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REFRIGERATION HVAC MECHANIC RED SEAL LICENSE EXAM REVIEWER - fan speed, static pressure, power, amperage
PROBLEM
The rpm of a 3 HP ducted fan moving 1,200 CFM is increased from 900 rpm to 1800 rpm. If the initial static pressure is 0.08 in. w.c., what is the resulting CFM, static pressure, and HP?
Given
cfm1 = 1200 cfm
sp1 = 0.08 in. w.c.
HP1 = 3 HP
rpm1 = 900 rpm
rpm2 = 1800 rpm
Unknown
cfm2
sp2
HP2
SOLUTION
Using the Fan Laws,
cfm2 =
cfm1 x (rpm2/rpm1)
cfm2
= 1200 x 1800/900
= 1200 x 2
= 2400 cfm
when the fan speed rpm is doubled, the cfm is also doubled
sp2 = sp1 x
(rpm2/rpm1)^2
sp2 = 0.08 x
(1800/900)^2
sp2 = 0.08 x (2)^2
sp2 = 0.08 x 4
sp2 = 0.32 in. w.c.
when the fan speed rpm is doubled, the static pressure is increased 4 times
HP2 = HP1 x
(rpm2/rpm1)^3
HP2 = 3 x
(1800/900)^3
HP2 = 3 x (2)^3
HP2 = 3 x 8
HP2 = 24 HP
when the fan speed rpm is doubled, the Power needed to drive the fan is increased 8 times
if the supply voltage is constant,
then the current draw (amperage)
is also increased 8 times
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