Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSIn Figure 1 for a single duct VAV system, the maximum airterminal unit load is 40,000 cfm with a diversity of 75 percent.This means that the supply fan is sized for only 30,000 cfm.Similarly the return fan is sized for 25,000 cfm instead of 35,000cfm and the coils, filters, and ducts can also be downsized.VENTILATIONCare must be used to assure that the AHU ventilation designcomplies with relevant codes and standards which are frequentlyrevised. Ventilation within a constant air volume system is oftena balancing task. During occupied, non-economizer periods ofoperation, the mixing dampers are positioned to bring in therequired OA cfm. The system balancing person determines thespecific minimum ventilation damper position.If this is done on a VAV system at design load, as the VAVboxes require less cooling and less cfm, the supply fan capacityreduces, and the inlet pressure to the supply fan becomes lessnegative as the fan unloads. As the filter inlet pressure becomesless negative, less OA is drawn into the system which isunacceptable from a ventilation and IAQ perspective. VAVsystems therefore require design considerations to prevent noneconomizeroccupied mode ventilation from varying with thecooling load. This may be accomplished in several ways.OUTDOORRELIEF (OR)RETURNAIR FANThe dampers may be set at design load as for a constant airvolume system, and the filter inlet pressure noted. Then thenoted filter inlet negative pressure can be maintained bymodulating the return air damper. Keeping this pressure constantkeeps the OA cfm constant. This method is simple but requiresgood maintenance on the OA damper and linkage, positivepositioning of the OA damper actuator, and the balancing personto provide the minimum damper position and the pressuresetpoints.Another positive method is to provide a small OA injectionfan set to inject the required OA cfm into the AHU mixing boxduring occupied periods (the OA damper remains closed). Thefan airflow quantity may be controlled by fan speed adjustmentor inlet damper setting and sensed by an airflow measuringstation for closed loop modulating control. This basic methodis positive and relatively maintenance free, but like the pressurecontrol method, it requires a balancing person to makeadjustments. This closed loop control method is more costlyand requires keeping the airflow pickup/sensor clean, but itallows simple setpoint entry for future adjustments.A minimum OA damper may be provided for the occupiedOA cfm requirement. An airflow measuring station in theminimum OA duct is required to modulate the minimum OAdamper in sequence with the RA damper to maintain a constantcfm of OA. This method is more costly than the first method,but it allows convenient software setpoint adjustments.EARA35,000 CFMCONSTANTREMOTEEXHAUST FANSUPPLYAIR FANOAFILTEROUTDOOR AIR5,000 CFMMINIMUMCOOLINGCOIL40,000 CFMCONSTANT2,000 CFMEXFILTRATION3,000 CFMINTERIOR ZONESCAV AIRTERMINALUNITSTTTTTTTTREHEATCOILSTTPERIMETERZONESC2663Fig. 2. Single Duct Constant Air Volume System.ENGINEERING MANUAL OF AUTOMATIC CONTROL250