AIR FLOW IN DUCTS Shaharin Anwar Sulaiman

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AIR FLOW IN DUCTS Shaharin Anwar Sulaiman
Now that you can size equipment, the next task would be to design the air distribution system. Shaharin Anwar Sulaiman

Duct design objectives
Occupant comfort Proper air distribution Economical heating and cooling system operation Economical duct installation

Supply duct systems Supply ducts deliver air to the spaces that are to be conditioned. The two most common supply duct systems for residences are: The trunk and branch system and The radial system

SUPPLY DUCT CONFIGURATIONS

Content Friction losses in: round ducts
rectangular ducts (aspect ratio) inlet / outlet of fans fittings Duct sizing methods: Equal Friction Method Static Regain Method

FRICTION LOSSES FROM AIR
FLOW IN DUCTS Like 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.

FRICTION LOSSES FROM AIR
FLOW IN DUCTS See Fig. 8.21 It’s quite similar to Fig. 8.13, 8.14, 8.15 used for piping

EQUIVALENT ROUND DUCT SIZES
For rectangular duct, need to do a conversion by referring to the chart for equivalent round duct sizes.

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.

FRICTION LOSSES FROM AIR
FLOW IN DUCTS Example-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.w Velocity = ?

FRICTION LOSSES FROM AIR
FLOW IN DUCTS Example-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”

ASPECT RATIO It 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.

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.

DUCT FITTINGS Like 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.

DUCT FITTINGS For loss coefficient method, the following equation is used: The values for loss coefficients can be found in Table

Example 8.22 & 8.23: own reading DUCT FITTINGS
Notice that answers are not accurately written. Hf = in both examples.

(Static pressure regain)
DUCT FITTINGS Example 8.23 also 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

System Effect Loss. Own reading (Sec. 8.14) PRESSURE LOSS AT
FAN INLET & OUTLET System Effect Loss. Own reading (Sec. 8.14)

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.

DUCT SYSTEM PRESSURE LOSS
System total pressure loss the total pressure loss through the duct path that has the largest pressure losses. Fan 700cfm 300 cfm 500 cfm

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.