Discussion Part I And Ii

TITLE OF EXPERIMENT Heat Exchanger OBJECTIVES OF EXPERIMENT 1. To study different types of heat exchanger operation. 2. To study the effect of flow rate on heat transfer. 3. To perform energy balance around a heat exchanger. 4. To study temperature profile across a heat exchanger. 5. To study and compare the heat losses, heat transfer coefficient and log mean temperature difference at different setting and exchangers. INTRODUCTION A heat exchanger is a piece of process equipment which enables heat exchange to happen between two fluids in and out at different temperatures. Both fluids may be contacted or separated by a solid wall. A heat exchanger is add heat to the cold fluid or remove heat from the hot fluid. There are two different configurations parallel flow will be used in this experiment which are co-current flow and counter-current flow. Co-current flow is if both of the fluids flow in the same direction whereas counter-current happens when they flow in the opposite direction. Heat exchangers are widely used in the industries applications such as power plants, space heating, chemical plants and air conditioning. In this experiment, we will focus on shell and tube exchanger and plate exchanger. Shell and tube heat exchangers comprise of a bundle of tube inside a shell. One of the fluid pass through the tubes and another fluid runs over the tube so that heat transfer can be happened. Moreover, plate heat exchangers are considering as the most efficient type of heat exchanger due to theirs low cost, high thermal transfer, flexibility and easy maintenance. They are usually used broadly in the food and beverage industries as they are easy to detach for cleaning purpose.

As the

temperature difference between two fluids is the main driving force of the heat transfer process. Log mean temperature difference is used to calculate the right

average temperature (Bengtson, 2010). There are some calculations and results will be discussed in the report. MATERIALS AND EQUIPMENT The four heat exxhangers supplied with the unit: A. Shell and Tube Heat Exchanger B. Spiral Heat Exchanger C. Concentric (Double Pipe) Heat Exchanger D. Plate Heat Exchanger

V20

SHELL & TUBE HEAT EXCHANGER

V4

V5

SPIRAL HEAT EXCHANGER

V19

V22

V6

V7

T2

CONCENTRIC HEAT EXCHANGER

V21

V24

V8

V9

PLATE HEAT EXCHANGER

V23 V25

V26 V11

V10 T1 T3

FI1 Water Inlet

T4

V27

V17

V15 FI2 V28

FT1

Hot Water Tank

FT2

V3 HEATER

Cold Water Tank

V14

V2

V13 V18

LEVEL SWITCH

V29

Water Inlet

LEVEL SWITCH

V16

V12

V1

To Drain

Pump 2

V30

To Drain

Pump 1 To Drain

Figure above shows Schematic Diagram for Heat Exchanger Training Apparatus

RESULTS AND CALCULATIONS

Table 1: Experimental data under co-current flow with constant hot water flow. Hot water’s flow rate, F|1( L/min)

Cold water’s flow rate, F|2 ( L/min)

T|1 (oC)

T|2 (oC)

T|3 (oC)

T|4 (oC)

0.5

2.5

52.4

39.0

29.3

30.0

0.5

3.0

52.6

35.9

29.4

30.0

0.5

3.5

52.7

35.9

29.5

29.9

0.5

4.0

52.8

36.5

29.5

29.8

0.5

4.5

52.8

36.7

29.5

29.8

Table 2: Experimental data under counter-current flow with constant hot water flow. Hot water’s flow rate, F|1( L/min)

Cold water’s flow rate, F|2 ( L/min)

T|1 (oC)

T|2 (oC)

T|3 (oC)

T|4 (oC)

0.5

2.5

49.9

41.1

30.1

28.7

0.5

3.0

49.7

40.5

29.9

29.0

0.5

3.5

48.2

40.8

29.4

29.1

0.5

4.0

51.8

38.1

30.0

29.1

0.5

4.5

52.3

38.9

29.8

29.1

Table 3: Summarize table for co-current flow under constant hot water flow Hot water’s flow rate, F|1( L/min)= 0.5 L/min Cold water flow

Heat transferred, 𝑄ℎ (W)

Heat absorbed, 𝑄𝑐 (W)

rate( L/min)

Heat loss,

∆𝑇𝑙𝑚

Uo

Ui

𝑄𝑙𝑜𝑠𝑠

(oC)

(kW/m2oC)

(kW/m2oC)

(W)

2.5

460.81

121.3

339.51

15.17

0.01599

0.3938

3.0

574.29

124.84

449.85

12.84

0.01945

0.5798

3.5

577.73

97.1

480.63

12.75

0.01523

0.5872

4.0

560.54

83.23

477.31

13.45

0.01238

0.5403

4.5

553.66

93.63

460.05

13.78

0.01359

0.5210

Table 4: Summarize table for counter flow under constant hot water flow Hot water’s flow rate, F|1( L/min)= 0.5 L/min Cold water flow

Heat transferred, 𝑄ℎ (W)

Heat absorbed, 𝑄𝑐 (W)

rate( L/min)

Heat loss,

∆𝑇𝑙𝑚

Uo

Ui

𝑄𝑙𝑜𝑠𝑠

(oC)

(kW/m2oC)

(kW/m2oC)

(W)

2.5

302.62

242.78

59.84

15.819

0.03070

0.2475

3.0

316.62

218.4

97.92

15.276

0.02860

0.2687

3.5

254.48

83.23

171.24

14.97

0.01110

0.2204

4.0

471.12

280.89

190.23

14.47

0.03870

0.4220

4.5

460.8

216.71

244.1

15.28

0.02836

0.3909

H eat rel eased agai n st col d w ater f l ow rate 700

HEAT RELEASED (W)

600 500 400 Co-current flow counter current flow

300 200 100 0 2.5

3

3.5

4

4.5

FLOW RATE OF COLD WATER (L/MIN)

Figure 1: A graph of heat released from hot water against the cold water flow rate under constant hot water flow rate.

H eat ab sorb ed agai n st f l ow rate of col d w ater 300

HEAT ABSORBED (W)

250 200 co-current 150

counter-current

100 50 0 2.5

3

3.5

4

4.5

FLOW RATE OF COLD WATER (L/MIN)

Figure 2: A graph of heat absorbed by cold water against the cold water flow rate under constant hot water flow rate.

H eat l oss agai n st f l ow rate of col d w ater 600

HEAT LOSS (W)

500 400 300

co-current counter-current

200 100 0 2.5

3

3.5

4

4.5

FLOW RATE OF COLD WATER (L/MIN)

Figure 3: A graph of heat loss against the cold water flow rate under constant hot water flow rate.

T e m p e r a t u r e ° C

(Co-Current) Temperature Profile with F|1=0.5LPM and F|2=2.5LPM 60

TH,I, 52.4

TH,O, 39

50 40 30 TC,I, 29.3

20

Cold

TC,O, 30

Hot

10 0 0

L Length

Figure 4: (Co-Current) Temperature Profile with F|1=0.5LPM and F|2=2.5LPM

T e m p e r a t u r e ° C

(Counter-Current) Temperature Profile with F|1=0.5LPM and F|2=2.5LPM 60 TH,I, 49.9

50

TH,O, 41.1

40 30 TC,O, 30.1

20

Cold

TC,I, 28.7

Hot

10 0 0

L Length

Figure 5: (Counter-Current) Temperature Profile with F|1=0.5LPM and F|2=2.5LPM

Calculation Sample calculation to obtain the energy balance, heat transfer coefficient and Log Mean Temperature Difference (LMTD). By using the experimental data recorded in Table 1 (co-current flow): ./When F|1 = 0.5 L/min, F|2 = 2.5 L/min,

Density of hot water at 52.4 °C, 𝜌ℎ = 986.89 kg/𝑚3 Density of cold water at 29.3 °C, 𝜌𝐶 = 995.85 kg/𝑚3 Heat capacity of hot water at 52.4 °C, 𝐶ℎ = 4181.5 J/kg.K Heat capacity of cold water at 29.3 °C, 𝐶𝑐 = 4178.6 J/kg.K

Mass flow rate of hot water, 𝑚ℎ ,= F|1 x 𝜌ℎ

= 0.5 L/min x 986.89 kg/𝑚3 x

1𝑚3 1000𝐿

x

1𝑚𝑖𝑛 60𝑠

= 8.224 x 10−3 kg/s Mass flow rate of cold water, 𝑚ℎ ,= F|2 x 𝜌𝑐

= 4 L/min x 995.85 kg/𝑚3 x = 0.0664 kg/s i) Energy balance: Heat equation: Q = mc∆T

Heat transferred by hot water: 𝑄ℎ = 𝑚ℎ 𝐶ℎ |(T|2 - T|1)| = 8.224 x 10−3 x 4181.5 x |(52.8-36.5)| = 560.54 W Heat absorbed by the cold water: 𝑄𝑐 = 𝑚𝑐 𝐶𝑐 (T|4 - T|3) = 0.0664 x 4178.6 x (29.8-29.5) = 83.23 W

Heat loss: 𝑄𝑙𝑜𝑠𝑠 = 𝑄ℎ - 𝑄𝑐 = 560.54 – 83.23 = 477.31 W

1𝑚3 1000𝐿

x

1𝑚𝑖𝑛 60𝑠

ii) Log mean temperature difference (LMTD): From lab manual, Log mean temperature difference (LMTD) can be obtained from the equation:

Tlm 

T1  T2  ln  T1 T2  

∆𝑇1= T|1- T|4 = 52.8 - 29.8 = 23 oC, ∆𝑇2 = T|2- T|3 = 36.5 – 29.5 = 7 oC

Tlm 

T1  T2  ln  T1 T2   23−7 =

23

𝑙𝑛 7

= 13.45 °C

iii) Heat transfer coefficient, U: From lab manual,

q  U o Ao Tm  U i Ai Tm Where, Ao= outside area of the tube, m2 Ai= inside area of the tube, m2 Tm= mean temperature difference, °C Uo= overall heat transfer coefficient based on the outside area of the tube, kWm-2°C-1 Ui= overall heat transfer coefficient based on the inside area of the tube, kWm-2°C-1

For Plate heat exchanger, Nominal surface = 0.5 m2 Plate width = 124.46 mm Plate length = 309.88 mm Plate channel = 43.18 mm Number of plates = 4

𝐴𝑜 = Nominal surface of plate heat exchanger = 0.5 m2 𝐴𝑖 = effective heat transfer area of plate heat exchanger = (No of plates – 2) x area per plate = (4-2) x (0.12446 x 0.30988) = 0.07714 m2

𝑈𝑜 =

𝑞𝑐 𝐴𝑜 ∆𝑇𝑙𝑚 83.23

= (0.5)×(13.45) x

1𝑘𝑤 1000𝑤

= 0.01238 𝑘𝑊/𝑚2 °𝐶

𝑈𝑖 =

𝑞ℎ 𝐴𝑖 ∆𝑇𝑙𝑚 560.54

= (0.07714)×(13.45) x

1𝑘𝑤 1000𝑤

= 0.5403 𝑘𝑊/𝑚2 °𝐶

DISCUSSION The valves arrangement to get counter-current flow for hot water flow is V27, V2, V3, V10 and V11 while for cold water flow is V28, V13, V14, V15, V26, V25 and V18. Moreover, the valves arrangement to get co-current flow for hot water flow is V27, V2, V3, V10 and V11 while for cold water is V13, V14, V17, V25, V26 and V16. The temperature indicators used to measure the inlet temperature of hot water is T1 while outlet temperature is T2. Besides that, the temperature indicators are used to measure the inlet and outlet temperature of cold water are T3 to T4 in counter-current flow while there are T4 to T3 in co-current flow. From Figure 4, it could be seen that the temperature of hot water decreases from TH,I,

52.4 oC to TH,O, 39.0 oC across the length. However, the temperature of the

cold water increases from 29.3 oC to 30.0 oC. This is applicable for co-current case in heat exchanger. The pattern is the same for hot water and cold water with increasing flow rate. When the direction of water flow is parallel towards each other, the temperature difference gets smaller and closer to one point in between TH,I and TC,I. Thus, heat transfer is constrained by the temperature of the cold inlet temperature (Nath, 2014). From Figure 5, the temperature of hot water in counter-current flow also decreases, from TH,I ,49.9 oC to TH,O, 41.1 oC. Meanwhile, the cold water temperature also increases from TC,I, 28.7 oC to TC,O, 30.1 oC. Although the temperature patterns for both streams are the same but, the direction of cold water is opposite to the direction of hot water. Inlet temperature of the hot water, TH,I coincides with the outlet temperature of the cold water, TC,O and vice versa. This results in two parallel-like lines. From Table 3 and Table 4, it could be seen that the heat transfer of co-current flow is larger than that of counter-current flow. Co-current flow has faster heat transfer due to high temperature difference at the initial state. It is also advantageous if we want to maintain a certain temperature or to prevent the temperature from exceeding certain temperature (Nath, 2014). From Figure 4 and Figure 5, the overall heat transfer is better in counter current flow.

Increasing flow rates will increase the heat absorbed by the water. Furthermore, this leads to increment of heat transfer and eventually leads to heat transfer coefficient. Increasing mass flow rate increases the velocity. Convection involves diffusion and bulk motion of molecules. At low velocity, dispersal dominates. Several improvements can be made to increase the accuracy of the results. Valve that controls the flow rate of the hot water should be fixed. Due to dysfunction of the valve, hot water flow rate is assumed to be less than 1 LPM throughout the experiment, Co-current and Counter-current. Besides, the cold water tank should always be filled continuously. High flow rate will results in fluctuation of the readings. The heat exchanger is also seemed to be rusty. This causes blockage in the pipeline. Therefore, the temperature is not that accurate. For better improvement, clean up the rusty part. Shell and tube heat exchanger and plate heat exchanger has their disadvantages and advantages. Shell and tube heat exchanger is more preferable due to its advantages compared to plate heat exchanger. First of all, it is cheaper than that of plate heat exchanger. Besides, it can withstand high pressure and temperature conditions. Furthermore, leakage in tube can be detected easily. Pressure drop is also lower than that of plate heat exchanger. Sacrificial anodes are used to deal with corrosion. Therefore, corrosion in shell and tube heat exchanger is lesser than that of plate heat exchanger. However, there is no doubt that plate exchanger also has its own advantages. Cleaning for plate heat exchanger is shorter compared to shell and tube heat exchanger. Shorter maintenance time means lesser loss in industry in terms of profit. This is because shell and tube heat exchanger requires enough clearance in one go in order to get rid of tube nest. Besides, plate heat exchanger has higher efficiency with higher capacity compared to heat and tube exchanger. In terms of spacing, shell and tube heat exchanger take more space. In industry, heat exchanger is used in petrochemical, nuclear and larger process industries. These industries deal with large amount of harmful chemicals. Heat exchanger is crucial to remove the heat produced. Industries involved include food and beverage industry, polymer industry and hydrocarbon production.

CONCLUSION Value of heat transferred by hot water and heat absorbed by cold water is different due to heat loss. Log mean temperature difference (LMTD) differs with Counter-Current flow higher than that of Co-Current flow. Counter flow is preferable due to the constant temperature difference between hot and cold water. Co-current flow is limited by the temperature of inlet cold water. Higher flow rates increases the heat transfer coefficient. The heat transfer coefficient of outer area of tube for Counter-Current is higher than that of Co-Current.

REFERENCES Bengtson, H. (2010). Heat Exchanger Theory and the Heat Exchanger Design Equation. [online] Brighthub Engineering. Available at: http://www.brighthubengineering.com/hvac/59900-fundamentals-of-heat-exchangertheory-and-design/ [Accessed 9 Mar. 2017]. Nath, S. (2014, December 29). Why is a counter flow heat exchanger better than a parallel flow heat exchanger? Retrieved March 9, 2017, from Quora: https://www.quora.com/Why-is-a-counter-flow-heat-exchanger-better-than-aparallel-flow-heat-exchanger