Che309-H3(report) - Abstract This experiment is about the condensation on vertical tubes If the temperature of the plate is below the saturation

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Unformatted text preview: Abstract: This experiment is about the condensation on vertical tubes. If the temperature of the plate is below the saturation temperature of the vapor, condensate will form on the surface and under the action of gravity will flow down the plate. The main objectives of this experiment are as follow: 1. To investigate heat transfer to water flowing inside a vertical tube from steams condensing on the outside of the tube. 2. To study the film and dropwise condensation mechanisms. 3. To determine the heat flux and surface heat transfer coefficient. 4. To study and investigate the effect of air in the condenser. However, if the liquid wets the surface, a smooth film is formed, and the process is called film condensation. Where if the liquid does not wet the surface, droplets are formed and fall down the surface in some random fashion. This process is called dropwise condensation. The experiment was run twelve times for both the filmwise and dropwise condensation. The experiment was performed by measuring the temperature of the steam, the tube surface, the water inlet and the water outlet. These temperatures were measured for different flow rates for each type of condensers. And it found that for the same flow rates, the heat transfer and the surface heat transfer coefficients in dropwise condensation are higher than that in filmwise condensation. Because of the higher heat transfer rate, dropwies condensation would be preferred than film condensation, but it is extremely difficult to maintain since most surfaces 1 become wetted after exposure to a condensing vapor over an extended period o0f time. The effect of air on the condensation was investigated, and it was found that the precence of air decreases the effectiveness of the condenser. Introduction: The transfer of heat, which accompanies a change of phase, is often distinguished by high rate. Heat flux high as 50 million Btu/h.ft2 has been obtained in boiling. Although condensation rates have not reached a similar magnitude, coefficient of heat transfer for condensation as high as 20,000 Btu/h.ft2.F have been reported. Condensation coefficients in ordinary commercial equipment are generally higher than in system where a change of phase does not occur. Condensation occurs when a saturate vapor comes in contact with a surface whose temperature is lower than the saturation temperature corresponding to the thermodynamic vapor pressure. However, heat transfer by condensation of vapors on a cold surface is a common industrial process. Condensation occurs usually on the outer surface of tubes in a shell and tube heat exchanger, and in most condensers the liquid inside the tubes which receives the heat is water. The vapor being condensed may consist of only one substance, as the case of steam being condensed in a power plant, or it may be a mixture of substances. The role of condensation using steam for power production, refrigeration and distillation, and to convey heat has a large history and its used in the industry and in these field are likely to continue 2 into foreseeable future. During condensation, very high heat fluxes are possible and provided the heat can be quickly transferred from the condensing surface into cooling medium. Condensation on solid surfaces occurs by tow method. One is known as filmwise condensation and other as dropwise condensation. Theoretical Background: 1.Filmwise condensation Filmwise condensation occurs if the liquid wets the surface and a smooth film is formed. In the film condensation process the surface is blanked by the film, which grows in thickness as it moves down the plate. A temerature gradient exissts in the film, and the film represents a thermal resistance to heat transfer. With couple of assumption, there are many theories explained this phonomena and stats its properities. Velocity pofile wuthin the boundry layer can be approximated by: y y2 u = U 2× − 2 δ δ Also, the mass of the condensed material that flow through the cross section at x per unit time is given by: ρ 2 gδ 2 f G = p f umδ = 3µ f p f gδ 2 um = 3µ f 3 The velocity at location x is The boundary layer thickness is obtained by 4µ f k f (Tsat − Tsur ) δ = 2 p f gh fg The average heat transfer coefficient is give by K 3 ρ 2 h fg g ff hmean = 0.943 × xµ f (Tsat − Tsur ) 2. Dropwise condensation dropwise condensation occure if the liquid does not wet the surface, and a droplets are formed and fall down the surface by the effect of gravavity force. In dropwise condensation a large portion of the area of the plate is directly exposed to the vapor; there is no film barrier to heat flow, and hgiher heat transfer rate are experienced.The surface heat transfer coefficient achieved during dropwise condensation is usually between 5 and 10 times greater than with filmwise condensation under the same conditions. For both types of condensation, the following can be applied: 1. The heat transfer rate is given by Q = m ×C p ×∆ T 2. The heat flux is given by φ = Q A 4 3. The correction for temperature drop across the shell is given by z ∆T = × Φ k 4. The experimental value of the surface heat transfer coefficient is give by Φ hexp = ∆ T Where: ΔT= Tsteam –TSurface Procedure: 1. the water level in the chamber should be corrected (about 30 mm above the top of the heater element) 2. all air was extracted 3. The unit was run for 5 minutes with saturation(steam) temperature T1 of 100 C and low condenser water flow rate (< 4gm/s for dropwise and 1nm/s for filmwise) 4. The steam temperature was selected to be 100 C. 5. The water was circulated through the dropwise condenser at low flow rate and the water to the filmwise condenser was turn off. 6. The heater input was adjusted to maintain the selected value of T1. 7. The cooling water flow rate and the temperature T1, T2, T3 and T4 were recorded. 5 8. The water flow rate was increased and the heater input was adjusted to bring T1 to 100 C. 9. Step [7] was repeated with changes in the flow rate. 10.Step [8] was repeated for other flow rate. 11.Air was extracted again like what did in step [2]. 12.All the above steps were repeated in a similar manner for filmwise condenser with different flow rates. The water flow rates and the temperature (T1, T5, T6, and T7) were recorded. 6 D iscussion of Results; The results of this experiment can be summarized in the following main points: 7 1. For each run in both filmwise and the dropwise condenser it is seen that as the flow rate increased, the heat transfer rate and the heat flux also are increased. 2. For the same flow rate, the heat transfer rate for dropwise is higher than that for filmwise. 3. For the filmwise condensate it was found that there is a huge difference between the theoretical rate of the heat transfer coefficient and the experimental one which can be seen from Fig. [1]. That indicates presence of a large error while the data had been taken. And the average error was found to be 144.7%. From the plot of heat flux vrs. ΔT, Fig. [2], it is seen that at the same corrected steam to surface temperature differences, the heat flux is much higher for dropwise than for filmwise. This result from the liquid film on the filmwise condenser which creates a thermal resistance. This resistance which account for large difference between the effectiveness of filmwise and dropwise condensation. However, the heat flux for filmwise in the graph does not appere as a stright line, which can be casused by error in the reading. From Fig. [3] which represent the plot of experimental heat transfer coefficient and ΔT for both condensers, it can be seen that the dropwise surface heat transfer coefficient is much higher than that for the filmwise at the same ΔT. The surface heat transfer coefficient decreases with increasing ΔT. These high values of the heat transfer coefficient indicate the higher effectiveness of the dropwise condenser. 4. 5. 6. It can be seen that the temperatures of the surface and the water inlet and outlet decreases with the presence of air for both condensers. This because air makes envelope befor it can 8 condense. The obstacle presented by the air caused a considerable reduction in the heat transfer coefficient. Conclusion and Recommendations: 1. In both condensers as the cooling water flow rate increases, the rate of heat transfer and enhance heat flux increases as well. 2. At the same flow rate, for example of 10x-3 Kg/s ,the rate of heat transfer of dropwise condenser is much higher than that for the filmwise. This indicate higher effectiveness of the dropwise condenser. 3. The surface heat transfer coefficient is higher for dropwise than that for filmwise condenser. 4. The presence of air decreases the surface heat transfer coefficient for both condensers, which in turn decreases their effectiveness. 5. For a dropwise condensed and at a given area and heat transfer rate, ther is a small temperature difference compare for that in filmwise. 6. Also, for a given temperature difference and heat transfer rate, a smaller heat transfer surface is associated with dropwise than a filmwise. After all of this calculations and analysis, I would like to recommend A wider range of flow rates should be included in order to clarify the the following: variation. 9 It is a good practice to test properties of dropwise and filmwise condensers It is important to use another fluid (oil) rather than water to check the affect at laminar and turbulent flow rats. of viscosity on condensation process Literature Cited: 1. Stephen Whitaker, ”Elementary Transfer Heat Analysis”, Pergamon Unified Engineering Series, 1976, 340-350. 2. E.R.G. Eckert, “ Introduction to Heat and Mass Transfer”, Mc Graw Hill , 1963, 196-203. 10 3. B. Gebhart, ”Heat Transfer” , Mc Graw,1961,303-327. 4. W.H. Mc Adams, ”Heat Transmission”, Mc Graw HILL. 5. F.A. Holland , R .M .Moores ,F .A. Watson and. Wilkinson,” Heat Transfer”,. Heineman Educational Books. Nomenclature: i. Notation A = surface area of condenser Cp = specific heat capacity (h/T), (J/Kg .K) = 4180 J/Kg.K – for water G = gravitational acceleration, (m/s2) h = surface heat transfer coefficient, (W/m2.K); specific enthalpy (J/Kg) K = thermal conductivity (W/ m 2. K) δ = film thickness (m) I = length of vapor space (m) M = mass flow rate (Kg/s) P = absolute pressure (N/ m2) or (Pa) Q = heat transfer rate (W) R = specific gas contestant (J/Kg.K) T = temperature ( C) V = volume (m3) v = specific volume (m3/Kg) X = height or length of condenser (m) =0.09 m Z = thickness of the shell (m) = 7.1 10-4 m D = diameter of condenser (m) =0.04127 m k = thermal conductivity of copper =360 W/m .K μ = viscosity (N .s/m2) 11 ρ = density (Kg/m3) Φ =heat flux (W/m2) ΔT = difference in temperature (K) u = velocity profile (m/s) U = velocity at the edge of the liquid films (m/s) um =average velocity at location x (m/s) Y = distance from the surface (m) I 0= shear stress (N/m2) G = mass of condensed material Nu = Nusselt number Re = Reynolds’s number =D v P/u ii. Subscripts a = refers to ambient conditions f = refers to properties of saturated liquid g = refers to properties of saturated vapor fg = refers to a phase change from saturated liquid to saturated vapor or vice versa Sat = refers to saturated conditions sur = refers to surface conditions d = dropwise e = filmwise Mean = average value 12 Appendix A1: Row Data A2: Sample of calculations 13 A3: Graphs A2: Sample of calculations (all for run#1): 1) Dropwise Condensation …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… 14 …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. 2) Filmwise Condensation …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… ………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. 15 …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. 3 16 ...
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This note was uploaded on 11/07/2010 for the course CHEMICAL E 300 taught by Professor Ahmedghazal during the Spring '10 term at King Fahd University of Petroleum & Minerals.

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