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Engineering Guide Displacement Ventilation Please refer to the Price Engineer’s

Engineering Guide Displacement Ventilation Please refer to the Price Engineer’s HVAC Handbook for more information on Displacement Ventilation. S E C T I O N J J-2 For more information on additional imperial and metric sizes please visit © Copyright Price Industries Limited 2016. priceindustries.com or contact your local Price representative. ENGINEERING GUIDE - DISPLACEMENT VENTILATION Displacement ventilation is an air distribution technology that introduces cool air into a zone at low velocity, usually also at a low level. Buoyancy forces ensure that this supply air pools near the floor level, allowing it to be carried up into the thermal plumes that are formed by heat sources. This type of air distribution is effective at delivering fresh air to occupants and removing many of the contaminants associated with heat sources, while creating a comfortable environment. This chapter focuses on the main design criteria for displacement ventilation systems as well as introduces its common applications. The following pages will go further into depth on the specific requirements of schools, theaters, health care and industrial spaces. Overview Air flow in ventilated spaces generally can be classified by two different types: mixing (or dilution) ventilation and displacement ventilation. Mixing ventilation systems (Figure 1) generally supply air in a manner such that the entire room volume is fully mixed. The cool supply air exits the outlet at a high velocity, inducing room air to provide mixing and temperature equalization. Since the entire room is fully mixed, temperature variations throughout the space are small, while the contaminant concentration is uniform throughout the zone. Displacement ventilation systems (Figure 2) introduce air into the space at low velocities, which causes minimal induction and mixing. Displacement outlets may be located almost anywhere within the room, but have been traditionally located at or near floor level. The system utilizes buoyancy forces in a room, generated by heat sources such as people, lighting, computers, electrical equipment, etc., to remove contaminants and heat from the occupied zone. By so doing, the air quality in the occupied zone is generally superior to that achieved with mixing ventilation. Benefits Flexibility - As load distribution changes within the space, a displacement system will be able to compensate. For example, if the space was designed to have a fairly even load distribution and now has the loads concentrated to one side, the system is able to compensate as the buoyant forces drive the supply system and will draw the supply air towards the loads. IAQ - Because fresh supply air is pooling at the floor level, personal thermal plumes draw fresh air up the body. All of the warm and polluted air is extracted at the high return. When properly designed, there should always be a greater amount of fresh air in the breathing zone when compared to a conventional dilution system, leading to higher ventilation efficiency. Green building rating systems, such as the LEED® program and Green Globes® have credits that are applicable to displacement ventilation systems. See the Green Tips for further information. Energy Savings - Displacement systems present many potential opportunities for energy savings. The lower pressure drop associated with displacement ventilation outlets and the corresponding selection of smaller fan components may allow for a reduction in fan energy. The supply air temperature is typically higher for displacement systems than for overhead mixing systems, and can lead to free cooling from increased economizer hours. Combined with a higher return temperature than overhead systems, the warmer supply temperature of DV systems can cause an increase in chiller efficiency. Due to a high ventilation effectiveness, the amount of outdoor air that must be conditioned can also be decreased when compared with a mixing system. This is especially significant in humid climates, where dehumidification of outdoor air is a significant cost. Displacement Ventilation Engineering Guide Introduction Figure 1: Mixing (Dilution) Ventilation Figure 2: Displacement ventilation Diffuser Induced Air/ Thermal Plume Limitations The size of displacement outlets can make selecting and locating diffusers difficult in areas where there is limited wall area. Ceiling and floor mounted diffusers may help alleviate this issue, where appropriate. DV systems are limited in their maximum cooling capacity, primarily due to stratification limits set by ASHRAE (2004a) and ISO (2005). The Price Engineer’s Handbook contains more information on how stratification affects the maximum cooling capacity of DV systems. Chen, Glicksman, Yuan, Hu, & Yang (1999) indicate a maximum cooling capacity of 38 Btu/hft2 [119 W/m2] while ensuring thermal comfort. Typical Applications Displacement ventilation is an effective method of obtaining good air quality and thermal comfort in the occupied space. Spaces where displacement ventilation has been successfully applied include the following. • Schools • Restaurants • Theaters • Industrial Spaces • Hospitals • Supermarkets • Casinos • Open Offices J-3 J-3 © Copyright Price Industries Limited 2016. For more information on additional imperial and metric sizes please visit priceindustries.com or contact your local Price representative. J-3 ENGINEERING GUIDE - DISPLACEMENT VENTILATION Displacement ventilation is usually a good choice if: • The contaminants are warmer and/or lighter than the room air • Supply air is cooler than the room air • The room height is 9 ft [2.75 m] or more • Low noise levels are desired Overhead air distribution may be a better choice if: • Ceiling heights are below 8 ft [2.4 m] • Disturbances to room air flow are strong • Contaminants are colder and/or denser than the ambient air • Cooling loads are high and radiant cooling is not an option Thermal Plume A thermal plume is a convection current caused by buoyancy forces that causes local air to warm and rise above the heat source, entraining surrounding air and increasing in size and volume as it loses momentum, as depicted in Figure 3. The maximum height to which a plume will rise is dependent on the strength of the heat source, as the initial momentum of the plume will increase. Also, a room with more stratification will reduce the relative density of the plume and, as a result, limit the height to which the plume will rise. The thermal plume generated from a point source acts differently than a thermal plume generated from large objects in the space. For example, a heated cylinder produces a boundary layer and the convective thermal plume takes a different shape than a point heat source. A point source type expansion of the thermal plume is still present, but at an altered height and with the thermal plume boundary layer included, shown in Figure 4. The cylinder is a better approximation of an occupant in the space than a point source. Room Air Flow Pattern Air flow patterns in a displacement ventilation system are quite different than in a mixing system. Because of the low discharge velocity of displacement outlets, the room air motion is largely driven by the convection flows created by heat sources such as people, equipment, and warm windows; or by heat sinks such as cold walls or windows. The convection flows within the room cause the formation of horizontal air layers. The warmest air layers are near the ceiling and the coolest air layers are near the floor. Concepts and Benefits Displacement Ventilation Engineering Guide Figure 3 & 4: Thermal plume from a point heat source and of a heated cylinder Typical Design Parameters IP SI Supply Temperature 63-68 °F 17-20 °C Return Temperature 78-85 °F 26-29 °C Supply Face Velocity – Mainly Sedentary Occupancy 40 fpm 0.2 m/s Ventilation Effectiveness (ASHRAE, 2004b) 1.2 1.2 Table 1: Typical DV parameters Figure 5: Air layers Draft Temperature (˚F) 30 25 20 15 5 10 0 Room air moves horizontally across the floor due to momentum from the supply outlet and suction from thermal plumes. It then passes vertically through the thermal plumes to a high level in the room where it is returned or exhausted. Vertical air movement (see Figure 5, next page) between layers is caused by stronger convection forces associated with heat sources or cold sinks. Heat sources such as people, computers, lights, etc. create a rising convection flow known as a thermal plume. The strength of the thermal plume is dependent on the power and geometry of the heat source. The strength of the thermal plume will determine how high the convection flows can rise before the momental is fully dissipated. Cold sinks such as exterior walls or windows can generate convection flows down the wall and across the floor. J-4 For more information on additional imperial and metric sizes please visit © Copyright Price Industries Limited 2016. priceindustries.com or contact your local Price representative. ENGINEERING GUIDE - DISPLACEMENT VENTILATION Displacement Ventilation Engineering Guide Displacement Ventilation Characteristics Figure 6: Obstruction Figure 8: Cooling air flow pattern Figure 7: Irregular room geometry Couch Partition Supply Air Diffuser Supply Air Diffuser Air Flow Penetration A displacement system supplying cool air through a diffuser will deliver air along the floor in a thin layer typically less than 8 in. [0.20 m] in height. The supply air spreads across the floor uploads/Industriel/ displacement-ventilation-engineering-guide.pdf