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HVAC Feature of the Month: Duct Sizing Options

Friday, April 1st, 2011

Design Master HVAC can size ducts using three different methods: constant pressure drop, constant velocity, and static regain. All of these sizing methods are fairly straightforward. The also all include a minor adjustment you can make to the sizing.

Constant Pressure Drop: For this sizing method, ducts are sized so that the pressure drop rate is the same in all ducts.

As the volume of air increases, the duct size also increases to keep the pressure drop the same. However, the duct size does not increase as quickly as the air volume does. As a result, the more air you have in a duct sized for a specific pressure drop, the higher the velocity is in that duct.

Sizing for a 0.08 pressure drop, here are the sizes and velocities DM HVAC comes up with for various airflow volumes:

1,000 CFM, 16”, 716 FPM
10,000 CFM, 36”, 1415 FPM
100,000 CFM, 82”, 2727 FPM

To keep air velocities in check, there is also a “Maximum Velocity” setting when using this sizing method. When the default value of 0 is used, the velocity is not checked. If a value is specified, the duct will first be sized to the maximum pressure drop value. Then, the velocity in the duct is checked. If it exceeds the maximum velocity setting, the duct is resized so that the velocity is less that the maximum value.

Constant Velocity: For this sizing method, ducts are sized so that the velocity is the same in all ducts.

You can also specify a minimum velocity for the ductwork. When you do this, the duct is first sized to use the same size as the duct previous to it. The velocity in the duct is checked. If it is between the maximum and minimum velocities, it is not changed. Only when the velocity falls below the minimum is a new size selected. Using this setting reduces the number of transitions in the ductwork.

Static Regain: Static regain is the least common form of duct sizing. It is typically used to size the medium and high pressure ductwork in a VAV system. This sizing method was actually our first HVAC feature of the month.

One setting that we added in the latest release for static regain is the minimum velocity value. Static regain works by slowly reducing the velocity in the ductwork as it moves from the fan to the diffusers. The goal is to keep the static pressure constant and have all of the pressure drop come from the velocity pressure. However, if the ductwork runs are long or the starting velocity is low, eventually the velocity reaches 0 and there is no more velocity pressure left. In these cases, the ducts get really big.

Setting a minimum velocity prevents this situation. When the minimum velocity is reached, the ducts are not increased in size beyond that. The result at the ends of the runs is ductwork sized using the constant velocity method, rather than static regain.