Centrifugal Fans

Topics: Pressure, Centrifugal fan, Static pressure Pages: 10 (2916 words) Published: March 8, 2013
Table of Contents

A. Objective2
B. Theory and Analysis2
C. List of Apparatus3
D. Procedure4
E. Set-up of Apparatus4
F. Final Data Sheet6
G. Sample Computation6
H. Test Data Analysis7
I. Related Studies8
J. Conclusion18
K. Recommendation19
L. References19

A. Objective

1. To be familiar with the operating procedure and principle as well as the efficiency of centrifugal fans.

B. Theory and Analysis
Tests of ventilating fans or blowers are made frequently by a very simple method, that is, by determining the necessary data for a calculating efficiency by measuring the work done by the fan “on the air” in giving velocity, and the horsepower required to drive the fan alone, excluding bearing friction. The fan is preferably operated by an electric motor, of which the efficiency can be readily determined by a Prony Brake Test. The power required to overcome the bearing friction of the shaft of the fan may well be first determined by measuring the power input or a series of speeds when the keys fastening the fan to its shaft have been removed and the fan itself has been “blocked” in its casing or still better, has been removed from the shaft. After the fan is attached again to the shaft, the input to the motor and the work done by the fan “on the air” should be determined for various speeds. Then, obviously, the ratio of the work done by the fan divided by the power required to drive it after correction for the efficiency of the motor and bearing friction is the actual efficiency of the fan. In general terms this may be started as follows: f = input to motor to drive motor and shaft of fan in bearings i = input to motor to drive motor and fan, in kW

e = efficiency of motor for motor input of 1kW and at speed of test e’ = efficiency of motor for motor input of 1kW and at speed of test Then if in is the net work in horsepower to drive fan alone,

in= ei-e'f0.740
Velocity measurements are usually made with a Pitot tube, consisting of two tubes with openings at the end, arranged so that one of them “faces” in the direction of flow and the other extends in a radial section.

C. List of Apparatus

1. Centrifugal fans
2. Hg Manometer
3. H2O Manometer
4. Pitot Tube
5. Tachometer
6. Amprobe
7. Set of Orifice
8. Thermometer
9. Stop watch

D. Procedure

1. Place the desired orifice diameter at the end of the air tunnel. 2. Start the motor by switching the main plug to the main AC source. 3. Make simultaneous readings on the manometer, current on the amprobe suction and discharge temperature and the speed using the tachometer. 4. Use two minutes duration for each trial.

5. Compute the necessary requirements needed to complete the data.

E. Set-up of Apparatus
Actual Centrifugal Fan used in the experiment

Different orifices with varying diameters mere used for every trial

The fan’s performance was measured several times during trial to ensure accuracy

F. Final Data Sheet
TRIAL| Orifice Diameter (inch)| Speed RPM| Orifice Pressure| Temperature| | | | Total| Vel.| Suction| Discharge|
1| 2 ½| 1764| 2 ¾| 1 ½| 29.8| 30.4|
2| 4 ½| 1759| 2 ¼| 1 ¼| 29.3| 30.1|
3| 7| 1763| 1 ¾| 1| 30.1| 30.3|

TRIAL| Current| Input| Output| %|
1| 3.42| 1.10| | |
2| 3.57| 1.15| | |
3| 3.64| 1.17| | |

G. Sample Computation

ρa=PRT=101.3325Kpa0.287(30.4+273)=1.164 kg/m3

Ha=hwδwδa=243inHg(101.325Kpa29.92inHg)1.164(9.81)(11000)=815.58m air

Output=Qwh ; Q=AV

A=π4d2=π4(221inch x 1ft12 inch x 1m3.28084)2=0.003167m2
v=2gh=2 x 32.2 x 121inch x 1ft12 inch x 1m3.28084=1.5664 m/s Output=(0.003167m2 x 1.5664 m/sx(1.164(9.81)(11000)x 815.57m)(1HP/0.746KW) Output= 0.0457HP...
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