ASHRAE Journal - April 2010 - 31

loads. Some types of local equipment, such as chilled ceiling Rp = outdoor airflow rate required per person, cfm/perpanels or chilled beams, must operate dry and avoid condensason (L/s · person) tion, which limits their ability to handling sensible loads only. Pz = largest number of people expected to occupy the Figure 1 shows several example DOAS configurations. Some zone during typical usage deliver the conditioned outdoor air (CA) directly to each zone,3 Ra = outdoor airflow rate required per unit area, cfm/ while other configurations deliver the air to the intakes of loft2 (L/s · m2) cal, single-zone equipment (such as fan-coils, water-source Az = occupiable floor area of the zone, ft2 (m2) heat pumps, dual-duct VAV terminals, small packaged rooftop units, or single-zone air handlers) or to centralized, multipleNext, Equation 6-2 and Table 6-2 from Standard 62.1 are used zone equipment (such as floor-by-floor VAV air handlers or to account for zone air-distribution effectiveness (Ez), and to self-contained units). calculate the design outdoor airflow for the zone (Voz). This is In addition, many types of dedicated outdoor air equipment the outdoor airflow that must be provided to the zone by the air are available (Figure 2). Dehumidification is usually provided by distribution system (that is, through the supply-air diffusers). direct-expansion (DX) refrigeration, a chilled-water coil, a desFinally, for a 100% outdoor air system in which one air iccant-based dehumidification handler supplies only outdoor device, or some combination air to one or more zones, Conditioned Outdoor Air Delivered Directly to Zones of these technologies. Often, Equation 6-4 from Standard Dedicated Dedicated OA OA OA Unit OA Unit the dedicated outdoor air unit 62.1 is used to calculate the CA CA CA Local includes an exhaust-air energy system-level outdoor air inHVAC Units recovery device (such as a take flow (Vot), by summing CA CA SA SA RA RA SA SA total-energy wheel, fixed-plate the zone outdoor airflows of heat exchanger, coil runaround all zones served by the dediLocal RA RA HVAC Units loop, or heat pipe), which can cated outdoor air unit: reduce energy use and allow Conditioned Outdoor Air Conditioned Outdoor Air Delivered to Intakes of Central, for downsizing of the cooling Delivered to Intakes of Local, Vot = Σall zones Voz (2) Multiple-Zone Units Single-Zone Units and heating equipment. In OA Dedicated Dedicated fact, ASHRAE Standard 90.1 In some system configuraEA OA VAV Terminals OA Unit OA Unit requires the use of an exhausttions, the dedicated outdoor SA Local CA HVAC CA air energy recovery device for air unit provides conditioned SA RA SA RA Units MA SA RA many DOAS applications.4 OA to the intakes of local or RA SA SA Floor By centralized HVAC equipment, Floor VAV SA RA SA RA Determining Design Airflow rather than directly to each Air Handlers MA In most applications, the zone. In these configurations, design airflow for a dedicated Figure 1: Example dedicated outdoor air system configurations. the dedicated OA unit must be outdoor air unit is dictated by sized to deliver the sum of the the amount of ventilation air required by industry standard or local outdoor air intake flows (Vot) required by each of the systems code.5 In some cases, the owner or design team may choose to being served. deliver more than code-minimum ventilation airflow to improve Makeup Air Applications. In some applications, the design indoor air quality or to earn the “Increased Ventilation” credit airflow for the dedicated OA unit is dictated by the need to when certifying a project using LEED. Finally, in applications replace air that is being exhausted from the building. This is with very low ventilation requirements or very high indoor latent common in laboratories, commercial kitchens, or other applicaloads, the design engineer may choose to increase the airflow tions with large exhaust requirements. In this case, the design delivered by the dedicated outdoor air unit so that the conditioned airflow is the sum of all exhaust airflows plus any air needed outdoor air can be delivered at a higher dew point (not as dry). for positive building pressurization. Table 6-1 of ASHRAE Standard 62.1-20076 prescribes two ventilation rates for each occupancy category: one for people- Safety Factor for Zone Population related sources of contaminants (Rp) and another for buildingThe first common use of a safety factor occurs when derelated sources (Ra). Equation 6-1 from Standard 62.1 is used termining Pz, the number of people expected to occupy the to determine the minimum outdoor airflow (Vbz) that must be zone. The definition of this term in Standard 62.1 states that delivered to each breathing zone: this is “the largest number of people expected to occupy the zone during typical usage.” This is not the largest number of Vbz = Rp × Pz + Ra × Az (1) people that could conceivably be in the zone under any special where circumstance. Vbz = outdoor airflow required in the breathing zone of If actual zone population is not known, or if the owner and the occupiable space, cfm (L/s) design team are not comfortable estimating it, Table 6-1 from
April	2010	 ASHRAE	Journal	 31
ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010
