10 CRITICAL STEPS TO CORRECT DUCT SIZING
08/25/2010 13:45
10 CRITICAL STEPS to Correct Duct Sizing
- Size the load with the Manual J approach. For duct sizing purposes, you must calculate the Btuh for each room.
- Size the equipment using Manual S techniques. You must determine the total airflow based on your equipment selection. Use the nominal medium speed cfm for your duct design.
- Calculate the cfm per room based on the cooling sensible Btuh room load.
- Determine the design type (trunk and branch, radial, etc.) and lay out the ductwork in the most effective manner.
- Compute the total effective length of the duct system. Add the longest effective length of the supply and return to determine the longest critical path.
- Determine available static pressure by deducting air side devices in the critical path from the design static pressure.
- Calculate friction rate per 100 ft. by applying the available static pressure times 100 and dividing it by the total effective length.
- Apply friction rate with the calculated room cfm to determine duct size (Use the appropriate duct calculator or friction chart for the chosen duct material).
- Check chosen duct size for appropriate velocity.
- Reduce friction rate to 0.04-in. wc or 0.05-in. wc for return sizing calculation to ensure adequate sizing and permissible velocity.
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HVAC Comfortech This article is based on the presentation, Duct Design 101, which Paul Wiebolt gave at HVAC Comfortech 2006, held in Baltimore, MD, Sept. 13-16, 2006. For more information about HVAC Comfortech 2007, which will be held September 26-29 2007 in St. Louis, MO, call 216/931-9550 or watch for updates on the show website: www.hvaccomfortech.com. |