The Dedicated outdoor air systems´ (DOAS) main function is to provide ventilation air to achieve acceptable indoor air quality. One advantage of using a DOAS is that the ventilation air can be directed and balanced to point of use. This is harder to achieve with all air multi-zone systems. The most common way to establish ventilation airflow is to follow ASHRAE Standard 62.1.
In addition to ventilation, the DOAS can also be called upon to deliver outdoor air to offset local exhaust such as bathroom exhaust, provide building pressurization and, with chilled beams or radiant cooling systems, provide dehumidification (latent cooling) and primary air to make the beams function properly.
For a typical office application, the ventilation air will likely be around 0.11 to 0.15 cfm/ft2 (0.55 to 0.75 L/s·m2). Centralizing the DOAS to a single unit means connecting every occupied space in the building back to a single location with ductwork. This can be costly but it also impacts fan power requirements and thus energy usage. In Europe, it is far more common to decentralize the DOAS into smaller local systems and thus lower the design external static pressure requirement.
Read an interesting article on “Design Considerations For Dedicated OA Systems!” by Hugh Crowther, published in the ASHRAE Journal March 2016. Read full article here (link).