Lab Heat Exchanger

1.0 ABSTRACT The aim of this experiment was to demonstrate the heat transfer process using shell and tube heat transfer, to evaluate and study the heat load and head balance, LMTD and overall heat transfer coefficient, to calculate the Reynolds numbers at the shell and tubes sides and lastly to measure and determine the shell and tube sides pressure drop. The experiment was begun with general startup of the apparatus which had been done by the lab assistance. Student only need to manipulated the data by change the water flow rate for hot water and cold water. Firstly, student only set the hot water flow rate at 10 LPM and the cold water flow rate at 2 LPM. After 10 minutes, students recorded all temperature and the pressure drop at the monitor. The cold water flow rate were change by increasing 2LPM until it become same flow rate as hot water. The experiment was repeated by fixed the cold water flow rate at 10 LPM and varies the hot water flow rate. At the end of the experiment, it is found that, the head loss of cold water and hot water changed when the water flow rate adjusted. The LMTD also depends on the flow rate either hot water of cold water. When the flow rate increase, the LMTD also increase. The Reynold Number also has been calculated and it was found that the flow in the apparatus was laminar flow. Overall of the experiment was succeeded.

1.0

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2.0 INTRODUCTION A heat exchanger can be defined as any device that transfers heat from one fluid to another or from or to a fluid and the environment. There are several types of shell and tube heat exchanger: A heat exchanger can be defined as any device that transfers heat from one fluid to another or from or to a fluid and the environment. There are several types of shell and tube heat exchanger:

Figure 1: Heat exchanger with fixed tube plates (four tubes, one shell-pass)

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Figure 2: Heat exchanger with floating head (two tube-pass, one shell pass)

Figure 3: Heat exchanger with hairpin tubes

Basic Considerations in Choosing a Mechanical Arrangement of Heat Exchanger. There are four basic considerations in choosing a mechanical arrangement that provides for efficient heat transfer between the two fluids or vapors, while taking care of such practical matters as preventing leakage from one into the other. They are:    

Consideration for differential thermal expansion of tube and shell Means of directing fluid through the tubes Methods of controlling fluid flow through the shell Consideration for ease of maintenance and servicing

Advantages of Heat Exchanger The main advantages of shell-and-tube heat exchangers are:    

 

Condensation or boiling heat transfer can be accommodated in either the tubes or the shell, and the orientation can be horizontal or vertical. The pressures and pressure drops can be varied over a wide range. Thermal stresses can be accommodated inexpensively. There is substantial flexibility regarding materials of construction to accommodate corrosion and other concerns. The shell and the tubes can be made of different materials. Extended heat transfer surfaces (fins) can be used to enhance heat transfer. Cleaning and repair are relatively straightforward, because the equipment can be dismantled for this purpose.

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Applications of Heat Exchanger Shell and tube heat exchangers represent the most widely used vehicle for the transfer of heat in industrial process applications. They are frequently selected for such duties as:       

Process liquid or gas cooling Process or refrigerant vapor or steam condensing Process liquid, steam or refrigerant evaporation Process heat removal and preheating of feed water Thermal energy conservation efforts, heat recovery Compressor, turbine and engine cooling, oil and jacket water Hydraulic and lube oil cooling Many other industrial applications Shell and tube heat exchangers have the ability to transfer large amounts of heat in relatively low cost, serviceable designs. They can provide large amounts of effective tube surface while minimizing the requirements of floor space, liquid volume and weight

3.0 OBJECTIVES i. To demonstrate the heat transfer process using shell and tube heat transfer. 4

ii.

To evaluate and study the heat load and head balance, LMTD and overall heat transfer coefficient To calculate the Reynolds numbers at the shell and tubes sides. To measure and determine the shell and tube sides pressure drop.

iii. iv.

4.0 THEORY This part of calculation is to use the data in Table 1 to check the heat load Q H and QC and to select the set of values where Qc is the closet to QH Hot water flow rate (HH) QH =mHCpH(THi-THo) Cold water flow rate Qc=mCCpC(TCo-TCi)

Where:QH =Heat load for hot water flow rate QC =Heat load for cold water flow rate mH =Hot water mass flow rate mC =Cold water mass flow rate THi =Hot water inlet temperature THo =Hot water outlet temperature TCo =Cold water outlet temperature TCi =Cold water inlet temperature

LOG MEAN TEMPERATURE DIFERENT (LMTD) 5

T T (¿ ¿ Hi−T Co ) ln (T Ho−T Ci) (¿ ¿ Hi−T Co )−(T Ho−T Ci ) ¿ LMTD=¿ Where all variables are same with the above section:

Overall Heat Transfer Coefficient, U Overall heat transfer coefficient ,U can be calculated by using equation by using equation below. In this experiment, the total heat transfer area, A has been given and equal to 2.93 m2. Q=U . A . LMTD

U=

Q A x LMTD

Where : U= Overall heat transfer coefficient Q = Heat rate with respect to the average head load

Reynolds Number Calculation Re 

DV 

Where  = density of liquid (kg/m3) D =Diameter (m) V=Volumetric flow rate (m3/s) =Viscosity of fluid (kg.s/m) Area of Shell,As= 0.029 ft2 Area of tubes ,At= 0.02139 ft2 Pressure drop 6

This part would determine the following: HW :

The measured tube-inside pressure drop DP (tube) which will be corrected and is expected to be more than calculated tube-side pressure drop.

CW :

The measured shell-inside pressure drop DP (shell) which will be corrected and is expected to be more than calculated tube-side pressure drop.

Notice that, both calculated pressure and also measured pressure are considered in unit mmH2O. In this case, since calculated pressure drop in both of shell and tube side have been obtained during the experiment, so it’s only required conversion factor to change the value into unit of mmH2O. Conversion factor: .

x.bar 

1  105 Pa 1mmH2O  1bar (9.81) Pa

Where x is the calculated pressure value in unit bar.

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5.0 APPARATUS

SHELL AND TUBE HEAT EXCHANGER

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6.0 PROCEDURE Step i) 1. All the pump suction valves (for PH, PC1, and PC2) are checked so that they are fully opened all the time. 2. BVC2 is opened fully but CV2 is closed fully so that PC2 shall operate as a back-mixing pump for tank T2 in the next experiment. Both CV1 and BVC1 are opened fully. Only PC1 shall be used here to pump CW into the Heat Exchanger in the next experiment. Do not switch on any CW pumps (PC1, PC2) yet. 3. HV is closed fully but BVH is opened fully. 4. Pump PH for HW is started to circulate around tank T1 via only BVH. 5. The heaters are started and TlC5 is noted. When the HW in tank T1 is almost 70 ͦC/158 F ͦ (see TlC5), HV is opened fully. The HW flowrate is quickly adjusted to about 25 USGPM by regulating its by-pass valve BVH. 6. Both the CW pumps, PC1 and PC2 are switched on. The CW flowrate is quickly adjusted to about 10 USGPM by regulating the by-pass valve BVC1. 7. The DP Selector Switch is switched to the DP (Shell) position. Step ii) a) The first set of temperature and flowrate readings are taken. CW: Temperature - inlet/outlet, T13*(T1), T14*(T2): Flowrate FC at FI(C*) HW: Temperature - inlet/outlet, TI1*(T1), T12*(T2): Flowrate FH at FI(H*) Note that the CW inlet temperature (T1) is increasing gradually. The CW outlet temperature (T2) varies together with the HW inlet/outlet temperatures t1/t2. It is important that all the temperature and flowrate readings be taken almost simultaneously. These readings are recorded appropriately in Table 1. Also the respective inlet pressure and inlet pressure drop of the CW and HW flow streams are recorded. For the pressure drop readings, 9

DP (shell), DP (tube) at the panel amount DPI*, use the DP signal Selector Switch appropriately as explained below: CW: PG-C; DPI* for DP (shell) with the DP Selector Switch at the DP (shell) position. HW: PG-H; DPI* for DP (tube) with the DP Selector Switch at the DP (tube) position. To take the DP readings at DPI*, they are waited till they are fairly steady. The DP reading is then taken at its highest reading (i.e. peak reading) just when it starts to decrease. b) The second and third sets of the above readings for RUN 1 are continued and taken consecutively. The last set of temperature readings should be taken when all the temperatures are fairly steady. Step iii) 1. RUN 1 is completed, with three sets of the above readings. 2. All the CW pumps PC1 and PC2 are stopped. 3. The heaters are kept on for the next RUN. 4. With the HW pump PH still running, the discharge valve HV is closed fully but the by-pass valve BVH is opened fully. 5. The DP Selector Switch is switched to the equalizing (vertical or “0”) position.

Step iv) 1. RUN 2, 3, 4 and 5 are repeated at different recommended nominal flowrate of CW (i.e. FC) and HW (i.e. FH) using the following procedures check-list. To continue with the next run 10

The HW pump PH is checked so that it is running with BVH fully opened and HV fully closed. With the heaters ON, the HW in tank T1is heated till it almost 70 ͦ C/150 F ͦ (see TlC5). HV is fully opened. The HW flowrate is adjusted until FH at Fl (H*) is almost at the recommended nominal flowrate for the RUN. This is done by regulating the by-pass valve BVH with HV fully opened. (However, if the flowrate is still too high even when its by-pass valve is fully open, its discharge valve, HV is gradually closed to get the required HW flowrate). The CW pumps PC1 and PC2 are started with CV1/BVC1/BVC2 are fully opened but CV2 is fully closed. FC at Fl(C*) is noted. FC is adjusted to the recommended nominal flowrate for the RUN by regulating the by-pass valve BVC1 with CV1 fully opened. (However if the CW flowrate (FC) from PC1 is still inadequate even when its by-pass valve BVC1 is fully closed, use the second CW pump (PC2) by gradually opening CV2 and simultaneously closing BVC2 to get the required CW flowrate). The DP Selector Switch is switched to the DP (shell) position. The various readings for the RUN are taken.

To end a RUN after getting 3 sets of readings All the CW pumps, PC1 and PC2 are stopped. The DP Selector Switch is switched to the equalizing (vertical or “0”) position. With the HW pump PH and the heaters stillON, HV is closed fully nut BVH is opened fully.

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PLANT SHUT-DOWN 1. The heaters are switched OFF. 2. All the pumps (PH, PC1 and PC2) are checked so that they are switched OFF. 3. The DP Selector Switch is switched to the equalizing (vertical or “0”) position. 4. The main power supply to the plant at the front of the panel/cubical is switched OFF. All the pumps suction valves, discharge valves (HV, CV1 and CV2) and by-pass valves (BVH, BVC1 and BVC2) are opened.

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7.0 RESULT FT1 FT2 (LPM) (LPM)

TT1 (C)

TT2 (C)

TT3 (C)

TT4 (C)

DPT1 (mm H2O)

DPT2 (mm H2O

10

2

43.2

30.8

47.8

50.1

104

12

10

4

38.0

29.8

46.8

49.0

105

19

10

6

35.8

29.3

45.6

48.6

104

66

10

8

34.2

29.3

44.5

48.5

105

136

10

10

34.0

29.6

45.3

49.7

104

180

FT1 (LPM)

FT2 (LPM)

QH (KJ/s)

QC (KJ/s)

Q (kJ/s)

10

2

8.65

0.32

4.49

11.20

0.1368

10

4

5.73

0.62

3.175

13.78

0.0786

10

6

4.54

1.26

2.9

14.48

0.0683

10

8

3.42

2.23

2.83

14.75

0.0655

10

10

3.08

3.07

3.07

-

-

TABLE 1

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LMTD

U

TABLE2 FT1 (LPM) 2

FT2 (LPM) 10

TT1 (C) 31.5

TT2 (C) 29.5

TT3 (C) 40.0

TT4 (C) 48.8

DPT1 (mm H2O) 5

DPT2 (mm H2O 180

4

10

31.9

29.5

43.0

48.9

1

176

6

10

32.6

29.6

44.3

49.8

22

177

8

10

33.4

29.5

44.2

48.6

51

177

10

10

34.8

29.5

44.5

48.3

108

176

FT1 (LPM)

FT2 (LPM)

QH (KJ/s)

QC (KJ/s)

Q (kJ/s)

LMTD

U

2

10

0.279

6.138

3.209

13.618

0.08

4

10

0.670

4.115

2.393

15.183

0.054

6

10

1.256

3.836

2.546

15.917

0.055

8

10

2.176

3.069

2.622

14.949

0.06

10

10

3.697

2.881

3.289

14.237

0.079

8.0 CALCULATION 14

For the first reading

FT1 (LPM) 2

FT2 (LPM) 10

TT1 (C) 31.5

TT2 (C) 29.5

Head load: QH

=mHCpH(THi-THo) = (0.033kg/s)(4.185KJ/Kg.K)(31.5-29.5) = 0.2762 KJ/s

Qc

=mCCpC(TCo-TCi) =(0.167kg/s)(4.185kJ/kg.K)(48.8-40.0) =6.138 kJ/s

Log mean temperature different T T (¿ ¿ Hi−T Co ) ln (T Ho−T Ci) (¿ ¿ Hi−T Co )−(T Ho−T Ci ) ¿ LMTD=¿ LMTD=

(29.5−40)−(31.5−48.8) (31.5−48.8) ln (29.5−40)

=13.62C

Overall heat tranfer coefficient U=

Q A x LMTD

U=

3.2071 2.93 x 13.62

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TT3 (C) 40.0

TT4 (C) 48.8

DPT1 (mm H2O) 5

DPT2 (mm H2O 180

=0.08

Reynold Number Re 

DV 

For shell : Re 

(1000)( 0.06)( 0.03) 4(0.8007)

=0.98 For Tube : Re 

(1000)(0.08)(0.167) 4(0.5494)

=6.97

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9.0 DISCUSSION In this experiment, the objectives are to evaluate and study the heat load and head balance, LMTD and overall heat transfer coefficient, to calculate the Reynolds numbers at the shell and tubes sides and to measure and determine the shell and tube sides pressure drop. At the end of the experiments, all objectives are met although maybe there are some errors. The reason the head loss of this experiment was being calculated were because student can observe the energy loss when the heat being transferred from one medium to another medium. Form the table 1, when the flow rate of the cool water being increase, the head loss for cold increase but for head loss for the hot water decrease. This show that at low flowrate of cold water, more energy being absorb by the cold water but less energy loss for hot water. Same goes to table 2, when flow rate of hot water decrease, it need more energy to transfer the heat to reach the required temperature at the output. The pressure drop also depends on the water flow rate. In the table 1, when the cold water was at low flow rate, the pressure drop also low, but it increase when the cold water flow rate increase. It was also happened in the table 2 when the hot water flow rate increase, the pressure drop increase. This is because, at low flow rate, the shear stress of The logarithmic mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold feeds at each end of the double pipe exchanger. The larger the LMTD, the more heat is transferred. From both table, LMTD increasing when the flow rate increasing. But at table 2, the LMTD decrease suddenly which might cause from the error or miscalculation. The overall heat transfer coefficients are also calculated in this experiment to determine the total thermal resistance to heat transfer between two fluids. The resistance can be reduced by increasing the surface area, which will lead to a more efficient heat exchanger. The overall heat transfer were depends on the value of LMTD. If the LMTD increase, the overall heat transfer coefficient will be decrease. It mean that, when the rate of heat transfer is higher, the resistance for the water to absorb heat is low . The calculated Reynolds Number is to determine whether the flow of water in shell and tube heat exchanger is turbulent flow or laminar flow. Form the experiment, it was found that the flow is laminar flow. But according to theory, the flow show be in turbulent flow as the water 17

flow rate increase. All the calculation were based on the data result. So if there are some errors when handling the experiment, the calculation also will have an error.

10.0 CONCLUSION From the experiment, all the objectives had been achieved. Student have ,demonstrate the heat transfer process using shell and tube heat transfer, evaluated and studied the heat load and head balance, LMTD and overall heat transfer coefficient, calculated the Reynolds numbers at the shell and tubes sides and lastly measured and determined the shell and tube sides pressure drop. Although the Reynold Number was not accurate, but overall of the experiment was succeeded.

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11.0 RECOMMENDATION From the experiment, there are few recommendation that can be make so ensure the experiment can have better result. First, the cold water which was used as the cooling agent can be replaced with oil which has highest heat capacity than water. Oil also has high viscosity which allowed the mass flow much slower to avoid much waste. Second recommendation is, the spiral heat exchanger can be replaced with other types such as plate heat exchanger so that student can varies the different between heat exchanger. Student may learnt on which heat exchanger is better to use in industry or for large scale. Last recommendation is, the time or period to take the data can be reduce to less than 10 minutes. This is to ensure the students didn’t waste much time on looking at the apparatus. Student may do other experiment during the 10 minutes times so that they can learn how other apparatus in the lab operating.

12.0 RFERENCES I. II.

Coulson and Richardson; Chemical Engineering; Volume 1, 6th edition. Max S. Peter & Klaus D. Timmerhaus; Plant Design and Economic for Chemical

III.

Engineering; 4th edition; Page 576. Rase, Howard F; Chemical Reactor Design and for Process and plants; Volume 1; 1st

IV.

edition. G.C DRYDEN; The Efficient Use of Energy; 1st edition.

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13.0 APPENDICES

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