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 Duct
Heat Gain |
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Design Criteria Duct System Costs Duct Construction Leakage Calculations A New Problem for Rectangular Duct System Spiral and Flat Oval Duct Duct Heat Gain Noise Control The first 90 feet of the round duct system in Figure 1 has an average velocity of about 3470 fpm and conveys 55°F air through a 115°F uncooled attic space. Each system is insulated with 2 inch, 0.75 lb/ft3 exterior insulation (U is obtained from Figure 3). Compare the heat gain in the 90 foot duct segment for each type I duct.  fig3 ROUND DUCT (Equations from ASHRAE)  FLAT OVAL DUCT (Average Velocity = 3050 fpm)  RECTANGULAR DUCT (Average Velocity = 2875 FPM)  90 FOOT OF DUCT SYSTEM HEAT GAIN COMPARISON (with round) Round spiral duct - 6766 Btuh heat gain Flat oval duct - 9293 Btuh or 2527 Btuh (37%) greater Rectangular duct - 12,250 Btuh or 5484 Btuh (81%) greater TOTAL DUCT SYSTEM HEAT GAIN Round Spiral duct - 23,825 Btuh (7.0 kWh) Flat oval duct - 32,820 Btuh (9.6 kWh) Rectangular duct - 43,100 Btuh (12.6 kWh) DUCT SYSTEM OPERATION COSTS Based on actual values established earlier for each type of system, yearly operation costs may be based on the following assumptions: VAV percentage of maximum=90% Fan efficiency = 75% Drive efficiency = 75% Annual hours of operation = 5000 Electrical costs = $0.08/kWh Heat gain (50% of maximum) = $0.04/kWh Equation: $ Cost = (cfm x S.P) .x( 0.746 kW/Bhp x $/kWh x hr/yr) / 6356 x Ef x Ed With above numbers substituted: $ Cost cfm x S.P. x 0.076 Round Spiral Duct System Fan operation cost = 20,338 x 2.90 x 0.076 = $4483/year System heat gain = 7.0 x 5000 x $0.04 = $1400/year Total = $5883/year Flat Oval Duct System Fan operation cost= 20,388 x 2.92 x 0,076= $4525/year System heat gain = 9.6 x 5000 x $0.04 = $1920/year Total = $6445/year (or 10% greater than round duct) Rectangular Duct System Fan operation cost = 20,887 x 3.0 x 0.076 = $4762/year System heat gain = 12.6 x 5000 x $004 = $2520/year Total = $7282/year (or 24% greater than round duct) If the rectangular duct system was lined with spray coated 1 inch, 1.5 pound per square foot fibrous glass instead of being externally insulated, the yearly operation costs would change to: Fan operation cost = 20,887 x 4.33 x 0.076 = $6874/year System heat gain = 30.0 x 5000 x $0.04 = $6000/year Total = $12,874/year (or 119% greater than round duct) |
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