In the aial flow ventilator the air must blow in line with the axis of rotation and there must be blades with space between them. With few exceptions, the axial blower or fan must not have a lot of back pressure and will work a lot better if the blade is shrouded. One obvious exception is the turbine compressor used in jet engines, but this requires very tight fittings and very high speed so the air is being rammed in against the outgoing air much faster than it can move - it uses a lot of power and makes a lot of noise. Shrouding works better because the greatest loss in an unshrouded fan is the air coming off near the tips and immediately turning out and back circulating only through the fan and not through the room. The most common shrouded fan people see is muffin style fan in computers. An axial blower can have multiple sets of blades and the blades can even over lap so light is not visible through the fan. Fans are used most often for moving large quantities of air between spaces at atmospheric pressure.
Centrifugal blowers come in a number of forms of which the most common is the squirrel cage blower. The common factor is the air is moving perpendicular to the axis of the part that is moving the air. Typically, the air is drawn in through the side of the unit, turns at low speed to encounter the inside of a spinning cylinder with ribs or vanes around the wall of the cylinder which fling the air out and away where it is captured and aimed through an outlet much smaller than the inlet. Think of taking an open tin can, drilling a hole in the center of the bottom to take a bolt as an axle and cutting slots up the sides, bending the material of the slots in, then spinning it with a drill. A variation looks like a plate with ribs running from the center and increasing in height toward the rim, the air being thrown off the rim. The name squirrel cage comes from the appearance like the tread wheel small rodents use for exercise - I suspect it is not called a hamster tread wheel because that is too precise - or childish. Blowers can have higher output pressures, but not a lot higher and they lose capacity rapidly with increasing pressure. In centrifugal fan, as opposed to axial flow fan, the air is moving perpendicularly to the fan shaft. Often called a "squirrel cage" (due to its similarity in appearance to exercise wheels for pet rodents), the centrifugal fan has a moving component (called an impeller) that consists of a central shaft about which a set of blades, or ribs, are positioned. Centrifugal fans blow air at right angles to the intake of the fan, and spin the air outwards to the outlet (by deflection and centrifugal force). The impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing. A centrifugal fan produces more pressure for a given air volume, and is used where this is desirable such as in leaf blowers, hair driers, air mattress inflators, inflatable structures, and various industrial purposes. They are typically noisier than comparable axial fans. A mixed flow fan is an air moving device in between axial flow fan and centrifugal fan. Mixed Flow fans are an excellent choice for return air, supply, or general ventilation applications where low sound is critical. As compared to similarly sized tubular centrifugals and vane axials, a mixed flow fan will be 5-20 dB quieter! In addition, the mixed flow wheel is extremely efficient and will cut down on operating expenses.
Aside from the small squirrel cage blowers used on some electrical equipment, the most common blowers of this type are the units used for drying carpet after cleaning or a fire, the blowers used for leaf sweeping and the unit hidden inside a vacuum cleaner.
Caution is necessary in selecting blowers for series installtion. It is generally accepted that most pre-engineered blowers are capable of withstanding twice their catalog rated static pressure. In other words, where two blowers are installed in series the second blower housing should withstand the higher pressure. There is, however, a need for concern when the combined pressure exceeds twice the rated static pressure for one blower. In series applications involving three or more blowers, or in any system in which positive or negative pressure exists prior to the consideration of the blowers, special housing reinforcement may be necessary.
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Process Ventilating Engineer
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Process Ventilating Engineer
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