UNIVERSITY OF MINNESOTA DULUTH DEPARTMENT OF CHEMICAL ENGINEERING ChE CONVECTIVE HEAT TRANSFER

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1 UNIVERSITY OF MINNESOTA DULUTH DEPARTMENT OF CHEMICAL ENGINEERING ChE CONVECTIVE HEAT TRANSFER OBJECTIVE The purpose is to measure heat transfer in cases where convection is a significant mechanism. Empirical relationships will be determined between the dimensionless groups that describe heat transfer, free convection and forced convection. INTRODUCTION Whenever a temperature gradient exists in a fluid, the density of the fluid will also vary within the fluid. The buoyancy effects arising from the variation in the density induce a flow of fluid called natural or free convection. In cases where free convection is significant, the rigorous analysis of heat transfer is very complicated. For design purposes empirical relationships are often used. THEORY In the general case in which heat is transferred by free and forced convection, empirical relationships are of the general form: Nu = f( Re, Pr,Gr) (1) where Nu is the Nusselt number, Re is the Reynolds number, Pr is the Prandtl number and Gr is the Grashof number. For cases where free convection is the dominant mechanism, the empirical relationships are of the general form: Nu = f( Pr,Gr) (2) 1

2 When forced convection is the dominant mechanism: Nu = f( Pr, Re ) (3) EXPERIMENTAL You will study convection effects in water flowing vertically inside a copper tube. Heat is transferred to the water from the condensation of steam on the outside of the tube. You will measure the following: (1) the mass flow rate of water (coolant, tubeside), (2) the mass flow rate of condensate (shellside), (3) the temperature of coolant entering the tube, T ci, (4) the temperature of coolant leaving the tube, T co, (5) the temperature of the outside wall at the bottom of the tube, T wi, and (6) the temperature of the outside wall at the top of the tube, T wo. CALCULATIONS The Reynolds number should be calculated at the average coolant temperature. The Grashof and Prandtl numbers should be calculated at the average wall temperature of the pipe. The temperature difference in the Grashof number should be taken as (T co - T ci ); the linear dimension should be the internal diameter of the pipe, D. The coefficient of volume expansion should be calculated using densities at T co and T ci. For the calculation of the Nusselt number use the individual heat transfer coefficient (tubeside) based on the log mean temperature "driving force". Plots should be made to find relationships of the form in eq 1 & 2. REFERENCES McCabe, W.L., Smith, J.C., Marriott, P., "Unit Operations of Chemical Engineering", 4th Edition, McGraw-Hill, 1985 McAdams, W.H., "Heat Transmission", 3rd Edition, McGraw-Hill,

3 EQUIPMENT gallon plastic bottle 2. Stop watch 3. Top loading balance ml beakers CHEMICALS/MATERIALS 1. Tap water 2. Deionized water 3. Steam EXPERIMENTAL PROCEDURE A diagram of the forced and free convection apparatus (model 9054) is shown in Figure 2. Familiarize yourself with the piping network of the experimental apparatus (Model 9054 and 9058) prior to class. The test chamber is comprised of a tubular glass steam chest (5" O.D. x 36" high) enclosing a 1/2" nominal, type L copper condenser tube (0.625" O.D. x 0.040" wall). The test portion of this copper tube, exactly 24" long, is shrouded by a brass expanded metal cylinder which has been installed to eliminate unwanted thermal convection radially induced between the tube and the glass envelope. First Lab Period Flow data will be collected at weir heights of -3/4", -3/8", 0", 0.5", 1.0", 2.0", 3.0" while the experimental apparatus is at room temperature. Steam will then be admitted to the chest and flow and temperature data will be collected while running the experiment at the weir heights listed above with the steam chest pressure held at atmospheric pressure. If time permits additional measurements should be made at other weir heights below 0". Start with a weir setting of -3/4" and increase the weir settings to 3". Flow data will be collected by hand and a computer data acquisition program will be used to collect the temperature data. Procedure for setup of equipment and data acquisition. 1. Load the computer data acquisition program. See instructions listed on page Close W-1, W-3, S-1 and S-2. Open the valve (Watts, green handle) on the main water feed line into the system located below the apparatus. 3

4 3. Open valve W-2, the drain from the constant head tank, and W-4, the cooling water drain valve if not already open. 4. Open the valve on the top of the constant head tank 5. Open W-1 a quarter turn and allow the head tank to fill. Adjust the weir overflow cup height in the constant head feed tank to a setting of -3/4". Ask the lab services coordinator to show you the proper procedure used to adjust the weir height. Do not use the indicator disk as an aid in adjusting the weir height. 6. At each weir height listed above collect three sets of cold water flow data. To do this, record the weight of the 4 L plastic bottle using the top loading balance. Collect the water in the 4 L plastic bottle from valve W-4 for a pre-determined time period. Record the weight of water collected and the bottle using the top loading balance. Also record the temperature of the water (from a digital thermometer) at each weir height. 7. Once the flow rates have been determined for each weir height with the system at room temperature, move the weir height back to -3/4". 8. Refer to the diagram of the Phase Heat Boiler, Model 9058, in Figure 2. With the metering valve closed, fill the feed tank with DEIONIZED WATER until it is 7/8 full (to the mark). Pressurize the feed tank (10 to 15 psi), by turning the three way valve on the compressed air line to the correct position. Open the metering valve and admit water into the boiler. Fill the boiler to 3/4 full (to the mark). Close the metering valve, fill the feed tank if needed. To fill the feed tank, depressurize the tank by turning the three way valve on the compressed air line to bleed the air. Open the fill cap and fill with deionized water. When the tank is full of water again pressurize the feed tank to 10 to 15 psi. 9. Fill the seal pot at the top of the convection experiment unit with water until it overflows into the steam chest. Open stopcock S-3 (horizontal position) to drain condensate from the steam chest as it forms during the heat up period. Close drain valve V-2. OPEN VALVE V-1 located at the bottom of Model 9054 if it is not already open. 10. On the Phase Heat Boiler, turn on the five liquid zone heaters. The water in the boiler will start to boil in approximately 8 minutes. Shut the valve on the steam condensate drop leg and allow steam to flow into the apparatus freely until it reaches steady state. 11. As steam is produced, the liquid level in the boiler will begin to drop slowly, When this happens tweak open the metering valve on the feed line from the feed tank to transfer water into the boiler to maintain the level in the boiler at a level of 1/2 to 2/3 full. Remember that too much water flowing into the boiler will inhibit steady state 4

5 operation. NOTE: Do not let the liquid zone and vapor zone temperatures go above 260C. Do not allow the pressure in the boiler to go above 15 psi. 12. As steam enters the test chamber, condensate forming on the glass wall of the steam chest will render the test chamber almost invisible. After about 10 to 15 minutes of heating, however, when the system is operating nearly at steady state conditions, the glass walls will begin to clear and the test chamber will become visible once again. Monitor the water and steam side temperatures by watching the readout on a digital thermometer or by using the data acquisition program until steady state operation has been reached (about 10 minutes). Water temperatures will oscillate. 13. Open valve S-2 to bleed the condensate line and then close it. Once steady state operation has been obtained at the weir setting of -3/4" begin computer data acquisition of the temperatures. The computer will acquire the temperature data once every second from each thermocouple for 10 minutes. This results in 600 temperature data points collected for each thermocouple. During this 10 minute period collect 3 sets of condenser tube condensate, steam chest condensate, and cooling water to determine the mass flow rates. This can best be done by recording the weight of two 250 ml beakers (one each for the condenser tube condensate and the steam chest condensate), and one plastic 4-liter wide mouth bottle (for the cooling water) using the top loading balance. Collect the liquid for a predetermined period of time (at least one minute) and again record the weight of the container plus liquid. Record the length of time to collect the sample. 14. Repeat the procedure at weir settings of -3/8", 0", 0.5", 1.0", 2.0", and 3.0". If time permits more measurements should be made at other weir heights below 0". After the tests are over and all necessary data recorded for the lab period, shut down the system as follows: 1. Turn off the 5 liquid zone heaters on the Phase Heat Boiler. Close the metering valve on the boiler unit. 2. Allow the unit to cool so steam is not being produced. 3. Depressurize the boiler by venting the steam to atmosphere through the condensate leg (open the valve). 4. Open drain line valve V-2 5

6 5. Wait 5 to 10 minutes and then shut off the main water inlet to the Model 9054 and the metering valve W-1. Open stopcock, S-1, to drain the water from the system. 6. Clean up. Second Lab Period The purpose here is to carry out the same procedure as in the first period, except that the coolant water is passed through the apparatus under mains pressure. In this way, you obtain higher flow rates and study the heat transfer under turbulent conditions. Experimental Procedure The procedure is essentially the same with the following exceptions. 1. The valve W-2 is closed so the incoming water does not flow over the weir. 2. The valve (not shown in Fig. 1) on top of the constant head feed tank is closed. With these two valves closed, all the water is directed through the condenser tube. You should use at least six different flows that span the transitional to turbulent regime. The highest Reynolds number that you can expect to obtain is about 15,000; which is limited by the funnel on the effluent side of the steam chest. Water will spill over the funnel if the flow rate is too high. 6

7 Figure 1. Model 9054, Forced and Free Convection Apparatus and Model 9058, Phase Heat Boiler. 7

8 SAFETY NOTES 1. Pressure in the phase heat exchange boiler should not exceed 15 psi. 2. Do not allow the level in the boiler sight glass to drop below 2 inches from the bottom of the sight glass. 3. Do not allow the water level in the boiler to become too high as liquid will overflow into the sight gauge entrapping vapor bubbles. Should this occur, shut off the liquid feed to boiler until the level drops sufficiently so that no overflow occurs, then resume normal operation. 4. Be very careful of unlagged steam lines and valves. Wear thermal mitts when adjusting the steam flow and adjusting the vent. Be careful of the condensate and the collection containers as they may be hot. 5. When first admitting steam to the system, open the metering valve cautiously and check all connections for leaks. 6. Be sure the steam chest is vented (vent stopcock S-4 is open) when first admitting steam. Check the liquid seal to be certain it is not plugged before closing the vent stopcock S-4 (if directed) after the steam is on. 7. If a valve appears tight or doesn't yield to hand force, do not attempt to turn it with a wrench. Turn off the steam, permit the system to drain and cool down, then loosen the packing bonnet slightly and try again. 8. Keep the laboratory working space clean and uncluttered. Be aware of potential hazards such as wet spots or debris on floors that could cause slips or falls. WASTE DISPOSAL PROCEDURES 1. There is only waste water associated with this experiment. Only steam and cold water are used and they can be sent to the drain. 01/07 8

9 APPENDIX A Convective Heat Transfer Experiment Computer Data Acquisition Instructions The temperature data is collected with a DATAQ DI-1000TC Instrumentation module for Temperature measurement. Two Software programs will be used. WinDaq/Lite for data acquisition and Microsoft Excel. The data will be imported to an Excel spreadsheet.. 1. Connect the thermocouple (TC) wires to the DI-1000TC box. 2. Connect the USB cable from the DI-1000TC box to the USB port on the portable computer. 3. Turn on the computer and select the student account in Windows XP. Enter student as the password. Wait for Windows XP to finish loading. 4. Double click on the WinDaq 1000TC shortcut icon on the desktop to load the data acquisition software. 5. A window opens (DI-1000 Acquisition 0) showing an oscilloscope tracing of the temperatures for 5 thermocouples. The data as it is displayed moves from left to right. When the cursor reaches the right side of the window it starts again at the left side of the graph. The mode of data display can be changed but this is probably the best way to view the data on the monitor. 6. The software has been configured to collect 1 data point/sec/tc (5 samples/sec). Over a 10 minute period 600 temperature data points are collected for each thermocouple. The number of data points/sec/tc and the length of the sample time can be changed but for this experiment, this is the best configuration. 7. Set up the Free and Forced Convection unit in the proper configuration so it is operating to your satisfaction. 8. During the 10 minute period you collect temperature data you will also collect 3 sets of water samples as specified above. 9. On the computer desktop, double click on the Excel icon to start Excel. 10. A small dialogue box appears on the spreadsheet (towards upper right area). This dialogue box is named WinDaq- XL. Click on the circle in this box to open up another dialogue box. a. Under Select the WinDaq Device click on DI-194 Serial Device (xx) 9

10 b. In the Starting Cell entry field, enter the cell designator for the upper left corner where you want to begin putting the data. A1 is the default. If you want to begin in another row or column enter that cell designator. c. In the Rows to Fill entry field enter data points will be collected over the 10 minute period (10 min x 60 sec/min x 1 data point/sec) for each thermocouple. When 600 data points have been collected the program will stop putting the data in the spreadsheet. d. Click on start to begin data acquisition. 11. The program automatically puts EC in each of the columns in the first row of the designated starting cell. 12. The thermocouple names for each column is listed below. They are the same as channels 1, 2, 3, 4, & 5 shown on the oscilloscope chart in the data acquisition program. Column 1 Column 2 Column 3 Column 4 Column 5 Temperature water in Wall Temperature bottom Temperature water out Wall Temperature top Steam Temperature 13. The data from additional experimental runs can be put to the right of the previous run. Data can not be collected in columns beyond column HZ. Data from each experimental run can also be put into a new sheet. 10

11 Department of Chemical Engineering Stockroom Checkout slip Convective Heat Transfer ChE 4211 Name: Group No.: Date: (print name) Lab No.: Lab 1 12:00-2:50 PM Lab 2 3:00-4:50 PM (circle one) Equipment Out In Equipment Out In Stopwatch ml beaker Digital Thermometer Name: (Signature) 11