By Matt Dodds
The use of dedicated outdoor air systems (DOAS) has increased in recent years to meet higher ventilation requirements and to improve indoor air quality (IAQ). While IAQ has been a growing topic over the last decade, the prevalence of requests for clean indoor environments is now unprecedented due to concerns related to COVID-19, and other airborne contaminants and diseases. To help meet the exponentially increased demand, HVAC engineers and contractors, and facility managers and building owners are turning to DOAS to meet the most stringent ventilation and energy efficiency conditions while providing more comfortable environments and better air quality.
IAQ Starts With Ventilation
Most ventilation requirements originate with carbon dioxide (CO2) management for occupied spaces. When humans breathe in oxygen, they exhale CO2. And when 20, 30, or 50 people occupy a classroom or workspace for several hours, the CO2 levels invariably rise, sometimes dramatically. Concentrations of CO2 in the outside air average around 400 parts per million (ppm), and in heavily occupied, poorly ventilated spaces these can elevate to as high as 2,000 to 5,000 ppm. Proper ventilation in these spaces keeps CO2 levels low and IAQ high.
Ventilation standards like ASHRAE 62.1 have helped designers incorporate outdoor air loads to match building use and occupant types—to promote good CO2 management and provide sufficient air changes per hour, which is vital for reducing the spread of pathogens and removing impurities in the air. Volatile organic compounds (VOCs), for example, enter occupied spaces through off-gassing processes from furniture and building materials, or activities in a facility such as cooking. ASHRAE 62.1-2019 Addendum aa provides a list of appropriate targets to consider to manage VOC contamination.
The first line of defense for controlling CO2, pathogens, and pollutants is proper ventilation, and DOAS technology is a powerful tool to that end.
Decoupling With DOAS
There are several benefits that push designers to use DOAS over central station ventilation; this includes ventilation quality, decoupled energy efficiency, and temperature and humidity control. Central station air handlers and rooftop units typically process outdoor and recirculated return air by mixing, conditioning, and supplying air to the space all in one piece of equipment, with outdoor air as a fractional percent of supply air. For many years, this has been the standard method for outdoor air distribution in variable air volume systems.
The fundamental problem with these systems is the inability to measure or control where the outdoor air ends up. Did it get evenly distributed to all occupied spaces? Are the clean air changes in each space equal? Decoupling the ventilation air from the central recirculated air with a DOAS unit guarantees that filtered, fresh air is always available and equally distributed throughout a building.
Additionally, by decoupling the fresh air to a DOAS unit, the ventilation air can be delivered only to areas of a facility that are being used, which saves energy when serving low- or mixed-occupancy spaces. This level of control cannot be achieved with centrally distributed ventilation systems.
Furthermore, ventilation air is the most expensive air that HVAC systems condition. From hot and humid to frigid climates, it takes a lot of energy to condition and supply ventilation air to spaces, and to do it without disrupting occupant comfort. Extreme temperatures and latent loads can overwhelm traditional central station or small terminal conditioning equipment, resulting in spaces with temperature control issues, and excessive humidity and moisture.
A DOAS is designed to handle the extremes of processing ventilation air and to do it efficiently. By decoupling the ventilation air from the recirculated system, facility operators can manage space humidity and temperature independently by using the DOAS as the primary dehumidification tool for the space and the recirculating equipment for temperature control. This promotes thermal comfort and ensures that fresh air is delivered to occupied spaces.
Packaged For Efficiency
DOAS delivers the above benefits by leveraging the latest refrigeration, control, and energy-recovery technology. Most DOAS units are packaged direct expansion (DX) systems that incorporate efficient variable-speed or multi-stage compressors for the precision temperature and humidity control required for operation. A wide range of compressor capacity modulation is necessary to provide the optimal comfort while expending a minimum amount of energy. Plus, by coupling variable-speed DX technology with energy recovery solutions, such as enthalpy wheels, plate heat exchangers, and hot gas and liquid sub-cool reheat systems, high efficiency is possible even in demanding circumstances.
As ventilation and air quality continue to be a primary focus for facility managers and building owners, a need that is only expected to increase in the coming years, DOAS systems is becoming a more prevalent choice to promote healthy buildings in an energy efficient package.
Dodds is the product manager for rooftop and self-contained systems at Daikin Applied. Responsible for product development and management, he has specialized expertise in variable speed rooftop equipment. Dodds holds a Bachelor’s degree in mechanical engineering from the University of Minnesota and an MBA from the University of St. Thomas in St. Paul, MN.
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