As we all know, ducts are generally larger than pipes, so design engineering is very tricky. The obvious concerns are headroom and metal corners where you can hit your head, while the less obvious include the details of specifying fans that can compromise system performance and serviceability if fan configuration specifics are overlooked during the design. Let’s look at some key concerns.
AMCA Standards. I’d venture a guess that one out of every three utility fan sets doesn’t get specified correctly or, if they do, the pertinent details are changed and missed by the engineer during the shop drawing phase. Starting with the fan discharge arrangement, it is critical to specify the most efficient discharge outlet on a centrifugal fan. For example, if the design calls for a clockwise, top angular upblast, and the fan submittal mistakenly shows the fan to be counter-clockwise, top angular upblast, then there is a good chance that the fan discharge pressure drop could be as high as an inch. If the fan was initially calculated at 0.25 in. discharge static pressure, the fan will be furnished and installed with 0.75 in. extra pressure drop. If the fan was scheduled for 2 in total static pressure, it will actually be 2.75 in., and the design CFM may not be met due to this approximately 40% added system resistance.
In addition to fan rotation, we have to keep in mind motor rotation as well as motor location. Is the motor location arrangement 10, W, X, Y, or Z? Big difference if the engineer is not paying attention to the shop drawing submittal or, worse yet, if the designer left the motor rotation and location selection up to the equipment manufacturer.
Fan drive arrangement is also an important detail that needs to be specified and not left up to the equipment manufacturer to decide. Serviceability can be compromised based on each aspect of fan data.
Airside economizer exiting into the equipment room. At first glance, dumping conditioned building exhaust into a fan room can seem like a good idea. Heck, the air has either been heated or air-conditioned for the occupied space, so why not use that air one more time before letting it exit the building? Usually, the fan room is looking for cooler air, so air-conditioned air from the occupied space seems like a good idea versus providing the fan room with its own air-conditioning unit. However, the airside economizer occurs when the outdoor temperature is cooler, and usually that means the fan room doesn’t need that conditioned air when the airside economizer is available. And what about air quality? The supply air to the occupied space is filtered, but the airside economizer air being dumped into the fan room will most likely not be filtered and will contain some level of dust and lint. If the building is a hospital, infection control may not allow this conditioned air to be supplied to the fan room at any time during the year.
Air intakes. This topic seems self-explanatory: the designer does not want the outdoor air to also introduce building exhaust air that could be con-taminated and/or toxic based on exhaust fan discharge and prevailing winds.
Fan exterior walls. Often, fan systems require duct attenuation to reduce the air noise within the duct system, and these attenuators are located close to the fan discharge and not close to where they exit from the fan room. This ductwork, between the attenuator and the fan wall, can pick up room noise that can travel down the sheet metal system and become a space comfort concern.
Whether the engineer is designing a chiller room, boiler room, or fan room, they all share common issues and concerns, and each has application-specific issues about which we have only scratched the surface.