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Flowcharts and Ventilation Systems - Fan Types
Introduction
Types of Fans
Fan Components
Practice Problems
Objective
- Describe the different types of fans and fan wheels used in industrial processes.
The fan is the heart of the system. Fans control the gas flow rate at the point of pollutant generation in the process equipment and through the air pollution control devices. Fans provide the necessary energy for the gas stream to overcome the resistance to gas flow (measured as pressure drop) caused by the ductwork and air pollution control devices. Data concerning fan performance is important during inspections and all other technical evaluations of system performance.
There are two main types of fans: axial and centrifugal. Most fans used in air pollution control systems are centrifugal fans.
An axial fan is shown in Figure 1. The term, axial, refers to the use of a set of fan blades mounted on a rotating shaft. A standard house ventilation fan is an axial fan.
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#1
- For what applications are axial fans useful and why? For what applications are axial fans not useful and why?
A centrifugal fan has a fan wheel composed of a number of fan blades mounted around a hub. As shown in Figure 2, the hub turns on a shaft that passes through the fan housing. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates as it passes over the fan blades. The term, centrifugal, refers to the trajectory of the gas stream as it passes out of the fan housing.
Centrifugal fans can generate high-pressure rises in the gas stream. Accordingly, they are well-suited for industrial processes and air pollution control systems. The remainder of this Module concerns centrifugal fans.
The major components of a typical centrifugal fan include the fan wheel, fan housing, drive mechanism, and inlet dampers and/or outlet dampers. A wide variety of fan designs serve different applications.
The fan drive determines the speed of the fan wheel and the extent to which this speed can be varied. The types of fan drives can be grouped into three basic categories:
- Direct drive
- Belt drive
- Variable drive
In a direct drive arrangement, the fan wheel is linked directly to the shaft of the motor. This means that the fan wheel speed is identical to the motor rotational speed. With this type of fan drive, the fan speed cannot be varied.
Belt driven fans use multiple belts that rotate in a set of sheaves mounted on the motor shaft and the fan wheel shaft. This type of drive mechanism is illustrated in Figure 3.
The belts transmit the mechanical energy from the motor to the fan. The fan wheel speed is simply the ratio of the fan wheel sheave diameter to the motor sheave diameter as indicated in Equation 1.
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Where:
Fan wheel speeds in belt-driven arrangements are fixed unless the belts slip. Belt slippage normally reduces fan wheel speed several hundred rpm and creates a noticeable squeal. If it is necessary to change the fan wheel speed in a belt-driven arrangement, the motor and/or fan wheel sheaves must be replaced with units having different diameters. However, there are very definite safety limits to the extent to which the fan speed can be increased. If the fan rotational speed is excessive, the fan can disintegrate.
Variable drive fans use hydraulic or magnetic couplings that allow operator control of the fan wheel speed independent of the motor speed. The fan speed controls are often integrated into automated systems to maintain the desired fan performance over a variety of process operating conditions.
Fan dampers are used to control gas flow into and out of the centrifugal fan. These dampers can be on the inlet side and/or on the outlet side of the fan. Dampers on the outlet side simply impose a flow resistance that is used to control gas flow. Dampers on the inlet side are designed to control gas flow and to change how the gas enters the fan wheel at different operating conditions. Inlet dampers conserve fan energy due to their ability to affect the airflow pattern into the fan.
The fan wheel consists of a hub and a number of fan blades. The fan blades on the hub can be arranged in three different ways:
- Forward
- Backward
- Radial
Forward-curved fans [Figure 4(a)] use blades that curve toward the direction of rotation of the fan wheel. These are especially sensitive to particulate and are not used extensively in air pollution control systems.
Backward-curved fan blades [Figure 4(b)] use a straight plate, a curved plate, or a curved airfoil. These types of fan wheels are used in fans designed to handle gas streams with relatively low particulate loadings because they are prone to solids build-up. Backward-curved fans are more energy efficient than radial blade fans.
Radial blades are fan wheel blades that extend straight out from the hub. A radial blade fan wheel, as shown in [Figure 4(c)], is often used on particulate-laden gas streams because it is the least sensitive to solids build-up on the blades.
Practice Problems
Fans - Fan Types
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Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.