Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsFan typesVAV air-handling units are typically available with several choices for fantypes and sizes. This affords the opportunity to select a fan that optimizes thebalance of energy efficiency, acoustics, and cost.The most common type of fan used in VAV systems is a centrifugal fan, inwhich air enters the center of the fan wheel (axially) and follows a radial paththrough it. A centrifugal fan may be characterized by the shape of the fanblades, whether it contains a fan scroll (housed) or not (plenum), whether it isbelt-driven or direct-drive, and whether one or multiple fan wheels are used.For more information on fan types, referto the Trane Air Conditioning Clinic titled,“Air Conditioning Fans” (TRG-TRC013-EN).• Shape of fan bladesA forward curved (FC) fan has blades that are curved in the direction ofwheel rotation. These fans are operated at relatively low speeds and areused to deliver large volumes of air against relatively low static pressures.Due to the inherently light construction of the fan wheel, FC fans aretypically used in smaller systems that require static pressures of 4 in. H 2O(1000 Pa) or less. FC fans are typically less costly, but usually lessefficient, than the other types.Systems that require greater than 3 in. H 2O (750 Pa) of static pressure areusually best served by the more-efficient backward curved (BC), backwardinclined (BI), or airfoil-shaped (AF) fan blades. In larger systems, thehigher fan efficiencies can result in significant energy savings.A BI fan has flat blades that are slanted away from the direction of wheelrotation, while a BC fan has shaped blades that are curved away from thedirection of wheel rotation. Their rugged construction allows them tooperate at higher speeds than FC fans, and makes them suitable formoving large volumes of air in higher static-pressure applications. Also,BC and BI fans are typically more efficient than FC fans.A refinement of the BI fan changes the shape of the blade from a flat plateto that of an airfoil, similar to an airplane wing. The smooth airflow acrossthe blade surface reduces turbulence and noise within the wheel. Theresult is that an AF fan typically requires less input power than other fantypes (Table 6).Table 6. Impact of fan type on input power (brake horsepower) 1Fan type and sizeInput power,bhp (kW)Rotational speed,rpmHoused FC, 22.375 in. (568 mm) 16.07 (11.98) 979Housed AF, 25 in. (635 mm) 14.65 (10.92) 1433Belt-drive plenum AF, 35.56 in. (903 mm) 16.15 (12.04) 1108Direct-drive plenum AF, 30 in. (762 mm) 14.85 (11.07) 13881 Based on a typical VAV air-handling unit configuration (OA/RA mixing box, high-efficiency filter, hot-waterheating coil, chilled-water cooling coil, and draw-thru supply fan with a single discharge opening off the fansection) operating at 13,000 cfm (6.1 m 3 /s) and 3 in. H 2O (750 Pa) of external static pressure drop.32 Chilled-Water VAV Systems SYS-APM008-EN