ABSTRACT
This Film and Dropwise Condensation Unit is used to observe the process of heat transfer during condensation, as well as gather experimental data for a better theoretical understanding. The unit itself contained bench top unit, with an integrated steam generator and air extraction system. The main components in the unit are the specially designed condensers for the observation of both filmwise and dropwise condensation about the differences of them. In this experiment, we want to study the heat transfer coefficient and the heat flux. Besides that, we were study the effect of air inside the chamber. Condensation occurs when vapour changes to liquid state with a large heat-transfer coefficient. Filmwise condensation occurs on a vertical or horizontal plane when a film of condensate is formed on surface and flows by action of gravity. Dropwise condensation occurs when small drops formed on surface. The heat transfer coefficient can be calculated using Nussselt equation. . Nusselt assumed that the heat transfer that occurs from the vapour through the film and to the wall is conduction. Since the process is conduction is well known, therefore calculation on the heat-transfer coefficients can be done. In this experiment, obtained data are for Tsat, Tsurf , Tin and Tout.
INTRODUCTION
Filmwise and Dropwise are two forms of condensation. In filmwise condensation a laminar film of vapour is created upon a surface. This film can then flow downwards, increasing in thickness as additional vapour is picked up along the way. In dropwise then flow downwards, accumulating static droplets below them along the way. When the rate of condensation is low (e.g., a noncondensible gas is present) or when the liquid does not "wet" the wall, dropwise condensation occurs. In most engineering components where condensation is a required part of an industrial process film condensation is expected, because of the large mass flux of condensed liquid per unit length of wetted area. Dropwise condensation was first recognized by Schmidt et al. (1930), and much interest was stimulated by their report that heat transfer coefficient were between 5 and 7 times those found with film condensation. Over the years there have been a few demonstrations of successful applications on an industrial scale. This experiment would be used in by any industry which is trying to increase the efficiency of heat transfer. An example of this is any vapour power cycle such as the rankine cycle. By increasing the efficiency of the condenser, its operational pressure can be reduced and the overall efficiency of the cycle can be increased. Dropwise condensation is difficult to sustain reliably; therefore, industrial equipment is normally designed to operate in filmwise condensation mode.
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In all application, the steam must be condensed as it transfer heat to a cooling medium which could be cold water in a condenser of generating station, hot water in a heating calorifier, sugar solution in a sugar refinery and etc. during condensation very high heat fluxes are possible and provided that the heat can be quickly transferred from the condensing surface into the cooling medium, the heat exchangers can be compact and effective. The SOLTEQ Film & Dropwise Condensation Unit (Model: HE163) is designed to help student to understand several key aspects in condensation topic, in particular the process of filmwise and dropwise condensation. It allows students to visualize both phenomena and perform a few experiments to demonstrate both concepts and how their a pplied and give benefit in industry.
OBJECTIVES 1. To demonstrate the filmwise and dropwise condensation. 2. To describe filmwise and dropwise condensation 3. To demonstrate the effect of air on heat transfer coefficient of condensation 4. To demonstrate the filmwise heat flux and surface heat transfer coefficient ay constant pressure. 5. To determine the dropwise heat flux and surface heat transfer coefficient at constant pressure.
THEORY
Mechanism of Condensation
Condensation of a vapor to a liquid and vice versa, both involve a change of a fluid with large heat-transfer coefficients. Condensation occurs when a saturated vapor such as steam makes a contact with a solid whose surface temperature is below the saturation temperature, to form a liquid such as water. When a vapor condenses on a surface , for example vertical or horizontal tube or other surface, a film of condensate is formed on the surface and flows over the surface because of gravity. It is this film of liquid between the surface and the vapor that produce the main resistance of heat transfer. This is called filmwise condensation. Another type of condensation is dropwise condensation. Dropwise condensation occurs when small drops are formed on the surface. These drops grow and mix together, and the liquid flows from the surface. Large areas of tube are devoid of any liquid and are exposed directly to the vapor during condensation. Very high rates of heat-transfer occur on these bare areas. The average heat transfer coefficient for dropwise condensation is five to ten times larger than the filmwise coefficient. 2
Dropwise condensation can be promoted by making the surface non-welting by coating. However, dropwise condensation is difficult to maintain in industrial applications because of oxidation, fouling and degrading of coating, and finally film condensation occurs. Therefore, condenser designs are often based on the assumption of filmwise condensation.
Film-condensation coefficients for vertical surfaces Film type condensation on a vertical wall or tube can be find analytically by assuming laminar flow of the condensate film down the wall. The film thickness is zero at the top of the wall or tube. It increases in thickness as it flows downward as a result of condensation. Nusselt assumed that the heat transfer from the condensing vapor at Tsat, through this liquid film, and at the wall at Tw, was by conduction. Equating this heat-transfer by conduction to that from condensation of the vapor, final expression can be obtained for the aver age heat-transfer coefficient over the w hole surfaces.
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APPARATUS
Equipment Prefer: SOLTEQ MODEL: HE 163 (Film and Dropwise Condensation Unit)
PROCEDURE
General Start-up 1. The main switch is ensured in its off position. 2. The power regulator knobs are turned fully anti-clockwise to set the power to minimum. 3. Valves V1 to V6 are checked to ensure its closed. 4. The chamber is filled with distilled water until the water level stays between the hater and baffles plates. The heater is ensured fully immersed in the water throughout the experiment. The chamber is filled with water through the drain valve with the vent valve, V4 opened. Then the vent valve V4 is closed.
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5. The water flow rate to the condenser is adjusted by controlling the control valve according to experimental procedure. 6. The main switch and the heater switch are turned on. The heater power is set by rotating the power regulator clockwise to increase the hater power. 7. The water temperature reading is observed where the water temperature should increase when its start heat-up. 8. The water is heated up to boiling point until the pressure reaches 1.02-1.10 bar. Immediately valve V1 is opened and follow by valve V5 for 1 minute to vacuum out the air inside condenser. Then both valve V1 and V5 is closed. 9. The system is let to stabilize. Then all relevant measurement is taken for experimental purposes. Adjustment is made if required.
General shut-down 1. The voltage control knob is turned to 0 Volt position by turning the knob fully anti-clockwise. The cooling water is kept flowing for at least 5 minutes through the condenser to cold them down. 2. The main switch and power supply are switch off. Then, the power supply cable is unplugged. 3. The water supply is closed and the cooling water connection tubes are disconnected if necessary. Otherwise, the connection tubes are leaved for next experiment. 4. The water inside the chamber is discharged using the discharge valve.
A) Demonstration of filmwise and dropwise condensation 1. The basic procedure is followed as written in the general set-up. The equipment by make sure connected to the service unit.
B) The filmwise heat flux and surface heat transfer coefficient determination at constant Pressure
1. Cooling water is circulated through the filmwise condenser starting with a minimum value of 0.1 LPM. 5
2. The heater power is adjusted to obtain the desired pressure at 1.01 bar. 3. When the condition is stabilized, the steam ( ) and surface temperature ( ) , (T1) and
(T2), and flowrate are recorded.
C) The dropwise heat flux and surface heat transfer coefficient determination at Constant pressure
1. Cooling water is circulated through the dropwise condenser starting with a minimum value of 0.4 LPM. 2. The heater power is adjusted to obtain the desired pressure at 1.01 bar. 3. When the condition is stabilized, the steam ( ) and surface temperature , (T3), (T4) and flowrate are recorded.
RESULTS Experiment 1: Demonstration of filmwise and dropwise condensation Pressure is kept constant at 1.02-1.10 bar Tin= 30.2 Tout=29.7 Tsat=61.8 6
Tsurf=31.0
Figure 2: (Left) The Filmwise Condensation and (right) The Dropwise Condensation
Experiment 2: Table 1: The filmwise heat flux and surface heat transfer coefficient determination at constant pressure Tin Tout Tsat Tsurf ΔTm ɸ Flowrate (LMP) Power (P) ( C) q U ( C) ( C) ( C) ( C) 0.1
185
31.9
32
66.9
30.7
34.95
0.70
172.82
4.94
0.2
205
34.3
34.6
70.4
33.3
35.95
4.19
1036.91
28.84
0.3
230
34.6
34.9
70.8
34.1
36.05
6.28
1555.36
43.14
0.4
259
34.7
34.9
71.6
33.8
36.80
5.58
1382.54
37.57
0.5
269
34.6
34.8
70.8
34
36.10
6.98
1728.18
47.87
0.6
274
34.7
34.9
71.4
33.8
36.60
8.37
2073.82
56.66
7
2500.00 2000.00 x u l f t a e H
1500.00 1000.00 500.00 0.00 30
35
40
Tsat-Tsurf
Graph 1: Heat Flux vs. Tsat-Tsurf for filmwise without air
Heat coefficient against Tsat-Tsurf 60.00 50.00 t n e i c i f f e o C t a e H
40.00 30.00 20.00 10.00 0.00 30
32
34
36
38
Tsat-Tsurf
Graph 2: Heat coefficient against Tsat-Tsurf for filmwise without air
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Experiment 3: Table 2: The dropwise heat flux and surface heat transfer coefficient determination at constant pressure Tin Tout Tsat Tsurf ΔTm Flowrate (LMP) Power (P) ( C) q ɸ U ( C) ( C) ( C) ( C) 0.4
347
34.6
35.7
68.8
41.6
33.65
30.70
7605.52
226.04
0.6
365
34.4
35.2
71.4
44.2
36.60
33.49
8296.93
226.70
0.8
487
34.9
35.9
72.2
44.6
36.80
55.82
13828.21
375.79
1
562
34.4
35.2
72
45
37.20
55.82
13828.21
371.74
1.2
588
35.1
35.9
71.8
45.2
36.30
66.99
16593.86
457.15
1.4
623
34.4
35.2
71.6
45.7
36.80
78.15
19359.50
526.09
Heat Flux vs. Tsat-Tsurf 25000.00 20000.00 x u l F t a e H
15000.00 10000.00 5000.00 0.00 25.5
26
26.5
27
27.5
28
Tsat-Tsurf
Graph 3: Heat Flux vs. Tsat-Tsurf for dropwise without air Heat Coefficient vs. Tsat-Tsurf 600.00 500.00 t n e i c i f f e o C t a e H
400.00 300.00 200.00 100.00 0.00 25.5
26
26.5
Tsat-Tsurf
9
27
27.5
28
Graph 4: Heat coefficient against Tsat-Tsurf for dropwise without air
Heat Flux vs. Tsat-Tsurf 25000.00 20000.00 x u l F t a e H
15000.00 Dropwise
10000.00
Filmwise 5000.00 0.00 25
30
35
40
Tsat-Tsurf
Graph 5: The comparison of heat flux vs Tsat-Tsurf for dropwise and filmwise in the absence of air Heat coefficient vs Tsat-Tsurf 600.00 500.00 t n e i c i f f e o C t a e H
400.00 300.00 Dropwise
200.00
Filmwise
100.00 0.00 20
25
30
35
40
Tsat-Tsurf
Graph 6: The comparison of heat coe fficient vs Tsat-Tsurf for dropwise and filmwise in the absence of air
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Experiment 4: Filmwise
Flowrate (LMP)
Table 3: The effect of air inside chamber for filmwise Tin Tout Tsat Tsurf ΔTm Power (P) ( C) q ( C) ( C) ( C) ( C)
ɸ
U
0.1
248
33.1
35.1
68.3
33.7
34.19
13.96
3457.05
101.112
0.2
275
33.1
33.6
69.9
32.5
36.55
6.98
1728.53
47.293
0.3
298
33.1
34.9
69.7
33.9
35.69
37.68
9334.04
261.513
0.4
308
33.3
33.4
69.8
32.1
36.45
2.79
691.41
18.969
0.5
326
33.3
33.4
69
32.1
35.65
3.49
864.26
24.243
0.6
339
33.4
33.5
70.9
32.1
37.45
4.19
1037.12
27.693
Heat coefficient vs Tsat-Tsurf 300.000 250.000 t n e i c i f f e o C t a e H
200.000 150.000 Heat coefficient
100.000 50.000 0.000 34
35
36
37
38
39
40
Tsat-Tsurf
Graph 7: The heat coefficient vs Tsat -Tsurf for filmwise with 1% air
Dropwise
Flowrate (LMP)
Table 4: The effect of air inside chamber for dropwise Tin Tout Tsat Tsurf ΔTm Power (P) ( C) q ɸ ( C) ( C) ( C) ( C)
U
0.4
351
33.4
34.3
71.3
32.6
37.45
25.12
6222.70
166.17
0.6
499
33.4
34.4
70.5
32.4
36.60
41.87
10371.16
283.38
0.8
599
33.6
34.4
71.2
32.6
37.20
44.66
11062.57
297.39
1
383
33.6
34.5
71.4
32.7
37.35
62.80
15556.74
416.53
1.2
571
33.8
34.7
72.8
32.9
38.55
75.36
18668.09
484.28
1.4
665
34
34.9
71.7
33
37.25
87.92
21779.44
584.71
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Heat coefficient vs Tsat-Tsurf 700.00 600.00 t n e i c i f f e o C t a e H
500.00 400.00 300.00 Heat coefficient
200.00 100.00 0.00 38
38.5
39
39.5
40
Tsat-Tsurf
Graph 8: The heat coefficient vs Tsat-Tsurf for dropwise with 1% air
Heat coefficient vs Tsat-t surf for filmwise 300.00 250.00 t n e i c i f f e o c t a e H
200.00 150.00
without air
100.00
with air
50.00 0.00 34
35
36
37
38
39
40
Tsat-Tsurf
Graph 9: The heat coefficient vs Tsat -Tsurf for filmwise with and without 1% of air
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Heat coefficient vs Tsat-t surf for dropwise 700.00 600.00 t n e i c i f f e o c t a e H
500.00 400.00 300.00
without air
200.00
with air
100.00 0.00 20
25
30
35
40
Tsat-Tsurf
Graph 10: The heat coefficient vs Tsat-Tsurf for dropwise with and without 1% of air
DISCUSSIONS
In this experiment we are mostly investigate about the film boiling condensation by using the SOLTEQ Film and Dropwise Condensation Unit (Model: HE 163). There are 4 objectives that must be accomplished. For the experiment 1, we have to demonstrate the filmwise and dropwise condensation. From this experiment, we are able to describe the characteristics of filmwise and dropwise condensation. In filmwise condensation, most materials used in the construction of heat exchangers are “wettable” and during the condensation a film condensate spreads over the surface.
More vapour condenses onto the outside of this film will increases its thickness and causes the flow downward and drip from the lowest points. The heat given up by the vapour during condensation is conducted through the film. During filmwise condensation a layer of condensate covers the cool surface and this will causes the resistance to the transfer of heat. However, for the dropwise condensation the material used in the construction is “non-wet table” .When the steam condenses, a large number of
spherical forms on its surface. These beads become larger and then the trickle downwards. The moving bead gathers all the static beads along its downward path, becomes larger, accelerates and leaves a virtually bare surface in its trail. For experiment 2 and 3, theoretically there is a big difference from the graph for the dropwise and filmwise condensation based on its heat transfer coefficient and temperature difference. For dropwise there is a relatively larger area heat transfer coefficient that proposes a larger value of heat transfer during condensation. As for filmwise there is a smaller value of heat transfer and the larger area on the graph that shows the larger margin of heat transfer value are at larger heat transfer value. But for the errors that has occur, they didn’t match for the desirable data.
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For experiment 4, theoretically, the clear difference between both heat transfer coefficient and temperature difference for filmwise and dropwise condensation is for dropwise with the present of air there is a larger value of heat transfer coefficient at a small temperature difference rather than for filmwise that shows a smaller value of heat transfer coefficient and at a more larger difference of temperature. The analysis for the data collected is that heat flux shows a clear difference due to the presence of air. With the presence of air the heat flux value is smaller. Dropwise condensation is far more efficient for condensation process due to the specifications for each plate surface. However we are not able to achieve the desired outcome due to some errors. The errors involve that effect the results is due to fluid involve for the process of condensation to occur. The impurities that contain within the fluid involve has effect the density and boiling temperature that occur at a much lower temperature. Thus condensation occurs at a less precise order and resulting in impairment data.
CONCLUSION For experiment 2 and 3, we have concluded that the dropwise condensation heat flux and surface heat transfer coefficient at constant pressure occur at a relatively larger value at smaller temperature difference than for filmwise condensation. Thus efficient condensation For experiment 4, we have demonstrate the effect of air on heat transfer coefficient of condensation for dropwise and filmwise, it concluded that dropwise has a more stable and larger heat transfer value for condensation process thus a mor e efficient condensation process.
RECOMMENDATIONS 1. Avoid error in taking readings and make sure eyes of observer are parallel to the meniscus 2. Make sure the valve are close tightly when the film condensation equipment is turn off. 3. Make sure all the valve close at the beginning of the experiment 4. Allow the cooling water to flow at the end of the experiment before the equipment is shut down to avoid the cracking of cylindrical tube.
REFERENCES 1) Aksan, S. N. and Rose, J. W. (1973). Dropwise condensation —the effect of thermal properties of the condenser material. Int. J. Heat Mass Transfer, 16 , 461-467. 2) Blackman, L. C. F., Dewar, M. S. J. and Hampson, H. (1957). Compounds for promoting dropwise condensation of steam. J. Appl. Chem., 7, 160-171. Date of search: 27/5/2013
http://www.freepatentsonline.com/5048600.html 14
http://www.p-a-hilton.co.uk/English/Products/Heat_Transfer/heat_transfer.html
http://wins.engr.wisc.edu/teaching/mpfBook/node9.html
Date of search: 30/5/2013
http://www.focus-science.com/pahilton-heattransfer-h910.html
http://www.springerlink.com/content/46322536602154p1/
http://www.scribd.com/doc/22992765/Dropwise-and-Fimwise-Condensation-Lab-Report
http://www.husni.net/Labs/Files/HeatTransfer/Exp6.pdf
http://www.springerlink.com/content/u2882206203n3788/
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