Exhaust ventilation systems for H & V
Figure 3.73 Static loss in duct tapers
• 10 If 20 29 SO 19
CMteranc« in velocity pressure Pa
Velocity increases towards fan.
Design ducts on equal friction per metre, allowing 12.5 to 15 m/s in duct at fan inlet. Calculate resistance as described and add 25% margin.
Quietness is the important factor. With ducts connected to fan inlet box (i. e. without other plant items) design for 6 to 7.5 m/s in main.
If quietness is vital, keep down to 7.5 m/s in main at fan inlet, and 2.5 to 4 m/s in branches.
Even at low velocity internal acoustic treatment of ducts may be necessary.
In Figure 3.71 the flow of air at B is less than at A by the amount passed through the outlet pop. Hence the velocity at B is less than at A and so a static pressure regain results. In passing from B to C there is a fall in static pressure as flow is restricted by the taper.
Many of these take-off outlets and tapers occur in a duct system and, as already shown, the effects are neglected in the generally accepted method of calculating duct resistance. Hence it is obvious that these gains and losses must cancel approximately because long experience has shown tat the accepted method is satisfactory.
Tests show that at normal velocities, the static loss in a taper is relatively small but the aggregate of many taper in along main can be a considerable item in the resistance.
For example, in a long duct on an installation at a textile mill it was estimated that the tapers represented about 700 Pa. The IHVE guide at that time gave the loss in a taper as 0.2 x velocity pressure in the small end, and on this basis the main at the mill calculated at 840 Pa. In contrast, American sources gave the loss as 0.04 to 0.05 of the velocity pressure in the small end, and this caused confusion. In fact, this was the loss in total pressure.
In a duct taper with included angle up to 10°, the conversion of velocity is complete and the loss occurs in the duct immediately after the taper due to slight turbulence at the walls in regaining the full flow area, see Figure 3.72. Friction loss is negligible.
Static pressure for taper
Figure 3.72 Loss in contractions
Analysis of tests shows that it is not satisfactory to calculate the static pressure loss from a single factor multiplied by the velocity pressure in the small end. In some cases this gives results which are less than the difference in velocity pressure at the entrance and exit.
Tests have shown that the best approximation for practical work is given by a variable factor multiplied by the difference in velocity head in the taper. This loss of static pressure becomes greater in proportion when the differential is very small, as shown in Figure 3.73.
In the normal design of ducting, the pop or take-off is not fitted on the taper, but is at the end of the duct before taper. Experiment has shown that the regain of static pressure is very much higher than would be expected. It varies with velocity to some extent and is greater when velocity is very low in the duct. The regain is estimated as a percentage of the difference in velocity head before and after the take-off.
For practical purposes it is suggested that average values may be taken at:
90% when ducts are designed at 0.82 Pa/m
82.5% when ducts are designed at 1.63 Pa/m
75% when ducts are designed at 2.45 Pa/m
It is of course risky to overestimate this regain in a commercial calculation.
Effect of change in volume
Whilst small variations in air volume passed through a duct system may be calculated as the square, it is not advisable to try this when the change is very considerable. The change in regain and taper losses from a very underloaded condition to an overloaded condition might upset the accuracy of this estimation. There will also be a change in Reynolds Number.
Large changes in flow on a given system are unusual, but cases are known where ducts have been installed to deal with some future condition in a factory. One must also remember that some systems with a variable fans speed can cope with a 10:1 reduction.
Posted in Fans Ventilation A Practical Guide