1:49 PM a bbappsrv.pmu.edu.sa STC 4G Evercise1 A centrifugal blower operating at

Question

1:49 PM a bbappsrv.pmu.edu.sa STC 4G Evercise1 A centrifugal blower operating at 15,000 rpm compeesses air from 68 F and 14.7 psia to 10 psig at 1350 cfm and requires 80 hp.(a) Determine the efficiency of the blower at this point. (b) What are the speed, flow rate and shaft power if it is desired to increase the discharge pressue to 12psig, (c) lfthe suction is throttled to 12 psia, what the discharge pressure and flow rate 7 (referred to 68 F and 14.7 psia) Eucercice2 If the above blower is operated in winter with an inlet air semperature of 40"F, determine the discharge pressure, flow rate (referred to 68"F and 14.7 psia) and shaft power If the outside diameter D ofthe above blower impeller is trimmed by 5%, what is the probable discharge pressure and brake horsepower operating at the conditions specified in Prob. 1? Eacercice A radial vane compressor with dimensions as shown, is operated at 12,000rpm, with air entering the inlet at 75 kPa, 290 K and 10 m's snder design condition. If the slip factor at the discharge is 085, adiabatic efficiency is 0.82 and mechanical efficiency is 0.98 determine the discharge volumetric flow rale, static pressure and brake horsepower Neglect the blockage effects due to blade and boundary layer thickness Dr. Aymen Jendoubl 80 mm> K h90 220 mm 380 mm Escercice 5 A centrifugal compressor rotating at 35,000 rpm is teslod in the open atmosphere of 14.7 psia and 60°F. The mass fow rade of 1.4 lb/s, discharge stagnation pressure of 45 psia and temperature of 280°F, are measured. If mechanical efficieney associated wih the bearing nd seal friction losses is estimated to be 99% (ut leakage is neglected determine the overall compressor efficiency and the brake horse power. Also detcrmine the expected discharge pressure and brake hp if the rotating speed is redaced to 30,000 rpm

Explanation / Answer

Answer a)

HP = [144P1Vk/(33000(k-1))]*[(P2/P1)(K-1)/K-1]

Where,

HP = horsepower

K = 1.41 = adiabatic expansion coefficient (the compression is assumed to be adiabatic

P1 = 14.7 psia

P2 = Absolute pressure = 14.7+10 = 24.7 psi

V = 1350 cfm

Therefore, HP = 47.6 hp

Installed power is 80 hp.

So, motor is running at (47.6/80) = 59.5%

Answer b)

From the fan laws, we have the following equations

From Boyle's law,

P1V1 = P2V2

P1 = 10 psig

P2 = 12 psig

V1 = 1350 cfm

10*1350 = 12*V2

V2 = 1125 cfm

We have, volume flow capacity,

V1/V2 = n1/n2

1350/1125 = 15000/n2

n2 = 12500 rpm

Shaft power, P1/P2 = (n1/n2)3

P1 = 80hp

P2 = ?

Therefore, 80/P2 = (15000/12500)3

P2 = 46.29 hp

Answer C)

Again,

From Boyle's law,

P1V1 = P2V2

P1 = 14.7 psia

P2 = 12 psia

V1 = 1350 cfm

V2 = ?

Therefore, V2 = 1653.75 cfm

For discharge pressure,

Pressure, dp1/dp2 = (V1/V2)

10/dp2 = (1350/1653.75)

dp2 = 12.34 psig

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