Flow Control for Blowers: Dampers, Inlet Vanes and Speed Control

Almost no blower runs at its design duty all the time. Loads change with production, weather, and the state of filters and beds, and the way you reduce the flow decides how much energy you waste. A blower that runs flat out against a partly closed damper burns power to create resistance, while one that slows down follows the load and saves most of it. This guide compares the three main control methods, works through an energy example, and covers the points that matter when fitting a variable-speed drive.

The Three Methods

Dampers. A damper adds resistance to the system, and so moves the operating point back along the blower curve to lower flow. It is simple and cheap, and it wastes energy, because the blower still runs at full speed and the throttled pressure is thrown away.

Inlet vanes. Adjustable vanes at the blower inlet impart a swirl to the air before it enters the wheel, changing the blower’s characteristic so it delivers less flow and pressure at the same speed. They save more energy than an outlet damper, but less than speed control, and they add a moving mechanism to maintain.

Speed control. A variable-speed drive changes the motor speed, and the blower follows the fan laws: flow varies with speed, pressure with speed squared, and power with speed cubed. It saves the most energy, and it also allows soft starting.

The Fan Laws and Why Speed Control Wins

At a fixed system, cutting speed by 20% cuts flow by 20%, pressure by 36%, and power by about half. A damper cannot do that, because it leaves the wheel at full speed. See sizing a centrifugal blower and the fan laws.

Worked Energy Example

A blower absorbs 320 kW at full duty and runs 8,000 hours a year, with this load profile.

Share of hours	Flow needed	Power at cube law
30%	100%	320 kW
50%	80%	164 kW
20%	60%	69 kW

The time-weighted average is about 192 kW. Adding roughly 3% for drive losses gives about 198 kW.

Approach	Average power	Annual energy
Constant full speed	320 kW	2,560,000 kWh
Speed control following the load	about 198 kW	about 1,581,000 kWh
Saving		about 979,000 kWh (38%)

This assumes a system whose resistance is proportional to flow squared. Systems with a fixed static head save less, and the figure is an estimate for comparison, not a guarantee. Multiply the saving by your electricity tariff to see the annual value.

Comparison

Factor	Damper	Inlet vanes	Speed control
Energy at part load	Poor	Moderate	Very good
First cost	Lowest	Moderate	Highest
Control range	Wide, but inefficient	Moderate	Wide
Soft start	No	No	Yes
Moving parts	Simple	Mechanism to maintain	None on the blower
Response speed	Fast	Fast	Fast, with acceleration ramps

When Each Suits

• Dampers: rare or short-lived turndown, small blowers, and isolation.
• Inlet vanes: moderate turndown on larger blowers where a drive is not justified.
• Speed control: variable duty over long hours, large blowers, and any duty where energy cost matters.

For combustion air, speed control gives faster, more accurate burner modulation as well as saving electrical energy. See combustion air.

Fitting a Variable-Speed Drive

• Minimum speed. At low speed a blower may not develop enough pressure to overcome a static head or a burner’s requirement. Set a sensible minimum.
• Resonance. Check that the blower and its supports do not have a natural frequency within the speed range, and program the drive to skip critical speeds.
• Motor compatibility. Inverter supply can stress motor insulation and bearings, and long cables need filtering.
• Cooling. A motor cooled by its own fan cools less at low speed, so check its rating.
• Bypass. For critical duties, provide a means to run at full speed if the drive fails.
• Harmonics and heat. Drives generate heat and harmonics, so provide ventilation and check the supply.

Control Loops

A drive is only as good as the signal that controls it. Common loops hold pressure, flow, temperature, or furnace pressure. Choose a sensor and a location that reflect the real process variable, and tune the loop so the blower does not hunt.

Staging, Bypass, and Minimum Flow

Two other approaches are worth knowing. Staging uses several smaller blowers in parallel and switches them on and off as the load changes, which suits large, variable duties and gives redundancy, though it controls flow in steps. Bypass control recirculates or vents part of the flow, which keeps the blower near its design point but wastes all the energy in the bypassed air, so it is a last resort. Whatever the method, respect the blower’s minimum flow. Running far to the left of its curve, at high resistance and low flow, wastes energy, adds noise, and in some designs risks unstable operation. Set control limits so the blower stays in its stable range, and check the lowest flow the process will ever ask for.

Hot Gas and Speed Control

On hot-gas duty, speed control also moves the operating point relative to the temperature-derated speed limit. Never run above the maximum speed set for the temperature, and confirm how the control range fits within it. See high-temperature centrifugal blowers.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, suitable for damper, inlet-vane, or variable-speed control. Tell us the load profile and control approach, and we will confirm the arrangement.

Tell Us Your Load Profile

Send your load profile, and we’ll recommend a control approach and confirm the arrangement. Contact us for a free quote.

Frequently Asked Questions