Condenser

It was not possible to adequately assess the performance of the condenser during normal operation due to difficulties in stabilising and accurately measuring the reflux rate. Therefore, extensive tests were completed with hot water flowing through the inner pipe with various inlet temperatures and flow rates. The results of these tests are shown in Table 12.6. The chilled water flow could not be measured and was calculated via an energy balance. The heat transfer coefficients calculated from this data are not directly applicable to the expected operating conditions (e.g. no phase change in the inner pipe and different physical properties) but were used to estimate the fouling resistance of the condenser during normal operation. This data was then applied to the original design basis to calculate the heat transfer coefficient at the expected operating conditions.

The measured and predicted heat transfer coefficients for the test data for various values of the fouling resistance are displayed in Figure 12.4. This data indicates that no value of the fouling resistance provides a satisfactory agreement between the measured and predicted heat transfer coefficients: the HTC is generally overestimated where low and underestimated where high. The discrepancy is probably due to changes in the flow regime and Figure 12.5 shows that the predictions are mostly accurate for turbulent flows but optimistic for laminar and transitional flows (say, Re < 6000). Since the expected operating conditions for the condenser lie in the turbulent flow regime, a fouling resistance of 0.0003 m2.°C/W was assumed to predict the actual condensing performance. This corresponds to a heat transfer coefficient of 1274 W/m2/°C and a heat transfer rate of greater than 20 kW.

The calculated heat transfer duty that is available from the condenser greatly exceeds the design cooling duty (7.3 kW) and, therefore, suggests satisfactory condenser performance. However, the effects of small amounts of non-condensibles (e.g. nitrogen that remains in the column after purging) could not be accounted for and could possibly reduce the heat transfer rate considerably. It will be important to minimise non-condensibles by venting some of the distillate as vapour during start-up. Furthermore, the fouling resistance contributes 30-50% of the total heat transfer resistance and fouling with a higher resistance than that which is anticipated will cause a noticeable deterioration in condenser performance.

Table 12.6 - Heat Transfer Data for the Condenser

Inner Pipe

Outer Pipe

Inlet

Outlet

Inlet

Outlet

Flow

Temp.

Temp.

Flow

Temp.

Temp.

LMTD

Duty

HTC

(L/min)

(°C)

(°C)

(L/min)

(°C)

(°C)

(°C)

(kW)

(W/°C/m2)

0.62

47.8

11.0

57

7.2

7.6

15.4

1.52

412

0.83

49.6

12.5

44

7.0

7.7

17.9

2.35

548

0.88

42.9

13.9

36

8.5

9.2

15.5

1.77

478

1.12

43.7

11.7

51

7.2

7.9

15.1

2.52

698

1.45

38.2

11.0

56

7.3

8.0

12.6

2.65

877

1.46

40.0

13.8

35

8.6

9.8

14.1

2.65

785

1.78

44.6

13.7

42

8.7

9.9

15.4

3.81

1032

2.04

38.1

13.1

39

8.8

10.1

12.6

3.54

1167

2.29

63.5

14.4

56

7.4

9.4

23.0

7.57

1373

2.40

37.6

14.0

40

8.9

10.3

13.2

3.93

1239

2.60

49.5

15.3

30

8.3

11.3

18.4

6.33

1439

2.85

50.6

14.0

55

7.0

8.9

19.4

7.10

1525

3.47

38.0

13.8

31

8.0

10.7

13.9

5.83

1756

3.53

63.5

17.5

52

7.0

10.1

26.4

11.31

1792

3.72

36.3

13.2

53

7.1

8.7

14.2

5.91

1733

4.22

60.5

20.5

38

8.7

13.1

25.6

10.61

1731

4.46

63.5

20.4

53

7.6

11.2

28.1

13.26

1974

4.59

38.8

15.5

33

8.0

11.2

15.4

7.17

3000

i, 2000

"2 1500

tfl n

1 1000

500 0

0 500 1000 1500 2000 2500 3000 3500

Predicted HTC (W/m2/C)

Figure 12.4 - Predicted and Measured HTCs for the C onden^er H est Conditions)

3000

Figure 12.4 - Predicted and Measured HTCs for the C onden^er H est Conditions)

-300

I-500

ca 0

TRANSITIONAL

LAMINAR

TURBULBiT

-700

2000 4000 6000 8000 10000

Reynolds' Number (Inside)

12000

14000

Figure 12.5 - Effect of Flow Regime on the Heat Transfer Correlation

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