AMCA In Motion - Fall 2016 - 14

Outlet damper/system damper FTP - 1180 rpm 9 Fan total pressure (inches WG) 10 HP - 1180 rpm 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 100 200 300 400 500 600 CFM 700 800 Horsepower 10 0 900 1000 Figure 3. Effect of resistance control devices on centrifugal fan performance. Image courtesy of Howden. Variable inlet vanes FTP - 0% VIV closure FTP - 25% VIV closure FTP - 50% VIV closure FTP - 75% VIV closure HP - 0% VIV closure HP - 25% VIV closure HP - 50% VIV closure HP - 75% VIV closure 9 Fan total pressure (inches WG) 8 7 6 10 9 8 7 6 5 5 4 4 3 3 2 2 1 1 0 0 100 200 300 400 500 600 CFM 700 800 Horsepower 10 0 900 1000 SPEED CONTROL Figure 4. Effect of spin control devices on centrifugal fan performance. Image courtesy of Howden. Variable blade pitch FTP - 0% blade closure FTP - 25% blade closure Fan total pressure (inches WG) 9 FTP - 50% blade closure FTP - 75% blade closure 8 HP - 0% blade closure HP - 25% blade closure HP - 50% blade closure HP - 75% blade closure 7 6 10 9 8 7 6 5 5 4 4 3 3 2 2 1 1 0 0 100 200 300 400 500 600 CFM 700 800 Horsepower 10 0 900 1000 Figure 5. Effect of blade pitch on axial fan performance. Image courtesy of Howden. 14 Summer 2016 attack between the incoming airflow and the blade. Reducing the angle of attack reduces both the airflow and the load on the motor. Consequently, variable pitch blades can keep fan efficiency high over a range of operating conditions (Figure 5). Variable pitch blades can be a very efficient flow control option. They offer several performance advantages. Because variable pitch blades maintain their normal operating speed, they avoid resonance problems that can be problematic for certain fan types. Additionally, when properly selected, variable pitch blades can operate from a no-flow to a full-flow condition without stall problems. During startup, the fan blades can be shifted to a low angle of attack, reducing the torque required to accelerate the fan to normal operating speed. There is a difference between where the area of peak efficiency occurs for axial fans and where it occurs for centrifugal fans. Peak efficiency for axial fans occurs with the blades in a partially closed position, meaning the fan can be selected to have its maximum efficiency at normal operating conditions while still being able to achieve test block conditions at a somewhat lower efficiency. Peak efficiency for a centrifugal fan occurs with spin control in the open position at design speed, meaning the maximum efficiency for a centrifugal fan occurs at test block conditions if using variable inlet vane (VIV) dampers or parallel bladed inlet dampers (Figure 6). This means that variable pitch axial fans can maintain higher levels of efficiency at lower fan loads than centrifugal fans with spin control devices (Figure 1). Disadvantages of this flow control option include the higher maintenance costs associated with the moving parts of the operating mechanism. AMCA Speed control provides the most efficient means of controlling fan flow. By reducing fan rotational speed, less energy is imparted to the airstream, which means less energy must be dissipated by the system airflow control devices (Figure 7). There are two primary devices used to control fan speed: two-speed motors and adjustable speed drives (ASDs). Although both directly control fan output, two-speed motors and ASDs typically serve separate applications. ASDs allow fan rotational speed adjustments over a continuous range, avoiding the need to jump from speed to speed as required by multiple-speed fans. ASDs include several different types of mechanical and electrical systems. Mechanical ASDs are typically adjustable fill fluid couplings. Electrical ASDs include eddy current clutches, wound rotor motor controllers and variable frequency drives (VFDs). VFDs are by far the most popular type of ASD, largely because of their proven effectiveness in reducing energy costs. VFDs have advantages and disadvantages that are described in the sidebar "More About VFDs." Two-speed motors contain a different set of windings for inmotion W W W. A M C A . O R G http://WWW.AMCA.ORG

Table of Contents for the Digital Edition of AMCA In Motion - Fall 2016