THERMAL DESIGN OF FALLING FILM EVAPORATOR

Transcription

1 YMCA Institute of Engineering, Faridabad, Haryana.., Dec 9-10, 006. THERMAL DESIGN OF FALLING FILM EVAPORATOR Ashik Patel 1, Manish purohit, C. R. Sonawane 3 1, Department of Mechanical Engineering Students, Sankalchand Patel College of Engineering, Visnagar. 3 Department of Mechanical Engineering,Lecturer, Sankalchand Patel College of Engineering, Ambaji- Gandhinagar link road, Visnagar, pin , Gujarat. 3 Phone: , chandrak_sonawane@rediffmail.com Abstract Many familiar engineering applications involve condensation and boiling heat transfer phenomenon. In a household refrigerator, for example the refrigerant absorbs heat from the refrigerator space by boiling in the evaporator section and rejects heat to kitchen air by condensing in the condenser section. Evaporation is a process in which liquid is converted to vapor phase just like in case of boiling, but there are significant differences between these two phenomenons. Evaporation occurs at liquid vapor interface, when the vapor pressure is less than saturation pressure of the liquid at a given temperature. Boiling, on the other hand, occurs at solid liquid interface when a liquid is brought into contact with a surface maintain with a temperature Ts sufficiently above the saturation temperature. A falling film evaporator [3] is a one of the best efficient used heat exchanger. As the name suggest, falling film evaporator works on the principal of the film boiling in which a thin or agitated film of the fluid, which has to be evaporate is produces on the inner (or outer) periphery of the polished tubes, where saturated steam is condensed on the outer (or inner) periphery of the tubes respectively. Thus in this evaporator high velocity of film flow of the fluid is achieved and hence the high rate of heat transfer is achieved. Falling film evaporator used in the several numbers for achieving the best evaporation rates that is called multi-effect evaporator. Falling film evaporator is used in the various industries like pharmaceutical for producing the steam at the rapid rate. In food industries for evaporating the unwanted quantity of the water from the heat sensitive food as well as fruit juices as the heat transfer rates in this evaporator is quite high. It is also best suited and one of the option for producing milk power from the milk without loosing any important natural vitamins, proteins and carbohydrates. This paper critically discussed the thermal design aspect of the falling film evaporator used in the pharmaceutical industry. It is required to produce the steam a flow rate of 1500 kg per hour at saturated steam of 3 bar for the medicine tablet purpose from the available input data. A computer simulation program is developed for the total design of the falling film evaporator. The design consist both thermal and mechanical design aspects, but this paper critically discuss the thermal design only. Keywords: Falling Film Evaporator, Film, Evaporation 1.0 Introduction Evaporation is a process in which liquid is converted to vapor phase just like in case of boiling, but there are significant differences between these two phenomenon s. Evaporation occurs at liquid vapor interface, when the vapor pressure is less than saturation pressure of the liquid at a given temperature. Boiling, on the other hand, occurs at solid liquid interface when a liquid is brought into contact with a surface maintain with a temperature Ts sufficiently above the saturation temperature. The boiling processes in practice do not occur in the equilibrium conditions, and normally the bubbles are not in thermodynamic equilibrium with the surrounding liquid. That is, the temperature and a pressure of the vapor in the bubble are usually different then those of the liquid. The pressure difference between the vapor in a bubble and surrounding liquid is the driving force heat transfer between the two faces. When the liquid is at lower temperature than the bubble, heat will be transferred from the bubble into the liquid, causing some of the vapor inside the bubble to condense and the bubble to collapse eventually. When the liquid is higher temperature heat will transferred from the liquid to bubble, causing the bubble and rise to the top under influence of buoyancy. Boling is classified as Pool boiling or Flow boiling, depending on the presence of bulk fluid motion boiling is called pool boiling in the absence of bulk fluid flow and flow boiling (or forced convection boiling) in the presence of it. Pool and flow boiling are further classified as sub cooled boiling or saturated boiling, depending on the bulk liquid temperature Boiling is said to be sub cooled when the temperature of the liquid is below saturation temperature Tsat and saturated when temperature of the liquid is equal to Tsat. This paper critically discusses the evaporation process and the falling film evaporator to be used for the pharmaceutical industry. 1

2 .0 Evaporator Classification According to the orientation, the evaporator can be classified as [3][4]..1 Rising film type: Feed level is maintained up to the bottom of the tube sheet. By condensing steam around the tubes, the feed is made to boil. Vapors so produce rise up through the central core of tubes, forcing the liquid against the tube wall. Due to high velocity of the vapor, shear force is exerted on the liquid. This and the surface tension result in movement upward of liquid film up the tube wall, against gravity. These require high heat flux and high temperature driving potential, which may not be possible with heat sensitive liquid.. Falling film type: Feed is input at the top of the unit and is distributed in each tube by means of perforated distributor plate or nozzles. The liquid flows down due to the gravity. Therefore, it is thin and highly turbulent, giving a high heat transfer co-efficient. The vapor produce also travels down and is separated from the liquid at the bottom. The temperature driving potential required is low, and also the residence time is less than the rising film type. Hence, it can be used for heat sensitive materials, such as fruit juices. No recirculation is allowed. Inert gases may be some type injected in to the falling film evaporator to reduce the partial pressure required to vaporize the volatile component s. these often eliminates the need for vacuum operation, a minimum liquid flow rate is required to form film. After film is formed, the flow rate then may be reduced..3 Rising-Falling film type: It has the advantage of both rising film and falling film types. Liquid distribution is easy as in the rising film type, and the heat transfer coefficient is high as in the falling film type. The height of this unit is less than that of either of the rising film or in the falling film type..4 Working Principle Of Falling Film Evaporator: Falling film evaporator basically works on the two phenomenons s & solely depends upon the latent heat transfer. In this equipment sensible heat transfer is intentionally prevented. This equipment depends upon the heat transfer with the phase change. The two phenomenons are 3.1 Laminar Film condensation. 3. Falling film evaporation 3.0 Laminar Film Condensation [3][4] Condensation means the fluid in a gaseous or vapor changes to a liquid state with the liberation of heat from the vapor phase. When a vapor is in contact with a surface whose temperature ts is lower then the saturation temperature tsat corresponding to the vapor pressure, the condensation sets in & the vapor changes to liquid phase. The condensation of vapor librates latent heat & there is heat flow to the surface. The liquid condensate may get somewhat sub-cooled by contact with the cooled surface & that may eventually result in more vapor to condensate on the exposed surface or upon the previously formed condensate. The condensation process has been categorized into two types as i) Film condensation. & ii) Drop wise condensation. The liquid condensate wets the solid surfaces, spreads out &forms a continuous film over the entire surface. The liquid flow down the cooling surface under the action of gravity & the layer continuously grows in thickness because of newly condensing vapors. The continuous film offers thermal resistance &restricts further transfer of heat between the vapor & the surface. Film condensation usually occurs when a vapor relatively free from impurities is allowed to condense on a clean surface. The condensate condenses in the form of the droplets & does not wet the whole surface is called drop wise condensation. Governing equations of the film condensation are as per below. N Nu = h L/ k L = 1.13 [ρ L (ρ L - ρ V ) g λ L 3 / μ L k L T ] ¼ - Re < 1800 (laminar flow) (1) N Nu = h L/ k L = (g ρ L L 3 / μ L ) 1/3 (N Re ) Re > 1800 (turbulent flow) ()

3 YMCA Institute of Engineering, Faridabad, Haryana.., Dec 9-10, 006. Figure 1: Falling film evaporator tube 3. Falling film evaporation [3][4] In the film evaporation the bubble formation is very rapid, the bubbles blanket the heating surface & prevent the incoming fresh liquid from taking their place. Eventually the bubbles coalesce & form a vapour film which covers the surface completely. Insulating effect of the vapour film overshadows the beneficial effect of liquid agitation & consequently the heat flux drops with growth in temperature excess. Within the temperature range between the 50 & 150 the conditions oscillate between nucleate &film boiling and this phase is referred to as transition boiling, unstable film boiling or partial film boiling. Eventually the temperature differences are so large that radiant heat-flux becomes significant, rather controlling factor & the heat flux curve begins to rise upward with increasing temperature excess. That marks the region of stable film boiling. The phenomenon of stable film boiling is referred to as Leidenfforst effect. For making the unstable film boiling to stable film boiling we need to increase the velocity of the fluid in the tube & decrease the thickness of the film so resistance of the vapor film developed is negligible. For that the limiting values of film thickness is 0. to 0.5 mm & the velocity of the film should lies between the values 1. to 1.5 meter per second. Governing equations for the falling film evaporation is depends upon the work of several scientists N Nu = h L/ k L (3) = (1.3+ bd) (N Pr ) L 0.9 (N Re ) 0.3 L (N Re ) v 0.34 (ρ L /ρ v ) 0.5 (μ L /μ v ) Other equations based on the work of Mc Adams, Drew, Bays has given the following equations for calculating the heat transfer co-efficient for film flow of water only (within the limits of 18%). hi = 10 (w/ π d ) ⅓ (4) hi / (k 3 ρ g/ μf ) ⅓ = 0.01 [ (cμ/k) (4G'/μf) ] ⅓ - Re < 1800 (laminar flow) (5) hi = 0.67 [ (k 3 ρ g / μf ) (c μf 5/3 / k Lρ ⅔ g ⅓ ) ] ⅓ (4G'/μf ) 1/9 - Re > 1800 (turbulent flow) (6) The thermal design follows the general equation of heat transfer for the heat exchangers which is as per below. Q = U A (T S T L ) nt (7) For finding out the overall heat transfer coefficient use the equation given below. 1/U = [do/ (di hi)] + [(do-di)/ (do+di)] (do/k) +1/ho (8) 4.0 Features Relating To Thermal Design For thermal design of falling film evaporator is carried out on the basis of TEMA code [1] developed by Tubular Heat Exchangers Manufacturing Association. The TEMA has classified heat exchangers into 3 main categories i) R type heat exchangers : - Most stringent category. This category gives details about the design and manufacturing procedures to be followed for the heat exchangers which are to be used for the application like refineries, etc and pose a great danger to human life and property. This type has the most stringent tolerances. 3

4 ii) C type heat exchangers :- less strict as compared to the type R catagory, but nevertheless is close to the R type. This category gives details about the design and manufacturing procedures to be followed for the heat exchangers, which are to be used for the application like chemical industries petrochemicals, etc. iii) B type heat exchangers : - least rigid amongst the three and is applicable to a sizable amount of heat exchangers. This category gives details about the design and manufacturing procedures to be followed for the heat exchangers. Which are to be used for the commercial application and small industries? 4.1 Definition of problem Figure shows the schematic of falling film evaporator to be used for pharmaceutical industry. The falling film evaporator has the number of tubes provided in the vertical shell. The saturated water flows inside the S.S. tubes at the pressure of 3 bar, which should produces the falling film of turbulence nature. The steam is condenses on the outer periphery of S.S. tubes of 14 OD & 1 ID at 6 bar of pressure. Steam condensation also takes place in the form of film condensation. The flow rate of steam is maintained 1500 kg per hour. A Computer program is developed to carry out the mechanical & thermal design. The thermal design specially carry out to evaluate the length of the tubes necessary and to find out the no. of tubes required to produced the required flow condition by falling film evaporator. Figure : schematic of actual falling film evaporator showing the required flow conditions. 4. Design steps 1. First find out the inlet & exit condition of the fluid i.e. steam or water.. Then from the saturated properties table find out the fluid property at the given condition. 3. Then decide the tube material of which is used for the heat transfer. Here SS316 is given so that we can find out the required properties of the tube & shell material likewise conductivity, shear stress, tensile stress, yield stress, Brinell hardness number etc. 4. Then from the given condition of the saturated water & the saturated steam find out the total heat transfer required by the following equation. Q = mf * hfg 5. Then fix the saturated water flow for the evaporator. Take the input flow 30 to 40 % higher then the required output because of the effectiveness of the evaporator is not 100%. 6. Then fix the number of the tubes from the following equation. Here we have to choose the values of velocity of the film & thickness of the film in the range of 1. to 1.5 m/s & 0.1 to 0.5 mm respectively. mf = π /4 [di -(di-t) ] nt ρ L v Where t = thickness of the film; v = velocity of the film; nt = number of the tubes 7. Now consider the general thumb rule value of 5700 for the outer heat transfer co-efficient. 8. Then in the equation no. (3) Put all the other values except the length of the tube. 9. Then put the value of outer heat transfer co-efficient & the equation or inner heat transfer co-efficient in the following equation of the overall heat transfer co-efficient. 4

5 YMCA Institute of Engineering, Faridabad, Haryana.., Dec 9-10, /U = [do/ (di hi)] + [(do-di)/ (do+di)] (do/k) +1/ho 10. Now put the values of the overall heat transfer co-efficient, area of heat transfer surface, temperature difference, total heat to be transferred in the equation no. (7). 11. Now find the value of the length of the tube required by the trial & hit method. 1. After that find the value Reynolds no. of the outer film of condensed steam by the following equation. Re = 4/3 [ ρ L g /μ L ] {4 μ L k L L(Ts -Tw)/ρ L g h fg } 3/4 13. If the Reynolds no. is higher then the value of 1800 then uses the equation no. () Or use the equation no. (1) end find out the exact value of the outer heat transfer coefficient & match it with the thumb rule value. 14. After that try the equation no. (4), to find out the inner side heat transfer co-efficient following the same method as mention above. 15. After finding the values of length according to two methods take the value of length which is greater one. 16. Fix the triangular pitch for the tube sheet layout. For that use the TEMA standard class R or use the thumb rule equation that triangular pitch is 1.5 times the diameter of the tube. 17. After fixing the tube pitch draw the tube sheet diagram for the number of tubes assumed before. 18. Then from the tube sheet diagram take the appropriate value of inner shell pipe from the chart of standard pipes &outer shell pipe which is the next one of the previously selected. 19. After that select the scheduled number of the selected pipe diameter by the following equation of pressure vessel. t = (Pd / σ t ) + c 0. After that from the ASME code SEC.8 design the tube sheet & the flange. Design the end cover according to ASME SEC. 8 & select the appropriate gasket from the standard. 4.3 Thermal design Here the required output is 1500 kg/hr & we have to fix the input flow of the saturated water flow which is 30 to 40% higher then the required, so let us take it 35% more. By using table 1. [][5] mf = 1.30 x 1500 kg/hr. = 1950 say 000 kg /hr. Now take the value of velocity of falling film in the range of 1. to 1.5 m/s, so let us take it as 1. m/s. The thickness of the falling film is to be fixed below the 0.5 mm range to avoid the flow boiling & nucleation boiling, so let us take it as 0.15 mm. Table 1: property table Properties of saturated water & steam at 3 bar and at C: ρ L = kg/m 3 ρ v = kg/ m 3 h fg = kj/kg C Pl = J/Kg C C Pv = 10.5 J/Kg C μ L = 0.0x 10 3 Kg/m.sec μ L = 1.37x10-5 Kg/m.sec Pr L = 1.85 Pr L = Properties of saturated water & steam at 6 bar and at C: ρ L = kg/m 3 ρ v = 3.56 kg/ m 3 h fg = 083 kj/kg C Pl = 4340 J/Kg C C Pv = 40 J/Kg C μ L = 0.170x10-3 Kg/m.sec μ L = 1.434x10-5 Kg/m.sec Pr L = 1.09 ;Pr L = 1.05 K L = W/m C Kv = W/m C Material properties of SS 316 L at 500 C: ρ = 838 kg/m 3 C P = 468 J/Kg k K 300 = W/m k K 600 =16.64 W/m k σ t = 450 N/mm Properties of condensed film for calculation of Re & hi at 145 C: ρ L = 919.5kg/m 3 ρ v =.55 kg/ m 3 h fg = kj/kg μ L = Kg/m.sec μ L = 1.38 Kg/m.sec K L = W/m C Kv = W/m C After that we can find the total heat transfer by the equation Q = mf x hfg = (1500/3600) x (159500) = Now from the equation of the mass flow rate mf = π/4 [di -(di-t) ] nt ρ L v Here, di = 0.01 m t = m ρ L = kg/m 3 v = 1.5 m/s So, from the above equation we get the value of number of tubes, nt ~ 100 Total area of heat transfer can be given as A = π x di x L x nt = x 0.01 x L x 100 = x L 5

6 To design the film evaporator according to experimental dimensional equation, we will assume the value of the outer heat transfer co-efficient as 5700 w/m k. Now first of all we need the value of the tube length for the thermal design which is unknown, so we will put all the other data in the equation and we will put that in the equation of the overall heat transfer co-efficient equation. N Nu = h L/ k L = (1.3+ bd) (N Pr ) L 0.9 (N Re ) L 0.3 (N Re ) v 0.34 (ρ L /ρ v ) 0.5 (μ L /μ v ) Here b = 18 for the si system N Re = Reynolds no. for liquid & vapor = (ρ v di / μ) (for liquid) = (ρ v di / μ) (for vapour) = (98.15 x 1. x / 0.05 x 10-3 = ( x 1. x / x 10-5 = = N Pr = Prandtle no. for the liquid. = 1.85 (from the table) Hence, N Nu =(1.3+(18 x 0.014))x(1.8) 0.9 x( ) 0.3 x(1849.) 0.34 x(98.1/1.73) 0.5 x(0.05x10-3 /1.3475x10-5 ) x(0.6835/l) = ( / L) From the equation of overall heat transfer, 1/U = [do / (di x hi)] + [(do-di) / (do+di)] x (do/k) +1/ho = [0.014 / (0.01 x ( / L))] + [( ) / ( )] x (0.014 / 16.64) + (1 / 0.014) Hence, heat transfer can be given by, Q = U A (T S T L ) = {[0.014/(0.01x(497.3/L))]+[( )/( )]x(0.014/16.6)+(1/0.014)}x(3.76x L) x 5. The computer program is developed for trial & hit method to find the value of the length of the tube. L =.49.5 m. Now from the value of the length of tube we will find the exact value of the outer heat transfer co-efficient as: First of all we will find the value of Reynolds no. of the steam condensation as follows. Here the properties of the steam is evaluated at the mean film temperature 135 c Re = (4/3) x [(ρ L x g) / μ L ] x [(4 x μ L x k L x L x (Ts-Tw)) / ( ρ L x g x hfg)] 3/4 = 1.34x[( x9.81)/(0.19x10-3 ) x[(4x0.19x10-3 x0.68x.5x5.) / ( x 9.81 x 19500)] = The Reynolds number is higher then 1800 so we will use following equation, Ho = (g ρ L k L 3 / μ L ) 1/3 (N Re ) 0.4 = x [(9.81 x x ) / (0.190 x 10-3 ) ] 0.33 x (73.87) 0.4 = The value above given is the actual value of the heat transfer. Now we will find the value of the length of tube according to Macadam s experimental equations which will give the value in the range of 18%. Hi = 10 (w/ π d ) 1/3 Here w = weight flow of water per periphery in the lb/hr. d = diameter of the tube in ft. Hi = 10 x ( / ( x ) 1/3 = This equation gives the value in British system, so to convert it into SI system multiply it by the factor Hi = x = Now repeating the same method of finding the length of the tubes by using the following equations as: 1/U = [do / (di x hi)] + [(do-di) / (do+di)] x (do/k) +1/ho Q = U A (T S T L ) We can find the value of the length of the tube, L =.435 m. Now from the value of the length of tube we will find the exact value of the outer heat transfer co-efficient as: 6

7 YMCA Institute of Engineering, Faridabad, Haryana.., Dec 9-10, 006. First of all we will find the value of Reynolds no. of the steam condensation as follows. Here the properties of the steam is evaluated at the mean film temperature 135 c Re = (4/3) x [(ρ L x g) / μ L ] x [(4 x μ L x k L x L x (Ts-Tw)) / ( ρ L x g x hfg)] 3/4 =1.34x[( x9.81)/(0.19x10-3 ) x[(4x0.19x 10-3 x x.435 x 5.) / ( x 9.81 x 19500)] 3/4 = The Reynolds number is higher then 1800 so we will use following equation, Ho = (g ρ L k L 3 / μ L ) 1/3 (N Re ) 0.4 = x [(9.81 x x ) / (0.190 x 10-3 ) ] 0.33 x ( ) Conclusion After critically discussing the thermal design of falling film evaporator it can be concluded that the total length of the tube for the given condition is.5 m. The computer program developed is also useful to design the other falling film evaporator having the different input conditions. Also the falling film evaporator can attain a very high rate of heat transfer because of the higher value of heat transfer coefficient. The paper deals with the falling film evaporator to be used for the pharmaceutical industry. References 1. Tubular Exchanger Manufacturing Association (TEMA) standards.. Heat exchanger Design Handbook, Vol -4, Published by HEDH. 3. Donald Q. Karn, (1978). Process heat transfer, Mcgraw hill publication, 4. Yunis A. Cengel, (00). heat and Mass transfer, Tata Mcgraw hill publication, 5. Material Properties ASME sec II, Part D. 7