Vaporization of LPG by ambient air

Transcription

1 ก ก ก ก 2549 Vaporization of LG by ambient air anadda hu-akat, Suvit ia and Bunyaphat Suphanit Chemical Engineering Department, King Mongkut s University of echnology honburi 9 racha-uthit Rd., Bangmod hungkru hailand 4 el ext. 49, Fax , panadda_phu@yahoo.com Abstract his research aims to obtain thermal design equations to be used in sizing the ambient LG vaporizer for industrial use. he heat transfer coefficients of LG during vaporization and ambient air were studied in the laboratory-scale testing unit. Firstly, the boiling heat transfer coefficient of LG inside a vertical tube with an outer diameter of 25.4 mm and 4m long in the double pipe heat exchanger was investigated. he range of heat flux is between 5-35 W/m 2, and the mass flow rate of LG was up to 5 kg/hr and its propane composition was 2-6%mol. he boiling experimental data showed that the Mishra correlation provided the lowest absolute average deviation of 3.6%. In the second experiment, the heat transfer coefficient of ambient air was investigated. he proper correlation for boiling heat transfer coefficient of LG was used to determine the ambient air heat transfer coefficient, which was found to be higher than the conventional natural convection due to the condensation of moisture from the air on the heating surface.. Introduction Liquid etroleum Gas (LG) is a mixture of hydrocarbon gases; i.e. propane, butane and trace of other hydrocarbons. LG is widely used in many applications such as domestic fuel, industrial fuel, alternative fuel for automobile, and used as a refrigerant in refrigeration system. For industrial use, the LG consumption or delivery rate is rather high. he typical evaporation rate supplied by the regular household cylinder is not sufficient, thereby requiring external heat source for higher vaporization rate. he LG vaporizer is therefore necessary for industrial use. he electrical and steam type LG vaporizers are mostly used in hailand. Reducing the electricity and steam requirements for this purpose can relax the burden on the industry s energy bill. o achieve this, the free heat from atmosphere may be used to evaporate the LG instead of electricity or steam. he LG can be evaporated at ambient temperature and pressure with the properly-designed LG vaporizer. his research proposes to establish the design correlation for the industrial ambient LG vaporizer. he knowledge of heat transfer mechanism and heat transfer coefficient is necessary for designing the heat transfer equipment. Due to the complexity in phase change of multicomponents and the variation of natural convection of the ambient air, the forced convective boiling heat transfer coefficients of LG and the heat transfer coefficient of ambient air were investigated in this study, respectively. In spite of many former experimental research conducted on the boiling of mixture especially for refrigeration system, there are very few experimental study on the forced convective boiling for hydrocarbon mixture in vertical tube. Normally, the nucleate boiling heat transfer and the convective boiling heat transfer were the two important mechanisms in flow boiling heat transfer[]. he nucleate boiling is similar to the nucleate pool boiling which is characterized by the presence of active nucleation sites. he convective boiling related to the convective heat transfer between the heated wall and the liquid phase considered as a single phase forced convection across the liquid film. In nucleate boiling, the heat transfer is strongly dependent on heat flux while it is dependent on mass flow rate and vapor quality for the convective boiling. Many studies simplified the flow boiling heat transfer correlation in term of dimensionless groups; i.e. Boiling number (Bo) and Martinelli parameter (X tt )[2,3]. he effect of heat flux on nucleate boiling is characterized by Bo. he parameter X tt is used to represent the effect of convective boiling. Mishra (98)[4] carried out the experiments on flow boiling of binary mixture between R-2 and R-22 with various mole fractions to evaluate the local heat transfer coefficients in a horizontal tube. he results showed that the heat transfer coefficients of binary mixture were lower than the ideal values of the two pure

2 components. he proposed correlation from experimental data can be predicted within ±3% accuracy. his work presents the boiling heat transfer coefficient of LG inside a vertical tube and the suitable correlation from the experimental data. he flow boiling correlation of LG was used to determine the ambient air heat transfer coefficient of the vaporizer. 2. Experimental apparatus he experimental apparatus is divided into two parts. he flow boiling heat transfer of LG inside a vertical tube was studied in the first part and the ambient air heat transfer coefficient was investigated subsequently. 2. he flow boiling heat transfer of LG he boiling heat transfer coefficient on the LG side can be determined by the heat transfer experiment between LG and water in a double pipe exchanger. he water was selected as a heating medium due to its availability and high accuracy in the property determination. he schematic of experimental apparatus is shown in Fig.. he system consists of the test section, LG storage tank, water chiller, separator, electrical vaporizer, gas flow meter and flare. he test section is a vertical double pipe heat exchanger with the dimension shown in Fig.2. he inner tube material is aluminum. his tube is 4m in length. Eight internal longitudinal fins were installed to enhance the heat transfer inside the tube. he height (b i ) and thickness (δ i ) of each internal fin were 3mm. he outer tube is made of VC. his double pipe test section and all piping equipments were insulated to reduce the heat transfer from surrounding. he chilled water flowing in the annulus was used to evaporate LG. A total of thirty four type- thermocouples were installed to measure the temperature of the chilled water in the annulus and the LG in the inner tube at every 25 cm distance along the tube. he mass flow rate of LG was measured by a digital balance with ±. kg accuracy. he pressure of liquid LG from the storage tank was adjusted by a regulator. After passing through the test section, the partially vaporized LG entered a small separator to disengage the vapor from the remaining liquid. he liquid was drawn from the bottom of the separator tank, evaporated by an electrical vaporizer and finally burnt using a flare. he vapor left the top of the separator, then flowed through the gas flow meter and burnt in a second flare. he single or mixed-phase LG can be directly observed via a sight-glass at the exit of the test section. he test conditions are summarized in able. he temperature measurements were collected by a data logging system (Data taker model D55) with a maximum measurement of 6 channels. he data logging system was programmed to sample the data at every period of 5s. he steady state ENE condition was achieved when the dynamic temperature profile turned to approximately horizontal line. For all test runs, the steady state condition was reached after approximately 3 minutes. he test conditions were chosen by the actual LG vaporizer operations. he typical LG sold in hailand contains approximately 2-6%mole propane. he water flow rate was adjusted to a very small value to emulate the low heat transfer coefficient on the ambient air side. he heat transfer coefficient of chilled water on the annulus side was firstly determined by a water-to-water heat transfer test. he data were analyzed and the heat transfer coefficient was correlated by Modified Wilson s plot technique[5]. Once obtaining the reliable correlation of heat transfer coefficient on the annulus side, the boiling heat transfer coefficient on the LG side was then correlated according to the LG-water heat transfer experiment. Water Chiller Flowmeter Sight glass est Section Seperator Sight glass rap Regulator Regulator LG Electric Vaporizer LG Flow meter Fig. Schematic diagram of flow boiling heat transfer of LG apparatus LG Flare Flare Fig.2 he double pipe heat exchanger dimension able Experimental conditions arameter Range LG Operating pressure (ka) Mass flow rate (kg/hr) -5 Heat flux (W/m 2. C) 5-35 Composition of propane (%mol) 2-6 Chilled water Inlet temperature ( C) 5-2 Mass flow rate (kg/hr) 5-9

3 Fig.3 Schematic diagram of ambient air heat transfer apparatus Fig.4 he ambient LG vaporizer dimension 2.2 he ambient air heat transfer coefficient Due to the fact that the heat transfer coefficient of air is typically low, the heat transfer enhancement via fin installation along the outer tube surface is required. In general, the ambient air flow around the fin tube is considered as natural during normal operation. However, the natural convection heat transfer coefficient of air along the fin tube cannot be determined accurately by any existing correlations. For that reason, the correlation of heat transfer coefficient of air must be established for this specific case in order to correctly design the vaporizer at any given load. he schematic diagram of experimental apparatus is shown in Fig.3. Apart from the test section, the main piping and equipments of the system are similar to the previous set of apparatus. Besides, the test section was confined in an airconditioning box to control the air temperature. o simulate the natural convection condition, the air velocity inside this box was limited to.2 m/s or under. he test section was a 4-m length aluminum fin tube with 8 longitudinal fins on the outside tube surface. Similar to the previous test section, there were also 8 longitudinal fins on the inside tube surface for heat transfer enhancement on the LG side. he physical dimension of ambient LG vaporizer is shown in Fig.4. he mass flow rate of LG and the air temperature were varied in range of -5 kg/hr and -3 C, respectively. δ ENE Data reduction he thermodynamics properties of LG mixture at any given condition were calculated by the RK-Soave equation-of-state. he heat transfer rate in the double pipe heat exchanger can be determined from the heat balance between water and LG sides as follows; Q= m Cp ( ) () w,w w,in w, out Q= mlg(hlg,out HLG, in) (2) Where Q is the heat transfer rate (W/m 2 ), m w is the mass flow rate of water in annulus (kg/s), C p,w is the specific heat of water (J/kg. C), w,in and w,out are the inlet and outlet temperatures of water ( C), m LG is the mass flow rate of LG (kg/s), H LG,out and H LG,in are the inlet and outlet enthalpies of LG (J/kg). he flow boiling heat transfer coefficient of LG inside the vertical tube can be obtained from the thermal resistance equation as shown in Eq.(3) = Rw (3) hlg,iai UA hw,oa o Where h LG,i and h w,o are the heat transfer coefficients of LG inside the inner tube and of water in the annulus (W/m 2. C), UA is the overall thermal resistance (W/ C), A o is the outside heat transfer area of tube (m 2 ), A i is the inside heat transfer area of the tube (based on nominal diameter, m 2 ), LM is the log mean temperature difference( C), and R w is wall resistance of tube ( C.m 2 /W). he UA value can be obtained from the total heat transfer equation shown below; LM = (4) UA Q he heat transfer coefficient of water in annulus (h w,o ) was determined by the modified Wilson s plot technique. After obtaining the boiling heat transfer coefficients at various conditions, these values were correlated and then used to determine the ambient air heat transfer coefficient in the second experimental apparatus. he ambient air heat transfer coefficient can be determined by the overall thermal resistance of ambient air LG vaporizer equation as follows; = + + Rw (5) UAo η hair,o Ao hlg,iai Where η is the weighted area efficiency, h air,o and h LG,i are the heat transfer coefficients of air and LG (W/m 2. C), A o is the total outside heat transfer area (m 2 ), A i is the inside heat transfer area of tube (based on nominal diameter,m 2 ). he natural convective heat transfer coefficient can be determined by the simplified correlation[6] for long vertical tube as h natural ( ) 3 =.3 (6)

4 Where h natural is the natural convective heat transfer coefficient of air (W/m 2. C), is the difference between the wall temperature and the bulk air temperature( C). 4. Results and discussion 4. Boiling heat transfer of LG In this section, the experiment was conducted using LG with compositions of 2-6%mol propane. he effect of boiling (vaporizing) fraction on the heat transfer was first studied at any specified mass flux, and heat flux. For this experiment, the heat flux cannot be accurately fixed because the chilled water is used as a heating medium. Any little variation on the water temperature or flow rate can easily disturb the heat flux. he heat flux is therefore considered in a narrow range. For the LG with 2%mol propane, the heat transfer coefficients of LG were plotted against the vapor quality at heat flux of 9- W/m 2, and mass flux of 8.9 kg/m 2.s. he heat transfer coefficient slightly increased when the boiling increased from.28 to.43 of vapor quality as shown in Fig.5. he boiling pattern in this range is regarded as the nucleate boiling regime. During the boiling course, the velocity of mixed-phase will increase due to vapor expansion, and thereby enhancing the boiling heat transfer. For the LG with 58.4%mol of propane, the mass flux was fixed at 9.6 kg/m 2 while the heat flux was controlled in the range of 9-27W/m 2. At this condition, the effect of the vaporizing fraction on the boiling heat transfer was studied in the range of vapor quality as shown in Fig.6. When the vapor quality was higher than.8, the dry-out regime was observed. his regime gradually changed into single-phase vapor forced convection regime in which the heat transfer coefficient is low. he effect of heat flux on the heat transfer coefficient was considered as shown in Fig.7. he heat transfer coefficient increased with the increase of heat flux in the low vapor quality region. he variation of heat transfer coefficients with respect to mass flux were also studied as shown in Fig.8. he heat transfer coefficient was insignificantly affected by the change in the mass flux due to the narrow range of the operating mass flux (-5 kg/m 2.s). he comparison of the heat transfer coefficients from the experimental data and those from the opened literatures were shown in Fig.9. he variable of fluid-dependent parameter (F fl ) of.5 as recommend for butane by Melin[6] was used in the Kandlikar[8] correlation. he heat transfer coefficient of liquid phase only(h L ) was calculated by the Dittus-Boelter correlation. he average deviation and absolute average deviation between the experimental data and the predicted value from correlations present in able 2. he lowest absolute average deviation was 3.6% which obtained by Mishra[4] correlation. herefore, the Mishra[4] correlation(eq. 7) was chosen to predict the boiling heat ENE transfer coefficient of LG in the ambient air heat transfer calculation. he Mishra[4] correlation can be calculated as following h = 8295hL Bo X tt W/m 2. C (7) kl.8.4 h L =.23 ReL rl D (8) G( x)d ReL = µ (9) q Bo= G X tt L h LG.5..9 ρ L µ G x ρ G µ L x () = () Where h L is the heat transfer coefficient of liquid phase only (W/m 2. C), X tt is Martinelli parameter, Bo is boiling number, Re L is Reynold number of liquid phase only, r is randtl number of liquid phase, G is the mass flux (kg/m 2.s), D is the inside diameter of tube (m), x is vapor quality, q is heat flux (W/m 2 ), h LG is latent heat of vaporization (J/kg), µ L and µ G is dynamic viscosity of liquid and vapor phase (a.s), and ρ L and ρ G is density of liquid and vapor phase (kg/m 3 ). heat transfer coefficient(w/m 2. o C) vapor quality,x Fig.5 Boiling heat transfer coefficient of LG at q= 9- W/m 2, G = 8.9kg/m 2.s and 2%mol of propane. heat transfer coefficient(w/m 2. o C) vapor quality,x Fig.6 Boiling heat transfer coefficient of LG at q = 9-27W/m 2, G = 9.6kg/m 2.s and 58.4%mol of propane

5 heat transfer coefficient(w/m 2. o C) Heat transfer coefficient(w/m 2. o C) q=w/m2 q=8w/m vapor quality,x Fig.7 Variations of boiling heat transfer coefficient with respect to heat flux at G = 8 kg/m 2.s Mass flux(kg/m 2.s) Fig.8 Variation of boiling heat transfer coefficient with respect to mass flux at q = -2W/m 2. hexperiment(w/m 2. C) % 25% -25% -4% shah Gungor-Winterton Kandlikar Mishra h predict (W/m 2. C) Fig.9 Comparison of boiling heat transfer coefficient with various correlations. able 2 Comparison of experimental data and correlation from opened literature. Correlation Average dev.(%) Absolute dev.(%) Mishra [4] Shah [9] Gungor-Winterton [3] Kandlikar [8] Average Absolute dev. = dev. = n n ( ) n hexpriment hpredict hexperiment n ( hexpriment hpredict ) hexperiment ENE Ambient air heat transfer he relative of heat transfer coefficient of LG and ambient against the log mean temperature shows in Fig.. he heat transfer coefficient of ambient air (without radiation effect) and LG was 8-7 W/m 2. C and W/m 2. C, respectively. he boiling heat transfer boiling of LG is higher than ambient air heat transfer coefficient for 9-3 times. herefore, the overall heat transfer coefficient of the heat exchanger is controlled by the ambient air side. Increasing air side heat transfer such as using an assisted fan would be well to enlarge the heat transfer of the vaporizer. he comparison of ambient air heat transfer coefficient and simplified natural convection correlation shows in Fig.. he ambient air heat transfer coefficient was higher than the normal value from simplified natural convection correlation for 4 times. heat transfer coefficient of air (W/m 2. o C) LG Ambient air LM ( C) Fig. he heat transfer coefficient of air and LG versus LM Heat transfer coefficient of air (W/m 2. o C) Ambient air Natural convection correlation LM ( C) Fig. Comparison of ambient air heat transfer coefficient and simplified natural convection correlation.

6 herefore, the overall heat transfer coefficient of the heat exchanger is controlled by the ambient air side. Increasing air side heat transfer such as using an assisted fan would be well to enlarge the heat transfer of the vaporizer. he comparison of ambient air heat transfer coefficient and simplified natural convection correlation shows in Fig.. he ambient air heat transfer coefficient was higher than the normal value from simplified natural convection correlation for 4 times. Since the boiling point of LG mixture at testing condition was about o C, the moisture in ambient air around the vaporizer was condensed into liquid and falling down by gravity along the external tube surface, and hence raising the heat transfer of ambient air. his work proposed the ambient air heat transfer coefficient for the log mean temperature of heat exchanger of 3-27 C as following.2 h ambient air = 8.65 LM W/m 2. C (2) Hence, the thermal design for ambient air LG vaporizer can be carried out by using equation 7 and 2 to estimate the required heat transfer area for any given flow rate of LG. 5. Conclusion In present study, the boiling heat transfer coefficient of LG and ambient air had been studied for designing the ambient air LG vaporizer. he experimental results showed that the boiling heat transfer coefficient correlation by Mishra[4] provided the suitable approximation of boiling heat transfer of LG. he study of ambient air heat transfer showed that the heat transfer coefficient of ambient air was higher than the conventional natural convection due to the condensation of moisture from the air onto the tube surface. he heat transfer coefficient of ambient air including the condensing effect was introduced. ENE Mishra, M..H., Vama, H.K, Sharma, C.., Heat transfer coefficients in forced convection evaporation of refrigerant mixtures, Lett. in Heat and Mass ransfer, (98) Briggs,D.E and Young,E.H., Modified Wilson plot techniques for obtaining heat transfer correlations for shell and tube heat exchangers, Chem. Eng. roc. Symp., 65(969) Holman, J., Heat transfer, McGraw-Hill International editions, 8 th edition, (997) 7. Melin, M., Measurements and Modeling of Convective Vaporization for refrigerants in a Horizontal ube. h.d. hesis, Department of Heat and ower echnology, Chalmers University of echnology, Boteborg, Sweden, (996). 8. Kandlikar, S.G., A General Correlation for wo-hase Flow Boiling Heat ransfer Coefficient Inside Horizontal and Vertical ubes, Journal of Heat ransfer. 2(99) Shah, M.M., Chart Correlation for Saturated Boiling Heat ransfer: Equation and Further Study., ransactions of American Society of Heating, Refrigeranting and Air Conditioning Engineers 88(982) Acknowledgment he financial support from he Commission for Higher Education of hailand and Hi-en standard Co., Ltd. is greatly acknowledged. Reference. Collier, J.G., home, J.R., Convective boiling and condensation, Oxford University ress.(2) Gungor, K.E. and Winterton, R.H.S., A general correlation for flow boiling in tubes and annuli, Int. J. Heat and Mass ransfer. 29(3)(985) Gungor, K. E., Winterton, R. H. S. Simplified general correlation for saturated flow boiling and comparisons of correlations with data, Chem Eng Res Des (987)