Plain-English definitions of the centrifugal-fan terms that show up on a datasheet, a GA drawing or an enquiry — engineer to engineer.
The actual shaft power a fan draws at its operating point, before drive and motor losses; it must be established at the real duty and density so the motor is sized with adequate margin. read more →
Backward-curved blades with a wing-shaped hollow or solid profile that minimises turbulence, delivering the highest efficiency and lowest noise of any centrifugal wheel on clean-air duty. read more →
The adjustment of a fan's rated performance (quoted at standard air) for the actual gas density set by temperature, altitude, moisture and composition; pressure and power scale directly with density while volume flow does not. read more →
A classification of how often an explosive atmosphere is present — gas Zones 0/1/2 and dust Zones 20/21/22; a fan for a hazardous area is built and self-declared suitable for the specified zone rather than certified by a notified body. read more →
Blades that lean away from the direction of rotation, giving high efficiency and a non-overloading power characteristic that peaks and then falls with increasing flow. read more →
A designation such as G 6.3 or G 2.5 (per ISO 21940-11) that specifies the permissible residual unbalance for a rotor as the product of specific unbalance and service speed; a lower G number means a finer balance and smoother running. read more →
The flow-and-pressure combination at which the fan converts the most shaft power into useful air power; selecting the duty at or just to the right of BEP gives stable operation and the lowest running cost. read more →
Combustion air fan — the fan that delivers air to a burner for combustion, as distinct from FD (forced draught, air pushed into the furnace), ID (induced draught, gas pulled out), PA (primary air, carrying fuel) and SA (secondary air, completing the burn). On a burner train the CA fan sits inside the safety chain and must hold its operating point through ignition and load change. read more →
The tongue of the scroll nearest the impeller outlet that separates discharging air from recirculating air; its clearance influences pressure development, efficiency and blade-pass noise. read more →
Flow regulation by adding adjustable resistance — an outlet or inlet damper — which moves the operating point along the fan curve; simple and cheap, but it wastes energy as pressure drop across the damper. read more →
Effectively two impellers back-to-back on a common shaft drawing air through inlets on both sides, roughly doubling the flow of an equivalent SWSI wheel at the same pressure and speed. read more →
The standardised (AMCA) description of the bearing and drive configuration — for example an impeller overhung on the shaft with pedestal bearings, or belt- versus direct-driven — which dictates how the wheel is supported relative to the motor. read more →
The AMCA construction class (I, II, III, IV) defining the minimum pressure-and-outlet-velocity envelope a fan's build is designed to withstand; a higher class means heavier construction rated for more demanding duties. read more →
The manufacturer's characteristic plot of the pressure a fan develops versus volume flow at a fixed speed and density, usually shown with efficiency and power curves overlaid. read more →
Blades that lean toward the direction of rotation, moving a large volume at low speed and pressure but with a steeply rising (overloading) power curve and lower peak efficiency; the classic squirrel-cage wheel. read more →
The rotating assembly of blades, backplate and (optional) shroud that adds energy to the air; its blade type, diameter and speed set the fan's pressure, flow and efficiency. read more →
The inlet cone (inlet bell) guides air smoothly into the impeller eye; the radial overlap and running clearance between the cone and the wheel shroud strongly affect efficiency and leakage and must be set correctly on assembly. read more →
Adjustable radial vanes at the fan inlet that pre-swirl the incoming air in the direction of rotation, reducing the work done and shifting the fan curve to throttle flow more efficiently than a plain damper. read more →
The general international standard series for measurement and evaluation of machine vibration. For fans it applies above 300 kW; below that, fan vibration is accepted to the fan-specific standard ISO 14694 (category BV-3), measured to ISO 14695. read more →
The current international standard series for the mechanical balancing of rotors; Part 11 defines the balance quality grades used to specify residual unbalance for fan impellers. read more →
The hours at which 90% of a population of identical bearings is still running under a stated load and speed — a statistical survival figure to ISO 281, not a warranty period and not a service interval. A 40,000-hour L10 does not promise your bearing lasts 40,000 hours; it says one in ten would be expected to have failed by then. read more →
The single flow-and-pressure condition where the fan curve intersects the system curve; it is where the fan actually runs, which is not necessarily where it was selected. read more →
Blades that project straight out from the hub, tolerating high pressure, abrasive dust and material build-up at the expense of efficiency; the wheel of choice for material-handling and high-wear service. read more →
Construction that reduces the risk of a frictional or impact spark in a fan handling flammable atmospheres; AMCA defines Type A (all air-stream parts non-ferrous), Type B (non-ferrous impeller plus a non-ferrous rubbing ring at the shaft opening) and Type C (construction that prevents ferrous-to-ferrous contact between moving and stationary parts). read more →
The aerodynamic separation of airflow from the blade surface as the angle of attack becomes too steep at low flow, producing a dip or discontinuity in the fan curve, lost pressure and raised vibration. read more →
The potential (compressive) pressure a fan develops to overcome the frictional and dynamic resistance of ductwork, dampers and filters; it acts equally in all directions and is measured perpendicular to the airflow. read more →
An unstable, oscillating flow condition that occurs when a fan is driven far up the left (low-flow, high-pressure) side of its curve, causing periodic flow reversal, pressure pulsation, noise and mechanical stress. read more →
A centrifugal fan with one impeller drawing air through a single inlet eye; the standard configuration for most duties and the simplest to install and maintain. read more →
The plot of pressure a duct system requires versus volume flow; because losses are dominated by turbulent friction, required pressure rises roughly with the square of flow, giving a parabola through the origin. read more →
The peripheral velocity of the blade outer edge (pi times wheel diameter times rotational speed); it sets the maximum pressure a wheel can generate and is capped by blade material strength, especially at elevated temperature. read more →
The algebraic sum of static and velocity pressure, representing the total mechanical energy imparted to the air; fan total pressure equals the rise in total pressure from inlet to outlet. read more →
A VFD controls flow by varying motor speed; by the affinity (fan) laws, flow varies directly with speed, pressure with the square of speed and absorbed power with the cube of speed, making speed control the most energy-efficient method of turndown. read more →
The kinetic component of air pressure created by air in motion, always positive and acting in the direction of flow; it is a function of gas density and the square of velocity (VP is proportional to rho times V squared). read more →
The spiral-shaped fan housing that collects air leaving the impeller and converts part of its velocity pressure into static pressure as the cross-section expands toward the outlet. read more →
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
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