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Thermal Environment Design Strategies
Published in Chitrarekha Kabre, Synergistic Design of Sustainable Built Environments, 2020
The dedicated outdoor air system (DOAS) decouples air conditioning of the outdoor air from the conditioning of the internal loads. The DOAS introduces 100% outdoor air, heats or cools it, may humidify or dehumidify it, and filters it, then supplies this treated air to each of its assigned spaces. DOAS can accommodate an exhaust or relief airflow for heat recovery between the outdoor and exhaust or relief airflows. Often, the DOAS serves multiple spaces and is designed not necessarily to control space temperature but to provide thermally neutral air to those spaces. A second application, the conventional system, is responsible for offsetting building envelope, and internal loads and DOAS are responsible for the condition or deliver outdoor air. A common example may be a large apartment building with individual fan-coil units (the conventional system) in each dwelling unit, plus a common building-wide DOAS to deliver code-required outdoor air to each housing unit for good indoor air quality and to make up a bathroom and/or kitchen exhaust.
Room Air Distribution and Hybrid Secondary Systems
Published in T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl, Heating and Cooling of Buildings, 2016
T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl
VAV systems suffer from two other possible drawbacks. One is that for a multizone building, outdoor air in excess of the minimum required amount has to be drawn in since the outdoor air and return air-streams are mixed together, and the amount of air supply to individual zones is based on thermal space loads; hence, the relative outdoor air fraction to each of the spaces varies as the zone loads vary over the course of the year.* In order to compensate for such variations, ASHRAE 62.1 (2013) stipulates a higher outdoor air amount that has energy penalties associated with conditioning it. No such excess air is required for DOAS. DOAS, or some earlier variant of it, has been proposed in the early 1980s. The HVAC industry was slow to adopt the system until the last 10 years or so. DOAS are said to be the most reliable system for introducing proper amounts of outdoor air for multiple zones during the full operating range of the system. DOAS units are 100% outdoor air units that directly supply this air to the various zones as required. Thus, unlike VAV systems in multizone spaces, DOAS ensures 100% outdoor air requirements to all spaces at all times, thereby enhancing indoor air quality. This is a compelling advantage for assurance verification in a court of law. Further, microbial growth is inhibited since the ventilation air provides all the latent load.
Typical Pre-Commissioining and Start-Up Procedures
Published in Maija Virta, HVAC Commissioning Guidebook, 2021
Dedicated Outdoor Air Units (DOAS) that mix outdoor air and return air, are being applied in place of a traditional air handling and treated fresh air unit system. In humid climates, dehumidification requires air to be cooled below the mixed air’s dew point even if the occupancy of the building does not require such low temperatures to meet space temperature set points. A better approach for many buildings is consolidating all of the outdoor air treatment into a DOAS unit that supplies 100% outdoor air. Treated air (humidified or dehumidified) from these units can be supplied directly to occupied spaces or can be supplied into the other AHUs dedicated to temperature control. However, some models have an option to return the part of the extracted air to the supply side.
Feasibility study of retrofitting VAV system to VAV-DOAS system for outdoor airflow requirement and distribution in multi-zone VAV system
Published in Science and Technology for the Built Environment, 2021
Tianyi Zhao, Jiaming Wang, Chao Liu, Yu Zhao
Because the control strategies had limitations in solving this issue, a possible solution was to focus on the reconfiguration of the system setup. For decades, the dedicated outdoor air system (DOAS) has received considerable attention and has numerous applications due to its superior performance on outdoor air handling and IAQ improvement (Khan et al. 2015; Jeong, Mumma, and Bahnfleth 2003; Xiao, Ge, and Niu 2011). Therefore, it is reasonable to study the system configuration that combines the VAV system with DOAS. There are two parallel systems integrated in a complete DOAS setup, where the outdoor air system takes charge of partial sensible load and latent load, while the corresponding parallel system takes charge of the remaining sensible load.
Exploring the adequacy of mechanical ventilation for acceptable indoor air quality in office buildings
Published in Science and Technology for the Built Environment, 2022
Tareq Abuimara, Brodie W. Hobson, Burak Gunay, William O’Brien
In accordance with the study findings for both data analytics and the simulation-based investigation, the following recommendations can be put forth: HVAC designers should consider the dynamic nature of occupants and the variable occupant densities in buildings. In other words, attention should be paid to the heterogeneous distribution of people across building zones, which is common, and plan for accommodating variable ventilation requirements. Additionally, space/building management teams should be made aware of the safe capacity of rooms in terms of IAQ.Data from existing sensing infrastructure (e.g., motion detectors and CO2 sensors) should be used to control zone level ventilation, which can improve IAQ as well as save energy.For new building designs, especially public buildings such as commercial, institutional, educational, and governmental buildings, alternative HVAC systems that decouple ventilation from heating and cooling should be considered. For example, a dedicated outdoor air system (DOAS) is an HVAC system that can provide improved IAQ by meeting ventilation requirements regardless of thermal loads while simultaneously saving energy (Kim et al. 2016; Lim and Jeong 2018).The implementation of current ventilation-related codes and standards should be investigated, and more effective ventilation approaches should be adopted. For instance, standards, such as ASHRAE Standard 62.1, should mandate technologies and approaches that ensure the proportional distribution of outdoor air based on the actual distribution of occupants in buildings.
Effect of air supply mode on the airflow distribution inside soft sleeper compartments of air-conditioned trains
Published in Science and Technology for the Built Environment, 2020
Sun Liying, Yanan Zhang, Xinyu Zhang
In terms of acceptable indoor air quality inside the train compartment, W. K. Chow (2002) suggested that this can be achieved by increasing the fresh-air supply rate for dilution or provide a better air-distribution design. In order to improve the air quality and the thermal comfort in the compartments of high-speed trains, an underfloor air supply mode was presented by Xu and Yalin (2017) and compared with the traditional sidewall supply and bottom return mode. The simulation results showed that the underfloor air supply mode significantly improved the air quality and thermal comfort. Numerical simulation of airflow in the compartment of a passenger coach was carried out in summer by Mohammad, Morteza, and Khosro (2017) An experimental measurement was also conducted to verify the performance of the numerical model. Results showed that due to the inappropriate design of the compartment, thermal comfort conditions do not distribute evenly in the compartment, and this makes passengers uncomfortable. Moreover, some modifications were implemented in the compartment design to improve the comfort conditions around the manikin in the two cases. The outcome of the modifications showed that the airflow symmetrically enters the compartment and provides better thermal conditions for passengers seated/sleeping in the compartment. To quantify the influence of heat release on ventilation efficiency and thermal comfort parameters of displacement ventilation in a generic train compartment, a series of human subject studies was conducted by Daniel and Johannes (2017) for various mean room temperatures. The result showed that the heat removal efficiency (HRE) decreases with increasing mean cabin temperature, and variation of the sensible heat has a negligible impact on the flow velocities near the passenger dummies. Yingying et al. (2018) investigated train air conditioning filters, interior ventilation systems, tunnel environments, and platform air quality as factors affecting the concentrations of airborne particles inside trains. The results show that interior ventilation plays a key role in maintaining cleaner in-train air. Hansol and Jae-Weon (2019) proposed for high-speed train cabins a dedicated outdoor air system (DOAS) with thermoelectric radiant panels (TERPs) that prevents air quality problems and reduces operation energy consumption. The DOAS supplies cooled and dehumidified outdoor air for ventilation and enables the removal of latent loads. The results indicated that the proposed system can save 32.6% of the annual operation energy. A multi-objective optimization platform has been developed by Nan Li et al. (2019) using the nondominated sorting-based particle swarm optimization (NSPSO) algorithm for searching the trade-off optimal design of the ventilation system in a fully occupied high-speed train (HST) cabin. Different combinations of ventilation operation parameters were evaluated against performance in terms of thermal comfort, air quality and energy consumption. A multifidelity Kriging technique was also proposed. The result demonstrates that the presented approach is able to perform a multi-objective optimization for an indoor ventilation system design and yields an accurate Pareto-front result with up to 35.61% saving of computational time.