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Abstract

The objective of this experiment is to define the efficacy of a centrifugal fan based on two different experimental speeds. The input power supplied at the shaft determines the readings of the dynamic and static pressures. The load variation on the shaft is seen to bring some change to the fan Flowrate calculated in cubic feet per second. The efficiency of the fan can be explained based on the head losses to get the ideal work ratio from input work. It is notable that the plotted graphs look similar to the typical back blade angle centrifugal fan.

Introduction

The purpose of the centrifugal fans is to deliver air or gas from one location to another. These fans are generally limited to low pressure applications. The ducting system determines the way the selection is done since it requires volumetric flow rate, and resistance pressure. The main characteristics of the fans much in regard to the performance can be determined in this case. Making a good choice of fan can be helpful in the production of the required flow, operating near its peak efficiency point with the aim of minimizing energy costs.

In this experiment, the calculation of the parameters to develop a set of characteristic curves analogous to typical back blade angle performance characteristics. The parameters measured could include the dynamic and static pressure of the exiting airflow, and the input shaft torque and rotational speed of the fan. It is important to calculate the uncertainty of these measurements to keep the error propagation.

Theory

For the flow of incompressible fluid, Bernoulli’s equation is applied:

Where hs - shaft head hL head loss in the control volume due to friction.

, Since hL = 0

Where = ideal fan power

Efficiency is ratio between output to input shaft power: ; where = T *

T is torque and w is angular speed.

Measurement of the pressure in the experiment is done using manometer for static pressure rise () and measurement of dynamic pressure () is done using Pitot tube. The flow around the Pitot tube was calculated using Bernoulli’s theorem given as: ; Where V is .

Fan head is calculated from two pressures:

Flowrate is determined using continuity equation where constant duct diameter is used:

Procedure

Measurements are taken for the fan speed, pressure dynamic, pressure static and the load.

1. Adjust the dampers in the dust to close without leading any heat exchange and keeping the cone wide open.

2. Measure the length of the shaft arm and the duct diameter and respective uncertainty. Making the speed constant using dynamometer. The shaft torque is determined by measuring the reaction force (and its moment arm) required to maintain the motor in a level position as it drives the fan.

3. Using the On-Off buttons and a crank handle to control the speed.

4. Adjust the flow at exit and keeping the Flowrate maximum record the dynamic pressure, static pressure and their uncertainties.

5. In order to decrease the flow rate, adjust the exit cone inward. Check the motor level before recording the force, and adjust the fan rpm if necessary so it remains constant. Repeat same step until the cone is fully shut. At this point the dynamic pressure will be nearly zero and the static pressure will be at or near its peak.

6. Shut down the fan by first decreasing the speed with the crank handle and then pushing the off button.

Results and Discussion

Table 1: Parameters

Table 2: Measured Data for 1200 RPM

Table 3: Calculated Data for 1200 RPM

Table 4: Uncertainties

Figure 1: Static Pressure vs. Flowrate

From Figure 1 the static pressure shows linear increase when the flow rate is getting increased which is similar to the typical back angle blade.

Figure 2: Efficiency vs. Flowrate

From figure 2 efficiency increases exponentially with increase in Flowrate.

Figure 3: Input shaft power vs. Flowrate

From figure 3 the input shaft work has an exponential decreasing nature, as the input work decreases the Flowrate increases.

Figure 4: Fan Head vs. Flowrate

In figure 4 fan head increases linearly with the increase in the Flowrate. This is similar to the typical Back angle blade.

Table 5: Measured Data for 900 RPM

Table 6: Calculated Data for 900 RPM

Table 7: Uncertainties

Figure 5: Static Pressure vs. Flowrate

From Figure 5 the static pressure shows linear increase when the flow rate is getting increased which is similar to the typical back angle blade.

Figure 6: Efficiency vs. Flowrate

From figure 6 efficiency increases exponentially with increase in Flowrate at the beginning then it started to act weirdly at the end.

Figure 7: Input shaft power vs. Flowrate

From figure 7 the input shaft work has an exponential decreasing nature. As the input work decreases the Flowrate increases however, our measured numbers

Is slightly off because of the experiment it self.

Figure 8: Fan Head vs. Flowrate

In figure 8 fan head increases linearly with the increase in the Flowrate. This is similar to the typical Back angle blade.

Conclusion

Static pressure measured varies along the fluid Flowrate. The volumetric flow rate gets increased up on the reduction in the load. Due to change in static pressure volumetric Flowrate is changing linearly. The electrical power input supplied to rotate the shaft with certain RPM is calculated as the work with a variable load application, this gives shaft work. The input shaft work has an exponential decreasing nature, as the input work decreases the Flowrate increases. Loss in the head that is due to the reason that change in pressure from dynamic to static. When the flow rate is increasing the due to load reduction, this increases the pressure that causes loss of head. This factor affects the efficiency of the centrifugal fan wisely. Thus Output work increase due to load reduction, so as the reason efficiency increases. Finally, The uncertainty measured and calculated are included in the plot error bars to show the experimental error under which the results obtained are correct.

Appendix

Table 2: Measured Data for 1200 RPM

Table 3: Calculated Data for 1200 RPM

Table 4: Uncertainties

Table 5: Measured Data for 900 RPM

Table 6: Calculated Data for 900 RPM

Table 7: Uncertainties

Efficiency vs Flowrate 0.15569700818807899 0.15569700818807899 5.2632789042645397E-4 5.2632789042645397E-4 3.8317901344824202E-4 4.8842578746937802E-4 6.4994842264426802E-4 1.0286007066224599E-3 1.67921212859452E-3 0.72227542419526902 0.79467574099252902 0.89232905965814002 1.02145170066573 1.0991383871839191

efficiency

Flow rate cfs

Input shaft power vs Flowrate

SInput Shaft power (ft-lb/sec)

Flow rate cfs

Fan Head vs Flowrate

Fan Head (ft)

Flow rate cfs

Static Pressure vs Flowrate 0.1177739343628 0.1177739343628 0.02 0.02 0.21 0.22 0.24 0.26 0.28000000000000003 1.8599930013727051 1.9037635311518599 1.988416157981147 2.0696091544292159 2.1477349200667022

Static Pressure (inches-water)

Flow rate cfs

Efficiency vs Flowrate 0.1177739343628 0.1177739343628 4.4962572013946701E-3 4.4962572013946701E-3 4.1547660390848602E-3 4.4887907623836999E-3 7.0325835659221002E-3 1.46394900975843E-2 1.1014284711567899E-2 1.8599930013727051 1.9037635311518599 1.988416157981147 2.0696091544292159 2.1477349200667022

efficiency

Flow rate cfs

Input shaft power vs Flowrate

Input Shaft power (ft-lb/sec)

Flow rate cfs

Fan Head vs Flowrate

Fan Head (ft)

Flow rate cfs

Static Pressure vs Flowrate 0.15569700818807899 0.15569700818807899 0.02 0.02 0.38 0.46 0.57999999999999996 0.76 0.88 0.72227542419526902 0.79467574099252902 0.89232905965814002 1.02145170066573 1.0991383871839191

Static Pressure (in-water)

Flow rate cfs

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