ENCH295 Report Template Blower (Autosaved) PDF

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Experiment on blow of the University of Canterbury
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ENCH295 Cover Sheet

Rootes Blower Experiment Name: ID Number: Due Date and Time:

Declaration I confirm that the submission attached to this cover sheet is entirely my own work (apart from general verbal discussion with other students). Signed:

Marking Schedule

1. Summary:

/10

2. Introduction:

/15

3. Materials and Methods:

/15

4. Results/Discussion:

/20

5. Conclusions:

/5

7. Appendices including uncertainty calculations

/25

8. Presentation (Grammar, English and Structure)

/10

Total

/100

25% late penalty

Final mark: Comments on report by the demonstrator and /or Tutor:

/100

Rootes Blower Experiment

Siti Nuraini Athirah A.Aziz

ENCH295 Chemical Engineering Professional Practice 2017

University of Canterbury

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Abstract This objective of this experiment is to calculate the isentropic efficiency using two different methods and to compare each method. The methods used are enthalpy ratio and the power ratio. Both methods were closely examined to determine which ratio gives the closest pproximity to an actual system and it was found that enthalpy ratio gives much more accurate result for efficiency compared to power ratio. The reliability and the accuracy of each method to perform well under isentropic condition were tested. Although there are some faulty in the equipment, it can be concluded that enthalpy ratio has much better approximation to an actual process as it is temperature and pressure dependent while power ratio has poor representation towards actual process because it is highly dependent on the velocity of fluid which is obtained from averaging the velocity data. It was noted that calculation in power ratio could be improved by using numerical integration. On the other hand, the relationship between Actual Power and Torque current was found to be directly proportional with Output Power and Output Current to be constant.

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Table of Contents Abstract..................................................................................................................................................i 1. Introduction.......................................................................................................................................1 2. Method..............................................................................................................................................2 3. Results and Discussion.......................................................................................................................3 4. Conclusions........................................................................................................................................6 5. References.........................................................................................................................................6 6. Appendices........................................................................................................................................7 6.1 Appendix A..................................................................................................................................7 6.2 Appendix B...................................................................................................................................8 6.3 Appendix C...................................................................................................................................9 6.4 Appendix D................................................................................................................................11

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1. Introduction The aim of this experiment is to find isentropic efficiency of Rootes Blower in the Special Purposes Lab using two different methods and to analyse critically each methods. The Roots Blower delivers high flow rates at pressure above ambient to different room in the Special Purpose Lab. It involves two rotors which are controlled by a motor via a variable speed drive (VSD). A pitot tube is used to measure the flow rate across the blower while sensors measure the ambient temperature, inlet temperature, outlet temperature, ambient pressure, inlet pressure and outlet pressure. The main concern of Root Blower is its thermal efficiency as it produces high discharge temperature which reduces the power from the engine. For a reversible adiabatic compression, the system is said to be isentropic. However, in reality entropy increases while actual enthalpy change is greater than the theoretical enthalpy change since more power is needed to compress the air. To find the enthalpy change of the gas:

∆ H=

γ R(T 2−T 1 ) γ−1 (1)

To find the isentropic outlet temperature for a given compression ratio and inlet temperature:

T out ,isentropic =T inlet (

γ −1 γ

Pout ) P¿

(2)

Isentropic enthalpy change can be calculated using Equation (1) by changing

T 2 to isentropic

outlet temperature obtained in Equation (2). Isentropic power consumption is defined by:

´ isentropic =m ∆ H isentropic W

(3)

Actual work is defined as the shaft work done onto the system by the compressor. Since blower software only provide “torque current”, it is best to estimate actual work through this equation:

´ ´ ´ W actual =W shaft = W output (

I torque ) I output

(4)

Isentropic efficiency is the ratio of isentropic process to actual process which can be defined two ways:

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η=

∆ H isentropic ∆ H actual

(5a)

or

η=

W isentropic W´ isentropic = ´ actual W actual W

(5b)

The list of variables used in the equations can be referred in the appendix.

2. Method Before experiment Before turning on the equipment, all of the Rootes Blower air feed valves were checked to be in the correct position. The rest of the procedure was followed as in briefing sheet. Starting the experiment The computer software was set for the given measurement as provided in the briefing sheet and was followed properly. Taking measurements The output power was set to be approximately 5 kW and the blower was allowed to reach steady state. The temperature and pressure were recorded. The readings for torque current, output current, outlet temperature, inlet temperature, outlet pressure and inlet pressure. In the Particle Processing Lab (D175), the readings for air velocity and pressure across the pipe were taken by locating the pitot tube at different diameter of pipe. This step is repeated 7 times to obtain the velocity profile across the pipe. The output power was increased closely to 10 kW, 15 kW and 20 kW. The output power was not increased any further because the temperature would increase drastically and the pipe which is made of PVC was not able to withstand such high temperature. Closing down The software was shut down accordingly to the instruction provided in the briefing sheet.

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3. Results and Discussion Before performing calculations, a few assumptions were made: 1. System at steady state. 2. Incompressible fluid. 3. Density, temperature, velocity of fluid is constant. 4. Air is an ideal gas. 5. Cross-sectional area of the pipe is constant throughout the pipework

a) Calculate and compare isentropic efficiency obtained using both methods (5a and 5b/c) A detailed calculation is provided in the appendix. The summarised result is tabulated in table below. Table 1: Isentropic efficiency calculated using both methods

Efficiency using enthalpy ( ± 0.1 ) 1.03

Efficiency using power ( ± 0.1 ) 0.30

0.90

0.31

0.84

0.35

0.79

0.30

In calculating the efficiency with power ratio it was noted that velocity of fluid was taken from averaging velocity data. A more accurate method is to use numerical integration of the velocity profile along the pipe radius. For incompressible flow in a circular pipe, the average velocity is determined by R

uavg =

2 ∫ u (r )r dr R2 0

Where u(r) is the velocity measured at location r.

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b) Plot the torque current/output current ratio against the output power.

Torque /Output Ratio

Torque Current/Output Current Ratio vs Output Power 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 4

6

8

10

12

14

16

18

20

22

Output Power (kW)

Figure 1: Graph of torque/output current ratio against output power

The graph above shows a gradual increase in Torque/Output Current ratio and remains a constant

´ output ¿ value when Output Power( W proportionally with

is about 13 kW. Subsequently,

´ output increases W

´ actual when Torque/Output ratio is constant at roughly 0.83. W

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c) Efficiencies as a function of output power over the range of values used.

Efficiencies vs Output Power 1.2

Efficiencies

1 0.8 Enthalpy ratio Power ratio

0.6 0.4 0.2 0 4

6

8

10

12

14

16

18

20

22

Output Power (kW)

Figure 2: Graph of efficiency against Output Power

Efficiencies obtained using enthalpy ratio decrease linearly while efficiencies from power ratio levelled off at roughly 0.3. Note that the efficiency from enthalpy ratio gives a value of more than 1 at 5 kW which is irrelevant since it is violating the Second Rule of Thermodynamics. This was due to some faulty in the equipment. It was found out that there was a leakage in the pipeline that reduces the velocity reading in the pipe which subsequently diverts the efficiency much further from ideality if power ratio is to be used. A huge loss of energy has to be taken into account to have a better judgement between the two methods. By referring to the graph above, efficiency from enthalpy ratio has a gradual decrease as the Output Power increases. This is due to higher loss of energy as the blower produces higher power. These energy loses in the various of forms such as heat, kinetic and sound. Air was delivered at really high velocity causing the temperature of the compressed air to rise which contributed to huge heat losses. Although Figure (2) provides quite an unreliable relation for the efficiency from power ratio, it can be concluded that efficiency from enthalpy ratio is much closer to ideal system and is more dependable

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as most of the value calculated are cIose to 1. It was noted that enthalpy is highly dependent on temperature and pressure. These properties can be easily detected by the sensor in the blower. In addition, ambient temperature and ambient pressure does not interfere with fluid which makes it easier to do calculations. In comparison, finding enthalpy using power ratio is less reliably. It diverts away from ideal system as the values are nowhere near 100% efficiency. This is because it is dependent on the velocity of the fluid across the pipe. It is very likely to make human error when taking the velocity reading. The micro-manometer gives a significant difference in the value as it varies too often which makes the uncertainties for this measurement becomes larger. However, theoretically both methods should display the same efficiency under isentropic condition.

4. Conclusions To conclude, the enthalpy ratio provides a better approximation to an actual process compared to power ratio. Efficiency calculated using enthalpy ratio has much higher value than ones calculated using power ratio. Power ratio gives a huge error in calculation as it is dependent on highly changing variable although it can be improved by using numerical integration. It was also found that Actual Power is directly proportional to Torque Current in condition that Output Power and Output Current is constant.

5. References Smith, J., & Van Ness, H. (1987). Introduction to Chemical Engineering Thermodynamic (4th ed.). McGraw-Hill Book Company. SuperChargers Online Dot Com. (2017). Retrieved from Roots Type Charger Explained.

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6. Appendices 6.1 Appendix A List of variables and parameters

∆H

J mol−1

Change in enthalpy

T2

Temperature at point 2(outlet)

K

T1

Temperature at point 1(inlet)

K

R

−1

Universal gas constant

J mol K

γ

Ratio of gas constants ( C p /C v )

m ´

Molar flow rate of fluid

η

Isentropic efficiency

W

´ W

= 1.4

mols

Work done by the blower

K

Power requirement of the blower

W

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−1

−1

6.2 Appendix B Followings are the raw data recorded. Table 2: Recorded results

Poout (kW ) 5.00 10.83 15.20 20.80

Pout (kPa) 13.7 25.6 32.0 39.4

P¿ (kPa) -1.4 -2.0 -2.3 -2.7

I torque ( Amps) 18.0 28.3 34.2 41.0

I output ( Amps) 26.0 35.1 40.5 48.0

T out (℃) 27.1 38.5 46.2 53.7

T ¿ (℃) 16.7 17.1 17.9 17.5

Table 3: Recorded velocity at different Output Pressure

Readings 5.00

Velocity (ms −1 ) 10.83 (kW) 15.20 (kW)

(kW ) 1 2 3 4 5 6 7 8 Average

13 16 20 22 21 18 12 5 15.88

ENCH295 Rootes Blower

Uncertainties 20.80

(kW ) 13 24 28 32 32 16 19 15 22.38

19 25 32 35 38 38 38 22 30.88

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15 29 38 41 41 39 30 5 29.75

± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1

6.3 Appendix C Calculation for Isentropic Efficiency using Equation 5a 1. Output Power = 5 kW a. Inlet Temperature

T inlet =16.7+273.15 K =289.85

± 0.1 K

b. Outlet Pressure (absolute)

out ,|¿|=13.7 kPa+101.325 kPa=115.025 ± 0.1 P¿

kPa ( absolute )

c. Inlet Pressure (absolute)

(

P¿ = −1.4 mmWG ×

)

1 kPa =−0.01373 ± 0.1 kPa 102mmWG

¿ ,|¿|=− 0.01373 kPa+101.325 kPa=101.31± 0.1 kPa(absolute ) P¿ d. Outlet Isentropic Temperature Using Equation (2),

T out , isentropic =( 289.85 K )

(

115.025 101.31

)

T2 =

T out ,isentropic

1.4− 1 1.4

=300.56 ± 0.1 K

e. Isentropic Enthalpy Using Equation (1) and let

∆ H isentropic =

ENCH295 Rootes Blower

1.4 ( 8.314) ( 300.56−289.85 )=311.58± 0.1 Jmol−1 1.4 −1

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f.

Actual Enthalpy Again, using Equation (1) and let

∆ H actual =

T 2 = T out ,

1.4 −1 ( 8.314) ( (27.1+ 273.15)−289.85 )=302.63 ± 0.1 Jmol 1.4−1

g. Isentropic efficiency Using Equation (5a),

η=

311.58 =1.03 ± 0.1 302.63

Calculation for Isentropic Efficiency using Equation 5b/c a. Cross-sectional area of pipe Diameter of pipe = 8 cm 2

π 8 −3 2 ) =5.03 × 10 ±0.1 m Cross sectional area= ( 100 4 b. Average velocity across velocity pipe

uave=

(13 +16 + 20 + 22 + 21 + 18 + 12 +5) =14.67 ±0.1 ms−1 8

c. Mass flow rate −3 −1 −3 2 −1 ´ m=1.225 kgm ×14.67 ms ×5.027 ×10 m =0.0903± 0.1 kgs

d. Molar flow rate

´ m=

0.0903 kgs−1 −1 =3.37 ±0.1 mol s −1 0,029 kg mol

e. Isentropic efficiency Using Equation 5b/5c,

η=

1050.132 =0.3 ± 0.1 3.4615

For Output Power 10.83 kW, 15.20 kW and 20.80 kW the calculations were made in the excel spread sheet which is included in the overleaf.

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6.4 Appendix D Calculating Efficiency with enthalpy ratio

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Calculating Efficiency with power ratio

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