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David - OR 11-03-2009 10:16:05 208.67.204.213 812491 Report to Moderator

With all due respect to TBone's experience, I have to dispute two points: 1) 500 FPM in a metal branch duct is NOT excessively noisy. A very typical room application might be 100 CFM through a 6 inch metal duct, giving a flow velocity of 533 fpm. I live with such a system every day and it is fine from a noise perspective (very conservative actually). The typical "ductolator" allows 700 fpm and up through the ducts. 500 FPM at return grilles and registers MAY be excessively noisy, which is why they are sized to a lower flow velocity. But 500 fpm in a DUCT is fine. 2) Excessive friction loss in a duct system will NOT "cause the fan motor too (sic) fail because of too high of static pressure". An ordinary centrifugal fan or pump draws LESS power, not more, as flow resistance rises. There are many reasons to be concerned about excessive flow resistance, such as heat exchanger temperature, duct temperature, and overall SYSTEM efficiency, but failure of the fan motor due to too high of a flow resistance is not one of them (variable speed drives excepted). Look at any text on "fan curve" or "pump curve". Here is a quote from one of them: "As the static pressure imposed on a blower driven by a PSC motor increases, the airflow decreases, following along a pressure-flow line on a fan curve. Because the power consumed by a fan is proportional to the cube of the airflow, power consumption also decreases with increasing static pressure when PSC motors are used." one reference on fan curves [Reply]   [Send Email]