Analysis, comments and evaluation

Experiment aim 1,

comparison of parallel flow and counter flow operation. Heat transmission and representa- tion of temperature curves.

The comparison is made using the example of the WL 110.02 Plate Heat Exchanger. Experiments V7-02 (parallel flow) and V8-02 (counter flow) are included.

The following temperature curves are produced using the data acquisition program. Due to the limited number of measuring points, the links between the feed and return temperatures are shown as simplified straight lines here.

In the figures, the designation of the water tem- peratures is enlarged.

Fig. 5.1 Temperature curve for experiment V7-02, WL 110.02, parallel flow

Fig. 5.2 Temperature curve for experiment V8-02, WL 110.02, counter flow

T₁ T₄ T₃ T₆ T₁ T₆ T₃ T₄

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Comments and evaluation:

• In counter flow mode, the heat transmission is better than in parallel flow mode. The values for the mean coefficient of heat transfer k_m are

2,58 for counter flow and

2,25 for parallel flow (see Tab. 5.2, Page 64).

As defined by Formula (4.20), Page 52 the mean heat flow rises as k_m increases . The values in Tab. 5.2, Page 64 confirm this rise. • The temperature curve from experiment V8-02

in Fig. 5.2, Page 66 confirms the assertion from Chapter 4.4, Page 53. In experiment V8-02 the outlet temperature of the heated fluid is higher than the outlet temperature of the cooled fluid. This is not possi- ble in parallel flow mode.

For this reason alone, the heat transmission must be better for counter flow mode than for parallel flow mode.

Experiment aim 2,

Investigation of heat transmission when changing the cold water and hot water flow rates.

The comparison is made using the example of the WL 110.01 Tubular Heat Exchanger in parallel flow mode. The experiments V1-01, V2-01 and V3-01 are analysed.

Fig. 5.3 shows the dependency of the mean kW m⁄( 2⋅K)

kW m⁄( 2⋅K)

Q·m

T₆=T_c,out T₃=T_h,out

Comments and evaluation:

Tab. 5.2, Page 64 and Fig. 5.3, Page 68 show that the mean coefficient of heat transfer k_m increases as the flow rates of cold water ( ) and hot water ( ) rise.

The cause of this increases is the greater turbu- lence caused by the increased flow rates on both sides of the „partition“ (in this case the inner tube). The greater turbulence produces higher coeffi- cients of heat transfer and , and thus lower heat transfer resistances 1/ and 1/ . As defined by Formula (4.14), Page 50 this results in a lower heat transfer resistance 1/k_m and there- fore greater heat transmission.

Fig. 5.3 Mean coefficient of heat transfer k_m as a function of cold water and hot water flow rates, for experiments V1-01, V2-01 and V3-01

0,8 1,0 1,2 1,4 1,6 1,8 2,0 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2

V·

V·

k

_m

k

_m

_{( )}V·

k

_m

_{( )}V·

V·c V·h αc αh αh αc

All rights reserved, G.U.N.T . Ge rätebau, B a rsbüttel, Germany 11/2011 Experiment aim 3,

Investigation of heat transmission when changing the hot water temperature.

The comparison is made using the example of the WL 110.01 Tubular Heat Exchanger in counter flow mode. The experiments V4-01, V5-01 and V6-01 are analysed.

The table below supplements the data from Tab. 5.2, Page 64. In addition to the measured values, the following calculated values are also set out:

• as defined in Formula (4.26), Page 54, here T₃-T₄

• as defined in Formula (4.27), Page 54, here T₁-T₆

• as defined in Formula (4.12), Page 49. • Mean heat flow , from measured value file.

Fig. 5.4 shows the dependency of the mean heat flow on the logarithmic mean temperature difference graphically. ΔT_max ΔT_min ΔT_lm Q·m Experiment SP(T₇) T₁ T₃ T₄ T₆ °C °C °C °C °C °C °C °C kW V4-01 70 67,1 54,4 15,3 29,5 39,1 37,6 38,3 1,31 V5-01 45 43,7 37,4 15,1 22,9 22,3 20,8 21,5 0,69 V6-01 20 20,6 19,8 15,0 17,5 4,8 3,1 3,9 0,16

Tab. 5.3 Parameters and measured values for experiments V4-01 to V6-01, calculated values for experiment aim 3 added

ΔT_{max Δ}T_{min Δ}T_lm Q·m

Q·m

Comments and evaluation:

Fig. 5.4 shows that the mean heat flow increases as the temperature T₄ (hot water feed) or rises. This increase is approximately lin- ear.

Formula (5.1) repeats Formula (4.20), Page 52, rearranged for :

(5.1)

The equation states that changes proportion- ally to if k_m and A_m are constant.

A_m is constant here as these three experiments were performed with the same heat exchanger

k_m should actually also be largely constant. This is generally the case in experiment V4-01 with

k_m=1,37 and in V5-01 with k_m=1,30 . However, in experiment V6-01

k_m=1,68 differs significantly.

Fig. 5.4 Mean heat flow as a function of for experiments V4-01, V5-01 and V6-01

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0 5 10 15 20 25 30 35 40

ΔT

_lm

Q·

m Q·m(ΔT_lm) Q·m ΔT_lm Q·m ΔT_lm Q·m Q·m = km⋅Am⋅ΔTlm Q·m ΔT_lm kW m⁄( 2⋅K) kW m⁄( 2⋅K)

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A possible explanation for this difference is that in experiment V6-01 there is a low temperature dif- ference between the cold and hot water. This increases the impact of any measuring inaccu- racy.

Experiment aim 4,

Comparison of heat transmission for the dif- ferent heat exchanger types.

Comments and evaluation:

The comparison of the average coefficients of heat transfer k_m is important. Analysing the mean heat flows is not useful here as the three heat exchangers have different heat transfer areas (see also Chapter 6.2, Page 81 onwards).

An evaluation can be carried out using Tab. 5.2, Page 64. This is done by comparing those exper- iments that only differ in terms of the heat exchangers, with identical flow rates and the same setpoint SP(T₇) for hot water. Tab. 5.4 shows the corresponding extract from Tab. 5.2: • For parallel flow mode (shown shaded in

blue) these are the experiments V2-01, V7-02 and V9-03.

The values for the average coefficient of heat transfer k_m rise in the following order of the heat exchangers: WL 110.03, WL 110.01 and WL 110.02. Q·m Experi- ment HE Flow direction k_m kW/(m²K) V2-01 01 PF 1,43 V4-01 01 CF 1,37 V7-02 02 PF 2,25 V8-02 02 CF 2,58 V9-03 03 PF 1,27 V10-03 03 CF 1,30

Tab. 5.4 Heat transmission of heat exchanger types

• For counter flow mode the experiments V4- 01, V8-02 and V10-03 are compared.

Once again, the values for the mean coefficient of heat transfer k_m rise in the order of heat exchangers WL 110.03, WL 110.01 and WL 110.02.

The best coefficient of heat transfer by some dis- tance is thus obtained using the WL 110.02 Plate Heat Exchanger.

It is notable that the best heat transmission in experiment V8-02 is linked to the highest differ- ence between the hot water setpoint SP(T₇) = 70°C and the hot water feed tempera- ture T₁= 61,2°C (see Tab. 5.2, Page 64).

The explanation for this is that the mean heat flow = 2,50kW is also at a maximum in this exper- iment. The remaining difference from the installed electric heating power of 3kW (see also Chapter 6.1, Page 79) corresponds to the heat losses from the hot water outside the heat exchanger (hoses, service unit etc.).

All rights reserved, G.U.N.T . Ge rätebau, B a rsbüttel, Germany 11/2011 5.2 Experiments with WL 110.04 5.2.1 Experiment aim

Recording the measured value time response.

5.2.2 General conditions

The adjacent figure repeats Fig. 3.37, Page 36. The experiment is performed as described in a), i.e. a defined volume of cold water inside the tank is heated by the hot water flowing through the heating jacket in batch mode.

The volume of cold water should:

• fill the tank well, so that a large proportion of the heating jacket area is covered.

• such that no water spills over during the exper- iment when stirring.

This defined volume of cold water can be adjusted by filling the tank using the cold water feed (item B in Fig. 3.38, Page 37) on the service unit. However, we recommend adding the cold water with a separate beaker and funnel (see Fig. 5.6). It is useful to measure the water into the beaker in advance. This results in greater accuracy and reproducibility for repeat experiments.

Fig. 5.5 WL 110.04, flow, schematic

A good fill is obtained with 1200g of water. The flow breakers (see also Fig. 3.36, Page 36) are then completely covered.

The other general conditions are selected to quickly achieve significant heating:

• Hot water setpoint SP(T₇)=70°C. • Hot water flow rate = 2,1ltr/min. • Operate the stirrer at maximum speed.

5.2.3 Experimental setup

Connected WL 110 Heat Exchanger Service Unit, commissioning carried out as described in Chapter 3.10, Page 40, in conjunction with the WL 110.04 Jacketed Vessel with Stirrer and Coil.

V·h

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5.2.4 Performing the experiment

The parameters to be set are set out in Chapter 5.2.2, Page 73.

1. Observe the safety instructions (see Chapter 2, Page 5).

2. Secure the WL 110.04 Jacketed Vessel with Stirrer and Coil on the base plate of the service unit as described in Chapter 3.9, Page 35 onwards and connect (see Fig. 5.7, Page 74).

3. Set the main switch (item 35 in Fig. 3.12, Page 19) to „1“.

4. Check the water level in the hot water tank (B) (see Fig. 3.6, Page 15).

– If the hot water tank (B) is empty: Add water until the low level is reached (level switch LSL1 trips and the low water warn- ing lamp (item 29 in Fig. 3.12, Page 19) goes out. Then add 0,5ltr of water with a beaker.

– If the hot water tank (B) is filled but with an unknown volume above the low level: Partially drain the hot water tank (B) (see Fig. 3.8, Page 16) until the low level is reached (level switch LSL1 trips and the low water warning lamp lights up). Then add 0,5ltr of water with a beaker.

5. Start the PC. Start the data acquisition pro- gram.

8. Set the desired hot water setpoint SP(T₇) on the TIC7 controller (28) (see also Fig. 3.13, Page 21).

9. Turn on the heater (H).

10. Measure 1200g of cold water into a sepa- rate beaker.

11. Wait until the hot water temperature T₇ has reached the setpoint SP(T₇).

12. Set the desired hot water flow rate using the regulator valve V1 (9).

13. Make settings for the measured value file. Start automatic measured value recording. 14. Add the content of the beaker to the

WL 110.04 (see also Fig. 5.6, Page 73). 15. Start the stirrer.

Set the maximum speed.

16. Wait until the temperature T₅ of the water in the WL 110.04 has approximately reached the hot water temperature.

17. Save a screenshot for the time response of the measured values in a file.

Give the file a name that will allow you to identify the values in the measured value file later.

18. When the experiment is complete, first turn off the heater (H).

19. Then stop the pump (P).

20. Close the regulator valve V1 (9). 21. Stop the stirrer.

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22. Stop recording and save the measured value file.

23. Set the main switch (35 in Fig. 3.12, Page 19) to “0”.

In document WL110e V1.1 Duplex (Page 73-85)