# Square to Round Duct Fitting

Author: B J Wernick PrEng BScEng

Date: 22 March, 2007

The square to round is a common duct fitting that can be found in almost any
duct system where rectangular ducting is connected to a tube-axial fan.

ASHRAE has assigned the square to round fitting the number SD 4-2. This
stands for

S - Supply Duct

D - Diameter (Round Duct)

4 - Transition type

2 - 2nd in category

Refer to the ASHRAE Fundamentals for more details on the naming convention.

The
pressure drop is based on the fitting loss coefficient multiplied by the dynamic
pressure.

Dynamic Pressure = ½ ρ V^{2}

where

ρ = air density, typically 1.2 kg/m^{3} at sea
level

V = air velocity, m/s

Fitting Pressure Drop = Co · ½
ρ V^{2}

In a supply system, the air discharging from the fan could easily be in the
order of 10m/s. This means that the pressure drop at sea level would be
60xCo. In a typical supply air system, a fan static pressure of 450
Pascals would not be unusual. It would be impractical to select a single
fitting with a pressure drop in excess of 1% of the total static. This
implies a loss coefficient of not more than 0.075. Looking at the loss
coefficient table, this means that we would exclude all fittings where Ao/Ai>=2
and θ >=90

The length L of the fitting would be based on the dimensions of the ducting.

### Example Calculation

Given a rectangular duct with the dimensions 915x700 going to a 700mm
diameter fan connection with an airflow of 6.5 m^{3}/s, calculate the
pressure drop where L=400mm.

Ao = π D^{2}/4 = 0.3848

A1 = H1 x W1 = 0.6405

Ao/A1 = 0.6

θ = 30°

From the loss coefficient table, Co = 0.06

Therefore the pressure drop is calculated as follows:

V = 10m/s

dP = Co · ½
ρ V^{2} = 3.6 Pascals

There is no reason why a shorter length should not be used but this would
result in higher pressure drops that are not normally allowed for in selecting
the fan.

### References

SAIRAC 2004 Technical Data Manual, Wernick B.J. Ed.

ASHRAE 2001 Fundamentals Handbook, Duct Design Chapter 34, pp 34.50

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