Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsAvoiding fan surge in a dual-pathAHUWhen a dual-path configuration is usedfor a VAV air-handling unit, preventingthe supply fan from operating in thesurge region can be more challenging.As supply airflow is reduced at part load,outdoor airflow through the OA path mayremain nearly constant (to ensureproper ventilation) while the recirculatedairflow through the RA path is reduced.Therefore, the pressure drop throughthe OA path remains high while thepressure drop through the RA pathdecreases. This high-pressure dropthrough the OA path can cause thesupply fan to surge at reduced supplyairflow.To help prevent the fan from operating inthe surge region:• Size the components in the OA pathfor a low airside pressure drop. Thismay involve increasing the casingsize for the top section (OA path) ofthe air-handling unit.• Carefully select the supply fan toreduce the potential for surge. Adirect-drive plenum fan (p. 35) oftenprovides the greatest flexibility forselection.• Implement the fan-pressureoptimization control strategy(p. 199), which allows the fan togenerate less pressure at part load.• Implement the ventilationoptimization control strategy(p. 204), which reduces outdoor-airintake flow during partial occupancy.With less intake airflow, the pressuredrop through the OA path decreases.Figure 7. Dual-path VAV air-handling unitSASource: Image adapted from Trane TOPSS programThe face area of a cooling coil is dictated by the design airflow throughthat coil, and the size of the coil typically dictates the footprint of the airhandlingunit: the larger the coil, the larger the AHU must be to house it.In a dual-path unit, because the RA cooling coil only conditions therecirculated air, rather than the mixture of outdoor and recirculated air, itcan be smaller than it would be for a single-path unit.Consider an example VAV air-handling unit that is sized for 13,000 cfm(6.1 m 3 /s) of supply air, of which 3,500 cfm (1.6 m 3 /s) is outdoor air and9,500 cfm (4.5 m 3 /s) is recirculated return air. In a single-pathconfiguration, the single cooling coil must be sized for the total 13,000cfm (6.1 m 3 /s). For this unit, a size 30 AHU casing results in a coil facevelocity of 435 fpm (2.2 m/s).Note: The unit “size” typically represents the nominal face area of thecooling coil, in terms of ft 2 . In this example, the face area of the size 30 airhandlingunit is 29.90 ft 2 (2.78 m 2 ).In a dual-path configuration, the RA (lower) cooling coil need only besized for the 9,500 cfm (4.5 m 3 /s) of recirculated air. For this path, a size 21AHU casing results in a coil face velocity of 456 fpm (2.3 m/s). The OA(upper) coil, which is sized for the 3,500 cfm (1.6 m 3 /s) of outdoor air,requires a size 8 AHU casing, which results in a coil face velocity of 438fpm (2.2 m/s). The overall footprint of the dual-path unit (size 8 casingstacked on top of a size 21 casing) is smaller than that of a dual-path unit(size 30 casing), although the dual-path unit is taller (Figure 8 andTable 3).OARA14 Chilled-Water VAV Systems SYS-APM008-EN