ASHRAE Journal Supplement - AMCA In Motion - Fall 2016 - 12

More About VFDs For many systems, VFDs offer a way to DISADVANTAGES improve fan operating efficiency over a Engineers should be aware that, in addition to their advantages, wide range of operating conditions. VFDs VFDs also have disadvantages. Disadvantages include higher also provide an easy and effective method of controlling airflow. Here are some of the advantages and disadvantages of selecting initial cost, low-flow conditions and resonance. First to note is the initial cost of a VFD, which is higher than many other methods of control. A second disadvantage of VFDs is their performance at low-flow conditions. If a fan is applied to a system that has a high degree VFDs as a means for fan control. of fixed resistance at lower flows, the fan controlled via speed ADVANTAGES may not be able to operate at some of the lower flow conditions Among the primary reasons for selecting VFDs are improved or may not have stable operation at these conditions. flow control and the ability to retrofit to existing motors. VFDs Also, fans, like most rotating machinery, are susceptible to also are chosen in order to eliminate both fouling problems resonance problems. Resonance is an operating condition in and the space required for other flow control devices. which the natural frequency of a component coincides with VFDs decrease energy losses by lowering overall system the frequency set up by the rotation. Fans are usually designed flow. By slowing the fan and lessening the amount of unneces- so that their normal operating speeds are not near one of these sary energy imparted to the airstream, VFDs offer substantial resonant speeds. However, decreasing the rotational speed savings with respect to the cost-per-unit volume of air moved. of a fan increases the chances of hitting a resonant speed. VFDs eliminate the reliance on mechanical components, The effects of operating at resonant speeds can be damag- providing an attractive operational advantage, especially in ing. Depending on which component of the assembly is in dust-laden airstreams. resonance with the fan rotational speed, the vibration can VFDs also have the benefit of lower airflow noise. Excess cause a wide range of problems, from annoying noise to fluid energy is primarily dissipated in the form of noise; con- destructive failure. Shafts, bearings and foundations are sequently, operating a fan at high capacity and then throttling particularly susceptible to problems with resonance. the airflow tends to generate high noise levels. Analysis of the resonant frequencies should be conducted Another system benefit of VFDs is their soft-start capability. by the fan manufacturer to ensure that no resonant frequencies During startup, most motors experience in-rush currents that are would be excited. If the fan cannot be designed to avoid reso- five to six times higher than normal operating currents. In contrast, nance problems, VFDs should be programmed to avoid operating VFDs allow the motor to be started with a lower startup current near resonant speeds. (usually about one and a half times the normal operating current), Although VFDs offer an attractive opportunity to reduce energy thus reducing wear on the motor windings and controller while consumption in many applications, all of these considerations also reducing the load on the electrical distribution system. should be included in any feasibility study or system analysis. the flow-versus-pressure fan curve (Figure 4). Because they can reduce both delivered airflow and fan load, inlet vanes or parallel bladed inlet box dampers can improve fan efficiency as compared to resistance control devices. Inlet vanes or parallel bladed inlet box dampers are particularly cost effective when the airflow demand varies between 80 and 100 percent of full flow; however, at lower airflow rates, they become less efficient. The efficiency savings of inlet box dampers is somewhat lower than inlet vanes because they are further removed from the fan wheel and less 12 Summer 2016 AMCA effective in pre-rotating the flow. However, inlet box dampers can be utilized in applications where particulate matter or corrosive environments would make inlet vanes a higher maintenance item. Inlet vanes are more commonly used with centrifugal fans than with axial fans. VARIABLE PITCH BLADES An option for some types of axial fans is the incorporation of a variable pitch feature for the fan blades. Variable pitch blades allow the fan blades to pivot, changing the angle of inmotion W W W. A M C A . O R G http://WWW.AMCA.ORG

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