Loss of pressure in hoods
The loss of pressure in an exhaust inlet is very much dependent upon its shape. It is mainly due to the contraction of the airstream which results in an increase in velocity at that point. In a bell mouthed entrance (Figure 3.44) there is virtually no contraction of the entering airstream. To create a flow of say 20 m/s at A or a velocity pressure of 250 Pa requires a static depression of 250 Pa in the duct.
Figure 3.44 Bell mouthed inlet
Thus if there are no losses:
Ps = Pv
When there is a contraction of the entering airstream then:
Ps=Pv + Pl where:
Pv = velocity pressure in the duct (Pa)
Extra static depression for the increased velocity (Pa)
1n Ј Q.
SHAPE \* MERGEFORMAT
The value of pL relative to the velocity pressure in the duct is:
But Ce as already shown =
°rpv=Ce ps Substituting for pv in the formula for relative pL:
Figure 3.45 shows this in graphical form for values of Ce from 0.6 to 1.0.
In practice, the estimation of this loss is required in the design of dust extracting plant. It is generally possible to estimate the value of Ce from some similar known example. In especial cases a model may be made and checked by a laboratory test.
Typical values of Ce are given in the paragraphs which follow. It may be appreciated that absolute accuracy in the figure is not required and is in fact impossible to achieve at the estimation stage. Results of tests have been given to three decimal places, but a rounded approximate figure may be all that is necessary.
Note: pL represents the mean facing tube reading as usually taken on the inlet side ducting of the fan. It is the equivalent of the resistance depression up to the point of
Measurement, but must be a mean over the area of flow.
Posted in Fans Ventilation A Practical Guide