Fans and Blowers

Blowers are positive displacement equipment, needing overpressure protection devices. Fans are otherwise.

I don't think thats correct!

Blower is a ducted air moving device but could use fans.

Not positive displacement as this would be a compressor.

There is no difference between a centrifugal fan and centrifugal blower, except maybe the output of the device.

A centrifugal fan/blower IS NOT a positive displacment pump.

It is the fan which moves the air through the entire duct system and conditioned space in HVAC applications. Broadly two types- Centrifugal fans and axial flow fans. Centrifugal fans are mostly with duct work as the static pressure drop is considerable. When there is no duct work propellers or axial flow fans can be used. Centrifugal fan can be further classified as backward and forward curved blade fans.

It was touched on earlier, but- in HVAC systems, virtually all of the fans used are centrifugal.

A centrifugal fan causes airflow because the spinning action imparts a continually increasing amount of energy into the air as it moves from the center inlet up to the tip of the wheel. At the end of the wheel, the air is thrust outward radially but, because of the friction caused by the fan blade ends, also "forward" toward the discharge nozzle.

There are basically three types of centrifugal fan styles- radial blade, forward curved and backward curved.

Radial blade is used in specialty applications, usually where some type of product is also moving with the air, and its blades are (as noted) installed in a radial fashion, beginning very near the center of the intake. They are essentially industrial (including their operating sound levels) and are virtually never used in residential (or most commercial) systems. Because of their applications, they also typically operate at relatively high rotational speeds- which further exaggerates the sound levels.

Forward curved fans have the blades very near the outer edge of the fan, with relatively smooth "walls" on either side. The inlet may be one one side or both sides. Their blades look like ice cream scoops, with the opening facing the discharge of the fan housing. Because of the blade style, the air is thrown not only OUT but also in a "forward" direction. This type of fan has a limited pressure capacity but is very efficient at low speeds and, because they run at low speeds- quiet, which is why they are used in well over 90% of all residential (and light commercial) applications.

Backward inclined fans have blades that are one of two styles- flat or airfoil- that are also installed near the outer edge of the fan wheel but- as indicated by the name- are installed with the outer edge "behind" the lead edge. These fans require a higher speed but also produce much higher pressures- great for larger commercial and industrial applications. The flat blades have a higher sound level and also- due to the entering turbulence at the "flat" blade edge- lower total air volume and pressure capacity. Sometimes, the blades are curved backward rather than just a flat plate which reduces noise a bit and provides slightly higher pressure capacity. Airfoil blades are always curved backward, and- as the name implies- are shaped with a curved leading edge and a reducing thickness to the discharge tip. They are the quietest in operation and can handle the highest air volumes and pressures because they impart the lowest parasitic pressure or airflow loads. Obviously, they cost more as well so are only used in large commercial and industrial HVAC systems.

In relative performance- the efficiency (HP required relative to work done) of a radial-blade fan will be about 30-45%, a forward-curved fan will be 50-60%, a backward-inclined blade will be 60-70% and an airfoil blade will be 65-80%, depending on the wheel diameter and speed. Generally, bigger diameters spinning slower have the highest efficiency- and quietest performance. Dual inlet fans are also higher efficiency than single inlet fans.

Dear Islam: It's mostly a matter of an individual's preference in terminology and either would be technically correct. Bottom line they are both air movers and all air movers are blowers or fans.

The term "centrifugal fan" describes a specific style of fan. The fan can be either a "centrifugal" or an "axial" style of fan. Centrifugal fans are typically used in HVAC where a system of duct work is connected to the fan to deliver air to various parts of a building. A centrifugal fan, by design, is capable of overcoming the resistence of the ducts and other components in the system.

Axial fans are used where less resistence is encountered such as being installed in an opening in the wall of a warehouse to exhaust hot air from inside the building to the outside.

I have oversiplified the axial fan. It is a family of fans which includes the propeller, tubeaxial and vaneaxial fans. The propeller fan is the most common of the axial family.

I'm going to refer you to an ASHRAE (American Society of Heating, Refrigerastion, Ventilation and Airconditioning Engieers) Handbook titled HVAC Systems and Equipment. It should be available in your local public library and studying the chapter titled "Fans" plus browsing through other chapters should help you get a good handle on the subject. There you will find illustrations and descriptions of the various types of fans.

Good luck.

Lou Bindner

thanks a lot for all of you and i really appreciate it