Lab4_HeatExchange
- 1. University of California, Merced Engr 135: Heat Transfer Lab #4: Heat Exchanger Efficiency Jonathan Ramirez, Derek Brigham, Eduardo Rojas-Flores Section 05L April 15, 2016
- 2. Abstract: The purpose of this experiment was to test and compare the efficiency of parallel and counter-flow concentric heat exchangers. Heat transfer equations were developed and an experiment was performed in order to accomplish this goal. It was found that as parameters such as the flow rate were changed, the efficiency of the heat exchanger changed as well. From the data calculated for both flow configurations, parallel flow had the higher efficiency values. The reasons for this were a larger temperature difference and a faster mass flow rate for parallel flow. Introduction: The purpose of this experiment was to test the effectiveness of parallel and counter-flow concentric heat exchangers. Two concentric heat exchangers composed of two copper tubes of different diameter (see Figures 1 and 2) were used. The smaller tube was inserted into the larger tube. Cold water flowed through the inner tube, and hot water flowed through the outer tube. The difference in temperature between the two flows caused heat transfer to occur from the hot water to the cold water. Figure 1 Figure 1: Schematic diagram of parallel flow heat exchanger Figure 2 Figure 2: Schematic diagram of counter-flow heat exchanger For parallel flow, both hot and cold water flowed in the same direction. But for counter- flow, hot water flowed one way, and the cold water flowed the opposite way. The question which this experiment sought to answer was which system had higher effectiveness. In order to solve for the effectiveness, first start with the heat transfer rate q = 𝐶ℎ(𝑇ℎ,𝑖 − 𝑇ℎ,𝑜) = 𝐶𝑐(𝑇𝑐,𝑜 − 𝑇𝑐,𝑖) ( 𝑬𝒒. 𝟏)
- 3. Where q was the heat transfer rate in units of 𝑊, 𝑇ℎ,𝑖 and 𝑇ℎ,𝑜 were the hot inlet and outlet temperatures, respectively, and 𝑇𝑐,𝑖 and 𝑇𝑐,𝑜 were the cold inlet and outlet temperatures, respectively. Also seen in Equation 1 were 𝐶ℎ and 𝐶𝑐, which are defined as 𝐶ℎ = (𝑚̇ ∗ 𝑐 𝑝) ℎ (𝑬𝒒. 𝟐) Where 𝐶ℎ was the hot thermal conductance in units of 𝑊 𝐾 , 𝑐 𝑝,ℎ was the hot water specific heat, and was given as 4180 𝐽 𝑘𝑔∗𝐾 . and 𝐶𝑐 = (𝑚̇ ∗ 𝑐 𝑝) 𝑐 (𝑬𝒒. 𝟑) Where 𝐶𝑐 was the cold thermal conductance, 𝑐 𝑝,𝑐 was the cold water specific heat, and was given as 4183 𝐽 𝑘𝑔∗𝐾 . The value of 𝑞 𝑚𝑎𝑥 was dependent upon the greatest value between 𝐶𝑐 and 𝐶ℎ. However, in general it was defined as 𝑞 𝑚𝑎𝑥 = 𝐶 𝑚𝑖𝑛(𝑇ℎ,𝑖 − 𝑇𝑐,𝑖) (𝑬𝒒. 𝟒) The effectiveness is defined as ε = 𝑞 𝑞 𝑚𝑎𝑥 = 𝐶𝑐(𝑇𝑐,𝑜 − 𝑇𝑐,𝑖) 𝐶 𝑚𝑖𝑛(𝑇ℎ,𝑖 − 𝑇𝑐,𝑖) (𝐄𝐪. 𝟓) The mass flow rate, 𝑚̇ , was defined as 𝑚̇ = 𝑉̇ 951 𝑘𝑔 𝑠 (𝑬𝒒. 𝟔) Equations 1-6 along with data produced from the experiment allowed for the effectiveness of both parallel and counter-flow heat exchangers to be found. Procedure: Referring to the apparatus shown in Figure 1 to initiate the experiment, let the apparatus sit and reach steady state conditions. The hot reservoir is then set to 30 degrees Celsius and the cold reservoir is set to 10 degrees Celsius. When steady state conditions are met, record the flow rates alongside with the inlet and outlet temperatures for the hot and cold fluid. Altering the flow rates and reaching steady state once more, record the new values for the cold flow rate and the inlet and outlet temperatures for the cold and hot fluid. Repeat until receiving a total of 3 separate cold flow rate values for the first hot flow rate. Repeat with new hot flow rate
- 4. values until receiving a total of 18 inlet and 18 outlet temperatures. Upon completion, repeat the entire process at the new station shown in Figure 2 for the counter flow configuration. After completing the recording of the station’s data, take the measurement of the diameter of the inner tube and outer tube. Then finish the experiment by lastly taking the measurement of the length of the heat exchanger pipe for both stations. 𝑉ℎ ̇ (GPM) 𝑉𝑐 ̇ (GPM) Th,i (C) Th,o (C) Tc,i (C) Tc,o (C) 19 4 30 27 12 22 19 10 30 27 11 17 19 20 30 25 11 15 21 10 30 27 11 17 21 20 30 26 11 15 21 15 30 26 11 16 15 10 30 26 11 17 15 15 30 26 11 16 15 20 30 25 11 15 Table 1: Recorded data for parallel flow. 𝑉ℎ ̇ (GPM) 𝑉𝑐 ̇ (GPM) Th,i (C) Th,o (C) Tc,i (C) Tc,o (C) 10 10 24.88 22.72 17.33 18.15 10 15 24.93 22.71 17.26 17.61 10 20 24.92 22.68 17.15 17.34 15 10 24.92 23.24 17.51 18.34 15 15 24.91 23.22 17.45 17.85 15 20 24.92 23.11 17.23 17.43 20 10 24.93 23.6 17.48 18.46 20 15 24.91 23.49 17.51 17.83 20 20 24.87 23.34 17.35 17.41 Table 2: Recorded data for counter flow. Results and Discussion: Upon finishing the lab experiment, the collected data was used to solve for the desired variables. The volumetric flow rate 𝑉𝑐 ̇ and 𝑉̇ℎ for both cold and hot temperature, respectively, were found by adjusting the flow rate meter at both ends of the heat exchanger tube. From the collected flow rate data, they were used to solve for the mass flow rate 𝑚 𝑐̇ and 𝑚ℎ̇ for both cold and hot temperatures using Eq. 6. Once the mass flow rate was found for both temperature values, it was used to solve for the thermal conductance Cc and Ch using Eq. 2 and 3. From there the heat transfer rate q and qmax can be found from the given thermal conductance values by plugging them in Eq. 1 and 4 along with the collected temperature values for the inlet and outlet areas. Since qmax refers to the largest heat transfer rate possible, Ch was used to calculate it because it has the largest mass flow rate values. For q, Cc was used since the
- 5. mass flow rate is at its smallest in which the variable in the data is denoted as qc to refer to the lowest heat transfer rate. The efficiency was then solved for once qmax and qc were determined and plugged into Eq. 5 where the ratio is the lowest heat transfer rate over the highest heat transfer rate. Based on the data found, the efficiency will be the indicator on which system is the most beneficial in terms of heat exchanger design. Tables 1 and 2 below were created to reflect the calculated values for both parallel and counter flow in the lab experiment. 𝑚ℎ̇ (kg/s) 𝑚 𝑐̇ (kg/s) Cc (W/K) Ch (W/K) qmax (W/m2*K) qc (W/m2*K) (%) 0.01997897 0.0042 17.6 83.5 317 176 55.6 0.01997897 0.0105 44.0 83.5 836 264 31.6 0.01997897 0.0210 88.0 83.5 1587 352 22.2 0.022082019 0.0105 44.0 92.3 836 264 31.6 0.022082019 0.0210 88.0 92.3 1671 352 21.1 0.022082019 0.0158 66.0 92.3 1254 330 26.3 0.015772871 0.0105 44.0 65.9 836 264 31.6 0.015772871 0.0158 66.0 65.9 1253 330 26.3 0.015772871 0.0210 88.0 65.9 1253 352 28.1 Table 1: Calculated values for parallel flow. 𝑚ℎ̇ (kg/s) 𝑚 𝑐̇ (kg/s) Cc (W/K) Ch (W/K) qmax (W/m2*K) qc (W/m2*K) (%) 0.010515247 0.010515247 44.0 44.0 332 36 10.9 0.010515247 0.015772871 66.0 44.0 337 23 6.8 0.010515247 0.021030494 88.0 44.0 342 17 4.9 0.015772871 0.010515247 44.0 65.9 326 37 11.2 0.015772871 0.015772871 66.0 65.9 492 26 5.4 0.015772871 0.021030494 88.0 65.9 507 18 3.5 0.021030494 0.010515247 44.0 87.9 328 43 13.2 0.021030494 0.015772871 66.0 87.9 488 21 4.3 0.021030494 0.021030494 88.0 87.9 661 5 0.8 Table 2: Calculated values for counter flow. In Table 3, the mass flow rates fluctuated and slightly increased for the hot temperature while it leveled off for the cold temperature. For the cold thermal conductance, the values steadily increased while the hot thermal conductance increased and then decreased. For qmax the values increased and for qc it increased then fluctuated at a constant rate. The efficiency found was relatively high then fluctuated a bit and leveled off. In Table 4 the mass flow rates increased for both hot and cold temperatures with the cold one fluctuating at the final values. For the thermal conductance both hot and cold values
- 6. increased as well with the hot values having a large increase. In qmax it steadily increased and in qc it remained almost constant then dropped dramatically. The efficiency was relatively low in which it decreased slightly then dropped very low at the final value. According to the data and based on the given lab questions, the reason qmax is shown as the form in Eq. 4 is because at the inlet of the heat exchanger, both temperatures for hot and cold are higher than their counterpart values in the outlet temperature. While in the heat exchanger, the mass flow rate for both the hot and cold water have been in contact with one another and heat transfer occurred. Due to this scenario the outlet temperature for both hot and cold changed due to their interaction with one another in the heat exchanger. At the inlet of the heat exchanger, the hot and cold water have not been in contact with one another because they haven’t gone through the tube yet, meaning their temperature values will be at either their highest or lowest which will in turn be greater than the outlet temperatures. From these conditions qmax is satisfied by the inlet temperatures not only because of their greater temperature values but also because Cmin correlates to the highest thermal conductance which is given by Ch because it has a higher mass flow rate. In Eq. 5, qc is used in the calculation because for qmax it is only for the highest heat transfer in which qc does not meet this requirement since qh has a higher heat transfer. It is also used in Eq. 5 to determine the efficiency which is the ratio between the hot and cold heat transfer rates. If qh was to be used in Eq. 5, the efficiency value will come out as 100% which is not possible and having qc in qmax will most likely produce efficiency values over 100% which is not possible as well. By changing parameters such as flow rate, it affects the efficiency of the heat exchanger because it correlates to the rate at which the temperature changes over time. If there is a large enough temperature change that means that heat transfer is occurring at a relatively fast rate and due to that the efficiency will be high. If the temperature change is low, then the efficiency will be low unlike the previous scenario. Changing the flow rate will also affect the efficiency because by increasing the mass flow rate it will also cause an increase in the heat transfer rate, therefore the efficiency will be higher and if there is a decrease in flow rate then the efficiency will be lower. Overall, changing any of the variables will affect the efficiency which is dependent on these variables and based on how it affects the behavior of the heat exchanger, it will determine the performance of the heat exchanger in terms of its design. In terms of which heat exchanger is the most efficient, it is based on the efficiency values where the highest values correlate to the best efficiency. From the data calculated in Tables 3 and 4 for both flow configurations, parallel flow has the highest efficiency values which makes it the most efficient. The reasons behind this is that there is a large temperature difference between the inlet and outlet temperatures and that the mass flow rate occurs at a much faster speed. The heat transfer that occurs in this configuration is also much higher in which due to the effect is produced higher efficiency values. For the counter flow configuration, the efficiency values were relatively low due to small temperature differences between the inlet and outlet and low
- 7. difference between opposing mass flow rates. Heat transfer at this configuration is also low which in turn produced low efficiencies. Error and uncertainty were present when it came to doing the lab experiment. Air pockets within the tubes where the water was flowing attributed to some inaccuracy of the data. The thermocouples attached to the heat exchanger were also sources of error because if one or a few are malfunctioning, it can skew the data and show values that are dramatically off to the theoretical values. Estimation on certain values such as flow rate can also lead to error since the flow rate meter given with the small metal ball inside had to be estimated based on where the middle of the ball landed on the meter. For the counter flow values, the heat exchanger at the outlet did not work well and has read values that were very off to what was supposed to be analyzed. Data from a previous experiment for the counter flow was provided and used as a replacement to calculate the desired variables since the heat exchanger wasn’t working properly for the configuration. Conclusion: The overall purpose of this lab was to test the effectiveness of a parallel and counter-flow of concentric heat exchangers. The two concentric heat exchangers were composed of two copper tubes of different diameter; refer to Figures 1 and 2. From the data recorded from the flow rate, the data was utilized to solve for the mass flow rate 𝑚 𝑐̇ and 𝑚ℎ̇ for both cold and hot temperatures by using Eq. 6. Those values then were used to solve for mass flow rate for both temperature values. Then it was used to solve for the thermal conductance Cc and Ch by applying Eq. 2 and 3. Final results were calculated to receive heat transfer rate q and qmax c by using thermal conductance values in Eq. 1 and 4 along with the collected temperature values for the inlet and outlet areas. The lab showed that by changing parameters such as flow rate, it affected the efficiency of the heat exchanger. There is a correlation among the rate at which the temperature changes over time. Where there is a large enough temperature change the heat transfer is occurring at a relatively fast rate and thus resulting in a high efficiency. If the temperature change is low, then the efficiency will be low. The lab’s counter flow encountered errors in output of data values. Upon further investigation it was concluded that the apparatus had a leak in the system and was skewing the results. Errors in the construction and set up in the second apparatus (Figure 2) are possible factors for skewed results.