Hello there, I'm an expert in the field of HVAC systems, particularly when it comes to ductwork. Duct size is a critical aspect of any heating, ventilation, and air conditioning (HVAC) system. It refers to the diameter or dimensions of the ducts that are used to distribute conditioned air throughout a building. Proper duct sizing is essential for ensuring that the HVAC system operates efficiently and effectively, providing the right amount of air to each space while minimizing energy consumption and noise.

Duct sizing is determined by a number of factors, including the air flow rate, the static pressure, and the friction loss within the duct system. The air flow rate is measured in cubic feet per minute (cfm), and it's a measure of how much air is being moved through the system. The static pressure is the force that the air exerts on the duct walls, and it's important for overcoming the resistance in the ductwork. Friction loss is the energy lost due to the air moving against the duct surfaces, and it's a crucial factor in determining the appropriate duct size.

For rectangular and circular ducts, there's a concept known as the equivalent diameter, which is used when comparing the flow capacity of different shaped ducts. The equivalent diameter is calculated in such a way that it represents the diameter of a circular duct that would have the same flow capacity as the rectangular duct under the same conditions.

When it comes to air flows between 100 to 50,000 cfm, the sizing of the ducts is often adapted to the equal friction loss method of sizing ventilation duct systems. This method takes into account the friction loss that occurs within the ducts, which is an approximate value of 0.8 inches of water gauge per 100 feet of duct, or roughly 6.6 Pascals per meter. This value is used to help determine the size of the ducts needed to minimize energy loss and ensure that the system operates efficiently.

It's important to note that duct sizing is not a one-size-fits-all approach. It requires careful calculation and consideration of the specific requirements of the building and the HVAC system. Engineers and HVAC professionals use various tools and formulas to calculate the appropriate duct size, taking into account the layout of the building, the number of air outlets, and the desired air flow rates.

In summary, duct size is a critical component of an HVAC system that must be carefully calculated to ensure optimal performance. It involves understanding the air flow rates, static pressures, and friction losses, and using these to determine the appropriate size and shape of the ducts. The goal is always to balance efficiency, effectiveness, and energy conservation.

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Equivalent diameter for rectangular and circular ducts - air flows between 100 - 50000 cfm. ... The rectangular dimensions and the air flow volume are adapted to the equal friction loss method of sizing ventilation duct systems. An approximate friction loss of 0.8 inches water gauge per 100 ft duct (6.6 Pa/m) is used.read more >>