A supply fan is operating at 30000 cfm and 4 inch of water with an efficiency of 50%. (a) Calculate the fan power at the current operating condition. (b) Calculate the pressure and power if the supply fan reduces its speed to deliver 20000 cfm.

Respuesta :

Answer:

The pressure and power of fan is 1.77 and 11.18 Hp respectively.

Explanation:

Given:

Discharge [tex]Q_{1} = 30000[/tex] cfm

Pressure difference [tex]\Delta P = 4[/tex] inch

Efficiency [tex]\eta = 50\%[/tex]

(A)

From the formula of fan power,

     [tex]P _{1} = \frac{Q \Delta P}{6356 \eta}[/tex]

     [tex]P_{1} = \frac{30000 \times 4}{6356 \times 0.5}[/tex]

     [tex]P_{1} = 37.76[/tex] Hp

(B)

Fan power and pressure is given by,

We know that pressure difference is proportional to the square of discharge.

    [tex]\frac{\Delta p_{2} }{\Delta P_{1} } = (\frac{Q_{2} }{Q_{1} } ) ^{2}[/tex]

   [tex]\Delta P_{2} = (\frac{20000}{30000} ) \times 4[/tex]

   [tex]\Delta P_{2} = 1.77[/tex]

Fan power proportional to the cube of discharge.

       [tex]\frac{P_{2} }{P^{1} } = (\frac{Q_{2} }{Q_{1} } )^{3}[/tex]

       [tex]P_{2} = \ (\frac{20000}{30000} ) ^{3} \times 37.76[/tex]

       [tex]P_{2} = 11.18[/tex] Hp

Therefore, the pressure and power of fan is 1.77 and 11.18 Hp respectively.