DP Report PDF - Heat Transfer Characteristics

HEAT TRANSFER CHARACTERISTICS OF A DOUBLE PIPE HEAT EXCHANGER (DP) By Group T5 Group Members: Necip Arda Yurduseven 2372381 Çağdaş Ozan Yayla 2440287 Yaren Uslu 2420578 Yıldız Kırbaç -2513075...

HEAT TRANSFER CHARACTERISTICS OF A DOUBLE PIPE HEAT EXCHANGER (DP) By Group T5 Group Members: Necip Arda Yurduseven 2372381 Çağdaş Ozan Yayla 2440287 Yaren Uslu 2420578 Yıldız Kırbaç -2513075 (Group Leader) Ch.E. 410 Chemical Engineering Laboratory II Middle East Technical University Submitted to: Assoc. Prof. Çerağ Dilek HACIBABİOĞLU Experiment Performed: October 22nd, 2024 Report Submitted: October 29th, 2024 NOMENCLATURE Symbol Definition Unit 𝜌 Density 𝑘𝑔/𝑚𝑙 𝐶! Heat Capacity 𝐽/𝑘𝑔. 𝐾 𝑄 Heat transfer rates W Individual heat transfer 𝑊. ℎ 𝑚" 𝐾 coefficient ∆𝑇#$ Log mean temperature K 𝑚 Mass Flow Rate 𝐾𝑔/𝑠 𝑁𝑢% Nusselt number Unitless Overall heat transfer 𝑈 𝑊/𝐾 coefficient 𝑃𝑟 Prandtl number Unitless ∆𝑃 Pressure difference Bar Re Reynolds Number Unitless 𝑘 Thermal conductivity W/m.K 𝜇 Viscosity Ns/𝑚" 2  ABSTRACT This study analyses the thermal dynamics of co-current and counter-current flow double pipe heat exchangers and investigates the effect of Reynolds number on heat transfer coefficients. In the experiments, the velocities of hot and cold fluids were adjusted to obtain different Reynolds numbers, and a comprehensive experimental setup was used to observe the effects of these flows. The experimental setup was equipped with eight thermocouples, two orifice meters for pressure measurement and various valves for flow control, allowing precise data collection under various operational conditions. The findings showed that higher Reynolds numbers increased turbulence and improved convective heat transfer. In addition, a strong relationship was found between Reynolds number and heat transfer coefficient. When compared, the expected situation is that the counter-current flow system provides a more efficient heat transfer by providing a more constant temperature gradient; while in the co-current flow, the temperature difference decreases throughout the system, revealing a decrease in thermal efficiency. However, since the experimental setup was not long enough and there were equipment errors, the experiments did not yield the expected results. Pressure and flow adjustments to maintain steady-state conditions have emerged as a significant challenge for accurate heat transfer rate evaluations. The research also yielded practical findings to validate the theoretical mass flow rate and pressure drop relationship. 3 Table of Contents 1. INTRODUCTION 5 2. EXPERIMENTAL PROCEDURE 7 3. RESULTS 9 4. DISCUSSION 16 5. CONCLUSION 18 6. APPENDIX 19 6.1 CALCULATIONS 19 6.2 RELATED TABLES 21 6.3 RAW DATA SHEETS 22 7. REFERENCES 25 4  1. INTRODUCTION Heat exchangers are systems used to transfer heat between two different temperature environments and allow thermal energy transfer without direct contact between the liquids. These systems, which are widely used in many industries such as heating, cooling, freezing, energy production, and waste heat recovery, are basically based on the principle of transferring heat from a hot liquid to a cold liquid. Heat exchangers can be classified according to their structure types such as shell and tube, double tube and plate and frame, or flow arrangements such as co-current and counter-current. Among these types, in one of the simplest forms, double-pipe heat exchangers, one pipe is placed concentrically inside the other. In this design, shown in Figure 1 and Figure 2, heat transfer starts with convection from the hot liquid to the wall, then continues with conduction on the inner pipe wall, and is finally transferred from the wall to the cold liquid by convection. They are often preferred in industrial applications due to their compact structure and resistance to high pressure and temperature. According to their flow arrangements, heat exchangers can be classified as co-current or counter- current. In co-current flow, both hot and cold fluids move in the same direction, while in counter- current flow, they move in opposite directions. Figure 1 and Figure 2 show these flow patterns and temperature profiles. Counter-current flow creates a higher temperature difference, allowing more heat to be extracted from the heating fluid, while co-current flow can be preferred in cases where a shorter pipe length is desired. Figure 1. Counter Current Flow Schematic and Temperature Graph 5  Figure 2. Co-Current Flow Schematic and temperature graph This report evaluates the effect of the Reynolds number on heat transfer coefficients by analyzing the interaction between co-current and counter-current flow configurations in a double-pipe heat exchanger. The research aims to investigate the dynamic behavior of the system under different conditions by determining the effect of changes in the Reynolds number on heat transfer coefficients. In addition, the laboratory experimental setup is designed to observe the differences between co- current and counter-current flow patterns in the heat exchanger. This experiment aims to explain the effects of these changes on heat transfer efficiency by observing how the temperature gradients between the two configurations change with time. 6  2. EXPERIMENTAL PROCEDURE The experimental system is designed to inspect the working principle of the double-pipe heat exchanger and the effect of the Reynolds Number on individual heat transfer coefficients. The experimental setup can be investigated in Figure 3. Figure 3. Experimental Setup for Double Pipe Heat Exchanger In the system, hot fluid flows through the outer pipe while cold fluid flows through the inner pipe. The double-pipe heat exchanger system consists of eight thermocouples to measure the temperature of both hot and cold streams at every transfer point to collect as much data as possible. Moreover, the experimental setup has ten valves to arrange the flow direction of hot and cold fluids and to accomplish both co-current and counter-current flows. One of the ten valves is a security valve that is used to protect the system from exceeding the maximum allowable pressure value. This valve is also called a spring-loaded safety valve, and it releases excess pressure and is crucial for the system. Pressure difference values of hot and cold streams are measured by an orifice meter, 7 and those pressure difference values are used to calculate the flow rate of the streams, which affects the Reynolds Number of the flow. The experiment starts with opening V9 to heat the water coming from V8, which will be opened right after V9. The steam engine will heat the water that is coming from V8, and the hot water is transferred to the double pipe heat exchanger system by opening the V7. V7 is opened until the desired value of pressure difference is read from the orifice meter, which was 0.22 bar and stayed constant for the whole experiment. On the other hand, V5 is opened to provide cold water to the system, and V10 is opened for the steam trap. The flow rate of cold water is arranged with an orifice meter, and pressure difference values are arranged as 0.20, 0.25, 0.30, and 0.35 bar to inspect the effect of the Reynolds number on heat transfer. Then, the experiment is continued by opening V4 and V1 to provide co-current flow for heat transfer, and temperature and pressure difference values are collected every 2 minutes until steady-state conditions are reached. Agreement of steady state is done by comparing data collected from thermocouples from T1 to T8 and making sure that the last three measurements are the same as each other and pressure difference values stay constant while the whole measurements. After a steady state is accomplished, the measurements are done for three other pressure difference values. Following that, valves V4 and V1 are closed, and V2 and V3 are opened for counter-current flow, and the same process is repeated for counter-current flow arrangement. While experimenting, the steam flow rate is measured at regular time intervals using a steam trap condensate outlet stream. Also, the cold water exit rate is measured to compare the measurement with the formula used to calculate flow rate by pressure difference. After collecting all the relevant data for the experiment, the experiment is finalized, and valves are closed one by one to stop the system. 8  3. RESULTS In the experiment 4 runs were made for two flow models, keeping the pressure difference of hot fluid constant while changing the cold fluid’s pressure difference. Performed calculations and the correlations used are given in Appendix 1. Table 1. Mass Flow rates of hot and cold fluids Next, the temperature change across the heat exchanger is plotted for two flow modes. The temperature data for the plots and ranges and approaches are given in Table 2 and Table 3. Table 2. Temperature data for co-current flow The plots for co-current runs are given below. 9 Figure 4. Temperature vs Length for Co-current Runs 1 and 2 Figure 5. Temperature vs Length for Co-current Runs 3 and 4 10 Table 3. Temperature data for counter current flow Plots for counter current runs are shown in figure 6 and figure 7. Figure 6. Temperature vs Length for Counter Current Runs 1 and 2 11  Figure 7. Temperature vs Length for Counter Current Runs 3 and 4 Then by using the flow rates Reynolds numbers are calculated for each stream, calculated values are given in Table 4. Table 4. Reynolds numbers of the fluids To estimate the individual heat coefficients and overall heat transfer coefficient Prandtl numbers are calculated. Physical properties like viscosity, thermal conductivity, and heat capacity are found by using temperature data in Table 2 and Table 3. 12 Table 5. Physical Properties of the fluids With physical properties data, Prandtl numbers are calculated. With the Reynolds and Prandtl numbers, using the suitable correlations Nusselt numbers are calculated. Prandtl numbers and Nusselt number values are given in Appendix 2. Then, individual heat transfer coefficients are calculated and given in Table 6. Table 6. Individual Heat Transfer Coefficients of the streams Overall heat transfer coefficients are calculated with thermal conductivity and heat transfer coefficients, then areas are calculated using pipe dimensions and length. According to Incropera (2011), the calculation of a U value is dependent on which side it’s calculated for. So, by dividing the multiplication of UA value to both areas, both overall heat transfer coefficients for cold and hot streams are calculated. 13  Table 7. Overall Heat Transfer Coefficients of the streams Next, the log mean temperature of the streams are calculated. ΔTLM and UA values that are used to determine theoretical heat transfer rates are calculated. Also, temperature differences of hot and cold fluids are calculated to find the experimental heat transfer rate. Below, in Table 8, those values are given. All calculated quantities are given in Appendix 2. Table 8. Theoretical and Experimental Heat Transfer Rates In addition, throughout the experiment cold stream outlet’s volumetric flow rate is measured with a cylinder. The time is measured for a certain volume of fluid, then it is used to calculate mass flow rate. These values are compared with the values obtained from the ΔP equation. Table 9. Cold stream outlet flow rates 14 Lastly, condensate’s volume is measured in the same way and volumetric flow rates are calculated, with shows difference in the mass flow rates assuming density is constant. Table 10. Condensate’s volumetric flow rate 15  4. DISCUSSION In this experiment, the primary objective is to investigate the effect of the Reynolds number on the heat transfer coefficient. In addition to that, temperature differences along the pipe of a double-pipe heat exchanger and the effect of configuration on heat transfer rate were easily observed in this experiment. In this configuration, the hot stream flows in the inside a pipe with a fixed direction and fixed flow rate while the flow direction and flow rate of the cold stream are changed. The flow rate of the cold stream is changed considering the pressure difference by adjusting it to 0.2, 0.25, 0.30, and 0.35 bars. Temperature data was collected with 2-minute time intervals to see if the system is at steady state. In this experiment, the system is assumed to be at steady state when the temperature data taken from the same thermocouple is the same three times in a row. When approach and range data were examined for co-current and counter-current double pipe heat exchangers, the range value of the co-current flow mode was smaller than the counter- current which was expected. Range equals to the difference between the outlet and inlet temperature of the same stream. In co-current flow streams, the temperature has a limited range due to the temperatures of the other stream, as it moves in the same direction the temperatures are getting closer. The approach is different between hot stream inlet temperature and cold steam outlet temperature. As expected, the approach of co-current is greater than the counter-current flow. Co current approach value is expected to be higher because as the streams move along the pipe, their temperatures approach each other and the exit temperature of one approaches the other, decreasing the approach value. Reynolds number which is a dimensionless parameter calculated from the formula in the appendix is directly proportional to the mass flow rate. Thus, deviation in Reynolds number for cold streams is expected in different runs. However, slight variations in Reynolds numbers have been observed in different runs with the hot stream. These deviations can be from the viscosity term in Reynolds number. Very minor differences in the average temperature of the hot stream cause different viscosity values. According to the equation derived in Incopera (2011), the Nusselt number should increase when the Reynolds number is increasing. According to Table A2.2 in Appendix 2, that increase was 16 observed. In addition to that, the individual heat transfer coefficient (h) is directly related to the Nusselt number shown in the formula in Appendix 1. And with the individual heat transfer coefficients calculated in Table 6, this relation is observed. For hot flow, the individual heat transfer coefficients are expected to stay constant, but all the numbers and formulas that have viscosity as a term change with temperature, therefore this causes inconsistency with the theoretical cases. Theoretically, counter-current flow is a more efficient way of heat transfer because the logarithmic temperature difference in counter-current flow is higher than co-current, according to Incopera (2011). However, this experiment was carried out in a small area and, therefore, with a short-length double-pipe heat exchanger. For this reason, there may be closeness in Q values. The main reason why counter-current flow is more efficient is that the temperature values do not get close to each other along the length. While working for a short length, that significant difference may not be observed. When we look at Table 8, as expected in theoretical Runs 1 and 4, the Q value transferred in counter-current exceeds co-current, while in Runs 2 and 3, the co- current value is higher. Based on this, we can say that a co-current flow heat exchanger can also be used while working for a short length. There are differences observed between theoretical Q and experimental Q values as not excepted. The primary reason for this difference can be the lack of insulation. Additionally, errors may occur due to a non-ideal environment for the experiment, damaged thermocouples, and the unsteady flow rate of the steam generator. The data taken from the condensate, which is shown in Table 10, shows the unsteady conditions in the steam generator. Finally, there is also a difference between the theoretical mass flow rate of the cold stream and the experimental one. Table 9, indicates that there are not too many differences between values. It shows that the formula derived from the pressure difference for mass flow rate gives results accurately. 17  5. CONCLUSION The purpose of the experiment was to investigate the effect of the Reynolds number on the individual heat transfer coefficient and the performance of co-current and counter-current flow heat transfer in a double pipe heat exchanger. In the experiment, it is necessary to open different valves to obtain co-current and counter-current flow. Valves numbered 1-4 are used for co-current, while valves numbered 2 and 3 are used for counter current. First, mass flow rates are calculated from the pressure difference of hot and cold water. Reynolds number is calculated for each stream using the calculated mass flow rates. Physical properties of the fluids and Prandtl number are calculated. Reynolds and Prandtl number are useful in finding the correct Nusselt number correlation. Thus, individual heat transfer coefficient values are calculated. It is seen that overall heat transfer coefficient (U) and individual heat transfer coefficient (h) numbers increase for both flow types as Reynolds number increases. When the amount of energy transferred in the heat transfer process is examined, it is expected to be observed that counter-current flow is more efficient than co-current flow, but this is found in run 1 and run 4. However, in this experiment, the main reasons such as the small area and the short length of the pipe were that the numbers were close to each other and the counter-current could not create a clear superiority chance in every experiment. In addition, the fact that the difference between the theoretical and experimental Q values found at the end of the calculations was very large is also an indication that the experimental setup was not insulated and that we experienced too much heat loss. 18  6. APPENDIX 6.1 Calculations Mass flow rates (kg/s) are calculated with pressure differences (bar); 𝑚 = 1.223 𝑥 (𝛥𝑃 )!.#$ For Reynolds number calculations formula below is used; 𝜌𝑉𝐷 4𝑚 𝑅𝑒 = = 𝜇 𝜋𝜇𝐷 To find Prandtl number; 𝜇𝐶! 𝑃𝑟 = 𝑘 From Table 8.4 (Bergman & Incropera, 2011,p.567) suitable correlation for Nusselt number is chosen, the parameters are given as below: 0.6 ≤ 𝑃𝑟 ≤ 160 10000 ≤ 𝑅𝑒 10 ≤ 𝐿 𝑁𝑢% = 0.023𝑅𝑒%&.( 𝑥 𝑃𝑟 &.) (For heating) 𝑁𝑢% = 0.0023𝑅𝑒%&.( 𝑥 𝑃𝑟 &.* (For cooling) Nusselt number is used for h calculations; 𝑁𝑢% 𝑥 𝑘 ℎ= 𝐷 For overall heat transfer coefficients circuit analogy is used; 1 𝑈+ 𝐴+ = 𝑈, 𝐴, = 𝑟 ln ( 𝑟+ ) 1 , 1 𝐴, ℎ, + 2𝜋𝑘𝐿 + 𝐴+ ℎ+ 19 Pipe inner and outer diameters are 0.0334 m and 0.05639 m respectively. For calculation of areas formulas given below is used; 𝐴, = 2𝜋(𝐷, /2)𝐿 𝐴+ = 2𝜋(𝐷- /2)𝐿 where 𝐷- = 𝐷+ − 𝐷, (Hydraulic Diameter) For the thermal conductivity of the pipe, carbon steel thermal conductivity (Cartagenas, 2022); 𝑊 𝑘!,!. = 45 𝑚𝐾 Theoretical and experimental heat transfer rates are calculated as below; 𝑄/-.+0./,123 = 𝑈𝐴(∆𝑇#$ ) 𝑄.4!.0,5.6/23 -+/ = 𝑚𝐶!,-+/ (∆𝑇) 𝑄.4!.0,5.6/23 1+39 = 𝑚𝐶!,1+39 (∆𝑇) For log mean temperature: ∆𝑇" − ∆𝑇: ∆𝑇#$ = ∆𝑇 ln (∆𝑇" ) : ∆𝑇: 1+ 1;00.6/ = 𝑇-,, − 𝑇1,, ∆𝑇" 1+ 1;00.6/ = 𝑇-,+ − 𝑇1,+ ∆𝑇: 1+;6/.0 1;00.6/ = 𝑇-,, − 𝑇1,+ ∆𝑇" 1+;6/.0 1;00.6/ = 𝑇-,+ − 𝑇1,, For cold stream’s and condensate’s flow rate calculation: 𝑉𝑜𝑙𝑢𝑚𝑒 (𝑚𝑙) 𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 = 𝑡𝑖𝑚𝑒 (𝑠) 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝜌= 𝑣𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 Density of water at 290°C is 0.7319, this is the value used in the calculations. 20  6.2 Related Tables Table A2.1 Prandtl number of the streams Table A2.2 Nusselt number of the streams Table A2.3 Multiplication of overall heat transfer coefficients and areas of the streams Table A2.4 Log mean temperatures of the streams 21 6.3 Raw Data Sheets 22 23 24  7. REFERENCES Bergman, T. L., & Incropera, F. P. (2011). Fundamentals of heat and mass transfer. Wiley. The engineering toolbox. Engineering ToolBox. (n.d.). https://www.engineeringtoolbox.com/ 25