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Best Practice Guide: Low Pressure-Drop HVAC Design for Laboratories
A fan’s brake horsepower is directly related to the pressure drop and airflow of the system it serves. Laboratories typically have HVAC systems designed at much higher airflows than other building types due to their ventilation requirements. As a result, reductions in pressure drop in these system produces significant brake horsepower savings, and, thus, energy savings. This webinar focuses on optimizing fan energy use by lowering the pressure drop in laboratory ventilation systems. Using an example laboratory, this webinar establishes the baseline pressure drop and explores the application of the fundamentals from the selection of equipment to the layout of the system.
Exhaust Stack Optimization
In traditional laboratory exhaust systems, multiple constant volume exhaust fans feed off of a central manifold and include redundancy; bypass dampers help maintain the desired static pressure within the exhaust duct. However, maintenance of these bypass dampers is often neglected, resulting in little to no duct static control, and exhaust systems can therefore account for up to 30 percent of the lab’s energy consumption.
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To tailor the design of the exhaust system to safely operate under a combination of variable air volume operation and staging, dispersion modeling can be used in its traditional sense to define the optimum design of the stacks and determine the appropriate number and size of fans to achieve the most energy-efficient sequence of operation for the exhaust system.
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Ultimately, the exhaust system should be designed such that the laboratory exhaust fans are used to control the duct static pressure rather than relying on a bypass damper. A variable frequency drive on an exhaust fan is much more responsive and reliable than a bypass damper. Also, maximizing the number of operating fans, including the redundant fan, can save energy, reduce carbon emissions, while also reducing wear and tear on the fans.
This presentation will discuss how dispersion modeling can be used to optimize the whole laboratory exhaust system, not just the stack height and diameter, through the proper placement of exhaust stacks and air intakes, defining the optimum number and size of fans to meet the building needs, and defining the most energy-efficient and safe sequence of operation. The result is a system that is not only less expensive to operate, but it is more sustainable, provides greater long-term resiliency, and could lower installation costs.
Exhaust Stack Optimization
In traditional laboratory exhaust systems, multiple constant volume exhaust fans feed off of a central manifold and include redundancy; bypass dampers help maintain the desired static pressure within the exhaust duct. However, maintenance of these bypass dampers is often neglected, resulting in little to no duct static control, and exhaust systems can therefore account for up to 30 percent of the lab’s energy consumption.
​
To tailor the design of the exhaust system to safely operate under a combination of variable air volume operation and staging, dispersion modeling can be used in its traditional sense to define the optimum design of the stacks and determine the appropriate number and size of fans to achieve the most energy-efficient sequence of operation for the exhaust system.
​
Ultimately, the exhaust system should be designed such that the laboratory exhaust fans are used to control the duct static pressure rather than relying on a bypass damper. A variable frequency drive on an exhaust fan is much more responsive and reliable than a bypass damper. Also, maximizing the number of operating fans, including the redundant fan, can save energy, reduce carbon emissions, while also reducing wear and tear on the fans.
This presentation will discuss how dispersion modeling can be used to optimize the whole laboratory exhaust system, not just the stack height and diameter, through the proper placement of exhaust stacks and air intakes, defining the optimum number and size of fans to meet the building needs, and defining the most energy-efficient and safe sequence of operation. The result is a system that is not only less expensive to operate, but it is more sustainable, provides greater long-term resiliency, and could lower installation costs.
The Myth of the Well-Mixed Space
People working in laboratories and critical workspaces are potentially exposed to airborne hazards. They rely on proper design and operation of the lab ventilation systems to keep them safe. Air changes per hour (ACH) is commonly specified to indicate the quantity of air required for safety, but there is little guidance on how to select an appropriate rate and then determine whether the air change rate provides effective dilution and removal of airborne contaminants.
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A risk-based, systems approach can be applied to evaluate hazardous activities, associate with a level of risk, and recommend appropriate air change rates. However, specification and application of air change rates assumes that the space of concern is well-mixed, meaning contaminants are diluted through disbursement and homogeneous distribution of concentrations throughout the volume of the room. Results of air tracer tests to evaluate ventilation effectiveness demonstrate that many spaces are not well-mixed and specification of air change rates without measurement of ventilation effectiveness can lead to a false sense of safety.
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This paper will describe why airflow patterns through a space are more important to air quality and occupant safety than the number of air changes per hour.