Single-Inlet vs Double-Inlet Centrifugal Blowers: Airflow, Width and Selection

A centrifugal blower can take its air in from one side or from both. The choice between single inlet (often written SWSI, single width single inlet) and double inlet (DWDI, double width double inlet) shapes how much air the machine moves for its size, how wide it is, where the bearings sit, and how it fits into the plant. We manufacture both, and this guide explains when each makes sense.

What Each Design Is

Single-inlet. Air enters one side of the wheel, through one inlet cone, and is discharged into the scroll housing. The wheel is supported on a shaft that extends out of the other side, or overhung from a bearing pedestal. One side of the wheel is free of ducting, so the drive can sit there.

Double-inlet. Air enters both sides of the wheel, through two inlet cones. The wheel is effectively two single-inlet wheels back to back on one shaft. The shaft passes through both inlets and is supported by a bearing on each side, and the drive connects at one shaft end.

Airflow for a Given Size

Because a double-inlet wheel takes air on both sides, it moves close to twice the air of a single-inlet wheel of the same diameter, at the same speed and pressure. Power is roughly double too.

Example. A single-inlet blower delivers 20 m³/s at 3,000 Pa and uses 60 kW of air power. A double-inlet blower of the same wheel diameter and speed delivers about 40 m³/s at the same 3,000 Pa, with about 120 kW of air power.

To get twice the airflow at the same pressure from a single-inlet wheel instead, the diameter would have to grow by about 41% (the square root of two) and the speed fall to about 71%, so the tip speed stays the same. That bigger wheel is heavier, needs a larger housing, and often a different motor speed. A double-inlet wheel reaches the same airflow with the original diameter and speed.

Comparison

Feature	Single-inlet	Double-inlet
Airflow at same size, speed, and pressure	Baseline	About twice
Width	Narrow	Wider, two inlet cones
Bearings	One side, or overhung wheel	Both sides, wheel between bearings
Free side for drive	Yes	Drive at one shaft end, both inlets need ducting
Rotor balance	Overhung wheel loads the bearing	Symmetrical, generally more stable
Space	Compact, one inlet duct	Needs room for two inlets
Typical duty	Moderate flow, higher pressure, compact installations	Large flow at moderate pressure

When to Choose Double-Inlet

• Large airflow at moderate pressure, such as large dryers, ovens, ventilation plenums, and heat-exchanger air supply.
• When the wheel diameter or speed must stay controlled, for noise, bearing loads, or tip-speed limits, especially at high temperature where material strength limits speed.
• When a symmetrical rotor and bearings on both sides suit the machine, as on large, heavy wheels.
• Where space allows two inlets, with straight, matched inlet ducts.

When to Choose Single-Inlet

• Moderate airflow at higher pressure, where a compact wheel at high speed does the job.
• Where one side must stay free, for a drive, an inlet box, or maintenance access.
• In tight installations, where a double-inlet blower’s width will not fit.
• For simpler, cheaper machines at smaller sizes.

A Worked Comparison of Diameter and Speed

A double-inlet blower has a wheel 1.0 m in diameter running at 1,480 rpm, giving a tip speed of about 77.5 m/s. To move the same air at the same pressure with a single-inlet wheel, the diameter must be about 1.41 m, and the speed about 1,047 rpm, which keeps the tip speed at the same 77.5 m/s. The larger single-inlet wheel is heavier, needs a bigger scroll housing, and runs at a speed that may not match a standard motor, so it may need a belt drive. Where the space and drive arrangement allow, the double-inlet blower is often the more compact and better-balanced answer for large flows at moderate pressure.

Inlet Conditions Matter

Both types are sensitive to the air arriving at the inlet. A bend or damper close to the inlet creates uneven flow, which lowers performance and raises noise. On a double-inlet blower, the two inlets should receive equal flow. If one side gets less air, for example because of a poorly designed inlet box, performance falls and the wheel is unevenly loaded. Provide straight inlet ducting and balanced inlet boxes.

Bearings, Drive, and Maintenance

Double-inlet blowers have bearings on both sides, so both need lubrication and monitoring, and the shaft passes through the inlet cones. Access to the bearings must be planned. Single-inlet blowers with an overhung wheel have the bearings on one side, and the drive on the same side or the other, depending on the arrangement. Direct drive and belt drive are both possible, and at high temperature the bearings sit outside the hot gas on either type. See bearings, balancing and vibration.

Control

Both types respond to speed control, and both can use inlet vanes. On a double-inlet blower, inlet vanes on both inlets must move together. Speed control avoids that complexity and saves more energy. See flow control for blowers.

Where Our Range Fits

NextAir Systems manufactures both single-inlet and double-inlet centrifugal blowers, up to 800 HP (about 597 kW) and 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels, and high-temperature designs for ovens, kilns, and furnaces. The choice depends on the airflow, pressure, temperature, and available space, and we confirm it for each selection.

Ask Which Inlet Type Suits Your Duty

Send your airflow, pressure, and available space, and we’ll recommend single-inlet or double-inlet. Contact us for a free quote.

Frequently Asked Questions