Industrial Centrifugal Blowers: Types, Pressure Classes, Applications and Selection

An industrial centrifugal blower is a fan whose impeller throws air outward, developing much more pressure than an axial fan of similar size. That makes it the right machine wherever air must be pushed or pulled through resistance: burners, dryers, ovens, filters, process lines, and long or restricted ducts. NextAir Systems manufactures centrifugal blowers up to 800 HP and up to 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels, single-inlet and double-inlet designs, and high-temperature blowers for furnaces (up to 800 °C), kilns (up to 650 °C), and ovens (up to 500 °C).

Blowers are general-purpose industrial equipment. The same machine family ventilates a plant room, feeds combustion air to a furnace, circulates hot air in an oven, cools a motor, and pulls dusty air through a filter. This guide explains how blowers work, how pressure classes map to applications, how the two wheel types compare, and what to specify, using worked numbers you can check.

What Is a Centrifugal Blower?

Air enters the impeller along the shaft, is accelerated outward by the rotating blades, and leaves at right angles to the shaft through a scroll-shaped housing that turns velocity into pressure. Because the air is thrown outward, pressure is developed by centrifugal action as well as by blade lift, which is why centrifugal machines cope with resistance far better than axial fans. A centrifugal blower is the natural choice when the system has filters, long ducts, burners, ovens, or process equipment in the air path.

Blower or Fan? What the Words Mean

The formal engineering definition separates machines by pressure ratio, the outlet absolute pressure divided by the inlet absolute pressure. Under the ASME convention, fans work up to a ratio of 1.11, blowers between 1.11 and 1.20, and compressors above 1.20. At atmospheric inlet, that puts the blower band at roughly 11 to 20 kPa, or about 1,130 to 2,070 mm WC.

Our maximum of 500 mm WC is about 4.9 kPa, a pressure ratio of about 1.05. On the formal definition, that makes our machines high-pressure centrifugal fans. On the plant floor, however, “blower” is the everyday word for a centrifugal fan built for higher pressure than ordinary ventilation fans, and buyers search for it that way. We use the word in that common industrial sense, and we state the pressure in mm WC so there is no ambiguity. Our guide to centrifugal blower versus centrifugal fan covers this in detail.

Pressure Classes and What Each Is Used For

Blowers are commonly grouped into four pressure classes. Static pressure here is what the machine develops against the system.

Our standard maximum of 500 mm WC (4,903 Pa, 19.7 in. w.c.) covers the low, medium, and high classes completely, and the first part of the very-high class. Pressure blowers above that, which serve some high-pressure conveying and deep filter-bed duties, are outside our range. Positive-displacement blowers or multistage machines generally serve those. See blower pressure classes and centrifugal versus positive-displacement blowers.

What 800 HP Means for Airflow

800 HP is about 597 kW. Power, pressure, and efficiency together limit the airflow a blower can move: air power equals flow multiplied by total pressure. The table shows the theoretical ceiling for 597 kW of shaft power at standard air density. It is an upper bound from power alone, and real limits also come from wheel size and speed.

At low pressures the same power would imply an airflow no single wheel could move, so the 800 HP figure matters mainly at higher pressures. At 500 mm WC, a full 800 HP blower has a theoretical ceiling of roughly 328,000 m³/h at 75% efficiency.

Backward-Curved and Forward-Curved Wheels

We manufacture blowers with backward-curved and forward-curved wheels, and the choice follows the application.

A non-overloading power curve is a practical safety feature: if the system resistance drops, for example a damper opens or a duct disconnects, the motor is not driven past its rating. Forward-curved wheels do the opposite, so the motor must be sized for the highest flow the fan can reach. Radial-blade wheels, used for heavy material handling and very abrasive gas, are not part of our range. See backward-curved versus forward-curved blower wheels.

Single-Inlet and Double-Inlet Blowers

A single-inlet blower draws air in from one side of the wheel. A double-inlet blower draws air in from both sides, and is effectively two single-inlet wheels back to back on one shaft. That lets it move close to twice the airflow at the same pressure and speed for a similar wheel diameter, at the price of a wider housing, bearings on both sides, and more space. Choose single inlet for compact, lower-flow duties and where one side must be free for a drive, and double inlet for large flows at moderate pressure where diameter or speed must stay controlled. We manufacture both. See single-inlet versus double-inlet blowers.

Where Centrifugal Blowers Are Used

Fan Curves, System Curves and the Operating Point

A blower curve plots pressure against flow at fixed speed, and the system curve plots what the ducts, filters, burners, and equipment need at each flow. The blower runs where the two cross. For most systems, resistance rises with the square of flow. Because resistance changes over time, for example as filters load or a damper is adjusted, the operating point moves, so check the blower at the clean and the worst-case resistance. See blower curves, system curves and operating point drift.

The Fan Laws, With a Worked Example

For a given blower in a given system, flow is proportional to speed, pressure to speed squared, and power to speed cubed.

Example. A blower delivers 60 m³/s at 4,000 Pa total pressure, about 408 mm WC. Air power is 60 × 4,000 = 240 kW. At 75% total efficiency, shaft power is 320 kW, about 429 HP.

Two lessons follow. Speeding up by 10% pushes pressure to 494 mm WC, right at our 500 mm WC limit, and raises power by a third. And slowing down by 20% halves the power. The laws hold for systems whose resistance rises with flow squared, and they need checking against the curve for systems with a fixed resistance. See gas temperature, altitude and density correction.

Gas Temperature and Air Density

Blower ratings are quoted at standard air density unless stated. At temperature or altitude, air is thinner, so at the same speed the blower moves the same volume but develops less pressure and absorbs less power.

This matters most for furnace, kiln, and oven duty. A blower that develops 4,000 Pa at standard density develops only about 1,520 Pa of actual pressure in 500 °C oven gas, 1,270 Pa at 650 °C, and 1,090 Pa in 800 °C furnace gas. Meanwhile the same mass of hot gas occupies more volume, up to 3.7 times as much at 800 °C as at 20 °C, so the system usually asks for more pressure at the same time as the blower can supply less. Hot-gas duties are therefore selected from a density-corrected curve, and the motor is sized for the coldest, densest condition, including cold start. See gas temperature, altitude and density correction and high-temperature centrifugal blowers.

Controlling Output

Most blowers run below their design duty most of the time, so control decides energy use. The main options are dampers, inlet vanes, and speed control with a variable-speed drive.

Example. A blower absorbing 320 kW at full duty runs 8,000 hours a year, at full flow for 30% of the time, at 80% flow for 50%, and at 60% flow for 20%. Following the load by speed control averages about 192 kW, or about 198 kW with 3% drive losses. That compares with 320 kW at constant full speed.

This assumes a system whose resistance is proportional to flow squared. Dampers waste far more energy than speed control, and inlet vanes fall in between. See flow control for blowers.

Motors, Drives and Starting at 800 HP

At 800 HP, about 597 kW, the motor is a major component. Starting a motor of this size directly on line draws a large inrush current, so soft starters, variable-speed drives, or fluid couplings are commonly used, and the supply must be able to cope. Motors at this size are often supplied at medium voltage. Check the motor and starting method against the supply available, and against the load the blower places on the motor at start-up. See motors, drives and starting large blowers.

Filtration Systems: Where the Blower Sits

Dust collection is one application among many, but it has one design point that shapes blower selection: where the blower sits. In many systems the blower is placed on the clean side, after the filter. That lets a higher-efficiency backward-curved wheel be used and keeps the ducting under negative pressure, so any leaks draw air in and not dust out. A blower placed before the filter handles dusty air, which calls for more abrasion protection and heavier construction. See dust collection system design.

Hazardous Atmospheres

NextAir Systems does not currently manufacture ATEX-certified or AMCA 99 spark-resistant blowers. Where a blower would sit in a classified hazardous area, or would handle a flammable gas, vapour, or combustible dust, it should come from a specialist supplier. A standard blower is only suitable where the blower and its motor sit outside any classified zone and the air handled is not explosive. See combustible dust and hazardous atmospheres.

Standards That Apply

See bearings, balancing and vibration and blower performance testing.

What We Manufacture

NextAir Systems manufactures its own centrifugal blowers, engineered to the customer’s duty.

• Power: up to 800 HP (about 597 kW)
• Static pressure: up to 500 mm WC (about 4.9 kPa), our standard maximum
• Wheels: backward-curved and forward-curved, chosen for the application, flow, and pressure
• Inlet: single-inlet and double-inlet designs
• High-temperature blowers: furnaces up to 800 °C, kilns up to 650 °C, and ovens up to 500 °C

Airflow and drive arrangement are confirmed for each selection, and at high temperature the materials, bearing cooling, and speed limit are engineered for the duty. Duties outside this range, such as pressures above 500 mm WC, radial-blade material-handling duty, very large process fans above 800 HP, and ATEX or spark-resistant blowers, are not part of what we build, and we will say so early instead of forcing a selection.

What to Provide When You Request a Quotation

1. Airflow and whether it is at actual or standard conditions.
2. Static or total pressure required, in mm WC or Pa, and the system resistance at clean and worst-case conditions.
3. Gas: type, temperature, humidity, dust load, and any corrosive content.
4. Site: altitude and ambient temperature range.
5. Duty: continuous or intermittent, fixed or variable.
6. Control: dampers, inlet vanes, or speed control.
7. Hazardous-area classification, if any, so we can tell you whether we can supply the blower.
8. Drive and installation: direct or belt drive, motor voltage, available space, and orientation.
9. Noise limits and documentation required.

Where Blowers Fit in Industry

Specify the Right Blower for Your Process

Send your airflow, pressure, gas conditions, and application, and we’ll help you select and size a centrifugal blower that performs in your real system. Contact us for a free quote.

Frequently Asked Questions