VFDs also reduce airflow noise at lower speeds and air volumes. Throttling airflow with dampers causes increased noise levels that may diminish worker comfort.
Despite the typical benefits of reduced electricity demand and maintenance costs, VFDs aren’t suitable for all applications. Further, fitting a VFD depends upon several critical fan design considerations: 1) natural resonant frequencies over the full operating speed range; 2) couplings; 3) bearings; and 4) system static pressure.
The various components of air movement systems have natural resonant frequencies. If excited during operation, these frequencies may cause vibratory stress and fatigue, resulting in noise and possibly cracking or destroying certain components such as impellers, shafts, bearings and foundations. Resonant impact testing can determine natural resonant frequencies. A key one is the torsional resonant frequency, which is calculated using data collected from components in the air movement system, including the fan, coupling and motor assemblies.
Fans typically are designed to have normal operating speeds above or below any natural resonant frequencies, including torsional frequencies. If the normal operating speed exceeds these frequencies, VFDs run the risk of hitting them as the rotating speed is decreased. The first fan speed at which a shaft lateral resonant frequency is reached is called the first critical speed.
Various system alterations can shift resonant frequencies outside of the fan’s operating speed range and thereby avoid hitting the first critical speed. The most common choice for altering torsional resonant frequencies is to change couplings — e.g., to elastomeric block-type couplings or other couplings with high damping characteristics. Modification to the fan rotor also can alter resonant frequencies in the system, enabling the preferred operating speed range. In addition to mechanical alterations, it’s possible to program the VFD to lock out speeds that are near the first critical speed or other speeds that may excite natural frequencies. A VFD professional should be involved in this process.
Bearings also require careful selection to match the air movement system. Sufficient operating speeds are necessary for the proper operation of some bearing types, including sleeve bearings; antifriction bearings typically perform well at any speed.
In systems utilizing duct outlets fitted with outlet dampers that open during operation, sufficient static pressure is required for the dampers to properly function. If pressure levels are too low when the fan’s rotational speed decreases, the dampers won’t open. So, examining the minimum operating speed needed to fulfill pressure requirements is essential to determine if a VFD is the best fit for these situations.
The sidebar, Basic VFD Selection Criteria, (http://www.chemicalprocessing.com/articles/2008/211.html) summarizes basic VFD selection criteria.
Make The Right Choice
Only one point exists at which the power efficiencies for inlet dampers, outlet dampers and VFDs are essentially the same — maximum speed. With maximum flow, power consumption is relatively similar for all three options. As flow demand decreases, VFDs typically offer the best efficiency. However, inlet dampers may be a viable alternative if system flow demands remain consistently in the 80%–100% range. For most systems, outlet dampers aren’t a viable means of flow control. Remember, though, choosing the best flow regulation device for an operation demands careful analysis.
Tom Kuli is chief engineer for Robinson Industries, Inc., Zelienople, Pa. E-mail him at firstname.lastname@example.org.
“Improving Fan System Performance – a sourcebook for industry,” U.S. Department of Energy and Air Movement and Control Association International, Inc., Washington D.C., April 2003. Available online at: