Plant Sizing Duct Sizing - Page 1 2 3 4 5 6 7 8

Duct Sizing Using Equal Pressure Drop Method

The following is a step by step approach to duct sizing by keeping the pressure drop the same in straight lengths.

1. Choose a rate of pressure drop and keep this constant for the whole system e.g. 1.0 Pa per metre run.

2. Size ductwork using Figure 1 (Duct Sizing Chart) if the volume flow rate of air is known.

This will give the duct diameter.

3. Determine the equivalent size of rectangular duct if required by calculation or by using CIBSE guide C (2007) Table 4.16.

4. Calculate the actual air velocity from:

Air velocity (m/s) = Volume flow rate (m3/s) / CSA


CSA = Cross sectional area of duct (m2)

Fittings Pressure Loss

5. Determine the velocity pressure factors ( z zeta) for the fitting(s) in each section of ductwork from CIBSE Guide C (2007) Section 4.10.

6. Determine the velocity pressure (V.P.) by calculation or by using CIBSE Guide C (2007) Table 4.19.

The actual air velocity will be that obtained from section 4 above.

V.P. = 0.5 x r x v2

Where: V.P. = Velocity pressure (Pa)

r = Density of air (1.2 kg/m3)

v = Air velocity (m/s)

7. Multiply z factors x V.P. to give total pressure loss for fittings.

Pressure loss for fittings (Pa) = z factors x V.P.


V.P. = Velocity pressure (Pa)

z factor = Pressure loss factor for a fitting from CIBSE guide C (2007) Section 4.10.

Total Pressure Drop in Section

8. Pressure loss in straight duct (Pa) = Rate of pressure drop (1.0 Pa per metre run) x length of section (m).

9. Total Pressure drop in Section (Pa) = Pressure loss for fittings (Pa) + Pressure loss in straight duct (Pa)  




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