Various types of fans are used in air-conditioning applications and are classified as propeller, tube-axial, vane-axial, and centrifugal. The propeller and tube-axial fans consist of a propeller or disk-type wheel mounted inside a ring or plate and driven by a belt or direct drive motor.
Avane-axial fan consists of a disk-type wheel mounted within a cylinder. A set of air-guide vanes is located before or after the wheel and is belt-driven or direct drive. The centrifugal fan is a fan rotor or wheel within a scroll-type housing. This type of blower is better known as a squirrel-cage unit. Whenever possible, the fan wheel should be directly connected to the motor shaft. Where fan speeds are critical, a belt drive is employed, and various size pulleys are used.
The various devices used to supply air circulation in air-conditioning applications are known as fans, blowers, exhausts, or propellers. The different types of fans may be classified with respect to their construction as follows:
A propeller fan consists essentially of a propeller or disk-type wheel within a mounting ring or plate and includes the driving-mechanism supports for either belt or direct drive. A tube-axial fan consists of a propeller or disk-type wheel within a cylinder and includes the driving-mechanism supports for either belt drive or direct connection. A vane-axial fan consists of a disk-type wheel within a cylinder and a set of air guide vanes located before or after the wheel. It includes the driving-mechanism supports for either belt drive or direct connection. A centrifugal fan consists of a fan rotor or wheel within a scroll-type housing and includes the driving-mechanism supports for either belt drive or direct connection. Figure 3-28 shows the mounting arrangements.
Fan performance may be stated in various ways, with the air volume per unit time, total pressure, static pressure, speed, and power input being the most important. The terms, as defined by the National Association of Fan Manufacturers, are as follows:
? Volume handled by a fan is the number of cubic feet of air per minute
expressed as fan-outlet conditions.
? Total pressure of a fan is the rise of pressure from fan inlet to fan outlet.
? Velocity pressure of a fan is the pressure corresponding to the average velocity determination from the volume of air-flow at the fan outlet area.
? Static pressure of a fan is the total pressure diminished by the fanvelocity pressure.
? Power output of a fan is expressed in horsepower and is based on fan volume and the fan total pressure.
? Power input of a fan is expressed in horsepower and is measured as horsepower delivered to the fan shaft.
? Mechanical efficiency of a fan is the ratio of power output to power input.
? Static efficiency of a fan is the mechanical efficiency multiplied by the ratio of static pressure to the total pressure.
? Fan-outlet area is the inside area of the fan outlet.
? Fan-inlet area is the inside area of the inlet collar.