Centrifugal Blowers for Ventilation and Exhaust: Rooms, Cabinets and Process Enclosures

A centrifugal blower is the right ventilation machine when air has to move against more resistance than an axial fan handles well: filters, louvres, coils, long or restricted ducts, or a sealed room or cabinet that must be kept under pressure. Most ventilation duties sit in the low and medium pressure classes, below about 76 mm WC (747 Pa), where compact forward-curved wheels do the job quietly. When filters, heat exchangers, or long duct runs push the resistance up, backward-curved wheels take over.

This page covers how to size a blower for ventilation and exhaust, when to use each wheel type, and the design points, such as leakage and pressurisation, that decide whether a system works.

Where Blowers Fit in Ventilation

Duty	Typical pressure	Why a blower
Cabinet and enclosure cooling	Low, tens of Pa to a few hundred Pa	Compact, works against grilles and filters
Room pressurisation	Low to medium	Steady flow against leakage and filters
Process enclosure exhaust	Low to medium	Handles hoods, ducts, and stack losses
Supply and extract with filters or coils	Medium to high	Tolerates rising filter resistance
Long or restricted duct systems	Medium to high	Keeps flow as resistance grows

Where the system is a short duct or an open wall with very low resistance, an axial fan is often the more efficient tool, and centrifugal blowers earn their place as soon as resistance is significant.

Choosing the Wheel: Forward-Curved or Backward-Curved

Forward-curved wheels suit clean-air ventilation at low pressure. They are compact, run at low speed, and are quiet, which is why they are common in enclosures and building services. Their efficiency is lower than backward-curved wheels, indicatively 55 to 65%, and their power rises with flow, so the motor must be sized for the highest flow the blower can reach. One manufacturer’s forward-curved line stops at around 620 Pa, so they belong to low-pressure duty.

Backward-curved wheels suit higher resistance, higher efficiency, and longer running hours. Their power curve is non-overloading, which protects the motor if a duct opens up or a damper is fully open. They also tolerate a modest amount of dust or moisture better than forward-curved wheels. See backward-curved versus forward-curved blower wheels.

Worked Example 1: Cooling an Electrical Cabinet

Heat removal by air follows the relation: airflow equals heat load divided by air density times specific heat times the allowed temperature rise.

Example. A cabinet dissipates 5 kW, and you allow a 15 K rise between the incoming and outgoing air. With air at 1.2 kg/m³ and 1.006 kJ/kg·K:

Airflow = 5 ÷ (1.2 × 1.006 × 15) = 0.276 m³/s, or about 994 m³/h.

The blower must deliver that flow against the resistance of the inlet filter, the cabinet’s internal layout, and the outlet grille. Filters load with dust, so size the blower for the fouled filter and check that the operating point stays within the blower’s stable range. See blower curves, system curves and operating point drift.

Worked Example 2: Exhausting a Process Enclosure

An enclosure exhaust has to draw air inward across its openings fast enough to keep contaminants from escaping. That is a capture velocity requirement.

Example. An enclosure has an opening of 1.2 m by 0.8 m, and you need 0.5 m/s of inward velocity across it.

Airflow = 0.5 × 1.2 × 0.8 = 0.48 m³/s, or about 1,728 m³/h.

If the exhaust duct, stack, and hood together need 250 Pa (about 25.5 mm WC), the air power is 0.48 × 250 = 120 W. At 60% total efficiency, the blower needs about 200 W at the shaft. Small numbers like these are why compact forward-curved blowers are so common in enclosure exhaust.

Pressurising a Room or Cabinet

Positive pressure keeps dust, fumes, and hot air out of control rooms, electrical rooms, and clean areas. The blower supplies filtered air, and the air escapes through leaks and door gaps, so the flow needed depends far more on how leaky the space is than on its volume.

Example. A room has an effective leakage area of 0.5 m², and you want 25 Pa of overpressure. Using a flow coefficient of 0.6:

Airflow = 0.6 × 0.5 × √(2 × 25 ÷ 1.2) = 0.6 × 0.5 × 6.45 = 1.94 m³/s, or about 6,970 m³/h.

Halving the leakage area halves the airflow needed. Sealing the room is therefore cheaper than buying a bigger blower, and it also reduces the energy the blower uses. Provide a differential-pressure gauge across the room and the filter, so you can tell whether the room is holding pressure and when filters need changing.

Static Pressure, Total Pressure, and Measurement

Blower catalogues quote either static or total pressure, and mixing them is a common specification error. Static pressure is what the system’s resistance consumes. Total pressure adds the velocity pressure of the air leaving the blower. Where the discharge is ducted, compare on total pressure at the blower, and where it discharges freely, remember that the velocity pressure is lost. State the basis in every specification. See how to size a centrifugal blower.

Single-Inlet or Double-Inlet for Large Flows

For large building supply and extract duties, a double-inlet blower moves close to twice the air of a single-inlet wheel of similar diameter, at the same pressure and speed. That keeps the wheel diameter and speed down, which reduces noise, and it suits air-handling arrangements with a central plenum. Single-inlet blowers suit compact installations and duties where one side of the wheel must stay free. See single-inlet versus double-inlet blowers.

Noise and Vibration

Ventilation blowers often sit near people. Noise depends on speed, operating point, and installation, and it rises sharply with speed, so a slower, larger blower is quieter and uses less energy. Flexible connections and vibration isolators stop noise travelling through ducts and structure. Silencers add resistance that must be included in the system curve. See blower noise and silencers.

Drive Arrangement

Direct drive is compact and low-maintenance, and it suits small blowers at motor speed. Belt drive allows the blower speed to be set independently of the motor, which helps when the system resistance turns out different from the calculation, and keeps the motor out of a hot or contaminated airstream. State which arrangement you need, and whether the motor sits inside or outside the airstream.

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, with backward-curved and forward-curved wheels and single-inlet and double-inlet designs. Airflow and drive arrangement are confirmed for each selection. Ventilation and exhaust of flammable vapours in classified hazardous areas needs specialist equipment, because we do not currently manufacture ATEX-certified or AMCA 99 spark-resistant blowers.

What to Specify

1. Airflow required, and how it was derived: heat load, capture velocity, or leakage.
2. System resistance at clean and worst-case (fouled filter) conditions.
3. Pressure basis: static or total.
4. Air conditions: temperature, altitude, humidity, and any contaminants.
5. Wheel type preference, or the duty so we can recommend one.
6. Drive arrangement and motor position.
7. Noise limits and any hazardous-area classification.

Get a Blower Selection for Your Ventilation Duty

Send your airflow, resistance, and air conditions, and we’ll size the blower and confirm the wheel and drive. Contact us for a free quote.

Frequently Asked Questions