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Direct drive Arrangement 8 fans offer much of the flexibility of belt driven fans and are even preferred in systems that require large volumes of air and are over 250-300 HP due to drive limitations. A major objection to direct-drive arrangements in the past was the inability to adjust fan speed if system requirements changed. With the advent of variable frequency drives (VFDs) the speed, and therefore performance, of direct-drive fans can now be adjusted to meet varying requirements. Given the variety of operating speeds available, choices in wheel width, and its effect on wheel safe speed limits, the only cost-effective method for selecting today’s direct-drive fans is software selection fan programs.
Belt driven fans, where the fan wheel is supported by a shaft and bearings and driven by belts, allow for a wider range of sizes and applications, and offer a greater selection of accessories and modifications. However, relative to direct-drive fans, they have a larger footprint for a given fan size and have a higher first cost. Without additional controls, belt-drive fan performance can be easily altered in the field by changing sheaves and belts.