热交换器实验

1、The studies on heat transfer across heat exchangers in different modesName: CHAO CHEN Pair: 07 Group: 1C CID Number: 00548719AbstractThis experiment investigates the operation of a tubular heat exchanger and a shell and tube heat exchanger. The two modes studied are co-current and counter-current. T

2、he heat exchangers are in small scale and the unit Armfield HT31 is used to provided the water as medium. The efficiency of tubular heat exchanger is not directly related to the hot water flow rate while the efficiency of shell and tube heat exchanger is inversely proportional to the flow rate of ho

3、t water. The efficiencies of the tubular heat exchanger are 94.5% in counter-current mode and 92.8% in co-current when hot water flow rate is 2 l/min and cold water flow rate is 1 l/min. On the other hand, the efficiencies of the shell and tube heat exchanger are 1.07% in counter-current and 1.22% i

4、n co-current. This indicates that the counter-current mode is better in tubular heat exchanger while no conclusion can be deduced for shell and tube heat exchanger since the efficiency is unreal. However, according to the overall heat transfer coefficient of both types of heat exchangers, shell and

5、tube heat exchanger is better than tubular heat exchanger in general. IntroductionA heat exchanger is a device built for efficient heat exchanging from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact. 1 It can heat and

6、 cool two streams simultaneously by transferring heat from the hot stream to the cold stream. Consequently it maximises the utilisation of energy in processes and reduces the cost because it recovers the waste heat and put it to use. The heat exchanger is widely used in industry and commerce. It pla

7、ys an important role in manufacture field and has a lot of applications like refrigeration, air conditioning, oil refining and food processing. In this experiment, the heat transfer in co-current and counter-current modes across both the tubular heat exchanger and the shell and tube heat exchanger w

8、ere investigated. The tubular heat exchanger consists of two concentric tubes carrying hot and cold fluid so that heat can be transferred from the hot stream to the cold stream. Usually the hot fluid flows in the inner tube and the cold fluid flows in the outer tube. There is a metal wall which sepa

9、rates the two fluids. The shell and tube heat exchanger consists of a number of parallel tubes wrapped by an outer cylindrical shell. This heat exchanger also includes baffles which can increase the velocity of the fluid in order to increase the rate of heat transfer. Both the heat exchangers can be

10、 operated in counter-current (flow in opposite direction) and co-current (flow in same direction) modes.The objective of this experiment is to compute the overall heat transfer coefficient for both heat exchangers base on the energy balances. In addition, a graph of outlet, inlet and midpoint temper

11、atures for both hot and cold streams during steady state operation in both co-current and counter-current modes using the tubular heat exchanger was plotted. As regards the shell and tube heat exchanger, a graph of inlet and outlet temperatures for both streams in co-current and counter-current mode

12、s was plotted. MethodologyThe tubular heat exchanger and shell and tube heat exchanger were used in experiment A and experiment B respectively. These two heat exchangers can be connected to the Armfield HT30X service unit, which supplied controlled hot and cold water feeds. The exchangers would be m

13、ounted on the service unit when they were in use. The service unit consisted of a control console which can set the temperature of hot stream. In addition, the console showed the volumetric flow rate with switch to select the cold or the hot stream. A temperature display was also showed on the contr

14、ol console which gave the values of temperature in different stream and different position. The figure 1 illustrated the Armfield HT30X service unit including the detailed sections. Fig.1 Armfield HT30X Heat Exchanger Service Unit1. Drain valve10. Cold-water supply inlet19. Thermocouple T72. Base/pl

15、inth11. Strainer20. Flexible-tube fitting3. Heat-exchanger mounting12. Adjust knob21. Turbine flow meter (hot)4. Base plate13. Pressure regulator22. Flexible-tube fitting5. Overflow14. Cold-water control valve23. Mani fold block6. Priming vessel15. Hot-water outlet24. Cold-water outlet7. Hot-water c

16、irculator16. Hot-water return25. Turbine flow meter (cold)8. Control console17. Hot-water control valve9. Main switch18. Flexible-tube fitting2As regards the tubular heat exchanger, the cold water inlet passed through in order of a pressure regulator (13), a manual control valve (14) and a turbine f

17、low meter (15) then it was delivered to the heat exchanger. The outflow simply went to the drain. The reservoir mounted on the top of the service unit circulated the hot water through the heat exchanger and back to the reservoir via a manual control valve (17) and a turbine flow meter (21) in order

18、to dispel any bubbles inside the pipes. The reservoir was thermostatically-controlled so that the recycled hot water can be kept in constant temperature. The experiment A used the tubular heat exchanger and it was investigated in both co-current and counter-current modes. In the counter-current oper

19、ation, the temperature controller was adjusted to a set point of 60 after switching on the unit. Then the cold water stream was firstly adjusted to give maximum flow by turning the valve fully in order to get rid of the air inside the pipes. However, in the measurement stage, the cold water flow rat

20、e would be adjusted to give 1 l/min and this flow rate would be used throughout the experiment, namely only hot water stream would be varied. In the measurement procedure, a group of temperatures of both streams in inlet, outlet and midpoint would be collected when the hot stream flow rates were 3 l

21、/min, 2 l/min and 1 l/min respectively. In each case, the system should be allowed to reach the steady state before logging data. These data can be recorded either using a computer or reading the data manually by switching to select the temperature sensor. As the flow rates of two streams may be dif

22、ferent from the initial set value when it was in the steady state, the new flow rates should be recorded and used in the calculation of energy balance. After finishing the operation in the counter-current mode in tubular heat exchanger, another set of measurements would be performed in co-current mo

23、de when the hot water flow rate was 2 l/min and cold water flow rate was 1 l/min. The apparatus was shut down firstly by closing the cold water valve and then turning off the main switch. The schematic diagram figure 2 illustrated the co-current and counter-current mode of tubular heat exchanger; it

24、 also showed the position of the thermocouples T1 to T6Fig.2 Schematic diagram of co-current and counter-current mode using a tubular heat exchanger shown in (a) and (b) respectivelyT6T6T1T1ColdCold T2T5T5T2HotHotT3T3(b)(a)T4T4The co-current and counter-current were also studied in experiment B, but

25、 in this case, a shell and tube heat exchanger was used instead of a tubular heat exchanger. The procedure of experiment B was very similar to the experiment A. They both had a set point of 60 and the measurements were both performed when the cold water flow rate was 1 l/min and hot water flow rates

26、 were 3 l/min, 2 l/min and 1 l/min in counter-current mode. The difference was that there were only four thermocouples used in the experiment B while the six of them used in the experiment A. Consequently only the temperatures of inlet and outlet of both hot and cold streams would be measured in the

27、 shell and tube heat exchanger. The shut down procedure of the shell and tube heat exchanger was exactly same as that of the tubular heat exchanger. The schematic of the fluid flow system was shown in the figure 3Cold Fig.3 Schematic diagram of co-current and counter-current mode using a tubular hea

28、t exchanger shown in (a) and (b) respectivelyT4T4HotHotT1T2T1T2Cold T3T3(b)(a)The software of the computer logged the temperatures of entry and exit of both hot and cold streams in shell and tube heat exchanger at 5 seconds intervals. Regarding the tubular heat exchanger, the software logged an addi

29、tional midpoint temperature. However, the computer logged the data at 30 seconds intervals when the tubular heat exchanger was used. In addition, the flow rates of both streams in both experiments were recorded by the software. Due to the limitations of the service unit, the hot stream cannot actual

30、ly achieve 3 l/min; as a result, the real flow rate would be obtained from the computer or the control console screen in order to perform the energy balance. The details of the raw data were showed in the appendix. Results and DiscussionFor each of the four data sets, the temperatures of inlet, Ti (

31、k), midpoint, Tm (k) and outlet, To (k) of the hot and cold streams were plotted against the position when the tubular heat exchanger was used both in co-current and counter-current modes. The temperature profiles of the shell and tube heat exchanger was also plotted in the two modes, but in this ca

32、se, only the temperatures of outlet and inlet were plotted. The four diagrams were showed in figure 4. The table 1 and 2 showed the data used in plotting graphs.Table.1 Selected data for plotting diagrams of tubular heat exchangerFhot/Lmin-1 ±0.005Fhot/Lmin-1 ±0.005ModeT1/ ±0.05T2/ &#

33、177;0.05T3/ ±0.05T4/ ±0.05T5/ ±0.05T6/ ±0.052.830.98counter59.856.654.311.619.426.12.000.98counter59.755.952.811.818.524.71.001.00counter59.953.848.911.516.521.62.001.00co60.055.653.024.818.911.8Table.1 Selected data for plotting diagrams of shell and tube heat exchangerFhot/Lmin

34、-1 ±0.005Fhot/Lmin-1 ±0.005ModeT1/ ±0.05T2/ ±0.05T3/ ±0.05T4/ ±0.05T5/ ±0.05T6/ ±0.052.401.00counter60.355.516.328.4N/AN/A2.001.00counter60.955.617.128.5N/AN/A1.001.00counter61.753.216.625.8N/AN/A2.001.00co61.756.726.914.7N/AN/AFig.4 Temperature variations for

35、 the counter-current tubular heat exchanger, co-current tubular heat exchanger, counter-current shell and tube heat exchanger and co-current shell and tube heat exchanger were showed in (a), (b), (c) and (d) respectivelyTemperature/(a)Temperature/(b)Temperature/(c)Temperature/(d) In (a) and (c), all

36、 the streams flowed from left to right hand side, in (b) and (d), the flow directions were indicated by the arrows.From the diagrams in figure 4, the two heat exchangers had very similar temperature profiles in each operation mode. In the co-current mode, the temperature differences between the hot

37、and cold streams in both heat exchangers were decreased. However, the temperature differences between two streams kept almost constant in counter-current in both heat exchangers. In addition, the temperature change became larger in hot water stream and became smaller in cold water stream when the ho

38、t water flow rate decreased. The power absorbed by the cold water flow and the power emitted from the hot water flow were computed and they are labelled and respectively, both of them were measured in Watt. The percentage efficiency for heat transfer was calculated using the formulawhere is the effi

39、ciency. In addition, the overall heat transfer coefficient for both co-current and counter-current operation at hot water flow rates of 2 litres per minute was determined for both tubular and shell and tube heat exchanger. The overall heat transfer coefficient U was given by the following equationwh

40、ere A is the heat transmission area measured in m2 and DTm was the mean temperature difference measured in K between the hot and cold surfaces. The DTm was given by the equationaccording to the logarithmic mean temperature difference (LMTD) method. However the transmission area was calculated in dif

41、ferent ways for each heat exchanger. As regards the tubular heat exchanger, the heat transmission area was given by dL, where d was the effective diameter of the inner tube and L is the heat transmission length both measured in m. On the other hand, the transmission area was approximated by pdmL for

42、 shell and tube heat exchanger, where dm was the arithmetic mean diameter given by dm=0.5(do+di) and L=n´ l was the heat transmission length, in which l is the heat transmission length of each tube and n was the number of tubes. The samples of calculation were showed as following.When Fcold=1 l

43、/min, Fhot 2 l/min, Cp(water)=4219 Jkg-1k-1 for tubular heat exchanger in counter-current mode=mCpDT=0.98×4.219×13.3=55.0KJ/min=917W=mCpDT=2×4.219×6.9=58.2 KJ/min=970WHence the efficiency wasA= pdL=3.14×0.0089×0.66=0.0185 m2W/m2KThe efficiencies of each case and the ove

44、rall heat transfer coefficient were calculated and summarised in the table 3.Table.3 Tabulated values for each operation conditionHeat ExchangerMode of OperationHot water volumetric flow rate (L/min) ±0.005 L/minQa (W)Qe (W) (%)Tm (K)U (W/m2K)TubularCounter-current2.831000109591.42.0091797094.5

45、38.013831.0071077391.8Co-current2.0091398592.837.31430Shell and TubeCounter-current2.40850810105.02.00802745107.035.411571.00647598108.0Co-current2.00858703122.037.71023From the table 1, it is interesting to see that the efficiency of the tubular heat exchanger in counter-current mode fluctuated wit

46、h the decrease of hot water flow rate. The lowest efficiency was 91.4% when the hot water flow rate was 2.83 l/min, then the efficiency increased and reached the peak at 94.5% when the hot water flow rate was 2.00 l/min. However it decreased to 91.8% when the hot water flow rate reduced further. The

47、 overall heat transfer coefficient is a measurement of thermal resistance; this means that the value of the coefficient is related to the performance of the heat exchanger. The coefficient of tubular heat exchanger was 1383 W/m2K in counter-current mode which was lower than 1430 W/ m2K in co-current

48、 mode when the hot water flow rates for both were 2 l/min. This indicated that the tubular heat exchanger performed better in counter-current mode than in co-current. The efficiencies of them also proved that as it was lower in co-current mode. Regarding the shell and tube heat exchanger, however, a

49、ll the efficiencies were above 100%, which were impossible. The efficiency cannot larger than 100% because the power absorbed by the cold water was only from the power released by the hot water flow. This significant error could be caused by the failure in calibration of the flow meter or the thermo

50、couples. Consequently the flow rates and temperatures recorded would be actually incorrect. Hence the efficiencies and the overall heat transfer coefficients were meaningless as they all based on the incorrect data. However, if the error was assumed to be systematic error and caused same amount of i

51、ncrease in the efficiency, the performance can still be compared. For instance, the efficiency increased with the decrease of the hot water flow rate, this indicated that the shell and tube heat exchanger had a better performance when the hot water flow rate was close to the flow rate of cold stream

52、 regardless the absolute values of the efficiency. In addition, unlike the tubular heat exchanger, it had a higher efficiency in co-current mode than in counter-current mode under same conditions. In general, the shell and tube heat exchanger had a better performance according to the overall heat tr

53、ansfer coefficient. The overall heat transfer coefficient was calculated using the recorded data which involved some uncertainties. The example of the coefficient of tubular heat exchanger in co-current was used, which was 1430 W/m2K. The temperature used for calculated was read from the control con

54、sole which had a uncertainty of ±0.05. The flow rate also had a uncertainty of ±0.005 l/min. Thus the temperature difference had a uncertainty of ±0.1, consequently had a uncertainty of ±1×0.1+7×0.005=±0.135. In the end, the uncertainty of the coefficient in this c

55、ase was ±14, so the percentage error was 1430±1%. In the calculation of the overall heat transfer coefficient, the value of DTm was deduced by the LMTD method. It is used to determine the temperature driving force for heat transfer in flow systems. The LMTD is a logarithmic average of the

56、temperature difference between the hot and cold streams at each end of the exchanger. The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties. 3In this experiment, the system was assumed to be adiabatic and the pipe was

57、free from bubbles. However the pipe actually had some bubbles inside and it emitted heat to the surrounding. This contributed to the inaccuracies in flow rate and temperature. In order to improve it, the pipe should be covered using lagging materials. In addition, the flow rate should be adjusted gi

58、ving a high velocity so that all the bubbles can be removed. In the experiment, the measurement repeated three times for each heat exchanger using counter-current mode in order to investigate the relationship between efficiency and the hot water flow rate. However, three examples were not sufficient to get any conclusion for tubular heat exchanger. Thus more experiments