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Polytechnic University of Puerto Rico

Department of Mechanical Engineering

Spring 2011

Experiment #1

Centrifugal Fans

Team #2

ME 5151 40: Thermal Engineering Laboratory

Jeh Damiani Santiago #53752

Javier De la Cruz Linares #48850

Jose A. Mena Burgos #59207

Johnmael Vzquez Montes #58147

Prof. Abdiel De Jesus

Performed: March 23, 2012

Submitted: March 30, 2012


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Abstract

An experimental analysis of efficiency for two different types of centrifugal fans,

forward and backward bladed, was done. It is important to see the difference between them

because each impeller has its own operational characteristics, which can be useful depending

on the application. The fan conditions were measured using specific software for each type of

fan. This method consists of having different sensors across the pipe in which the fan is in, thus

obtaining the values needed to calculate efficiency. Results obtained from this experiment

showed that forward-curved impeller is more efficient than backward-curved impeller.

Forward-curved impeller also has higher fan total pressure and flow rate compared with the

backward-curved impeller. Results indicated in this reports will reflect that the highest

efficiency recorded was 32% for a forward-bladed, while the highest for backward-bladed in the

same conditions was 20.6%.


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Table of Contents

Abstract ................................................................................................................................. ii

Introduction .......................................................................................................................... 1

Theory ................................................................................................................................... 2

Method ................................................................................................................................. 8

Results ................................................................................................................................... 9

Computations ...................................................................................................................... 15

Discussion ............................................................................................................................ 17

Conclusion ........................................................................................................................... 17

Recommendations ............................................................................................................... 18

References ........................................................................................................................... 19

Appendix ............................................................................................................................. 20


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List of Figures

Figure 1: Operating characteristic curve for a centrifugal fan ............................................... 2

Figure 2

Operating Characteristics for forward-bladed centrifugal fan ............................ 11Figure 3 Operating Characteristics for backward-bladed centrifugal fan .......................... 14

List of Tables

Table 1 Measurements of forward-bladed centrifugal fan at 30 Hz ................................... 9

Table 2 Measurements of forward-bladed centrifugal fan at 40 Hz ................................... 9

Table 3 Measurements of forward-bladed centrifugal fan at 50 Hz. ................................ 10

Table 4 Measurements of forward-bladed centrifugal fan at 60 Hz. ................................ 10

Table 5 Measurements of backward-bladed centrifugal fan at 30 Hz. ............................. 12

Table 6 Measurements of backward-bladed centrifugal fan at 40 Hz .............................. 12

Table 7

Measurements of backward-bladed centrifugal fan at 50 Hz .............................. 13

Table 8 Measurements of backward-bladed centrifugal fan at 60 Hz .............................. 13

Table 9 Fan flow rate performance prediction ................................................................ 15

Table 10 Fan total pressure prediction ........................................................................... 16

Table 11 Fan Power Output prediction ........................................................................... 16


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Introduction

In this experiment, centrifugal fans will be compared to see the difference in operational

characteristics. Two different impellers will be evaluated: forward and backward curved impellers.

It is important to do this exercise because according to their orientation or modification, it will

provide the user with certain characteristics useful in different applications. It is valuable to identify

these characteristics during the experiment because they will inform us with experience as well as

visualization from the system itself.


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Theory

The mechanical fan is a machine that consists of a blade assembly powered by an electrical motor

used to move gases, most commonly used to move air [1]. Mechanical fans are commonly used in

HVAC systems to control temperature and machinery cooling systems. There are three types of

fans that has different applications: centrifugal, axial and cross flow. For this experiment, we are

only considering axial and centrifugal fans since mixed flow fan will not be used.

A centrifugal fan is a type of fan in which air flows in a radial, or perpendicular, direction

relative to the fan shaft [1]. These fans are better suitable for applications that require higher

pressure. Blades on a centrifugal fan may be forward, backward, or radial. Forward blades are

curved in the direction of fan rotation, and backward blades may be either curved backward or

straight. Centrifugal fans are used in many industrial processes like: heating, ventilation and air

conditioning (HVAC) systems, air pollution control systems, grain drying and greenhouse

ventilation. [1]

The best way to analyze the performance of a fan is using its characteristics curve (Figure 1). In this

curve we can find specific operational characteristics for a rotational speed such as fan total

pressure, efficiency, and power output.

Figure 1: Operating characteristic curve for a centrifugal fan


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To determine the actual work in a control volume, the 1st

Law of Thermodynamics establishes:

Eq(1)

= Work = densityP = static pressures at entrance and Exit

V = Velocity at entrance and exit

Fan total pressure can be expressed as the following:

() Eq (2)

ptF total fan pressure

v velocity

P Pressure (kPa)

Since fans creating a change a pressure, which makes the air to flow through the pipes, it is

important to calculate the flow rate. The primary equation used to determine the flow rate is:

Eq(3)

Q Flow rate

A Area


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However, in this experiment the air flows through an orifice. Depending on the geometry of the

orifice, the flow rate changes resulting in the following equation:

Eq(4)

Cd Orifice discharge coefficient

d Orifice discharge coefficient (mm)

For the present equipment, and [2].

The power consumed by the fluid in producing the total fan pressure (ptF) at a discharge Q is the

rate at which work is perform and is expressed as [2]:

Eq(12)

The overall efficiency of the device is a comparison to determine the amount of energy transferred

from the electric motor to the fluid:

Eq(13)


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Method

1. Turn the equipment and computer on (the impeller should be install, for are case thebackward curve was installed)

2. Run the FM10 software.3. Set the power to 30 Hz, with the diaphragm fully open take a reading using the

software, this will measure the max flow for this speed.

4. Close the diaphragm complete, this will be the minimum flow for this speed; take thereadings using the software.

5. Subtract the minimum flow rate from the maximum flow rate. Divide the result by 9,10, depending in the intervals need. The result from the division is a reference to try

and have uniform readings.

6. Use the increment to measure at least 10 readings.7. Repeat the steps 3 to 6 for speeds of, 50 Hz, 40 Hz and 30 Hz.8. After the three speeds for the impeller are measure and record, change the impeller and

go through the steps again.

9. Use the data obtained to construct the characteristics curve for the centrifugal fan.


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Results

Below are the corresponding tables and graphs for the forward and backward bladed centrifugal fans.

Table 1

Measurements of forward-bladed centrifugal fan at 30 Hz

Table 2 Measurements of forward-bladed centrifugal fan at 40 Hz

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*

SpeedAir*Temp

Motor*

Power

Volume*

Flow*rate

Fan*Total*

Pressure

Fan*Power

Output

Overall*

Total*

Efficiency

Variable dPo dPs n Ta Pe Qv ptF Pu Egr

(SPG2) (SPW1) (SS01) (STS1) (SWA1)

UNITS kPa kPa Hz Deg*C Watts m^3/s kPa Watts %

1 0.25 0.2 30.57 23.43 152.91 0.0541 0.2822 15.28 10.00

2 0.22 0.28 33.53 23.27 147.29 0.0512 0.3515 18.01 12.23

3 0.18 0.45 40.15 23.27 133.37 0.0460 0.5064 23.32 17.48

4 0.14 0.56 46.81 23.21 118.21 0.0400 0.6098 24.39 20.55

5 0.1 0.68 51.5 23.16 101.86 0.0344 0.7123 24.47 24.03

6 0.08 0.64 52.2 23.16 98.93 0.0310 0.7201 22.31 22.55

7 0.05 0.73 54.46 23.11 88.18 0.0235 0.7482 17.62 19.98

8 0.03 0.73 54.84 23.05 85 0.0200 0.7456 14.88 17.50

9 0 0.84 58.36 23.11 68.88 0.0027 0.8393 2.25 3.25

10

Calculated*VariablesMeasured*Variables

M

o

t

o

r

*

a

t

*

*3

0

H

z

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*

SpeedAir*Temp

Motor*

Power

Volume*

Flow*rate

Fan*Total*

Pressure

Fan*Power

Output

Overall*

Total*

Efficiency




1 0.47 0.35 40.45 22.46 204.93 0.0738 0.5034 37.15 18.13

2 0.38 0.55 46.43 22.41 183.19 0.0664 0.6745 44.82 24.47

3 0.29 0.69 50.91 22.41 159.75 0.0577 0.7823 45.12 28.24

4 0.19 0.74 53.61 22.46 139.96 0.0476 0.8075 38.42 27.45

5 0.14 0.79 55.57 22.57 124.33 0.0401 0.8347 33.5 26.95

6 0.08 0.81 56.83 22.52 110.89 0.0306 0.8382 25.65 23.13

7 0.03 0.84 58.42 22.57 96.73 0.0201 0.8535 17.16 17.75

8 0.01 0.84 59.44 22.46 88.42 0.0107 0.8392 8.94 10.11

9 0 0.89 59.59 22.52 88.18 0.0027 0.8916 2.37 7.69

10

Measured*Variables Calculated*Variables

M

o

t

o

r

*

a

t

*

*

4

0H

z


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Table 3 Measurements of forward-bladed centrifugal fan at 50 Hz.

Table 4 Measurements of forward-bladed centrifugal fan at 60 Hz.

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*

SpeedAir*Temp

Motor*

Power

Volume*

Flow*rate

Fan*Total*

Pressure

Fan*Power

Output

Overall*

Total*

Efficiency

Variable dPo dPs n Ta Pe Qv ptF Pu Egr(SPG2) (SPW1) (SS01) (STS1) (SWA1)


1 0.68 0.56 50.12 22.09 244.26 0.0888 0.787 69.91 28.62

2 0.52 0.7 52.64 22.3 217.28 0.0775 0.8727 67.63 31.04

3 0.4 0.79 54.58 22.3 194.68 0.068 0.9206 62.59 32.15

4 0.28 0.84 56.45 22.3 172.94 0.0569 0.9269 52.7 30.48

5 0.16 0.86 57.83 22.35 150.71 0.0436 0.9141 39.85 26.44

6 0.09 0.88 58.86 22.35 135.56 0.0327 0.9107 29.8 21.98

7 0.06 0.88 59.59 22.35 129.7 0.0262 0.9005 23.62 18.21

8 0.04 0.89 59.85 22.19 128.72 0.0204 0.8996 18.39 14.29

9 0.01 0.88 60 22.25 120.18 0.011 0.882 9.68 8.06

10 0 0.9 60 22.26 114.07 0.0027 0.9038 2.41 2.11


M

o

t

o

r

*

a

t

*

*

5

0

H

z

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*

SpeedAir*Temp

Motor*

Power

Volume*

Flow*rate

Fan*Total*

Pressure

Fan*Power

Output

Overall*

Total*

Efficiency


(SPG2) (SPW1) (SS01) (STS1) (SWA1)UNITS kPa kPa Hz Deg*C Watts m^3/s kPa Watts %

1 0.87 0.77 57.92 23.64 331.4 0.101 1.0595 106.99 30.28

2 0.73 0.84 58.36 23.64 311.92 0.0921 1.0802 99.57 31.97

3 0.56 0.9 58.86 23.7 289.94 0.0812 1.0815 87.83 30.29

4 0.43 0.93 59.38 23.7 274.06 0.0712 1.0773 76.666 27.97

5 0.31 0.94 59.85 23.75 264.53 0.0602 1.042 62.724 23.71

6 0.22 0.93 60 23.75 250.12 0.0506 1.002 50.74 20.29

7 0.14 0.93 60 23.81 234.98 0.0399 0.9757 38.89 15.16

8 0.08 0.92 60 24.18 217.39 0.0312 0.9515 29.64 13.63

9 0.04 0.91 60 24.24 206.16 0.0214 0.9213 19.69 9.55

10 0 0.94 60 24.18 186.86 0.0027 0.9451 2.52 1.35


M

o

t

o

r*

a

t

*

*

6

0

H

z


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Figure 2 Operating Characteristics for forward-bladed centrifugal fan

0

20

40

60

80

100

120

0 0.02 0.04 0.06 0.08 0.1

50 Hz

Efficiency

Fan Pressure

Power Output

0

20

40

60

80

100

0 0.02 0.04 0.06 0.08

40 Hz

Efficiency

Fan Pressure

Power Output

0

20

40

60

80

100

120

0 0.02 0.04 0.06 0.08 0.1 0.12

Flow Rate (m^3/s)

Power Output 60Hz

Efficiency 60Hz (x3)

Fan Total Pressure 60Hz

(x100)


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Table 5 Measurements of backward-bladed centrifugal fan at 30 Hz.

Table 6 Measurements of backward-bladed centrifugal fan at 40 Hz

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*Speed Air*Temp Motor*PowerVolume*Flow*

rate

Fan*Total*

Pressure

Fan*Power*

Output

Overall*Total*

Efficiency




1 0.15 0.29 40.57 22.68 66.19 0.0413 0.3372 13.9125 21.03

2 0.13 0.19 43.47 22.52 74.74 0.0382 0.2336 8.9311 11.95

3 0.08 0.26 46.69 22.46 68.64 0.0305 0.2855 8.7029 12.68

4 0.06 0.31 48.51 22.41 71.57 0.0254 0.3261 8.2856 11.58

5 0.04 0.36 51 22.35 73.77 0.0208 0.3761 7.8221 10.6

6 0.02 0.41 52.91 22.41 67.77 0.0153 0.421 6.4405 9.59

7 0.01 0.45 54.37 22.46 64.48 0.011 0.4561 5.0087 7.77

8 0 0.47 54.69 22.52 60.58 0.006 0.4671 2.782 4.59

9 0 0.52 55.37 22.52 55.45 0.0027 0.5156 1.3733 2.48


M

o

t

o

r

*a

t

*

*

4

0

H

z

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop


rate

Fan*Total*

Pressure

Fan*Power*

Output

Overall*Total*

Efficiency




1 kPa kPa Hz C Watts m^3/s kPa Watts %

2 0.08 0.09 30.57 21.66 77.67 0.03 0.1119 13.9125 21.03

3 0.07 0.16 36.2 21.39 73.28 0.0284 0.1853 8.9311 11.95

4 0.05 0.21 40.74 21.44 67.66 0.0241 0.2221 8.7029 12.68

5 0.04 0.26 43.76 21.44 79.63 0.0208 0.2714 8.2856 11.58

6 0.02 0.33 47.37 21.6 73.03 0.016 0.34 7.8221 10.6

7 0.01 0.38 49.45 21.76 58.38 0.0119 0.3855 6.4405 9.59

8 0.01 0.39 50.33 21.76 70.84 0.008 0.3924 5.0087 7.77

9 0 0.43 51.35 21.76 53.98 0.0038 0.4269 2.782 4.59


M

o

to

r

*

a

t

*

*

3

0

H

z


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Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop


rate

Fan*Total*

Pressure

Fan*Power*

Output

Overall*Total *

Efficiency

Variable dPo dPs n Ta Pe Qv ptF Pu Egr(SPG2) (SPW1) (SS01) (STS1) (SWA1)


1 0.24 0.18 50.74 21.66 76.94 0.0525 0.262 13.7628 17.89

2 0.18 0.25 51.65 21.87 73.52 0.0462 0.3099 14.3042 19.46

3 0.13 0.31 53.02 21.57 65.64 0.0395 0.3587 14.1569 20.63

4 0.11 0.35 54.34 21.92 64 0.0352 0.388 13.6589 21.34

5 0.07 0.4 55.75 21.92 74.5 0.0287 0.4255 12.1937 16.37

6 0.04 0.46 56.75 21.92 78.9 0.0218 0.4704 10.2457 12.99

7 0.02 0.5 57.33 21.92 76.94 0.0168 0.51 8.5831 11.16

8 0.01 0.52 57.77 21.98 68.39 0.01 0.5206 5.1839 7.58

9 0 0.55 58.21 22.03 72.55 0.0038 0.5529 2.0818 2.87

10 0 0.57 58.59 22.03 72.3 0.0027 0.5734 1.526 2.11


M

o

to

r

*

a

t

*

*

5

0

H

z

Desc:

orifice*

pressure*

drop

Comp.*

Pressure*

Drop

Motor*Speed Air*Temp Motor*PowerVolume *Flow*

rate

Fan*Total*

Pressure

Fan*Power*

Output

Overa ll*Total *

Efficiency




1 0.33 0.26 60 22.19 214.46 0.0616 0.3714 22.8715 10.66

2 0.25 0.34 60 22.3 211.53 0.0542 0.4226 22.8953 10.82

3 0.18 0.4 60 21.98 206.89 0.046 0.464 21.3526 10.324 0.13 0.44 60 22.09 194.68 0.0384 0.4841 18.583 9.55

5 0.08 0.48 60 21.98 196.87 0.0305 0.5051 15.3862 7.82

6 0.04 0.53 60 21.98 193.45 0.0223 0.5434 12.0991 6.25

7 0.02 0.57 60 22.25 189.55 0.0143 0.5436 8.3187 6.33

8 0 0.62 60 22.25 184.17 0.0027 0.5802 8.3187 4.39

9 0 0.59 60 22.25 188.57 0.0065 0.5884 3.8371 2.03


M

o

t

o

r

*

a

t

**

6

0

H

z


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Figure 3 Operating Characteristics for backward-bladed centrifugal fan

0

20

40

60

80

100

120

0 0.02 0.04 0.06 0.08 0.1

50 Hz

Efficiency

Fan Pressure

Power Output

0

20

40

60

80

100

0 0.02 0.04 0.06 0.08

40 Hz

Efficiency

Fan Pressure

Power Output

0

20

40

60

80

100

120

0 0.02 0.04 0.06 0.08 0.1 0.12

Flow Rate (m^3/s)

60 Hz

Power Output 60Hz(Watts)

Efficiency 60Hz (x3)

Fan Total Pressure

60Hz (x100, kPa)


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ComputationsPerformance prediction using affinity equations:

since D1=D2

Here is a chart showing scaling for each parameter an their respective error percent

Table 9 Fan flow rate performance prediction

30

Q$[m^3/s] Q$[m^3/s] diferencia$[%] Q$[m^3/s] diferencia$[%] Q$[m^3/s] diferencia$[%]

0.0541 0.072133333 2% 0.090166667 2% 0.1082 7%

0.0512 0.068266667 3% 0.085333333 9% 0.1024 10%

0.0460 0.061333333 6% 0.076666667 11% 0.092 12%

0.0400 0.053333333 11% 0.066666667 15% 0.08 11%

0.0344 0.045866667 13% 0.057333333 24% 0.0688 13%

0.0310 0.041333333 26% 0.051666667 37% 0.062 18%

0.0235 0.031333333 36% 0.039166667 33% 0.047 15%

0.0200 0.026666667 60% 0.033333333 39% 0.04 22%

0.0027 0.0036 25% 0.0045 144% 0.0054 296%

40 50 60

Forward@curved$Impeller


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Table 10 Fan total pressure prediction

Table 11 Fan Power Output prediction

30

ptF&[kPa] ptF&[kPa] diferencia&[%] ptF&[kPa] diferencia&[%] ptF&[kPa] diferencia&[%]

0.2822 0.501688889 0% 0.783888889 0% 1.1288 6%

0.3515 0.624888889 8% 0.976388889 11% 1.406 23%

0.5064 0.900266667 13% 1.406666667 35% 2.0256 47%

0.6098 1.084088889 26% 1.693888889 45% 2.4392 56%

0.7123 1.266311111 34% 1.978611111 54% 2.8492 63%

0.7201 1.280177778 35% 2.000277778 54% 2.8804 65%

0.7482 1.330133333 36% 2.078333333 57% 2.9928 67%

0.7456 1.325511111 37% 2.071111111 57% 2.9824 68%

0.8393 1.492088889 40% 2.331388889 62% 3.3572 73%

40 50 60

Forward?curved&Impeller


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Discussion

After conducting the experiment we can rapidly notice the difference between both blade

designs, forward blade and backward, just by observing the results. Each blade will behave

differently and thus each will have a different application. They have different characteristics in

which we can analyze their performance; among these are the efficiency, fan output power and

the fan total pressure.

For the forward blade fan we can observe its behavior by observing the following results:

To state the most efficient in respect to flow produced, when the fan was set at 50 Hz, it

peaked at 32% (0.068 m3/s). This follows the fact that the more power is applied to this

system, the more effective it will become. In an efficiency comparison versus the flow rate, the

40hz setting proved to be the most convenient if the desired flow was close to 0.0401 m3/s, this

is a range were all of the results coincided.

The pressure became fairly constant with respect to the decrease in flow rate around the

0.0212 m3/s, this suggests a type of vacuum is being formed and was thought to be reason why

the motor speed increased inversely proportional to the flow rate but further research is

needed on the subject to prove this theory. Another possible explanation for this is that, with

the reduction in the exit orifices diameter, flow is forced to circulate within the pipe and

consequently directed towards the fan, increasing the speed but not the output power.

In power, efficiency is the main concern but not the highest recorded value for output. This was

found to be 106.99 watts with a flow of 0.1010 m3/s in the 60 Hz while obtaining an efficiency

of 30.28%. This was the maximum value for flow rate that was encountered during the test.

Temperature during the test remained constant with the exception of the slight increase when

the motor was using the maximum power but it is not sufficient to have a big impact on the

results, although the increase of density due to temperature would, but more information is

needed to proceed with that analysis.


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Subsequently we also have the backward blade fan, which its behavior differs greatly from the

forward blade fan. The results where the following:

From the behavior of the total pressure we can observe that when the flow rate starts to

increase the fan total pressure diminishes. This is due to the fact that as you open the valve the

air will have no resistance in the exit. As a result we are going to have an increased flow rate

with low pressure. In this case what will increase will be the velocity and not the pressure.

In terms of the efficiency, the highest efficiency occurs when the motor speed is 50Hz and a

flow rate of 0.0352 [m^3/s]. But it is important to focus in the fact that as we decrease the

motor speed we notice that the system increases its efficiency. So we can recall that due to a

low input, we can obtain a high efficiency. Also as the flow rate increase, we have a higher

efficiency as an output. We can state that as we lower the motor speed and increase the flow

rate, a higher efficiency will be obtained.

When observing the power, it can be recalled that as the flow rate increases, the output power

increases as well. Also it is important to keep in mind that the output power will be

proportional to the motor speed, as it is seen in the Figures 2 and 3. We can then state that the

best application for this type of blade is to due to a input flow rate, produce a high power

output.

Looking at the prediction made in computations section, it can be recalled that the predicted

values, which can be considered as theoretical values, are close enough to the experimental

values. However, as flow rate increases, error percent increases too; it can be concluded that

affinity equations does not consider friction losses in pipes causing this little discrepancies. As

flow rate increases, recalling Bernoullis energy equation, friction losses increases, affecting all

parameters measured.


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17

Conclusion

To conclude, objectives for this experiment was accomplished since operation curve

could be plotted using experimental information. Comparing the experimental curve with the

one provided by the handbook, they are very similar. Based on experimental data and further

analysis made from operating curves, it can be concluded that forward-curved impeller is more

efficient than backward-curved impeller. However, using this data, it cannot be determined

which impeller would be suitable for certain applications; it only depends on the designers

criteria.


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Recommendations

For this experiment it is important to see if each sensor (pressure, temperature, etc.) is

connected and works properly. The outlet pipe should be have some reference for the students

to close the valve in the same interval for each task; this will give the experiment more

precision because the student could make a better comparison between impellers.


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References

1. WiseGEEK.com. What is a mechanical fan?http://www.wisegeek.com/what-is-a-mechanical-fan.htm

2. Manual of Thermal Engineering Laboratory. Carranza, J.E. Polytechnic University ofPuerto Rico
http://www.wisegeek.com/what-is-a-mechanical-fan.htmhttp://www.wisegeek.com/what-is-a-mechanical-fan.htmhttp://www.wisegeek.com/what-is-a-mechanical-fan.htm


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Appendix

Software Validation:

using data from Table 1, first reading:

Power output using the same data set:

Efficiency:

Student Report+Contribution

Jehu+Damiani+Santiago 25%

Javier+De+la+Cruz+Linares 25%

Jose+A.+Mena+Burgos 25%

Johnmael+Vazquez+Montes 25%

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