When a fan is selected for a given application, the expectation is for steady air flow and pressure with little fluctuation. Certain conditions, however, can produce extreme variations in air flow, leading to a variety of issues that include excessive noise, structural metal fatigue, and a reduction in efficiency. One such condition is known as fan stall.
Air is deflected when a fan blade passes through an airstream. If the attack angle of the blade is increased, the amount of air deflection is likewise increased, generating higher air pressure. But if the attack angle becomes too severe in relation to CFM, air will no longer flow uniformly and will separate from the blade’s surface. Centrifugal force causes the air to move outward toward the tip of the blade, rather than continuing forward, resulting in a drop-off in pressure. This is what is known as the stall point. Fans operating in stall are extremely noisy, at times emitting a loud hammering sound, and are highly prone to sustaining mechanical damage.
Fan stall usually results when CFM is too low – in other words, the selected fan is too large for the system. This is a mistake often made by underqualified or inexperienced designers who specify a larger fan than needed to compensate for possible calculation errors. Not only does this increase the risk of fan stall, but it also wastes energy. Experienced HVAC designers and contractors are better equipped to make appropriate fan selections and can take additional steps to avoid stall.
A temporary fix that can often eliminate stall is to increase airflow through the fan by creating a “leak” downstream or by running a duct from the outlet back to the inlet. This, however, will reduce system efficiency. If fan stall is a persistent problem, replacing the fan with a properly-sized unit is probably in order.
MSC Mechanical Service Corp. is an HVAC service subspecialist expert in fan stall diagnostics, providing effective solutions to elusive fan stall issues. Contact MSC at 973-884-5000 to schedule a service call or for more information on fan stall.