# Duct Design - biasew.net

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Duct Design - biasew.net

Duct Design Presented by Dave Janquart Overview of Topics • Duct design • Duct insulation • SMACNA Standards Factors Influencing Duct Design • • • • • Equipment Losses Air Velocity Duct Material Duct Size & Shape Length of Duct Friction Losses SUPPLY A coil .25” sp FAN COIL OR FURN ACE R.A. BOX RAG CONT G35W S.P. 0.045 RETURN AIR SAG CONT 32W S.P. 0.04 FILTER S.P. 0.07 FURNACE NO SCALE Air Distribution System Components • • • • • • Supply fan Supply Ductwork Transition fittings Discharge grills Return grills Return ductwork Calculate Air System Pressure Losses • The air pressure losses from both the supply and return duct systems plus the loss through external a/c coil is often referred to as the external static pressure. • Manufacturers generally publish fan ratings based on external static pressure (internal losses due to coils, filters, etc. are usually accounted for in the cataloged fan ratings). Air Velocity in Duct Design • As Air Velocity Increases Noise Increases • Increasing Air Velocity Causes More Turbulence • Recommended Velocity 700-900 FPM Duct Material Roughness – Duct material roughness refers to the inside surface of the duct material. – The rougher the surface, the higher the friction loss. – Most duct sizing tables use the roughness factor for smooth, galvanized sheet metal as the reference value. – The back side of the Duct Calculator has a conversion table for converting the material roughness from smooth sheet metal to other materials. Select Duct Material (sheet metal, fiberglass, flex) • Decide which duct construction material to use. • Common choices: fiberglass duct board, galvanized sheet metal, “flex” vinyl coated with helical wire core. • Each material has advantages. – – – – First cost (price & ease of installation) Performance (friction loss, fan energy) Acoustic properties Thermal properties (heat gain & loss; internal/external insulation) Round vs. Rectangular Duct of Equal Friction Rate Ratio of Perimeter Perimeter to Area 9.03 ft2 40.7 in. 3 ft. 9.0 ft2 Equivalent Round Duct Friction at 15,000 CFM 10.65 ft. 1.18:1 40.7 in. .07 in. / 100 ft. 12 ft. 1.33:1 39.4 in. .086 in. / 100 ft. 13 ft. 1.45:1 38.7 in. .095 in. / 100 ft. 15 ft. 1.67:1 37.2 in. .113 in. / 100 ft. 20 ft. 2.22:1 34.5 in. .156 in. / 100 ft. 3 ft. 9.0 ft2 2 ft. 4-1/2 ft. 1-1/2 ft. 1 ft. 9.0 ft2 6 ft. 9 ft. Figure 8 Fitting Influences • Selection of the proper fittings is critical Design Friction Rate • When sizing a duct system the designer will usually choose a design friction rate. • This is the desired friction loss in inches w.g. per 100 ft. of equivalent length (in. w.g./100 ft. E.L.) of duct. • The design friction rate is generally determined based on the velocity of the air in the first section of ductwork. • Most designers use a design friction rate somewhere between 0.08 and 0.10 in. w.g. per 100 ft. E.L. Determining Friction Rate/100 ft. • Equipment pressure drop • Duct pressure drop • Terminal device pressure drop Terminal Device Pressure Drop • Pressure loss for supply air grill • Pressure loss for return air grill Friction Losses SUPPLY A coil .25” sp FAN COIL OR FURN ACE R.A. BOX RAG CONT G35W S.P. 0.045 RETURN AIR SAG CONT 32W S.P. 0.04 FILTER S.P. 0.07 FURNACE NO SCALE Equal Friction Method • • • • Same friction loss per foot of ductwork Most commonly used method Accomplished by use of a Ductulator Requires less balancing – Important to have balance dampers at each outlet • Results in reasonable duct sizes Velocity Scale Duct Calculator Airflow (CFM) Velocity Pressure Friction Loss Round Duct Diameter Equivalent Rectangular Duct Sizes Figure 7 Duct Calculator Duct Calculator (Reverse) Recommended & Maximum Duct Velocity Ranges Design toward smaller end of range for quieter systems and toward larger end of range to minimize duct sizes. Things to Consider • Aspect ratio no more than 3:1 • Residential 2 times # of 6” RD take offs plus 2” for ducts that are 8” high • .5 cfm per Sq Ft • No more than100 cfm per 6” RD Round vs. Rectangular Duct of Equal Friction Rate Ratio of Perimeter Perimeter to Area 9.03 ft2 40.7 in. 3 ft. 9.0 ft2 Equivalent Round Duct Friction at 15,000 CFM 10.65 ft. 1.18:1 40.7 in. .07 in. / 100 ft. 12 ft. 1.33:1 39.4 in. .086 in. / 100 ft. 13 ft. 1.45:1 38.7 in. .095 in. / 100 ft. 15 ft. 1.67:1 37.2 in. .113 in. / 100 ft. 20 ft. 2.22:1 34.5 in. .156 in. / 100 ft. 3 ft. 9.0 ft2 2 ft. 4-1/2 ft. 1-1/2 ft. 1 ft. 9.0 ft2 6 ft. 9 ft. Figure 8 90 SQ FT 190 SQ FT 50 SF 90 SQ FT 450 SQ FT 120 SQ FT 120 SQ FT WASHER (N.I.C.) 50 SQ FT GAS DRYER (N.I.C.) 60 SQ FT 160 SQ FT 120 SQ FT 90 SQ FT 45 CFM 190 SQ FT 95 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM WASHER (N.I.C.) 50 SQ FT 25 CFM GAS DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM 90 SQ FT 45 CFM 190 SQ FT 95 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM WASHER (N.I.C.) 50 SQ FT 25 CFM GAS DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM 90 SQ FT 45 CFM 190 SQ FT 95 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM WASHER (N.I.C.) 50 SQ FT 25 CFM GAS DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM 90 SQ FT 45 CFM 190 SQ FT 95 CFM 50 SF 25CFM 8/8 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 18/8 120 SQ FT 60 CFM 12/8 WASHER (N.I.C.) 50 SQ FT 25 CFM GAS DRYER (N.I.C.) 8/8 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM 90 SQ FT 45 CFM 190 SQ FT 95 CFM 8/8 450 SQ FT 225 CFM 50 SF 25CFM 90 SQ FT 45 CFM 120 SQ FT 60 CFM 12/8 120 SQ FT 60 CFM 8/8 WASHER (N.I.C.) 50 SQ FT 25 CFM GAS DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM Select and Adjust Fan • The total static pressure is the pressure that the fan must overcome to deliver the required amount of air. This total amount is also used to select the required fan motor size or fan speed. • Perform air balance next. Per Comm 23.18 • Measured airflow values should be compared against the required values. • Adjust fan and balancing dampers as necessary to deliver the correct amount of air to each zone. Duct Insulation • 22.17 UDC Duct system insulation. (1) except as provided in sub. (4), all heating and cooling duct systems, or portions thereof, that are located in unheated or uncooled spaces respectively, shall be provided with insulation with a thermal resistance of at least R-5. Duct Insulation cont. • R-4.2 and R-6 are common industry standards for insulated flex • R-5 and R-8 are common industry standards for duct wrap Installation of Underground Ducts SMACNA Standards Installation of Underground Ducts cont. Installation of Underground Ducts cont. • 23.08 UDC supply air ducts shall be insulated with a moistureproof material having a resistance value of a least R-5. Underground Duct cont. Underground Ducts cont. • Uninsulated duct will require 152 btuh per linear foot of duct for heating • 1” insulated duct will require 91 btuh per linear foot of duct for heating (40% less energy) • 2” insulated duct will require 74 btuh per linear foot of duct for heating (51% less energy)