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.
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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. [1] Gatley also describes the application of DOAS for delivering dehumidified air to buildings to improve the indoor air quality and thermal comfort. [2] [3] [4] 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. [5] 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. [7] 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: [8]
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. [9]
There are two main ways to design a DOAS when using an air-based system as the parallel system: [10]
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:
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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.[ further explanation needed ] The effectiveness of the total energy wheel is an important factor for improving the efficiency of DOAS.[ further explanation needed ]
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. [13] 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 m2 ) office space in an educational building in Pennsylvania. [9] 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. [14] The building model was developed to be consistent with typical new construction and meet the ASHRAE Standard 90.1 (ASHRAE 90.1) requirements. [15] The simulation results indicated that the full DOAS resulted in the annual HVAC energy cost savings ranging from 21% to 38%. [14]
Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.
Ventilation is the intentional introduction of outdoor air into a space. Ventilation is mainly used to control indoor air quality by diluting and displacing indoor pollutants; it can also be used to control indoor temperature, humidity, and air motion to benefit thermal comfort, satisfaction with other aspects of the indoor environment, or other objectives.
Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is used to reduce the heating and cooling demands of buildings.
An air handler, or air handling unit, is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, furnace or A/C elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU, sometimes exhausting air to the atmosphere and bringing in fresh air. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork
Displacement ventilation (DV) is a room air distribution strategy where conditioned outdoor air is supplied at a low velocity from air supply diffusers located near floor level and extracted above the occupied zone, usually at ceiling height.
Variable air volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system. Unlike constant air volume (CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant or varying temperature. The advantages of VAV systems over constant-volume systems include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.
Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.
Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. Ducts commonly also deliver ventilation air as part of the supply air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.
Constant air volume (CAV) is a type of heating, ventilating, and air-conditioning (HVAC) system. In a simple CAV system, the supply air flow rate is constant, but the supply air temperature is varied to meet the thermal loads of a space.
Room air distribution is characterizing how air is introduced to, flows through, and is removed from spaces. HVAC airflow in spaces generally can be classified by two different types: mixing and displacement.
A chilled beam is a type of radiation/convection HVAC system designed to heat and cool large buildings through the use of water. This method removes most of the zone sensible local heat gains and allows the flow rate of pre-conditioned air from the air handling unit to be reduced, lowering by 60% to 80% the ducted design airflow rate and the equipment capacity requirements.
A fan coil unit (FCU), also known as a Vertical Fan Coil Unit (VFCU), is a device consisting of a heat exchanger (coil) and a fan. FCUs are commonly used in HVAC systems of residential, commercial, and industrial buildings that use ducted split air conditioning or central plant cooling. FCUs are typically connected to ductwork and a thermostat to regulate the temperature of one or more spaces and to assist the main air handling unit for each space if used with chillers. The thermostat controls the fan speed and/or the flow of water or refrigerant to the heat exchanger using a control valve.
Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced each hour. Perfectly mixed air refers to a theoretical condition where supply air is instantly and uniformly mixed with the air already present in a space, so that conditions such as age of air and concentration of pollutants are spatially uniform.
HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.
Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system. UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air to supply outlets, located at or near floor level within the occupied space. Air returns from the room at ceiling level or the maximum allowable height above the occupied zone.
ANSI/ASHRAE/IES Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings is an American National Standards Institute (ANSI) standard published by ASHRAE and jointly sponsored by the Illuminating Engineering Society (IES) that provides minimum requirements for energy efficient designs for buildings except for low-rise residential buildings. The original standard, ASHRAE 90, was published in 1975. There have been multiple editions to it since. In 1999 the ASHRAE Board of Directors voted to place the standard on continuous maintenance, based on rapid changes in energy technology and energy prices. This allows it to be updated multiple times in a year. The standard was renamed ASHRAE 90.1 in 2001. It has since been updated in 2004, 2007, 2010, 2013, 2016, and 2019 to reflect newer and more efficient technologies.
Airflow, or air flow, is the movement of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperature, and composition.
Radiant heating and cooling is a category of HVAC technologies that exchange heat by both convection and radiation with the environments they are designed to heat or cool. There are many subcategories of radiant heating and cooling, including: "radiant ceiling panels", "embedded surface systems", "thermally active building systems", and infrared heaters. According to some definitions, a technology is only included in this category if radiation comprises more than 50% of its heat exchange with the environment; therefore technologies such as radiators and chilled beams are usually not considered radiant heating or cooling. Within this category, it is practical to distinguish between high temperature radiant heating, and radiant heating or cooling with more moderate source temperatures. This article mainly addresses radiant heating and cooling with moderate source temperatures, used to heat or cool indoor environments. Moderate temperature radiant heating and cooling is usually composed of relatively large surfaces that are internally heated or cooled using hydronic or electrical sources. For high temperature indoor or outdoor radiant heating, see: Infrared heater. For snow melt applications see: Snowmelt system.
Demand controlled ventilation (DCV) is a feedback control method to maintain indoor air quality that automatically adjusts the ventilation rate provided to a space in response to changes in conditions such as occupant number or indoor pollutant concentration. The most common indoor pollutants monitored in DCV systems are carbon dioxide and humidity. This control strategy is mainly intended to reduce the energy used by heating, ventilation, and air conditioning (HVAC) systems compared to those of buildings that use open-loop controls with constant ventilation rates.
Cooling load is the rate at which sensible and latent heat must be removed from the space to maintain a constant space dry-bulb air temperature and humidity. Sensible heat into the space causes its air temperature to rise while latent heat is associated with the rise of the moisture content in the space. The building design, internal equipment, occupants, and outdoor weather conditions may affect the cooling load in a building using different heat transfer mechanisms. The SI units are watts.
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