Air whose density is 0.078 lbm/ft3 enters the duct of an air-conditioning system at a volume flow rate of 450 ft3/min. If the diameter of the duct is 10 in, determine the velocity of the air at the duct inlet and the mass flow rate of air.

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Answer:

The velocity of air is approximately 825.059 meters per second.

The mass flow rate of air is 35.1 pounds per minute.

Explanation:

From Dimensional Analysis and under the consideration that air velocity is constant, we get that velocity ([tex]v[/tex]), measured in feet per minute, is:

[tex]v = \frac{4\cdot \dot V}{\pi\cdot D^{2}}[/tex] (Eq. 1)

Where:

[tex]\dot V[/tex] - Volume flow, measured in cubic feet per minute.

[tex]D[/tex] - Inner diameter of the air duct, measured in feet.

If we know that [tex]\dot V = 450\,\frac{ft^{3}}{min}[/tex] and [tex]D = \frac{5}{6}\,ft[/tex], then air velocity is:

[tex]v = \frac{4\cdot \left(450\,\frac{ft^{3}}{min} \right)}{\pi\cdot \left(\frac{10}{12}\,ft \right)^{2}}[/tex]

[tex]v \approx 825.059\,\frac{m}{s}[/tex]

The velocity of air is approximately 825.059 meters per second.

And the mass flow ([tex]\dot m[/tex]), measured in pounds per minute, is now calculated:

[tex]\dot m = \rho \cdot \dot V[/tex] (Eq. 2)

Where [tex]\rho[/tex] is the density of air, measured in pounds per cubic feet.

If we know that [tex]\rho = 0.078\,\frac{lbm}{ft^{3}}[/tex] and [tex]\dot V = 450\,\frac{ft^{3}}{min}[/tex], then mass flow is:

[tex]\dot m = \left(0.078\,\frac{lbm}{ft^{3}}\right)\cdot \left(450\,\frac{ft^{3}}{min} \right)[/tex]

[tex]\dot m = 35.1\,\frac{lbm}{min}[/tex]

The mass flow rate of air is 35.1 pounds per minute.

Velocity of the air in duct and the mass flow rate of air is 825 ft/min and 35.1 pound.

Given that;

Density of material = 0.078 lbm / ftÂł

Volume flow rate = 450 ftÂł/min

Diameter of duct = 10 inch

Find:

Velocity of the air in duct and the mass flow rate of air

Computation:

[tex]V = \frac{v}{\pi D^2/4 } \\\\V = \frac{450}{(3.14) 10^2/4 } \\\\V = \frac{450}{(3.14) 100/4 } \\\\V = 825 ft/min[/tex]

Velocity of the air in duct = 825 ft/min

Mass flow rate of air = (Density of material)(Velocity)

Mass flow rate of air = (0.078)(825)

Mass flow rate of air = 35.1 pound

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