Centrifugal vs Positive-Displacement Blowers: Choosing by Pressure and Flow

Blowers divide into two families that work on different principles. Centrifugal blowers accelerate the air and turn its velocity into pressure. Positive-displacement (PD) blowers, such as rotary lobe machines, trap a fixed volume of air and push it through. The two overlap in some duties and are worlds apart in others, and choosing the wrong family is one of the costliest mistakes in blower selection. This guide sets out how each behaves, where each wins, and where our centrifugal range stops and a PD machine takes over.

How Each Family Works

Centrifugal. A rotating wheel throws air outward, and the housing converts velocity into pressure. Flow and pressure are linked by the fan curve, so as the system resistance changes, the flow changes with it. Speed sets the pressure the wheel can develop.

Positive-displacement. Lobes or vanes trap air in a fixed volume between the rotors and the casing, and carry it from inlet to outlet. Each revolution moves nearly a fixed volume regardless of the pressure it has to push against. Flow depends mainly on speed, not on resistance.

Behaviour Compared

Feature	Centrifugal	Positive-displacement
Flow versus resistance	Flow falls as resistance rises, along the fan curve	Flow stays nearly constant as resistance rises
Pressure capability	Up to our 500 mm WC standard maximum for single-stage designs	Higher pressure at lower flow
Flow capacity	Large flows	Moderate flows
Airflow character	Smooth, no pulsation	Pulsating, needs silencers
Efficiency	High at the design point	High at higher pressures
Noise	Moderate	Higher, needs enclosures and silencers
Oil-free air	Yes	Yes, in oil-free designs
Clearances	Generous	Tight, sensitive to dust and wear
Typical uses	Ventilation, combustion air, drying, filtration, low-pressure conveying	Pneumatic conveying, aeration, high-pressure duties

Where Each Wins

Centrifugal wins when:
– The flow is large and the pressure moderate. – The system resistance varies and the flow can vary with it. – Smooth, pulsation-free air matters, as in combustion. – The gas is warm, dusty, or wet, and generous clearances help. – Efficiency at high flow matters.

Positive-displacement wins when:
– The pressure is high and the flow moderate, above what a single-stage centrifugal blower can deliver. – A constant flow is needed regardless of changes in resistance, as in some conveying. – The system needs to build pressure quickly against a closed or restricted path.

The Pressure Boundary

Our standard maximum is 500 mm WC (about 4.9 kPa). The ASME blower band, a pressure ratio of 1.11 to 1.20, runs from about 1,130 to 2,070 mm WC, and single-stage centrifugal machines exist across part of that band. Beyond it are multistage centrifugal machines, geared high-speed machines, and compressors. For guidance:

Pressure required	Usually served by
Up to 500 mm WC	Single-stage centrifugal blower (our range)
About 500 mm WC up to 2,000 mm WC	High-pressure single-stage centrifugal or PD blowers, depending on flow and duty
Above about 2,000 mm WC	Multistage centrifugal, PD blowers, or compressors

The middle row depends heavily on flow. At high flow and moderate pressure, a centrifugal machine still wins. At low flow and high pressure, PD does. Ask for both options where a duty sits near the boundary.

The Conveying Example

Pneumatic conveying shows the difference clearly. A short dilute-phase line with light loading, and an air slide, need low pressure, which a centrifugal blower handles well. A long line or a dense loading needs much higher pressure and a flow that stays steady as the line fills, which a PD blower provides. The pressure required grows with distance and loading, and it can move a duty from one family to the other. See low-pressure pneumatic conveying.

Fan Curve Versus Constant Flow

A centrifugal blower’s flow moves with resistance, and it is stable on a smooth curve. A PD blower’s flow barely moves, so if a line is blocked or closed, it keeps pushing air, and pressure climbs until a relief valve opens or the motor trips. A PD blower therefore needs a safety valve and protection against overpressure, while a centrifugal blower simply delivers less flow when resistance rises. That built-in self-limiting behaviour is a safety advantage of the centrifugal family.

Energy and Control

Centrifugal blowers follow the fan laws, so speed control saves energy with the cube law. PD blowers deliver flow roughly in proportion to speed, so speed control adjusts flow directly, and power varies more nearly with flow and pressure. For variable flow at moderate pressure, a variable-speed centrifugal blower is usually the most efficient. See flow control for blowers.

Maintenance and Contamination

PD blowers have tight clearances between rotors, so dust, moisture, and wear reduce their performance and can damage them. They need clean, dry air and good filtration. Centrifugal wheels, with generous clearances, are more tolerant, though deposits still unbalance them. Where the gas is warm, damp, or lightly dusty, the centrifugal type is usually the more forgiving choice.

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. We do not manufacture positive-displacement blowers or compressors. If your duty needs more than 500 mm WC, or a constant flow against varying resistance, we will say so early, and explain which machine type suits it, instead of forcing a centrifugal selection.

Ask Which Machine Suits Your Duty

Send your flow, pressure, and application, and we’ll tell you whether a centrifugal blower suits it or another type does. Contact us for a free quote.

Frequently Asked Questions