Dedicated outdoor air system

A dedicated outdoor air system (DOAS) is a type of heating, ventilation and air-conditioning (HVAC) system that consists of two parallel systems: a dedicated system for delivering outdoor air ventilation that handles both the latent and sensible loads of conditioning the ventilation air, and a parallel system to handle the (mostly sensible heat) loads generated by indoor/process sources and those that pass through the building enclosure.
Traditional HVAC systems, such as variable air volume (VAV) systems serving multiple zones, have potential problems in terms of poor thermal comfort and possible microbial contamination. Depending on the environment and the parallel system involved, in a DOAS setup the outdoor air system will handle some of the sensible load in addition to the latent load, and the parallel system will handle the remaining sensible load. The main point of a DOAS system is to provide dedicated ventilation rather than ventilation as an incidental part of the process of conditioning interior air. DOAS is a term given to a system that has been used extensively in Europe and in various forms in the US.
William Coad proposed in 1999 to handle the OA (outdoor air) and return air separately in building HVAC systems. Gatley also describes the application of DOAS for delivering dehumidified air to buildings to improve the indoor air quality and thermal comfort. More recent research efforts have been conducted to study the basics of DOAS with emphasis on the potential advantages compared to the conventional HVAC systems. S.A. Mumma suggests that there are four main problems with conventional all air overhead mixing VAV HVAC systems. These issues of VAV systems highlight the corresponding advantages of DOAS systems. However, some disadvantages of DOAS include: potentially higher first costs, lack of use in the United States, and potentially higher complexity.
For a typical DOAS ventilation system, the outside air system can accommodate around 0-30% of the space sensible load. In order to create a comfortable indoor environment, the balance of the space sensible loads must be accommodated by many other optional equipment choices as follows:
Compared to other sensible cooling systems, radiant ceiling cooling panels are the best parallel system choice for use with the DOAS. Because the DOAS only accommodates the space ventilation and latent loads, it provides an opportunity to reduce the required floor-to-floor height by reducing the size of the duct system and the required fan power. There are numerous advantages of a radiant ceiling cooling system coupled with a DOAS. The general evaluation section in 2008 ASHRAE Handbook gives a brief description as follows:
The main advantages are:
The main disadvantage is related to higher initial costs.
Besides the advantages presented above, parallel radiant cooling panels offer other advantages as well, such as compact design, vertical shaft space area savings, and quick accommodation of dynamic controls. Energy savings in DOAS/radiant ceiling cooling panel system can by linked to: cooling coil load reduction, chiller energy reduction, pumping energy consumption and fan energy consumption reduction. In general, due to the total energy recovery and small supply air quantity of DOAS, the chiller energy consumption can be reduced significantly compared to the conventional VAV system. In a study of a pilot DOAS/radiant ceiling cooling panel system, hourly energy simulation predicts that the annual electrical energy consumption of the pilot DOAS/radiant panel cooling system is 42% less than that of the conventional VAV system with economizer control.
There are two main ways to design a DOAS when using an air-based system as the parallel system:
In this setup, there is an outdoor air system that dumps preconditioned air (accounting for latent load and partial sensible load) directly into the space in its own duct/diffuser. There is a separate system (e.g. fan coil unit) that takes air from the space and conditions it to meet the remaining space sensible load.
Advantages:
Disadvantages:
Conditioned outdoor air is ducted to the terminal unit in the space. In this setup, the preconditioned outdoor air is ducted into the fan coil units directly, mixing with the return air from the space. This system is similar to a chilled beam setup.
Advantages:
Disadvantages:
With the increasing application of DOAS in many countries, there is also increasing demand for DOAS equipment, such as a total energy wheel that uses total energy recovery, a passive dehumidifier wheel, and other relevant equipment. The effectiveness of the total energy wheel is an important factor for improving the efficiency of DOAS.
The requirements in the design of a DOAS include:
Mumma proposed the following steps for designing the DOAS:
For DOAS with air-based system as parallel cooling system, the following steps were proposed: 1) calculating the sensible cooling load met by the DOAS supply air for each space; 2) calculating the sensible cooling load remaining on the parallel system for each space; 3) determining the supply air dry bulb temperature for parallel systems (above the space dew point temperature to avoid condensation); 4) determining the supply air flow rate for each parallel sensible cooling device.
Many studies have been conducted to demonstrate the energy and cost performance of DOAS in terms of simulations. Khattar and Brandemuehl simulated the parallel system and a conventional single system for a large retail store in Dallas, St. Louis, Washington DC, and New Orleans. The study demonstrated annual energy savings of 14% to 27% and 15% to 23% smaller equipment capacity for the parallel cooling system. Jeong et al. compared the energy and cost performance of a DOAS with parallel ceiling radiant panels to a conventional VAV system with air-side economizer for a nearly 3,000 square feet (280 m) office space in an educational building in Pennsylvania. A 42% reduction of the annual energy usage for the DOAS system with substantial savings in both fan and chiller energy use was reported in this study. Emmerich and McDowell evaluated the potential energy savings of DOAS in U.S. commercial buildings. The building model was developed to be consistent with typical new construction and meet the ASHRAE Standard 90.1 (ASHRAE 90.1) requirements. The simulation results indicated that the full DOAS resulted in the annual HVAC energy cost savings ranging from 21% to 38%.