20 As shown in Fig 1.8, mechanically ventilated systems move outdoor air via an air handling unit (AHU) from the rooftop into the building and distribute it via a main vertical shaft that splits off into horizontal ducts on each floor. A return air system brings the exhaust air back to the AHU with heat recovery inside. If there is no heat recovery system in place, the pressurized supply air may just be left to leave the building via haphazard exfiltration pathways. The cross-section of the main shaft is dictated by the maximum required air flow rate and the air speed in the duct. The latter is typically capped at 8m/s (1500ft/min) for resistance and noise reasons. Using Equ 1.1 for our Medium Office Boston, the shaft cross-sectional area for a minimum outside air system – including a 15% safety factor – is Aminimum air = (2 × 2.2m3/s ÷ 8m/s) × 1.15 = 0.63m2 Equ 1-2 The factor two accounts for supply and return air shafts. In addition to fresh air, an all air system has to also meet heating and cooling loads stemming from solar gains, envelope losses and internal gains. This means that these systems deliver air that is cooler than the cooling or warmer than the heating set points. Supply air may diverge from heating and cooling set points by up to 25ºF (14K) to avoid occupant discomfort. In other words, for an cooling set point of 75ºF (24ºC) the supply air may be as low as 24ºC − 14ºC = 10ºC or 50ºF. To calculate the air flow rate required to meet peak heating and cooling loads, both quantities need to be known. For the Medium Office Boston in climate zone 4a (CDDI-4), peak heating and cooling loads correspond to 353kW and 247kW, respectively. To meet these loads with an allowable temperature difference of 14K, the required air flow rate for heating, Qheating, is Qheating = Peak heating load ÷ (cp × d × DT) = 353kW ÷ (1006J/kgK × 1.2kg/m3 × 14K) = 20.9m3/s Equ 1-3 where cp and d are the specific heat capacity and density of air, respectively. An equivalent calculation leads to 14.6m3/s to satisfy peak cooling needs. Comparing the air flow rates required for fresh air, cooling and heating reveals that the latter dictates the overall size, i.e. the all air system needs to accommodate a flow rate of 20.9m3/s. Using Equ 1.2, the shaft size of the all air system, Aall-air,is Aall-air = (2 × 20.9m3/s ÷ 8m/s) × 1.15 = 6.0m2 Equ 1-4 The cross section of the all air system is thus an order of magnitude larger than that of its minimum outdoor air counterpart. The HVAC System Selector form provides similar calculations for commercial systems in different ASHRAE climate zones using reference peak loads from the Medium Office Boston and adjusting them by the conditioned floor area
RkJQdWJsaXNoZXIy MjA2MzQ5MA==