The heating, ventilation and air conditioning (HVAC) industry is constantly improving, and there is a relatively new system has been proven to work wonders.
The Dedicated Outdoor Air System, or DOAS, uses separate equipment to condition all outdoor air (OA) brought into a building for ventilation purposes. This is in contrast to the most commonly used technology to condition air temperature in a building, the Variable Air Volume (VAV) system, which provides all of the HVAC functions through a single unit.
In this post, we will introduce you to a typical DOAS system, have a closer look at a DOAS unit, and suggest how the whole system could be improved with the addition of radiant cooling systems or chilled beams.
The Dedicated Outdoor Air System
Although DOAS does not rely on any new tech, it uses conventional equipment in a system that is configured differently from a traditional all air VAV system. It handles OA separately from return air so it requires two separate sets of equipment. By splitting the equipment function in this way, DOAS effectively separates the sensible and latent loads, so they can be handled more efficiently. This efficiency leads to reduced energy consumption and additional cost savings.
Very briefly explained, sensible load is heating or cooling that causes a change in temperature, while latent load is heating or cooling that causes a change in state (solid, liquid or gas) without a change in temperature. In buildings, sensible heating or cooling translates into the temperature that one can feel. Latent heating or cooling refers to changing the relative humidity in the air without changing the temperature in the building, ultimately to improve indoor air quality.
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Returning to our DOAS system, rather than handle ventilation and AC with a single unit, the DOAS OA unit controls humidity by removing the latent load from ventilation air, while the DOAS return air unit removes the sensible load to control temperature. It can also remove any latent load generated within the building by simply providing ventilation air that is drier than the desired humidity level.
A DOAS Outdoor Air Unit
Now that you understand how a DOAS system is assembled, let’s have a closer look at the separate unit that handles the ventilation brought into a building. ASHRAE provides a detailed deconstructed view of such a unit which you can see.
The figure shows three different configurations. Typically, a unit consists of a filter at the beginning of the duct, followed by a series of cooling and heating coils and ends with a humidifier. Some of these components are not always necessary, it is possible to remove the humidifier as seen in configuration 3.
In addition to the coils, energy recovery is highly recommended and is typically handled by latent and sensible energy recovery (enthalpy) wheels to transfer heat and moisture between relief and supply air (configuration 2). In cold climates, OA is preheated to prevent frost from forming on the energy wheel. With two energy recovery wheels (configuration 3), the first wheel brings OA closer to the temperature and humidity of the relief air and as such lower the load on the cooling coil. The second wheel is then used to make a final temperature adjustment to prevent overcooling of the ventilation air, eliminating the need for a humidifier.
Adding Chilled Beams & Radiant Cooling Solutions
Active chilled beams and radiant cooling solutions, such as underfloor cooling, both use water as opposed to air to alter the temperature of room air. Water has a volumetric heat capacity 3500 times greater than air and as such is much more efficient at heating and cooling. For example, a one-inch water pipe can supply as much heating and cooling capacity as a 18×18” air duct. It also takes far less energy to move water through a pipe than it does to move air through a duct.
These systems should only be used for sensible heating and cooling, as they cannot remove moisture from room air. Humidity reduction is a latent process, as seen above, that involves condensation. With a DOAS system in place, latent loads are handled by the supply air that is now cool and dry thanks to the cooling coils within the DOAS unit. This means that the air volume supplied is often much less than in a VAV system because it only has to meet the ventilation and latent heat removal requirements.
Overall, chilled beams and radiant systems typically have equal or lower installation costs compared to VAV systems, and may reduce mechanical operating costs by 50%. And as an added benefit, such installations increase scores substantially on the way to obtaining LEED or BREEAM certifications.