AMCA In Motion - Fall 2016 - 11

RESISTANCE CONTROL DEVICES Resistance control devices include system dampers, outlet dampers and opposed bladed inlet dampers. There are two types of dampers, parallel and opposed blade, that can be oriented in different ways, as shown in Figure 2. Opposed bladed dampers generally provide more precise control than parallel bladed dampers. A system damper is a damper that is not located adjacent to the fan itself. It can be located anywhere in duct system upstream or downstream of the fan. All these dampers function in the same mann e r, r e g a r d l e s s o f t h e i r l o c a t i o n o r orientation. Dampers provide flow control by changing the restriction in the path of an airstream. As dampers close, they reduce the amount of flow and increase system pressure. By increasing system resistance, dampers force fans to operate against higher pressure, which reduces their output. As a fan works against higher pressure, its operating point shifts to the left along its performance curve (Figure 3). Fans operating away from their best efficiency points increase energy and maintenance costs for the fan owner. Inlet Box Damper Opposed Blades Cone Type Variable Inlet Vanes Outlet Damper Horizontal Opposed Blades System, Resistance Control Devices * Low * Low cost first cost maintenance * Moderate Spin Control Devices VariablePitch Blades Adjustable Speed Drives and TwoSpeed Motors first cost * Improved efficiency compared to resistance control * High efficiency over a wide operating range * High efficiency over a wide operating range * Reduced W W W. A M C A . O R G wear and noise at reduced speed Outlet Damper Horizontal Parallel Blades Cylindrical Type Variable Inlet Vanes Outlet Damper Vertical Opposed Blades Outlet Damper Vertical Parallel Blades Figure 2. Fan control devices. Image courtesy of AMCA International. Spin control devices include variable inlet vanes and parallel bladed inlet box dampers, and they change the profile of an airstream entering a fan. These devices create swirls that rotate in the same direction as a fan impeller. Rotating swirls at the Advantages Diffuser (Evase) Inlet Box SPIN CONTROL DEVICES Method Inlet Box Damper Parallel Blades Disadvantages inlet reduce the angle of attack between the incoming air and the fan blades, which reduces the load on the fan and, consequently, reduces fan pressure and airflow. By changing the severity of the inlet swirl, these devices essentially change Best Applications * Low * Least efficient way to control a fan energy fans * Fans that operate near wide-open damper position * Control at low flows may cause the fan to operate to the left of peak pressure in an unstable area of the performance curve * Fans * VIVs have higher maintenance costs than dampers * Higher first cost * Higher maintenance cost * Higher first cost * Obsolescence of electrical components over time AMCA with moderate variation in flow requirements Notes * Fans with low to moderate dust loading with wide variation in flow * VIVs are more efficient than parallel bladed IBDs but have higher maintenance requirements * Fans * Fans with low dust loading * Fans with wide variation in flow * Fans with high dust loading inmotion * Not normally used for retrofits * Two-speed fans make sense when the required performance has a few discreet operating points that are relatively widely spaced * See the sidebar for VFD disadvantages Summer 2016 11 http://WWW.AMCA.ORG

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