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

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 0.015 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 8.4 - 8.8.

Example 8.22 & 8.23: own reading DUCT FITTINGS Notice that answers are not accurately written. Hf = 0.035 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.

DUCT DESIGN METHOD If ceiling space is very limited, an option is to use small ducts with high air velocity. But this leads to high friction and fan operating costs.

Duct design can be classified into two groups: DUCT DESIGN METHOD Duct design can be classified into two groups: Low Velocity High Velocity Relate this with Q = VA

Low Velocity Ducting DUCT DESIGN METHOD The air velocity in this group is low. Typical range of design equal friction loss rates are from 0.08 to 0.15 in.w per 100 ft of duct Refer Table 8.11 for suggested velocities.

Recommendations within Low Velocity System DUCT DESIGN METHOD Recommendations within Low Velocity System

High Velocity Ducting DUCT DESIGN METHOD The air velocity in this group is high. Normally intended to reduce the duct size, due to space limitation. This type is generally costly.

The duct must also be able to withstand high pressure. DUCT DESIGN METHOD High Velocity Ducting High velocity ducting is related to noise, which must be treated by using thicker or internal insulation or special sound attenuation system (silencer). The duct must also be able to withstand high pressure.

Two available methods: DUCT DESIGN METHOD Two available methods: Equal Friction Method Static Regain Method

EQUAL FRICTION METHOD The same value friction loss rate per length of duct is used to size each section of duct in the system. This method is the simplest and the most popular.

EQUAL FRICTION METHOD A problem with equal friction method is that the outlet nearest to the fan may be over-pressured or too much draft. Need balancing using damper  quite difficult.

EQUAL FRICTION METHOD Refer to Example 8.27

STATIC REGAIN METHOD This method is widely used for high velocity systems. The concept is to have friction loss equals to SPR in each section. An initial velocity in the main duct leaving the fan is selected, in the range of 2500 to 4000 fpm.

STATIC REGAIN METHOD The velocities in each successive duct section is reduced to result in SPR that is enough to overcome the friction loss. When, SPR = friction loss, the static pressure in each section will be the same, thus pressure is almost balanced everywhere. REFER EXAMPLE 8.28

Static regain method is tedious to calculate manually. DISADVANTAGES Usually ducting using static regain method is bigger than if using equal friction method. Static regain method is tedious to calculate manually. END