CHE 415 - Heat Transfer – Steam to Water Final Report 2007

(Inspector) (Leader) (Data Reporter) Experiment #4, Heat Transfer – Steam to Water Date of Experiment: October 23rd, 2007 Section #1, Group #5 (Inspector) (Leader) (Data Reporter) Experiment #4, Heat Transfer – Steam to Water Date of Experiment: October 23rd, 2007 Section #1, Group #5 (Inspector) (Leader) (Data Reporter) Experiment #4, Heat Transfer – Steam to Water Date of Experiment: October 23rd, 2007 Section #1, Group #5 Ryerson University Department of Chemical Engineering CHE 415 Unit Operations II Lab Report Experiment #4: Heat Transfer – Steam to Water Experiment Performed on October 23rd, 2007 Report Submitted to Dr. 1. /Leader By Group # 5 Section # 1 2. / Data Reporter 3. /Inspector October 30th, 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 This report determines the overall heat transfer coefficients of a double pipe heat exchanger for saturated water and saturated steam in co-current and counter-current flow arrangements. Heat exchangers are widely used to transfer energy, where cold components can be heated or hot components are cooled. A double pipe heat exchanger was used, where it consists of two pipes, one inside another. Saturated water with a known flow rate flowed into the inner pipe, while the outer pipe had saturated steam. The temperature of the inlet and outlet of both streams were taken and various valves were opened to control the flow direction. The experimental overall heat coefficient was calculated using the first law of thermodynamics, where the overall heat coefficient transferred within the system is the same as the heat gained by the water. The theoretical overall heat transfer coefficient was determined by the overall thermal resistance equation. It was found that the overall heat transfer coefficient for co-current was 435.2 W/m2K experimentally and 441.0 W/m2K theoretically resulting in a 1.33% error. The overall heat transfer coefficient for counter-current was 461.2 W/m2K experimentally and 439.0 W/m2K theoretically resulting in a 5.08% error. The errors were a resultant from the assumptions used in the calculations, such as no heat loss within the system, negligible fouling, and constant thermodynamic properties. Overall, this experiment was successful at determining the overall heat transfer coefficient for co-current and counter-current double pipe heat exchangers. Table of Contents 22 44 44 88 99 99 1010 1111 1212 1313 1313 1515 15 List of Figures 55 55 88 8 List of Tables 1111 1212 12 Objectives and Experimental Design This experiment seeks to determine the overall heat transfer coefficient for a co-current and counter-current concentric tube heat exchanger. It will also verify that the two flow arrangements provide the same heat transfer coefficient. In this experiment, a concentric tube (also know as a double piping) heat exchanger will be used. It consists of two pipes, one inside another. The smaller pipe will have water flowing at a known constant rate, the outer pipe contains saturated steam. The temperature of the inlet and outlet for both steam and water are measured with thermocouples. Valves are placed at various locations to control the direction of flow, such that co-current and counter-current are achieved. Saturated water and saturated steam are used, such that thermodynamic properties of both substances are known. Introduction and Theoretical Background Heat transfer is the thermal energy in transit due to a temperature difference or temperature gradient [1]. There are three modes heat transfer occurs. Conduction is the transfer of thermal energy from more energetic particles to less energetic particles through their interactions and occurs when there is no bulk motion [1]. Convection is the transfer of energy through random motion (diffusion) and through bulk motion of a fluid [1]. Lastly, radiation is the energy emitted from a substance at a finite temperature [1]. In general, thermal energy (heat) is transferred from a high temperature to a low temperature. This phenomenon, known as the first law of thermodynamics, is greatly utilized in the industry as cold substances can be heated or hot substances are cooled. This is accomplished through heat exchangers. Heat exchangers are devices that separate two fluids with different temperatures by a solid wall [1]. The typical classification of heat exchangers is based on the flow arrangement and type of construction [1]. The flow arrangement is the direction the two fluids move, such as co-current, counter-current, and cross flow. The type of construction is the physical nature (the design) of the heat exchanger. Some examples are concentric tube (or double piping), shell-and-tube, plate, finned or unfinned, stacked and non-stacked, compacted or not compacted. In this experiment, the double piping heat exchanger will be investigated as it is the simplest model, where two fluids move in the same (co-current) or opposite (counter-current) direction in a concentric (cylindrical) tube as shown in Figure 1. There are two cylindrical pipes, one inside the other. Liquid water (cold component) is fed to the smaller/inner pipe, while the steam (hot component) is fed in the larger/outer pipe. Valves at various positions control the flow rate and the flow direction. The temperature distributions (temperature profile) for both the co-current and counter-current are shown in Figure 2. For Figure 2, “c” and “h” represents saturated water (cold substance) and saturated steam (hot substance) respectively, while “i” and “o” represents inlet and outlet respectively. Figure 1: Concentric tube heat exchangers a) co-current b) counter-current [1] a) b) Figure 2: Temperature profile in a concentric tube heat exchanger a) co-current b) counter-current [1] The inlet and outlet temperatures of each fluid will be measured along with the flow rate of the liquid. To calculate the overall heat transfer coefficient of the system, two relationships are used. Equation (1) relates the amount of heat, Q, gained by the liquid water from the steam [1]. Equation (2) relates the amount of heat transferred based on Newton’s law of cooling [1]. Equation (2) assumes heat loss to the surrounding, kinetic and potential energy changes, tube wall thermal resistance, and fouling factors are negligible. (1) (2) where Q – heat transferred/gained m - mass flow rate of liquid Cp, L - specific heat capacity of liquid ?TL – temperature difference between the inlet and outlet U – overall heat transfer coefficient A – area of heat transfer ?TLM – log mean temperature difference The log mean temperature difference, LMTD, is different for co-current and counter-current. Thus, an energy balance on differential elements coupled with a few assumptions (such as negligible axial conduction, negligible potential and kinetic energy, overall heat transfer coefficient and fluid specific heat are constant), gives the LMTD for the co-current and counter-current [1]. (3) For co-current: ?T1 = TS,IN – TW,IN and ?T2 = TS,OUT – TW,OUT For counter-current: ?T1 = TS,IN – TW,OUT and ?T2 = TS,OUT – TW,IN The subscript “w” and “s” represents water and steam respectively. Combining equation (1) and (2) allows the overall heat transfer coefficient to be determined: (4) To determine the theoretical overall heat transfer coefficient, the convection coefficient is utilized. For an unfinned tubular heat exchanger, the overall heat exchanger is shown in equation (5). (5) where hi – heat transfer coefficient for the inner tube ho – heat transfer coefficient for the outer tube k – thermal conductivity of the pipe ri – inner diameter of the pipe ro – outer diameter of the pipe The inner heat transfer coefficients hi can be determined using Nusselt’s number, which can be computed by the Dittus-Boelter equation for heating shown in equation (6). This is based on Chiltron-Colburn’s analogy, a classical expression for local Nusselt number for fully developed turbulent flow in a smooth circular tube [1]. (6) where Re – Reynolds Number; Pr – Prandtl Numbers, m – mass flow rate of liquid D – characteristic length ?L – viscosity of the liquid CpL – heat capacity of the liquid kL – thermal conductivity of the liquid Also, the general definition of Nusselt’s number is expressed in equation (7) [1]. (7) where hi – the inner heat transfer coefficient D – characteristic length k – thermal conductivity Combining (6) and (7) yields the inner heat transfer coefficient for the inner pipe. The outer heat transfer coefficient for the outer pipe can be calculated with dropwise condensation. The dropwise condensation heat transfer coefficient is much larger than the film condensation, about an order more [1]. For heat exchangers, the dropwise condensation dominates more than the thermal resistances [1]. Therefore, the heat transfer dropwise coefficient can be used for the outer pipe as the saturated steam losses heat causing some vapours to form droplets. For copper surfaces, the dropwise condensation heat transfer is correlated in equation (8), where the subcooling effect is neglected and no “crowding” occurs [1]. for 22ºC < Tsat < 100ºC for Tsat > 100ºC (8) Substituting the values of the two heat transfer coefficient (hi and ho) into equation (5) yields the theoretical overall heat transfer coefficient. Experimental Set-up Figure 3: Schematic Diagram of Experiment Set-up Simple Batch Distillation Experimental Procedure Before beginning the experiment, it was ensured that all inlet water and steam values were fully closed. The power was switched on, on the control panel, where all of the temperatures could be read. The main steam valve for the laboratory was turned on. Co-current operation For this operation, the valves V2 and V3 were opened, while the valves V1 and V4 were fully closed. This was to allow the steam to be directed into flowing in the same direction as the water inside of the heat exchanger. The system was left to settle to a steady-state, which was assumed to be when there were no fluctuations in the water outlet temperature. The temperatures of the steam and water streams in and out of the heat exchanger were recorded every 5 mins for 20 minutes after steady-state was reached. Counter-current operation For this part of the experiment, V1 and V4 were opened fully, was V2 and V3 were closed, and the steam was directed into a direction opposite to the flow of water. The same procedure for the co-current operation was then followed Laboratory Safety: During this experiment, steam was used as a heating fluid in a double-pipe heat exchanger. Care was taken when handling the valves connected to the steam pipes as they were very hot. Heat resistance gloves were used. As a precaution In the heat exchanger, a cool fluid is in close proximity to the hot fluid. It is possible that the pipes can crack or fissures in the pipes may result if the cool fluid is directed through the heat exchanger after the hot steam has been flowing through it. This is because the pipes may experience thermal shock. This is dangerous as the cracks may release hot steam in its vicinity. For this experiment, the cool water was allowed to flow through the heat exchanger before applying the hot steam to the system. Some of the valves and the pressure measurement gauge were very high on the wall where the equipment is located. A step-ladder will be used to reach these parts of the equipment safely. Safety and Environmental Concerns in Industrial Applications: Industrial applications of heat exchangers are very diverse. They can be used for transmission and engine cooling, wastewater heat recovery, evaporators and condensers in refrigeration systems to name just a few [3]. In nuclear power plants, large, phase-change heat exchangers are used to pass heat from the nuclear reactor plant to the steam plant, where water is converted into steam and used to power turbines, which is used to generate electricity [4]. In refrigeration systems, the cooling fluid may be chemical, such as R-14 (carbon tetrafluoride) or R-30 (Dichloromethane) [4]. These chemicals may be dangerous if released to the environment. There can be no leaks in the heat exchanger. In the food industry, heat exchangers can be used in the sterilization of many foods. However, if the parameters of the system is not set correctly, there may be over-heating of the food, and the food product may burn and build-up inside of the heat exchanger. For example, in the processing of milk, milk is pasteurized in a heat exchanger. Burnt milk proteins may build-up inside of the tube, which is termed fouling. This presents a safety issue because there may be a build-up of pressure inside of the heat exchanger, which may result in damage to the heat exchanger. Results and Discussion In this experiment, it was desired to calculate the overall heat transfer (HT) coefficient for a double-pipe heat exchanger. To determine the theoretical overall HT coefficient, the theory of thermal resistance was used. In order for this to be used, it was necessary to first determine the HT coefficient for both fluids, as well as the thermal conductivity of the pipe. The pipe was made out of copper, and the thermal conductivity was found in the literature to be 0.598W/mK. To calculate the HT coefficient of the inner pipe where water was flowing, Colburn equation, equation (6), was employed. The criteria for the Reynold’s number and the Prandtl number were satisfied. For each data point, a HT coefficient of the water was found, as shown in Table 1 below. The average HT coefficient was also calculated. Co-current Flow Time (min) ReD Pr NuD hi 0 0 7.556388 0 0 5 28508.78 7.556388 189.2502 4346.938 10 28508.78 7.556388 189.2502 4346.938 15 34210.54 7.556388 218.9684 5029.542 20 22807.02 7.556388 158.31 3636.264 Average 4339.921 Counter-current Flow Time (min) ReD Pr NuD hi 0 0 7.556388 0 0 5 39912.29 7.556388 247.7073 5689.654 10 17105.27 7.556388 125.7643 2888.713 15 28508.78 7.556388 189.2502 4346.938 20 28508.78 7.556388 189.2502 4346.938 Average 4318.061 Table 1: Inner HT Coefficient It was also necessary to calculate the HT coefficient of the steam on the outer surface of the pipe. It was assumed that there was dropwise condensation occurring on the surface of the pipe, which would lead to the temperature of the surface of the pipe being constant at any point along the pipe. The correlation used for this assumption was: In using this assumption, the HT coefficient on the outer surface of the pipe was calculated to be 188052W/m2K. The other objective of this experiment was to compare the overall HT coefficients between a co-current and a countercurrent heat exchanger. It was expected that the overall HT coefficient would be the same for both systems since the temperature on the outer surface of the heat exchanger pipe was assumed to be constant at any point along its length. Table 2 summarizes the results from this experiment. From both tables 1 and 2, it can be seen that the HT coefficient on the inner surface of the pipe was very similar, differing by only 21W/m2K. The Overall HT transfer coefficient was also very similar in both the experimental and theoretical values between the co-current and counter-current systems. Inner HT Coefficient, hi (W/m2K) Outer HT Coefficient, ho (W/m2K) Theoretical Overall HT Coefficient, Utheo (W/m2K) Experimental Overall HT Coefficient Uexp (W/m2K) % Error (%) Co-Current 4339.9 188052 441.0 435.2 1.32 Counter-Current 4318.1 188052 439.0 461.2 5.1 Table 2: Comparison of Results Error Analysis The percent error was 1.32% and 5.1% for co-current and counter-current respectively. The primary error was due to the assumptions. Firstly, it was assumed that the system would not have heat loss as in all the heat gained by the water was the same as the amount of heat transferred. This was not the case as the outer pipe was not insulated. Also, since the pipes were long, there was bound to be heat loss. Next, fouling within the pipe was assumed to be negligible. Although fouling is minimal here, this could contribute for the small percent error. Fouling could have occurred within the pipes as the copper piping could have oxidized over time or small particles from the steam and water inlet. It was also assumed that kinetic energy did not change from the inlet to the outlet. The steam and water occurred individually from the change of energy due to friction within the pipes and each other. Also, the kinetic energy of both changed because there was a transfer of energy. Furthurmore, the thermodrynamic properties of the water and steam are constant as assumed because of disturbances within the pipe and prior to the pipe. Finally, the assumption of the dropwise condensation plays a significant part for the difference of the co-current and counter-current. Based on visual inspection, the outlet steam had more steam in the co-current than the counter-current. This leads to the conclusion that the counter-current had more dropwise condensation than co-current. This could have lead to “crowding” of droplets known as constriction resistance, which was not accounted for [1]. Conclusion and Recommendations In conclusion, this experiment was successful at determining the heat transfer coefficient for both co-current and counter-current flow for a double piping heat exchanger. The values were 435.2W/m2K and 461.2 W/m2K experimentally. During this experiment, the only difference between the co-current and countercurrent systems was the direction of flow of steam over the heat exchanger pipe. All other parameters were kept constant between the runs for both systems, such as the pressure flowing through the outer pipe of the heat exchanger and the inlet temperature of the water. As was expected, the overall heat transfer coefficient for the co-current system was the almost the same as that for the countercurrent one, with a difference of only 26W/m2K. To improve on this experiment the following are recommended: Place a controller (such as PID controller) or replace valves to minimize variations of inlet steam and inlet water. Insulate the outer pipe such that no heat is loss Install a transparent pipe for the outer pipe to verify the dropwise condensation Use saturated water References: [1]. DeWitt, D.P., Incropera, F.P., Fundamentals of Heat and Mass Transfer, 5th Ed., 2002, John Wiley & Sons [2] Turcotte, G. CHE415 - Unit Operations II Lab Manual – Exp 3 – Steam to Water, Ryerson University, Fall 2007 [3]. Wikipedia, Heat Exchangers, Retrieved on October 19th, 2007 from _http://www.heatexchangersonline.com/_ [4]. Heat Exchangers Online, Heat Exchangers in the HVAC Industry, Retrieved on October 19th, 2007 from _http://en.wikipedia.org/wiki/Heat_exchangers__ Appendix: RAW DATA Co-current Flow Time (min) Steam Pressure (lbs/in) Rotameter Reading (gal) Steam Temp In Stem Temp Out Water Temp In Water Temp Out %Steam 0 4.0 416680 100.6 82.1 9.6 24.9 32 5 4.0 416780 100.4 79.2 9.6 24.3 32.3 10 4.0 416780 100.4 77.1 9.5 24 32.3 15 4.0 416840 100.4 77.6 9.5 23.6 32 20 4.0 416880 100.4 77.2 9.5 23.6 32.1                 Counter-current Flow Time (min) Steam Pressure (lbs/in) Rotameter Reading (gal) Steam Temp In Stem Temp Out Water Temp In Water Temp Out %Steam 0 4.5 416930 102.4 59.2 9.5 23.9 33.4 5 4.5 417000 102.4 61.7 9.4 24.3 33.9 10 4.5 417030 102.6 60.6 9.3 23.9 33.3 15 4.5 417080 102.8 61.7 9.6 24.4 33.9 20 4.5 417130 102.7 63.2 9.6 24.8 34.2 Table 3: Raw Data SAMPLE CALCULATIONS Inner Pipe Radius, ri=0.042708 ft Outer Pipe Radius, ro=0.046875ft Length of HE Pipe, L=9.2ft Heat Transfer Area = Surface Area of Pipe: Cross-Sectional Area of HE Pipe: Volumetric Flow Rate of Water: Mass Flow Rate of Water: To find inner-film heat transfer coefficient: Reynolds Number: Prandtl Number: Nusselt Number: Inner Heat Transfer Coefficient: To find outer-film heat transfer coefficient: Assuming that dropwise condensation of the steam occurs on the outer surface of the heat exchanger pipe: At 4psi, Tsat=340K=67ºC Theoretical Overall Heat Transfer Coefficient: Log-Mean Temperature: For Co-current Flow: & For Countercurrent Flow: & Experimental Overall Heat Transfer Coefficient: & Combining these equations: %Error: The average Overall HT coefficient was taken for the duration of each run. - 1 - - 1 - - 1 -