Blower Pressure Classes: From Ventilation to Pressure Blowers

Pressure is the single number that decides what a blower can do. A machine that ventilates a workshop and a machine that feeds a burner may both be centrifugal, but they differ by an order of magnitude in the pressure they develop. Manufacturers and buyers group blowers into four pressure classes, and knowing the classes tells you which applications belong where, which wheel type suits each, and where our range fits. This guide sets out the classes in millimetres of water column, pascals, inches of water column, and pounds per square inch, so you can convert whatever units a datasheet uses.

The Four Classes

Class	mm WC	Pa	in. w.c.	psi	Typical applications
Low	0 to 38	0 to 374	0 to 1.5	0 to 0.05	Basic ventilation, cabinet cooling, minor exhaust
Medium	38 to 76	374 to 747	1.5 to 3.0	0.05 to 0.11	Commercial HVAC and general air handling
High	76 to 381	747 to 3,736	3 to 15	0.11 to 0.54	Industrial drying, dust collection, combustion air supply
Very high (pressure blowers)	381 to 3,556	3,736 to 34,872	15 to 140	0.54 to 5.06	Pneumatic conveying, deep-bed filtration, high-resistance process lines

Class boundaries are conventions, not physical laws, and different suppliers draw them slightly differently, but the ranges above are widely used.

Converting Units

Datasheets mix units, so keep these conversions to hand.

– 1 mm WC = 9.807 Pa – 1 in. w.c. = 25.4 mm WC = 249.1 Pa – 1 kPa = 101.97 mm WC – 1 psi = 6,895 Pa = 703 mm WC

Example. A burner needs 350 mm WC. That is 350 × 9.807 = 3,432 Pa, or 13.8 in. w.c., or about 0.50 psi. It sits in the high class.

Which Applications Sit Where

Low class (to 38 mm WC). Short ducts, open grilles, cabinet cooling, and simple exhaust. Forward-curved wheels are common, and axial fans compete here as well.

Medium class (38 to 76 mm WC). General air handling with filters and coils, and commercial HVAC. Forward-curved and backward-curved wheels are both used.

High class (76 to 381 mm WC). The heart of industrial process air: dryers, ovens, dust collection, filtration, combustion air, and most scrubber systems. Backward-curved wheels dominate, because they combine the pressure with good efficiency and a non-overloading power curve.

Very high class (381 to 3,556 mm WC). Beds, long conveying lines, high-energy scrubbers, and other high-resistance duties. It needs high tip speeds, and beyond a point, multistage machines or positive-displacement blowers.

Where Our Range Sits

Our standard maximum is 500 mm WC (4,903 Pa, 19.7 in. w.c., 0.71 psi). That covers:

Class	Covered
Low	Fully
Medium	Fully
High	Fully
Very high	The first part, from 381 to 500 mm WC

So we cover essentially all ventilation, air handling, industrial drying, dust collection, and combustion air duties, up to 500 mm WC. The upper reaches of the very-high class, roughly 500 to 3,556 mm WC, including pressure blowers for dense conveying and deep filter beds, are outside our standard range. See centrifugal versus positive-displacement blowers.

Static Pressure, Total Pressure, and Which One the Class Refers To

Classes are normally stated in static pressure, the pressure the system’s resistance consumes. Total pressure adds the velocity pressure of the air leaving the blower. In a ducted system, use total pressure at the blower for selection, and where the blower discharges freely, note that the velocity pressure is lost. Always state which basis a figure uses. See how to size a centrifugal blower.

Pressure at Temperature and Altitude

Blower pressure ratings are quoted at standard air density. At high temperature or altitude, the actual pressure a blower develops is lower, so a duty in the high class at a hot site may need a blower rated in a higher class at standard air. A 408 mm WC duty at 20 °C, for instance, needs a 519 mm WC standard-air rating at 100 °C, which moves it into a higher class. See gas temperature, altitude and density correction.

Choosing the Class from the Application

1. Identify the application and estimate the system resistance from its parts: hoods, ducts, filters, burners, beds.
2. Add the resistances at the worst-case condition, such as loaded filters.
3. Correct for gas temperature and altitude.
4. Read off the class, and check the wheel type and construction that suit it.
5. Confirm the blower is within range, and if the total lands near a class boundary, look at reducing the resistance before buying a bigger machine.

Worked Example: Reading the Class from a Resistance Budget

An industrial dryer has these resistances at the worst-case condition.

Element	Resistance
Heater bank	400 Pa
Product bed	900 Pa
Plenum and distribution	500 Pa
Ducts and fittings	700 Pa
Total	2,500 Pa

2,500 Pa is about 255 mm WC, or 10.0 in. w.c., which is in the high class. If the product bed loaded and its resistance rose by 50%, the total would climb to 2,950 Pa, about 301 mm WC, still in the high class but nearer the top. A duty like this is well within our 500 mm WC standard maximum, and the blower would be a backward-curved single-stage machine.

Reducing Resistance Is Often Cheaper

A system that just crosses into a higher class can sometimes be brought back by enlarging ducts, easing bends, or choosing a lower-resistance filter. Resistance costs energy every hour the blower runs, so reducing it saves money as well as moving the duty into a cheaper class.

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. Send us your resistance and airflow, and we will tell you which class the duty falls in and whether it is within range.

Ask Which Class Your Duty Falls In

Send your airflow and system resistance, and we’ll confirm the class and whether the duty is within range. Contact us for a free quote.

Frequently Asked Questions