contoh lab heat exchanger
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ME 340 – Fall 2005 Lab 6: Thermal System The purpose of a heat exchanger is to transfer energy from one fluid to another. Many different types of heat exchangers exist to accommodate different fluid properties. One of the simplest designs is the concentric tube heat exchanger shown in Figure 1 which is normally used for heat exchange between two liquids. As it appears in the figure, the heat exchanger is operating in a counterflow arrangement in which the two fluids flow in opposite directions. An alternative flow arrangement for this heat exchanger is parallel flow in which the fluids flow in the same direction.
Figure 1: Concentric tube heat exchanger in counterflow arrangement
LMTD Analysis of Heat Exchangers Two different methods are used to analyze or characterize heat exchanger performance: the logmean temperature difference (LMTD) method and the effectiveness - number of transfer units (ε, Ntu) method. Both methods provide information which is useful for design or prediction of performance, but this information is provided in quite different format. LMTD is often used to size heat exchangers while ε, Ntu is often used to determine the amount of heat transferred with a given heat exchanger. We will look at the LMTD method here. For a concentric tube heat exchanger such as is shown in Figure 1, local heat flow from the hot fluid on one side of the tube to the cold fluid on the other side will obey Newton's law of cooling. & Q = U∆T A In this equation U, the overall heat transfer coefficient, accounts for the net effect of the various thermal resistances between the two fluids of interest. Detail regarding theoretical calculations of U can be found in heat transfer texts, however it can be noted here that U is dependent on such 1
parameters as the local fluid properties, tube size and surface conditions. Of course, over the entire length of the heat exchanger temperatures, and therefore fluid properties, can vary significantly as can the geometry of the heat exchanger itself. Thus it is likely that heat transferred from the hot to the cold fluid in one section of the heat exchanger can be very different from that transferred in another portion of the heat exchanger. In spite of this acknowledged complication, the goal of the LMTD analysis is to define a single overall, or effective, heat transfer coefficient U which will &, the total heat transfer rate from the hot to cold side of the heat exchanger, allow description of Q by Newton's law. &= UA∆Tlm Q Here A is the total area through which the heat flows from the hot to cold side and ∆Tlm will be defined below. Analysis for a parallel flow heat exchanger begins by considering the heat transferred in a small segment of the heat exchanger, as shown in Figure 2.
Figure 2: Segment of the concentric tube heat exchanger of length dx At steady state the only energy crossing the system boundary is the heat in the hot and cold streams. &h,i − Q &h,o ) + ( Q &c,i − Q &c,o ) 0= ( Q This can be written in terms of the mass flows, constant heat capacities and temperatures. −m &hcp,h Th − ( Th + dTh ) = m &ccp,c Tc − ( Tc + dTc ) Heat transferred to the cold and from the hot fluids is determined by an energy balance on the differential volume. &= − m dQ &hcp,hdTh = −ChdTh
&= m dQ &ccp,cdTc = CcdTc The capital letter C is called the capacity rate. Within this control volume one may also express the heat transfer as follows. &= U∆TxdA dQ ∆Tx = ( Th − Tc ) x The differential form of this equation d( ∆Tx ) = ( dTh − dTc ) x may be combined with Equations and to yield the expression & 1 + 1 d( ∆Tx ) = −dQ Ch Cc which when combined with Equation yields 1 1 1 d( ∆Tx ) = −U + dA ∆Tx Ch Cc Using the parallel flow configuration in Figure 2 and integrating yields the expression T −T 1 1 ln h,out c,out = − UA + Ch Cc Th,in − Tc,in In this integration ∆Tx is as measured from the hot side to the cold side at a particular location x, while the integration is along the length of the heat exchanger. A final substitution is accomplished using integrated forms of Equations and . T − T UA T −T ln h,out c,out = − T −T & ( h,out h,in ) ( c,out c,in ) T − T Q h,in c,in
For parallel flow we will assign the designations ∆T1 and ∆T2 to the temperature difference terms. ∆T1 = Th,in − Tc,in ∆T2 = Th,out − Tc,out Using the above temperature designations, rearranging and solving for the heat transfer rate yields &= UA ( ∆T2 − ∆T1 ) Q ∆T ln 2 ∆T1 We now identify the term ∆Tlm in Equation , called the log mean temperature difference. ∆Tlm =
( ∆T2 − ∆T1 ) ∆T ln 2 ∆T1
Equipment Description The Heat Exchanger Demonstration Panel contains two concentric tube heat exchangers (one aluminum and one copper), one shell and tube heat exchanger, one cross flow heat exchanger and one free-convection heat exchanger. A schematic of the concentric tube equipment is given in Figures 3. Type K thermocouples are located throughout the panel to measure hot side and cold side inlet and outlet temperatures for all heat exchangers. A rotary thermocouple channel selector is connected to digital temperature readout. Temperatures are displayed in degrees Centigrade. Cold water is the cold side fluid. The hot side fluid can be either water or steam. When using water on the hot side, a mixing valve is used to combine hot and cold water to obtain a range of operating temperatures. Water flows are measured by flow orifices connected to differential pressure transducers. Digital readouts give the water flow in gallons per minute. In this experiment, you will be taking measurements on the copper tube concentric tube heat exchanger parallel mode.
Dimensions and specifications for the copper concentric tube heat exchanger are: overall length = (1.21 * 2) m heat transfer area = (.06081 * 2) m2 copper inner tube inside diameter = .01384 m copper inner tube outside diameter = .015875 m copper outer tube inside diameter = .01994 m copper outer tube outside diameter = .02223 m conductivity of copper = 339.2 W/(m*C)
Figure 3: Schematic of the heat exchanger experimental setup copper concentric tube - parallel flow
Experimental Procedure General Test Procedure To perform a test you will set up cold and hot water flows through the heat exchanger. Once the valves are set to give the desired flow rates and temperatures, you will close off the cold water to the tube side and wait for the shell side temperatures to stabilize. When you are ready to collect transient data, you turn on the cold water flow to the tube side.
Pre-Start Check 1. Inspect the test equipment in room 108. 2. Sketch the system showing basic structure, the interconnections between components and where and how data is measured. 3. Note: Close all valves on the heat exchanger panel before starting a test. Handles in line with the tube indicate that the valve is open. 4. Check that the power switch in the upper left section of the heat exchanger panel is on. 5. Set the parallel/counter four-way valve to the parallel. The handle of the valve indicates the valve position (the red paint). Leave all other valves closed.
Testing 6. Check that the water supply valves located along the wall, near the floor, are open. 7. Open the following valves to set up cold water flow through the tube side of the copper exchanger. Refer to Figure 3. Open: CV5, CV12, CV14, CV8 8. Slowly open control valve NV2 to obtain a cold-water flow rate of about 1.0 gpm. The flowmeter (FM2) display is in units of gpm (gallons per minute). 9. Open the following valves to set up hot water flow through the shell side of the copper exchanger. Refer to Figure 3. Open: HW1, CV1, HW2, HW7, HW13, HW14, HW8 10. Slowly open control valve NV1 to obtain a hot water flow rate of about 1 gpm. 11. Adjust hot side inlet temperature to about 35˚ C by adjusting the mixing valve MX1. 12. Stop cold water flow by closing valve CV12. 13. Log onto the computer using your regular CAE account. Then login to the local network. Your instructor will give you the name and password. a. Start LabView using the icon one the desktop. b. Click on Open File. Select the Thermal System vi located in the LabView Data folder in My Documents. c. To monitor temperatures without saving data, click the button to disable saving a data file. Then click the Run arrow in the upper, left corner. 6
14. Let the system run until all temperatures have stabilized. a. Stop data monitoring using the Stop button. b. When you are ready to begin a transient test, click the button to enable saving a data file. Then click the Run arrow. When the dialog box comes up, select a folder on your disk space and enter a name for the data file. 15. Once the data system is running, start cold water flow to the tube side by opening valve CV12. 16. Stop data acquisition after one or two minutes. 17. Repeat the test using the same flow and initial temperature conditions. 18. Repeat the test using shell side cold water flow rates of 3 and 5 gpm. 19. Repeat the test using different initial hot water temperatures.
Shutdown 20. When all tests are completed close all yellow handled valves on the heat exchanger panel. 21. Close LabView and log off the computer.
Results 1. From the data you collected, develop differential equation models for the outlet temperatures as a function of flow. τ Where: τ ∆Thot K q
d∆Thot + ∆Thot = Kq dt
is the system time constant is the hot water temperature difference between the inlet and outlet is the static gain is the volumetric flow rate
2. Write your group lab report using this document and the above numbering scheme. Submit a single group lab report to your lab instructor. Include all of your names on the group lab report.
Conclusions Note that reports submitted are to be individual after this point. 3. What effect did flow rate have on your value of the time constant?
4. What effect did initial temperature have on your value of the time constant? 5. What physical aspects of this system are likely to have the most effect on the speed of response? 6. Why are the outlet temperatures are interrelated? 7. Write your individual lab report using this document and the above numbering scheme. Submit your lab report to the lab instructor.