Conditioned air delivered by a central air-handling unit must be distributed among laboratory spaces, offices, corridors, etc. Room air must be exhausted from the building or returned to the central unit, mixed with outside air, filtered, cooled or heated, and again delivered to the conditioned spaces. [AABC, 1965]
Air delivered to laboratory spaces serves the following major purposes:
Air moves through a duct system that comprises a number of single ducts connected in the shape of a tree. The air moves because of fan energy, which is lost as the air moves through the system for three following basic reasons:
Three major disciplines are needed to understand air flow in ductwork: aerodynamics, thermodynamics, and acoustics. A fourth discipline, economics, is also relevant.
Aerodynamics studies air/gases in motion. Aerodynamic laws describe the forces acting during contact between air and surfaces. Three fundamental laws govern air movement: conservation of mass, conservation of energy, and conservation of momentum. The major aerodynamic influences in a duct system are pressure, friction, and dynamic losses. The main difficulty with applying aerodynamic laws to duct design is identifying the true resistance of the air movement through a duct system.
Thermodynamics is concerned with the science of heat transfer. The major thermodynamic parameters in a duct system are temperature, air density, heat gain from a fan, and heat loss/gain through duct walls and from equipment. The two main thermodynamic parameters associated with a duct system are air-flow temperature and density at each section (part) of a duct and the amount of heat that is lost/gained through duct walls, which determines the thickness of insulation needed.
Acoustics is the science of sound. The major sound considerations in a duct system are noise generated by fans, fittings, and duct-mounted equipment. The major tasks are to determine if the noise generated needs to be abated, and if so, the necessary size, location of sound attenuators.
Economics allows designers to optimize the operating and capital costs of a system that will deliver the design air flow.
Air distribution ducts can be categorized by purposes (supply/exhaust/return), shapes (round/rectangular/flat oval), materials, sizes, internal pressures, and/or special requirements (e.g. transportation of chemically corrosive substances, dust, radioactive materials, or smoke from a fire). Duct construction is classified in terms of application and static pressure [ASHRAE, 1996]. Ducts are divided into three applications: residential, commercial, and industrial. Laboratories usually belong in the industrial category. Many laboratories have specific requirements to minimize dust collected inside the ductwork and other ventilation equipment that may require the installation of round ducts. Specific requirements for preventing the release of nuclear material, bacteria, or harmful gases outside a laboratory may mean dampers must be avoided in exhaust systems, and negative space pressures must be maintained under DBA conditions.
The layout of laboratories and other rooms in a building affects the available space for the duct layout, which in turn affects the performance and cost of a duct system.
A large number of different duct sizing methods use arbitrary initial parameters based on "engineering experience." These include initial velocity or pressure loss per unit of length. Two of the most user-friendly duct sizing methods presented in the ASHRAE 1997 Handbook [ASHRAE, 1997] are the Equal Friction and the Static Regain methods. For duct optimization the ASHRAE Handbook recommends the T-Method, which allows the user to select duct sizes and fan pressure to minimize life-cycle cost. There are two applications for the T-Method, optimization and simulation. However, no simulation method by itself produces an optimized air distribution system and no computer optimization program currently exists yet. Simulation methods can only model ductwork systems. A comprehensive explanation of the T-Method can be obtained from a number of publications, including Tsal, 1988.
Following is the list of American associations and companies that have sponsored the most important investigations in the air distribution field:
Heating, ventilation, and air conditioning (HVAC) engineers must know which information sources, standards, and/or code compliance obligations affect their design. The bases for air distribution can be obtained from two ASHRAE publications, The Fundamentals Handbook, Chapter 32, "Duct Design" [ASHRAE, 1997] and HVAC Systems and Equipment, Handbook, Chapter 16, "Duct Construction" [ASHRAE,1996] as well as HVAC Systems - Duct Design [SMACNA, 1990]. Smoke and fire control is covered in the HVAC Applications Handbook, Chapter 48, "Smoke Management" [ASHRAE, 1995].
