety function, and its instruments, location, timing and response are set by the burner supplier and the applicable standards for that installation. Generic articles, including this one, should never be used to set purge times, low-air trip points or flame-safety permissives.

• A pressure switch at the blower discharge may stay healthy while a downstream damper closes or a duct joint opens. The proving point must detect the real loss-of-air scenario.
• Damper actuators need position feedback and a defined fail position.
• A standby blower is not a safety function by itself. Whether firing may continue during a changeover, or must shut down, is decided by the burner management design and proved by test.
• Pre-purge flow and time come only from the approved burner documentation.

We also make sure the blower’s own protections, such as motor overload, discharge temperature on positive-displacement machines, vibration and filter differential pressure, are wired so that they alarm or trip correctly without defeating burner safety.

Installation details that cause trouble later

• Intake: site it away from hot exhausts, dust sources and rain; Indian summer dust loads filters quickly, so include a dirty-filter case and a differential-pressure gauge.
• Duct geometry: elbows and transitions close to the blower or flow meter distort readings and operating points; provide straight runs where measurement is needed.
• Loads: support ducts independently so the blower and burner nozzles do not carry duct weight or thermal growth.
• Noise and pulsation: positive-displacement blowers need inlet and discharge silencers and flexible connectors; check for duct resonance.
• Access: leave room to remove impellers or rotors, change filters and reach damper actuators, with proper isolation and lockout points.
• Instruments: fit pressure and temperature tappings at each measurement plane, across the filter and silencer, plus motor current and damper feedback.

Commissioning and troubleshooting

Commissioning should test minimum, normal and peak firing, recording airflow (where measurable), pressures at each plane, blower speed, damper position and motor power, all corrected to the inlet conditions on the day. Burner management tests are led by the burner supplier’s qualified personnel. Air proving and fuel isolation must never be bypassed to take a blower reading.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad since 2002, sells new and used/refurbished blowers, rents blowers and services them, covering Roots, screw, centrifugal and side-channel machines of many brands. We are an independent multi-brand specialist, not an authorised dealer of any maker. We have tie-ups with all major and reputed brands. For combustion-air duties we work alongside your burner supplier and process engineer rather than replacing them.

• We review the burner’s air-demand data and site conditions and check the inlet-density correction and pressure build-up for each firing case.
• We propose suitable blower types and sizes, and supply the machine, with silencers, filters and flexible connections as needed.
• Our team of more than 20 service engineers and technicians services, overhauls and repairs blowers across Telangana and Andhra Pradesh, and supplies spares across India, identified from the nameplate and confirmed before quoting.
• Where a blower has failed or a plant is being modified, rental blowers from our own fleet can help keep production running while the permanent solution is arranged, subject to the burner supplier accepting the interim arrangement.

What to send us

• Burner make and model, with the supplier’s air-demand data for low, normal and peak fire.
• Required pressure at the burner air inlet and expected chamber pressure or draught.
• Site elevation, summer and winter ambient temperatures and intake location.
• Duct layout sketch with lengths, bends, filter, silencer and damper details.
• Existing blower nameplate photo, motor rating and drive type, if replacing a machine.
• Description of the problem or the project timeline.

Frequently asked questions

How do I size a combustion air blower for a burner?

Start from the burner supplier’s air mass flow at low, normal and peak fire, not a single volume figure. Convert each case to actual inlet volume at your hottest site conditions, then add up the pressure losses from intake to combustion chamber for each case. Overlay those points on the blower’s performance map and check the stable low-flow limit, motor power at the coldest inlet condition and temperature limits.

Why does my burner trip on low air in summer?

Hot air is less dense, so the same blower delivers less air mass on a 45 °C afternoon than on a cool morning. Dust also loads the intake filter faster in summer, adding resistance. Check the filter differential pressure, damper travel, proving switch tubing and chamber pressure before deciding the blower is too small. If peak fire genuinely needs more air mass, the blower selection must be rechecked at summer conditions.

Can a Roots blower be used for combustion air?

Yes, for higher-pressure duties such as some kilns and incinerators, but it behaves differently from a fan. A Roots blower moves a nearly fixed volume per revolution, so closing a burner damper raises pressure and temperature rather than reducing flow. It therefore needs relief protection, silencing and an approved method of turndown, usually speed control within the maker’s limits, coordinated with the burner’s air control.

Is a VFD on a combustion air fan worth it?

Often, because reducing speed uses much less power than throttling with a damper, especially if the burner spends long periods at part fire. The drive must stay within the fan’s stable range and the motor’s cooling limits at low speed, and the burner supplier must approve the combined speed and damper control curve. The energy benefit should be estimated from your actual firing profile in kWh.

Where should the combustion air pressure switch be installed?

That decision belongs to the burner supplier and the burner management design, not the blower supplier. The proving point must detect the real loss-of-air cases, such as a closed damper or a disconnected duct, not just whether the blower is running. A switch placed only at the blower discharge may stay satisfied during a downstream blockage, which is why location must be reviewed carefully.

Can a standby blower keep the burner firing if the duty blower fails?

Only if the approved burner management design explicitly allows it and the changeover has been proved by test. A standby start command does not prove that air has been restored. Many burner systems are designed to shut down safely on loss of air and restart through a purge, and that remains the safer default unless the burner supplier has engineered and validated a different sequence.

Related pages

• Roots blowers
• Centrifugal blowers
• Side-channel blowers
• Positive-displacement blower pulsation and silencing
• Vacuum and blower performance acceptance
• Specialist compression and vacuum engineering centre