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 SAG CONT 32W S.P. 0.04 A coil.25 sp FAN COIL OR FURN ACE R.A. BOX RETURN AIR RAG CONT G35W S.P. 0.045 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 Perimeter Ratio of Perimeter to Area Equivalent Round Duct Friction at 15,000 CFM 40.7 in. 9.03 ft 2 10.65 ft. 1.18:1 40.7 in..07 in. / 100 ft. 3 ft. 9.0 ft 2 12 ft. 1.33:1 39.4 in..086 in. / 100 ft. 3 ft. 2 ft. 4-1/2 ft. 9.0 ft 2 13 ft. 1.45:1 38.7 in..095 in. / 100 ft. 1-1/2 ft. 6 ft. 9.0 ft 2 15 ft. 1.67:1 37.2 in..113 in. / 100 ft. 1 ft. 9 ft. 20 ft. 2.22:1 34.5 in..156 in. / 100 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 SAG CONT 32W S.P. 0.04 A coil.25 sp FAN COIL OR FURN ACE R.A. BOX RETURN AIR RAG CONT G35W S.P. 0.045 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 Airflow (CFM) Duct Calculator 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 Perimeter Ratio of Perimeter to Area Equivalent Round Duct Friction at 15,000 CFM 40.7 in. 9.03 ft 2 10.65 ft. 1.18:1 40.7 in..07 in. / 100 ft. 3 ft. 9.0 ft 2 12 ft. 1.33:1 39.4 in..086 in. / 100 ft. 3 ft. 2 ft. 4-1/2 ft. 9.0 ft 2 13 ft. 1.45:1 38.7 in..095 in. / 100 ft. 1-1/2 ft. 6 ft. 9.0 ft 2 15 ft. 1.67:1 37.2 in..113 in. / 100 ft. 1 ft. 9 ft. 20 ft. 2.22:1 34.5 in..156 in. / 100 ft. Figure 8
GAS 90 SQ FT 190 SQ FT 50 SF 90 SQ FT 450 SQ FT 120 SQ FT 120 SQ FT 50 SQ FT WASHER (N.I.C.) DRYER (N.I.C.) 60 SQ FT 160 SQ FT 120 SQ FT
GAS 190 SQ FT 95 CFM 90 SQ FT 45 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM 50 SQ FT 25 CFM WASHER (N.I.C.) DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM
GAS 190 SQ FT 95 CFM 90 SQ FT 45 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM 50 SQ FT 25 CFM WASHER (N.I.C.) DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM
GAS 190 SQ FT 95 CFM 90 SQ FT 45 CFM 50 SF 25CFM 90 SQ FT 45 CFM 450 SQ FT 225 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM 50 SQ FT 25 CFM WASHER (N.I.C.) DRYER (N.I.C.) 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM
GAS 190 SQ FT 95 CFM 90 SQ FT 45 CFM 50 SF 25CFM 8/8 450 SQ FT 225 CFM 18/8 90 SQ FT 45 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM 12/8 50 SQ FT 25 CFM WASHER (N.I.C.) DRYER (N.I.C.) 8/8 60 SQ FT 30 CFM 160 SQ FT 80 CFM 120 SQ FT 60 CFM
GAS 190 SQ FT 95 CFM 90 SQ FT 45 CFM 50 SF 25CFM 8/8 450 SQ FT 225 CFM 12/8 90 SQ FT 45 CFM 120 SQ FT 60 CFM 120 SQ FT 60 CFM 8/8 50 SQ FT 25 CFM WASHER (N.I.C.) 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)