Heat exchangers are one of the most common and most important pieces of process equipment you will come in contact with. A knowledge of how they operate and how to analyze their performance is crucial to your future success as a chemical engineer. This experiment involves the determination of overall heat-transfer coefficients as functions of water velocity for a concentric-pipe heat exchanger with steam or water in the inner tube and water in the annulus. Specific objectives for this experiment include:
1) Learning how to operate the heat exchanger.
2) Developing a set of experiments to obtain statistically significant trends for the overall heat transfer coefficient and the inside heat transfer coefficient as a function of water velocity.
3) Observing the difference between parallel-flow and counterflow operation of the heat exchanger.
4) Using the "Wilson Plot" method to analyze your data and obtain information on the inside and outside heat transfer coefficients.
5) Assessing the sensitivity of the Wilson Plot method to operating conditions of the exchanger.
6) Determining the precision of your measurements and assessing the accuracy of your measurements by comparing your results with published data and/or correlations.
7) Presenting a clear, concise, and complete report of your findings.
Table I presents characteristics of the equipment. Water-side and steam-side heat-transfer coefficients will be based on careful statistical analyses of your results. The data reduction will require the use of "Wilson plots" [see References (1), (3)].
Prior to the preliminary discussion you should review:
(1) Chapters 8, 10, and especially 11 of "Introduction to Heat Transfer" by Incropera and Dewitt. Know how to analyze heat exchanger performance via the LMTD and Effectiveness-NTU methods.
(2) What is meant by an overall heat transfer coefficient? What it is composed of?
(3) How a heat exchanger operates when a condensing or evaporating fluid is used.
(4) How the Wilson Plot method works.
During and following the preliminary discussion you should keep in mind:
(1) Do I know how to operate the equipment? Trace all lines and locate all important valves, gauges, etc.
(2) Do I know how the measurement systems work?
(3) Do I know the required safety precautions?
(4) What may go wrong during the experiment? Are there any calculations I can perform, on the fly, to insure the data I am taking is realistic?
You should be able to answer the following questions before your laboratory session starts.
(1) What types of experiments will I run? Parallel flow? Counterflow?
(2) How many experiments of each type must I run? 5, 10, 100?
(3) What kinds of data must I record? Temperatures? Flowrates? Pressures? etc.
(4) What variables do I manipulate during a run?
(5) What will I be calculating during the experiment?
(6) What do I calculate to develop a Wilson plot?
(7) What does a Wilson plot look like? What information can I get from it? What
are the limitations of the method, i.e. what conditions must I have for the
Wilson plot method to work?
You will be quizzed on these questions and on the operational procedures for the heat exchanger before you will be allowed to start up the experiment. Make sure you know what you are doing before the lab starts. Ask questions of the teaching assistants during the preliminary or the faculty in charge of the experiment before the laboratory session to clear up any problems or misconceptions.
Retrace all the lines and determine the function of each component to refamiliarize yourself with the equipment. Then proceed as follows:
1) Decide which form of operation you wish to run first (co- or countercurrent) and set up the heat exchanger directional valves (those directly within the heat exchanger set up).
2) Turn on the switch to the power strip. This turns on power to the computer and other equipment. The Labview program loads automatically. See Appendix I for details.
3) Close the cold inlet valve leading to the mixing chamber.
4) Open the cold inlet valve to the heat exchanger (gate valve labeled "cold water supply"). Open the cold water supply valves outside the heat exchanger set up.
5) Open the main steam valve and the steam supply valves to the heat exchanger. Open the valves nearest the heat exchanger first and the inlet valve last. Do not open the steam bypass valve. The bypass valve is located behind the steam regulator. Make sure that the steam pressure gauge reads less than 50 psi.
6) Open the air supply valves. Make sure that the air pressure gauge reads 20 psi.
7) Set the sample time and datafile name.
8) Insert your disk into the computer and start the Labview program by clicking on the leftmost arrow on the Labview toolbar.
9) Set the steam and water control valve openings. If you are making a steam run, this valve should be fully open. The minimum valve openings you should use are 20%. Why?
(B) The Experiment. Take sufficient data to fulfill the objectives of the experiment. Be sure to run the exchanger in a parallel-flow and countercurrent mode using condensing steam. Perform a countercurrent experiment using hot water with the hot water temperature set to 60 - 70 oC. You will want to make runs at selected values of water velocity for every combination. Take enough data to be statistically significant. During one run, preferably the first, switch to another water velocity, make the change very abruptly and record temperature data during the transient period. How long does it take the exchanger to reach steady state? Graph this data for discussion during the interim report. Each member of your group should supervise a run. You should calculate heat transfer coefficients, Reynolds numbers, etc. on the fly to determine if your data is good.
(C) Shutdown. When you have adequate data of good quality:
1) Close the three main steam valves, starting at the inlet and waiting after each one for at least 15 sec.. This will allow the pressure at the specific pressure gauge to decrease before you close the next valve. Also, during this time, cold water can continue to flush through the heat exchanger to cool it off.
2) Set the steam and cold water control valves to 0%.
3) Close the cold water supply valves.
4) Close the air supply valves.
5) Stop the Labview program by clicking on the stop button.
6) Choose Quit from the Labview File menu.
7) Exit Windows.
8) Turn off the switch on the power strip.
(1) Foust, A.E., et al., PRINCIPLES OF UNIT OPERATIONS, Wiley, New York, 1960, p. 228, 2nd ed., 1980, pp. 332-333.
(2) Incropera, F.P., and D.P. DeWitt, INTRODUCTION TO HEAT TRANSFER, 2nd ed., J. Wiley & Sons, New York, 1990 (General Reference).
(3) McAdams, W.H., HEAT TRANSMISSION, 3rd ed., McGraw-Hill, New York, 1954, pp. 343-346.
The interim report is your only opportunity for any mid-course corrections. It is vitally important that you are prepared for this discussion. Your data reduction and analysis should be complete. All charts, graphs and figures should be finished or at least in rough draft form suitable for presentation and discussion. If you are unprepared, the interim will be cancelled and you will be left, with no future guidance, to complete the final report.
At the "interim", you should be prepared to discuss:
(1) The theory behind heat exchanger operation. The nuances implied in parallel-flow or counterflow operation with and without a change of phase.
(2) The origins, derivations, assumptions, and limitations of all equations used in the reduction and analysis of your data.
(3) The quality and estimated errors of your results; focusing on the accuracy and precision of your data. How will accuracy and precision be determined?
(4) Comparisons of your heat-transfer coefficients with those obtained from correlations or data from the literature.
HEAT TRANSFER EXPERIMENT
2 sections each 40" (1.0 m) long
Dimensions of the Inner Pipe (copper)
O.D. = 0.625" (1.59 cm)
Wall Thickness = 0.040" (0.1 cm)
Dimensions of the Outer Pipe (copper)
O.D. = 1.125" (2.86 cm)
Wall Thickness = 0.050" (0.127 cm)
Labview Program Control
The heat exchanger is controlled and data is collected using the Labview software system. This is a sophisticated data acquisition and control package. The package runs under Windows. The Labview program loads automatically when the computer boots.
You now need to set parameters in the program as discussed in the procedure section of the handout and start the program running. The heat exchanger vi (virtual instrument) has several featurs of interest to you.
1) You can choose when to log data to disk (toggle on/off). Data is appended to the end of the datafile each time you log data. Alternatively, you can toggle data logging off, change the filename and resume data logging to the new datafile. Data logging is default off.
2) Data are stored in the following format, columnwise:
TC1 thermocouple #1
TC2 thermocouple #2
TC3 thermocouple #3
TC4 thermocouple #4
TC5 thermocouple #5
TC6 thermocouple #6
F1 flowmeter #1
F2 flowmeter #2
Water Valve % Opening
Steam Valve % Opening