Fonte (Source): Consulting – Specifying Engineer
Por (By): J. Patrick Banse, PE, Smith Seckman Reid Inc., Houston
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Dedicated outdoor air systems (DOAS) deliver dehumidified air to buildings to improve the indoor air quality and thermal comfort.
- Explain basic air system design techniques.
- List codes and standards relevant to dedicated outdoor air systems (DOAS).
- Apply these design techniques, with examples from two health care buildings.
There continues to be an increased demand for highly efficient HVAC system design and high-performance buildings that provide indoor air quality (IAQ) and thermal comfort that exceeds minimum code requirements and occupant expectations. Many jurisdictions are adopting the most recent ventilation standards and commissioning requirements to help achieve these design goals.
Air-handling units (AHUs) that provide pretreatment of outdoor air (OA; also referred to in some localities as pretreat units) have been standard in HVAC design for a long time. The unit takes in 100% OA and then filters, heats, cools, dehumidifies (or humidifies) the air, and introduces it into a mechanical room plenum where it is mixed with return air. With further filtration and treatment, the air is introduced into the space. It continues to be a good way to reduce the work of dehumidification of the air (moisture removal) a building air unit would need to perform and to achieve more effective energy use.
This approach also provides a positive introduction of required ventilation air, which results in positive building pressurization. This design approach is especially useful in humid coastal climates. One of the primary reasons for the air pretreatment is to decouple the latent and sensible cooling loads of the OA from the space’s cooling loads. This pretreated air is also an excellent method to supplement other devices such as active and passive-radiant and convective cooling technologies.
Ventilation standards such as ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality have been in use since 1973 (originally as Standard 62) and are now updated regularly using ASHRAE’s continuous maintenance process. The introduction of pretreated OA and minimum ventilation requirements go hand in hand. The pretreat unit nomenclature has been used for a long time, but is now more commonly known as the dedicated outdoor air system (DOAS). Some of the functions and methods of air introduction include:
- Direct introduction of treated OA to each occupied space (OA direct to space)
- Introduction of treated OA to air-handling unit inlet (OA to AHU inlet)
- Introduction of treated OA to mechanical room plenum (OA to mechanical room plenum)
- Using treated OA for active chilled beams and other equipment needing primary air (OA to equipment).
Each of these air-delivery approaches requires air balancing, air measurement, and controlled delivery of the air. The OA quantity is dependent in each case upon the required ventilation air for each space, meeting the exhaust air requirements and for proper building pressurization.
Calculating required ventilation air
ASHRAE 62.1-2010 allows for multiple ways to calculate and introduce the required ventilation air to the occupied zones. Two of the methods included in the ventilation-rate procedure are the 100% OA system and the multiple-zone recirculating system. The 100%-OA-system formula in paragraph 6.2.4 uses the ventilation required by each zone and sums all of the zones for the total air to be supplied by an air handler supplying only OA, which can be individually supplied as treated air to each zone or space.
The multiple-zone recirculating system uses one or more air handlers to supply a mixture of OA and recirculated air to more than one zone. The air formula noted in paragraph 184.108.40.206 calculates the primary OA fraction to be supplied based on the total ventilation air required divided by the total primary air required for proper treatment of the spaces. Further explanation and a ventilation system schematic are contained in Normative Appendix A of the standard.
OA direct to space
The first approach noted above (direct introduction of treated OA to each occupied space) requires a separate OA unit, separate supply duct system, air devices, and control strategy or sequence to maintain the correct air balance and air delivery. The air supply can be at space-neutral conditions or slightly cooler to assist in treatment of the space cooling requirements.
The goal with this approach is to provide the correct amount of ventilation air to the space that meets ASHRAE 62.1 requirements and/or satisfies the space-exhaust requirements. The supply air ductwork would be smaller than ducts for the total supply air volume; several similar room types could be combined and the air distributed through an air-terminal unit (ATU) to ensure the correct amount of air was being delivered at all times.
This approach is particularly suited to rooms that have individual recirculating units such as fan-coil units. Other spaces, such as conference rooms or libraries, that have fluctuating occupant loads and sometimes lengthy unoccupied periods may rely on occupancy sensors or demand-controlled ventilation (DCV) to meet the ventilation requirements for the space. DCV is defined by ASHRAE 62.1 as “any means by which the breathing-zone outdoor airflow can be varied to the occupied space or spaces based on the actual or estimated number of occupants and/or ventilation requirements of the occupied zone.” An ATU could be controlled to modulate closed in the unoccupied mode, and the central OA unit would then be able to decrease its air volume in response to the reduced space needs either from measured airflow or static pressure control.
A typical DOAS-unit configuration is shown in Figure 1. A DOAS-unit schematic diagram showing some potential control components is shown in Figure 2.
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