Duct resistance calculation

The design basis of friction per metre will be known at this stage. Prepare a scale layout diagram.

1. Examine this scale diagram and decide which is the lon­gest run from fan discharge to remote air outlet. The equivalent length of this longest run is the actual length in metres, as measured from the diagram, plus the equiva­lent length in metres for each bend in this run, plus the equivalent length of any junction.

Values for bends, junctions etc. are given in this Chapter and also in CIBSE and ASHRAE guides.

As mentioned in Section 3.5.1, if the piping is rectangular it is important to note the “way” of each bend and to use correct di­mension to work out the equivalent length. Ignore any resis­tance set up by duct tapers.

2. If the total equivalent length of the longest run calculated in metres is L, then duct frictional resistance is

L x friction Pa/m =Pa

3. If ducts are not in galvanised sheet steel, use the correc­tion factor as given in Table 3.7.

4. Add an extra 25% of duct resistance only as a margin for balancing. Do not include resistance of heaters, washers, coolers, filters etc. in this addition as these should be known more accurately.

General notes

It will be appreciated that when a duct is sized on equal friction per metre, the velocity is gradually reduced from the fan to the remote end of the system. Hence it might be expected that there would be a gain in static pressure due to this reduction.

It is normal to neglect any such gain, and this was advised by ASHVE. Some engineers allow a regain of half the difference in initial and final velocity pressure in the longest run of duct. This is deducted from the calculated frictional resistance.

Actually, as will be shown, the pressure changes in a duct sys­tem are extremely complicated, and cannot be assessed with accuracy in commercial work. Experience over years has shown that the simple method as given will provide a reason­able approximation to the actual working resistance when installed.

Before the design is finally approved it is necessary to check the overall resistance of the plant. This includes duct resistance (with margin), addition for any special type of air outlet or grille, fresh air inlet louvres, filters, heaters, etc.

If the calculated overall resistance is found to be excessive for the particular type of system, it would then involve too high a fan speed. Noise in operation must be considered, and also the power absorbed by the fan, both of which are related to overall resistance.

If resistance is too high, then either redesign the ductwork for lower velocity or increase the area of filters, heaters, etc. to re­duce their resistance.

Overall resistance values depend upon local conditions and ex­perience is necessary to judge. Table 3.8 may be used as a guide noting that these values may currently be viewed as low. However, in an energy conscious world we should be endeavouring to reduce system resistance.

Type of system

System resistance


Heating only

200 Pa to 300 Pa

H & V with washer

300 Pa to 750 Pa

Public buildings:

Ventilation only

100 Pa to 250 Pa

H & V

150 Pa to 300 Pa

H & V with washer

200 Pa to 350 Pa

HVACR with noise control

1000 Pa to 1500 Pa

Table 3.8 Typical static pressure loss in various systems

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