How to calculate the size of a rectangular ventilation duct for 150 pipes?

Hello. When calculating, one should proceed from the equivalent diameter, i.e. the diameter of the duct with a cross section in the form of a circle, where the friction loss is equal to the corresponding value in the duct of rectangular cross section with the same length.

The equivalent diameter for a rectangular section is calculated as follows: De = 1.30 x ((204 x 60) 0.625) / (204 + 60) 0.25) = 150, where De is the equivalent diameter in mm.

This means that the round duct has a sectional area smaller than the corresponding parameter of the duct with a section in the form of a rectangle with an equivalent diameter. Given that the cross-sectional areas - round and rectangular, are equal, the perimeter of the rectangular section should be greater than the circumference:

1. The value of the perimeter: 204 x 2 + 60 x 2 = 528 mm;
2. Circumference: 2 x π x R = 2 x 3.14 x 75 = 471 mm.

As you can see, the condition is not met, therefore, the duct 204 x 60 cannot replace a pipe with a diameter of 150.

Perhaps you will be useful this articlewhere you will find formulas and examples of calculations on them.
If the equivalent diameter is known, you can find without any calculation the sides of the rectangular duct according to the table attached below.