6 FRICTION LOSSES FROM AIR FLOW IN DUCTSLike piping, for convenience pressure loss due to friction for air flow is available in a chart (Fig. 8.21).Chart is however meant for straight round duct.The chart assumes clean galvanised steel material with about 40 joints per 100 ft, with air at standard conditions.
7 FRICTION LOSSES FROM AIR FLOW IN DUCTSSee Fig. 8.21It’s quite similar to Fig. 8.13, 8.14, 8.15 used for piping
8 EQUIVALENT ROUND DUCT SIZES For rectangular duct, need to do a conversion by referring to the chart for equivalent round duct sizes.
9 EQUIVALENT ROUND DUCT SIZES For rectangular duct, need to do a conversion by referring to the chart for equivalent round duct sizes.Can also use ductulator.
10 FRICTION LOSSES FROM AIR FLOW IN DUCTSExample-1:A 10” diameter round galvanised duct 200’ long has 150 cfm of air flowing through it. What is the pressure loss due to friction? What is the air velocity in the duct?Refer to Fig. 8.21:At 150 cfm and 10 in diameter, the friction loss is ft. w per 100 feet. Therefore for 200 ft pipe:Hf = 0.015/100’ x 200’ = 0.03 ft.wVelocity = ?
11 FRICTION LOSSES FROM AIR FLOW IN DUCTSExample-2:If the previous duct were to be replaced with a rectangular type, what would be the size if the maximum duct height is limited to 8” ?Refer to Fig. 8.23:Let the vertical scale at 8”, then the horizontal ones read 10.8”. Square CS cannot be obtained.Therefore size is 8” x 10.8”
13 ASPECT RATIOIt may be thought that equivalent round duct will have the same cross-sectional area as the rectangular duct for the same friction loss.However, actually for same friction loss, rectangular duct will have greater cross-sectional area.Because of the rectangle shape that has greater ratio of surface to cross-section, the flow experiences more friction.This is related to Aspect Ratio (AR).Aspect Ratio (AR) is the ratio of the two adjacent sides - height over width or vice versa.
14 ASPECT RATIO The higher the aspect ratio, the worse the friction loss. As a general rule, the AR of rectangular ducts should be kept as low as possible to minimise friction losses and save energy.Higher AR also means more material for ducting.The best option is to have square duct.But that is not always possible due to limited ceiling space.
15 DUCT FITTINGSLike in pipes, pressure loss also occurs in duct fittings.Duct fittings could be elbow, tee, transition, and dampers.Similar to piping system, the equivalent length could be used.Another option is the use of loss coefficient method.
16 DUCT FITTINGSFor loss coefficient method, the following equation is used:The values for loss coefficients can be found in Table
17 Example 8.22 & 8.23: own reading DUCT FITTINGS Notice that answers are not accurately written. Hf = in both examples.
18 (Static pressure regain) DUCT FITTINGSExample 8.23also shows the SPR(Static pressure regain)SPR: When velocity decrease, due to change in ducting size, there will be an increase in the static pressure
19 System Effect Loss. Own reading (Sec. 8.14) PRESSURE LOSS AT FAN INLET & OUTLETSystem Effect Loss.Own reading(Sec. 8.14)
20 DUCT SYSTEM PRESSURE LOSS In piping system, the pressure loss must be calculated to properly size the pump.Likewise, in duct, the duct pressure losses must be determined to identify the proper fan capacity.
21 DUCT SYSTEM PRESSURE LOSS System total pressure lossthe total pressure loss through the duct path that has the largest pressure losses.Fan700cfm300 cfm500 cfm
22 DUCT SYSTEM PRESSURE LOSS In analysing duct pressure losses, total pressure loss is preferred to static pressure loss.It gives clearer picture of the total pressure available at any point in the duct.Identification of total pressure loss is quite similar to the piping system (fluid mechs); i.e. everything must be included, including the filter.Go through examples 8.25 & 8.26.