CHE 415 -HEAT TRANSFER – Steam-to-Air Final Report 2007

(Data Reporter) (Leader) (Inspector) Experiment #4, HEAT TRANSFER – Steam-to-Air October 16th, 2007 Section #1, Group #5 (Data Reporter) (Leader) (Inspector) Experiment #4, HEAT TRANSFER – Steam-to-Air October 16th, 2007 Section #1, Group #5 (Data Reporter) (Leader) (Inspector) Experiment #4, HEAT TRANSFER – Steam-to-Air October 16th, 2007 Section #1, Group #5 (Data Reporter) (Leader) (Inspector) Experiment #4, HEAT TRANSFER – Steam-to-Air October 16th, 2007 Section #1, Group #5 (Data Reporter) (Leader) (Inspector) Experiment #4, HEAT TRANSFER – Steam-to-Air October 16th, 2007 Section #1, Group #5 Ryerson University Department of Chemical Engineering CHE 415 Unit Operations II Lab Report Experiment #4: HEAT TRANSFER – Steam-To-Air Experiment Performed on October 16th, 2007 Report Submitted to Dr. Ginette Turcotte / Kiran Shah 1. /Leader By Group # 5 Section # 1 2. / Data Reporter 3. /Inspector October 23rd, 2007 Date Report Submitted Marking Scheme Formatting Answered all 6 questions in each Report Section / 10 General Appearance; Grammar and Spelling / 5 Complete and Informative Tables and Graphs / 15 Contents Accuracy and Precision of Results / 20 Comparison with Literature Data / 10 Discussion on Influence of Procedural Design on Results / 10 Logic of Argumentation / 20 Sample Calculations / 10 _____ Total: / 100 Abstract Heat transfer devices, or heat exchangers, are used in many chemical engineering industrial applications. This experiment was performed to determine the heat transfer coefficient of air flowing over coils containing condensing steam and to investigate the relationship between the flow rate of air over the coils, the number of coils used, and the distance between the coils, on the local heat transfer of the film over the coils. The effect on the heat transfer coefficient of air flowing over different arrangements of coils was also considered. This was achieved by measuring the temperature of cool air before and after contact with the steam pipes, at different steam pipe arrangements and different mass flow rates of air. The overall heat transfer coefficient was calculated by equating the amount of heat loss from the condensing steam to the amount of heat transferred to the air. A relationship between the overall heat transfer coefficient and the heat transfer coefficient of the air was derived using the theory of heat transfer resistances. Also, multiple regression was used to develop a mathematical relationship between the heat transfer coefficient of the air and the number of coils of steam used, as well as the mass flow rate of air used. While the constants for the regression were calculated, the relations was found not to be valid because it did not include other factors that affects the heat transfer coefficient, such as the distance between the steam pipes. It was found empirically that the power coefficient for the mass flow rate of air and number of pipes used to be 1.46 and -0.88, respectively. While experimental data showed that average values to be 1.43 and -1.00. The general constant value was found to be -0.73, where meant distance played a factor in the power equation. In conclusion, the experiment was successful at determining the power coefficients but it must account for the differences in distance. Table of Contents 22 44 44 77 77 88 88 1010 1313 1515 1616 1717 17 List of Figures 55 77 1111 1111 1313 1313 1414 14 List of Tables 1010 1717 17 Objectives and Experimental Design The objectives of this experiment was to determine the heat transfer coefficient of air flowing over coils heated with condensing steam, and to investigate the relationship between the flow rate of air over the coils, the number of coils used, and the distance between the coils, on the local heat transfer of the film over the coils. The effect on the heat transfer coefficient of air flowing over different arrangements of coils was also considered. This was achieved by using a bank to pipes containing condensing steam. Cool air was blown over the outer surface of the steam pipes, and the temperature of the air before and after contact with the pipes were measured. The number of pipes used was changed, as well ad the distance between the pipes. Different air blower speeds were also investigated for each pipe arrangement. Introduction and Theoretical Background Heat transfer can occur in three ways: conduction, convection and radiation. Conduction can be thought of as the transfer of thermal energy from more energetic particles to less energetic particles [2], and occurs when there is no bulk motion of the substance. Convection is energy transfer through random motion, or diffusion, and through bulk motion of a fluid. Radiation is energy that is emitted from substances at a finite temperature. In this experiment, condensing steam is flowing through pipes, around which air that is to be heated is flowing. The direction of the steam is perpendicular to the flow of air on the outside of the tube. This arrangement of flow directions has been coined “cross-flow” [2]. Because the steam is at a higher temperature, there will be heat transfer from the steam to the air. This involves the convective heat transfer from the steam to the walls of the pipe carrying the steam. This thermal energy is then conducted to through the wall, where there is more convective heat transfer from the outer wall of the pipe to the cooler air. This is summarized in figure 1 below for each pipe used. Figure 1: Heat Transfer from the steam to the air through a pipe To calculate the overall heat transfer coefficient of the system, two relationships will be used. Equation 1 below relate the amount of heat being transferred, Q, to the mass flow rate of air and the specific heat capacity of the air [3]. Equation 2 is based on Newton’s Law of cooling and is used to calculate the rate of heat transfer transferred [3]. (1) where, m = mass flow rate of air Cp = specific heat capacity of air ?T = the temperature difference between the inlet and outlet (2) where, U = the overall heat transfer coefficient A = the area of heat transfer ?TLM = the log-mean of the temperature difference between the inlet and outlet The log-mean temperature is used because as the temperature of the air increases closer to the temperature of the steam condensing inside the pipe, ?T decreases [2]. The log-mean temperature is a more appropriate form to use. It can be calculated using equation 3 below. (3) where, Ts = the temperature of condensing steam Ti = the temperature of the air at the inlet Ti = the temperature of the air at the outlet Equating equations (1) and (2) yields equation (4): (4) The overall heat transfer coefficient of one of the tubes is related to the other parameters of the system as shown in equation (5) [2]. (5) where, hi = the heat transfer coefficient of condensing steam k = the thermal conductivity of air ho = the heat transfer coefficient of air Equation (5) is rearranged to achieve the heat transfer coefficient of the air film on the outside of the tube. In this experiment, the heat transfer coefficient for 1, 2, 3 and 4 coils are all investigated. The arrangement of 3 and 4 of these coils are also changed to establish a qualitative analysis on the effect of the heat transfer coefficient. At each coil arrangement, different air flow rates are also used to investigate the effect of the air flow rate on the heat transfer coefficient. The heat transfer coefficient is dependant on the number of tubes used in the system. Using the heat transfer coefficients calculated using equation (5) at different air flow rates and different number of coils, a mathematical relationship can be derived in the form of equation (6). The constants will be found using multiple regression analysis. (6) Experimental Set-up Figure 2: Schematic diagram of experiment apparatus Experimental Procedure During the experiment, the steam coils and valves were visually inspected for possible cracks or loose fittings. The main valve/switch to allow for steam flow was then turned on. Following this, all valves leading to and from the seven steam coils in the heat exchanger were closed, except for coil number one, which was ensured to be fully open. The air blower was turned on and set to 40V. The system was allowed to reach steady-state, which was considered to be when there were no fluctuations in the temperatures of the inlet and outlet air. These temperatures were then measured. The air blower voltage was increase at increments of 10V for each trial, from 40V to 60V. After these three trials, the arrangement of steam coils was changed. Whenever the arrangement was changed, the steam pressure gauge had to be adjusted to ensure the steam pressure was kept constant. The arrangements trialed were as follows: 1; 1,2; 1,2,3; 1,2,3,4; 1,3,5; 1,3,5,7, 1,4,7. Safety and Environmental Concerns during the Experiment: Care should be taken within the laboratory during the experiment to avoid contact with the coils as the ones in use will be very hot (due to them carrying condensing steam). Insulated gloves should be worn in the vicinity of the coils and also when closing and opening the steam valves. The coils should also be visibly checked over before beginning the experiment to ensure that there are no cracks or loose fittings that would allow the hot steam to escape and possibly harm (e.g. burn) the operator of the equipment. In addition to the above, the following attire should be worn at all times when working in the unit operations laboratory - safety goggles, hard hat, closed toe shoes and lab coat. Further, the equipment should never be left unattended while in operation. In the case of an emergency evacuation the main steam valve/switch and blower should be turned off to avoid leaving them on unattended Safety and Environmental Concerns in Industrial Applications: There exist many industrial applications for which heat transfer to or from a bank of tubes is relevant [2]. Further, numerous commercial heat exchangers are made with several rows of tubes where the fluid flowing within the tubes is at a right angle to the bank of tubes [3]. Examples of applications include a gas heater where a gas passing by the outside of the tubes is heated by the hot fluid within the tubes, an air conditioner where air is cooled in the coil and steam generation in a boiler, among others [2,3]. Within such industrial applications safety precautions similar to those outlines above should be adhered to. Precautions, such as insulated gloves and proper attire, should be taken when workers are coming into contact with hot equipment. Also, the equipment should be checked for defects to avoid any adverse effects. Further, if hot steam is being directly emitted into the environment, e.g. through a vent directly into the outdoors, care should be taken that the vent is located in a place where the hot steam will not harm anyone (person or animal) that comes into close proximity. In industrial heat exchangers the heat exchange surface should be kept clean by either chemical or mechanical methods to prevent fouling snd corrosion. Several cleaning procedures that could be utilized are: circulating hot water to wash out salt deposits, circulating hot oil or light distillate to remove sludge or other, soft deposits, use of commercial cleaning compounds to remove sludge and/or scale, high presssure water jet use, and scrapers or rotatign wire brushes to remove scale, cole and other deposits. The choice of procedure is dependent on the particular plant, for example the type of fluid used or resultant deposit [6].   Some precautions that should be taken when cleaning are to ensure that the cleaning compound is compatible with the metal of the exchanger, that the tubes are not damaged by any mechanical cleaning tools, and to avoids expansion of the any tubes that may be caused by steam [6].   Heat exchangers are typically made with materials that are corrosive resistant. But a heat exchanger being resistant to corrosion in a difficult characteristic to meet and therefore damage is typically inevitable. Types of materials used include exotic metallics and thermoplastics. Results and Discussion In this experiment, the heat transfer coefficient of the air flowing over a bank of various arrangement of coils was determined, as summarized in Table 1. Pipe arrangement Blower Voltage (DC) Pin, steam (psig) Tin, air (ºC) Tout, air (ºF) # of pipes used Air Mass Flowrate, m (kg/hr) ?TLM ho (W/m2K) 1 40 35 40.5 188 1 516.1 71.292575 1681.06 50 35 44.5 194 1 597.9 67.565715 2125.56 60 35 48.5 195 1 679.7 65.590354 2357.26 1,2 40 35 49 208 2 516.1 60.730909 958.48 50 35 53 211 2 597.9 58.064474 1128.60 60 35 54.5 212 2 679.7 57.110665 1304.58 1,2,3 40 35 51 204 3 516.1 61.40128 548.59 50 35 54 216 3 597.9 55.809114 778.99 60 35 57 216 3 679.7 54.655824 856.06 1,2,3,4 40 35 52.5 207 4 516.1 59.723899 417.47 50 35 56 212 4 597.9 56.52418 508.70 60 35 57 215 4 679.7 55.027974 610.90 1,3,5,7 40 35 53 220 4 516.1 54.650616 532.65 50 35 55 225 4 597.9 51.934977 670.77 60 35 56 230 4 679.7 49.539271 844.67 1,3,5 40 35 52.5 216 3 516.1 56.381255 677.71 50 35 53 218 3 597.9 55.425605 823.21 60 35 55 220 3 679.7 53.899981 961.40 1,4,7 40 35 52.5 219 3 516.1 55.227113 718.75 50 35 54 225 3 597.9 52.30166 935.19 60 35 55 231 3 679.7 49.480186 1212.56 Table 1: Results of h0 It was found that increasing the mass flow rate of air at each arrangement affected the heat transfer coefficient. As the mass flow rate of air was increased, the heat transfer coefficient of the air also increased. This may be because at higher mass flow rates, the air has a higher velocity, resulting in more turbulence being experienced. More turbulence allows more heat energy to be transferred per unit area, therefore increasing the heat transfer coefficient. This relationship is shown below in Figure 4. Also, the more piped used, the lower the heat transfer coefficient value. This is due to the greater surface area and less turbulence within the pipes. In summary, the more pipes used and the lower the mass flow rate of air yields lower heat transfer coefficient. Figure 3: Graph to shown relationship between mass flow rate of air and heat transfer coefficient of air The effect of the arrangement of the steam pipes was investigated by determining the heat transfer coefficient for three different arrangements of using three steam pipes. The arrangements trialed were as follows: 1,2,3; 1,3,5; and 1,4,7. There were differences between the heat transfer coefficients calculated for each of these arrangement as shown in Figure 5 below. Figure 4: Heat Transfer Coefficient for different arrangements of 3 steam pipes As can be seen from figure 5 above, for each mass flow rate of air, changing the arrangement affects the heat transfer coefficient of air. This is because the distance between the steam pipes were increased between the arrangements 1,2,3 and 1,3,5, and even more so between 1,2,3 and 1,4,7. The distance between each pipe was found to be 5.75cm. The increased distance between steam pipe lines results in more turbulent flow. This was due to the fluid passing over the first pipe, the increased distance allowed for more mixing of the fluid to occur causing an increased in turbulence. At smaller distances, the fluid does not mix as well, and instead reaches the second pipe and so on, where the fluid exhibits less turbulence. Log-log graphs were plotted to investigate the dependence on certain variables on the heat transfer coefficient, as in equation (6). In Figure 6, log(ho) was plotted against log(m), to determine the relationship between the heat transfer coefficient and the mass flow rate of air. As shown in Figure 6, the graph is linear for each set of data, i.e. for each arrangement of steam pipes. The slopes of these lines were found to be similar, ranging from 1.12 to 1.89, which indicates a strong correlation between the mass flow rate and the heat transfer coefficient. The slope for the arrangement 1,2,3,4 was omitted as an anomaly. The average value was 1.43. The actual value found through multiple regression was 1.46, which was within the region found by all the types of pipe arrangements. The difference was due to errors mentioned in the sources of error section, such as fouling. In Figure 7, log(ho) was plotted against log(N), where N was the number of steam pipes used. For each mass flow rate, there was a linear relationship between log(ho) and log(N). Therefore, there is a strong correlation between the number of steam pipes used and the heat transfer coefficient. The slopes between the different mass flow rates were very similar, ranging from -0.97 to -1.02, while the average value was -1.00. The actual value calculated through regression was -0.88. The difference was due to errors mentioned in the sources of error section, such as fouling. Figure 5: log(ho) vs log(m) Figure 6: log(ho) vs log(N) Finally, the constant (c value) was calculated through regression to be -0.73. However, this constant can not be negative as it results in a negative value for the heat transfer coefficient. This was because the distance between each piping should be accounted for. This was obvious as when 1,2,3 and 1,3,5 and 1,4,7 pipe arrangement were used. At a specified mass flow rate of air, the heat transfer coefficient calculated was different for each of these arrangements, which indicates that the heat transfer coefficient is not dependent on only the number of steam pipes and mass flow rate of air but also the distance should be accounted for. However, due to limit number of samples, an exact relationship between the distance and heat transfer coefficient could not be determined, as shown in Figure 7. Figure 7: Relationship between distance and the heat transfer coefficient with constant air flow rate Error Analysis During the experiment, there were a number of factors that influenced the data obtained. The thermocouple gauges for the inlet and out of the air were positioned in undesirable positions. To measure in the inlet temperature, the gauge was very near the base of the equipment, and to read it required the person performing this duty to be very close to the hot steam pipes. To measure the outlet temperature, the gauge was near the top of the equipment, out of reach of some of the students. The readings were therefore difficult to obtain accurately. Another error encountered within the experiment was the fact that the steam pressure kept shutting off and also escaped through leaks in the valves. The steam shutting off meant that it was passing through the tubes at a decreased pressure and therefore the temperature readings taken would be different from those that would occur at the correct pressure (the pressure value that was chosen and kept constant). The pressure gauge did not seem to immediately respond to the decrease in pressure when the steam flow shut off and therefore the pressure was difficult to control. Conclusion and Recommendations In conclusion, the experiment was successful at determining the power coefficients but the power equation should account for the differences in distance. The power coefficient for the mass flow rate of air and number of pipes was found empirically to be 1.46 and -0.88, respectively. While experimental data showed that average values to be 1.43 and -1.00. The general constant value was found to be -0.73, where meant distance was significant in the power equation. Recommendations include standardizing the temperature gauges to one unit, either in Fahrenheit of Celsius. The position of the gauges could also be changed so that more accurate readings can be obtained with maximum safety. The steam valves could be insulated to make it easier to open and close them, or have better insulating gloves available for use. References: [1]. Turcotte, G. CHE415 - Unit Operations II Lab Manual – Exp 4 – Steam to Air, Ryerson University, Fall 2007 [2]. DeWitt, D.P., Incropera, F.P., Fundamentals of Heat and Mass Transfer, 5th Ed., 2002, John Wiley & Sons [3]. Geankopolis, C.J., Transport Processes and Unit Operations, 3rd Ed., 2002, Prentice Hall [4] Incropera, F.P. and DeWitt D.P. Introduction to Heat Transfer 4th ed., John Wiley & Sons, 2002. [5] Chapra S.C. and Canale R.P. Numerical Methods for Engineers 5th ed. McGraw Hill, 2006. [6] ABC Craftsmen in Stainless Allegheny Bradford Corporation. Sanitary shell and Tube Heat Exchanger Specification. _http://www.rodem.com/files/Heat%20Exchangers%20&%20Homogenizers/Allegheny%20Bradford/Heat%20Exchanger%20Manual.pdf__ Appendix: RAW DATA Pipe Arrangement Blower Voltage (DC) Pin, steam (psig) Tin, air (ºC) Tout, air (ºF) 1 40 35 40.5 188 50 35 44.5 194 60 35 48.5 195 1,2 40 35 49 208 50 35 53 211 60 35 54.5 212 1,2,3 40 35 51 204 50 35 54 216 60 35 57 216 1,2,3,4 40 35 52.5 207 50 35 56 212 60 35 57 215 1,3,5,7 40 35 53 220 50 35 55 225 60 35 56 230 1,3,5 40 35 52.5 216 50 35 53 218 60 35 55 220 1,4,7 40 35 52.5 219 50 35 54 225 60 35 55 231 Table 2: Raw Data SAMPLE CALCULATIONS Calculating the overall heat transfer coefficient, U: For pipe 1 only, From equation (1), Q = mCpT From equation (2), Q = UATLM Combining (1) & (2): mCpT = UATLM Therefore, Given conditions: Mass flowrate of air: m = 1138 lb/hr = 516.1 kg/hr Pressure of steam in: Pin steam = 35 psig = 343 kPa Air temperature inlet: TI = 40.5ºC = 313.65K Air temperature outlet: TO = 194ºF = 363.15K Total area of bank: AT = 9.87ft2 From thermodynamic tables [4], @ Pin steam = 343kpa for saturated steam, Ts = 137ºC = 410.5K Cp = 2.21 kJ/kgK T = TO - TI = 363.15K – 313.65K = 46.17K Therefore, Calculating the outer heat transfer coefficient, ho: From equation (5), Therefore, Given conditions: Outer radius: ro = 0.3125in = 0.0079375m Inner radius: ri = 0.2725in = 0.0069215m Thermal conductivity of copper [4]: k = 410W/mK Inner heat transfer coefficient [4]: hi = 1500 btu/hrft2F = 8516.92 W/m2K Therefore, Calculating the constants, a,b,c: From equation (6), ho = c maNb log(ho) = log(c) + a log(m) + b log(N) using multiple linear regression by minimizing the sum of the square residuals and differentiating with respect to each unknown coefficient [5]: - 1 - - 1 - - 1 - - 1 - - 1 -