Letter of Transmittal
Dear, Sir December 3, 2003
The experiment is about determining the overall heat transfer coefficient of the heat exchanger in room EMCS 120.
Sincerely,
Phuong Mai
DETERMINATION OF THE OVERALL HEAT TRANSFER COEFFICEINT
University of Tennessee at Chattanooga
ENCH 435
Writer: Phuong Mai
Partner: Greg Kirton
Kevin Zitzow
Professor: Frank Jones
Jim Henry
Abstract
The purpose of this experiment was to determine the overall heat transfer coefficient of the heat exchanger in room EMCS 120. The experiment was conducted using the web lab and counter current flow and hot water flows in the tubes side of the heat exchanger. Temperature data from the computer and measured mass flow rates of hot water and cold water data were used to calculate the overall heat transfer coefficient. The overall heat transfer coefficient of the mass flow rates of hot water of 2.5 kg/s and of cold water of 4.8 kg/s was determined to be 1100W/m2K. For water flow rates of hot water of 1.6 kg/s and of cold water of 3.0 kg/s, the overall heat transfer coefficient was 520 W/m2K. The results showed that the overall heat transfer coefficient increased as the flow rate of hot water and cold water increased.
Table of contents
Introduction 5
Theory 6-7
Equipment 8-11
Procedure 12-13
Results 14-17
Discussion of Results 18
Conclusions 19
References 20
Appendices 21
Introduction
The purpose of this experiment was to determine the overall heat transfer coefficient (U) of the heat exchanger in room EMCS 120. Heat exchanger is the process of heat exchange between two fluids that are at different temperatures and separated by a solid wall. Heat is transferred from high temperature fluid to low temperature fluid. The two fluids can be run at the same or opposite directions. DAQ temperature data and measured mass flow rate of hot and cold water were used to calculate U.
Theory
Figure 1: Schematic Diagram of the Heat Exchanger
In Figure 1, Thot in and Thot out are the temperatures of the hot stream that enters and leaves the heat exchanger. Tcold in and Tcold out are the temperatures of the cold stream that enters and leaves the heat exchanger. Q is the amount of heat that transfers from one stream to the other.
If q is the total rate of heat transfer between the hot and cold fluids and there is no heat loss to the surroundings, as well as negligible potential and kinetic energy changes, the overall heat transfer coefficient can be determined from equation (1) below
(1)
where , U is overall heat transfer coefficient, q is heat rate, A is heat transfer area, and DTlm is the log mean temperature.
The log mean temperature can be found by using the equation (2) below
(2)
where Th,i and Th,o are hot temperature in and out respectively, and Tc,i and Tc,o are the cold temperature in and out.
The heat rate for each stream can be calculated from equation (3) below
(3)
where m is the water mass flow rate, cp is the heat capacity at constant pressure, and DT is the change in temperature, which is Thigh – Tlow.
Equipment
Heat exchanger is the process that heat exchange between two fluids that are at different temperatures. The heat exchanger system in room EMCS 120 was used in this experiment. Below are the pictures of the heat exchanger.
Figure 2: The Picture of the Heat Exchanger.
The picture of the heat exchanger system that used in this experiment is shown in Figure 2. This system is located in room EMCS 120.
Figure 3: The Schematic Diagram of the Heat Exchanger in Rom EMCS 120.
The letter symbols are defined as: FT is flow transmitter, S is solenoid, TT is temperature transmitter, CW is cold water, W/H is water heater, CWS is cold water supply, and CWR is cold water return.
Figure 3 is a schematic diagram of a heat exchanger with counter current flow and hot water in the tubes. Cold water supply is taken from pipe runs into two control valves, the flow transmitter, and then the temperature transmitter. It then runs into the heat exchanger where it receives heat exchange from the hot flow. Cold water stream is heated up and leaves the heat exchanger then goes through another temperature transmitter before goes into the drain. Hot water is taken from the water reservoir by the pump to the water heater where it is being heated. Hot water stream then leaves the water heater and goes through a flow transmitter and temperature transmitter to goes into the tubes in the heat exchanger. In the heat exchanger, hot water stream transfers heat to the cold stream and becomes cold when it leaves the heat exchanger. The hot water steam goes through another temperature transmitter and back to the reservoir. Hot water and cold water streams flow in an opposite way. Mass flow rates and temperatures of the two water streams are taken when water streams flow through the flow transmitter (FT) and the temperature transmitter (TT) respectively.
Figure 4: The Picture of the Heat Exchanger before Install into the System.
The picture of the heat exchanger in room EMCS 120 is shown in Figure 4. Hot water is entered in location 1 and cold water entering at location 2.
Figure 5: The Picture of Tube Bundle inside the Heat Exchanger.
The picture of the tubes inside the heat exchanger for this particular system is shown in Figure 5. Hot water flows into these tubes in the heat exchanger. There are 55 tubes and the heat transfer area of these tubes is 0.27 m2
Procedure
1). Go to this web page http://chem.engr.utc.edu/Webres/Stations/T-CONST.HTM
2). Click on Temperature and click on constant.
3). Enter the variables as show below
Constant Input / Temperature StationEnter this Information about you:
Name
/ Your location
/ E-mail address
Enter these Parameters for the Experiment:
Length of experiment (min):
Lab Wizard / Constant input value (%):
Lab Wizard
Enter this Configuration Information: / TEMPERATURE HELP
Control variable is
temperature (deg.C) / Flow Directions are
/ Hot Water Flow is in
Cooling Water Valve Info / Cooling Water Valve Opening % / Valve cycle time
(min)
#1 -- open %
#2 -- open %
4). Collect data from this web page http://chem.engr.utc.edu/329.
5). Use the hot water and cold water calibration equations to find the hot and cold water flow rates.
· Collect the mass flow rates of hot water and cold water from the web.
· Use the values of the hot and cold water flow rates for variable x in the equations that obtain from the hot and cold water calibrations.
· Calculate value of y for each value of x.
· Average the values of y and use these two average y values as the mass flow rates of the hot and cold water to calculate the overall heat transfer coefficient.
Results
Graph #1: Plot of Time versus Hot In Temperature
Graph # 1 is an example graph of temperature. It shows the steady state region where the average temperature is taken for calculation.
First Experiment
Table1: Mass Flow Rates of the Hot and Cold Streams
Mhot stream (kg/min) / Mcold stream (kg/min)2.5 ± 0.17 / 4.8 ± 0.28
Mass flow rates of the hot and cold streams values are listed in Table 1 with their two times standard deviation (or error).
Table 2: Experimental Temperature Values
Thot in (oC) / Thot out (oC) / Tcold in (oC) / Tcold out (oC)34.6 ± 0.6 / 54.2 ± 0.032 / 20 ± 0.25 / 37.2 ± 0.41
Table 2 contains the experiment temperature data with plus and minus two times standard deviation.
Table 3: Temperature Change of the Hot and Cold Streams
DThot stream (oC) / DTcold stream (oC)19.6 / 17.2
The change in temperature of the hot and cold streams is listed in Table 3. The change in temperature is calculated by taking high temperature minus the low temperature.
Table 4: Calculated Results of Total Heat Transfer Rate (q) and Overall Heat Transfer Coefficient(U)
Streams / q(kW) / U(W/m2K)Hot / 3.37 / 790
Cold / 5.76 / 1400
Average / 4.57 / 1100
Table 4 contains the calculated results of total heat transfer rates and overall heat transfer coefficients as an average with the hot water flow rate of 2.5 kg/min and the cold water flow rate of 4.8 kg/min. The calculation of these results is shown in appendix A1 and A2.
Second Experiment
Table 5: Mass Flow Rates of the Hot and Cold Streams
Mhot stream (kg/min) / Mcold stream (kg/min)1.5 ± 0.058 / 3.0 ± 0.10
Mass flow rates of the hot and cold streams values are listed in Table 5 with their two times standard deviation.
Table 6: Experimental Temperature Values
Thot in (oC) / Thot out (oC) / Tcold in (oC) / Tcold out (oC)52 ± 0.024 / 34.6 ± 0.6 / 19.3 ± 0.12 / 30.4 ± 0.2
Table 6 contains the experiment temperature data with their two times standard deviation.
Table 7: Temperature Change of the Hot and Cold Streams
DThot stream (oC) / DTcold stream (oC)17.4 / 11.1
The change in temperature of the hot and cold streams is listed in Table 7. The change in temperature is calculated by taking high temperature minus the low temperature.
Table 8: Calculated Results of Total Heat Transfer Rate (q) and Overall Heat Transfer Coefficient(U)
Streams / q(kW) / U(W/m2K)Hot / 2.25 / 510
Cold / 2.33 / 530
Average / 2.29 / 520
Table 8 contains the calculated results of total heat transfer rates and overall heat transfer coefficients as an average values with the hot water flow rate of 1.5 kg/min and the cold water flow rate of 3.0 kg/min. The calculation of these results is shown in appendix A3 and A4.
The Reynold numbers of the first and second experiment were calculated to be 650 and 300 respectively. The calculation of the Reynold number is shown in appendix A2 and A4.
These results are for the heat exchanger with counter current flow and hot water on tube side.
Discussion of Results
The results showed that the overall heat transfer coefficient of the first experiment is higher than the second experiment. The first experiment was run at higher mass flow rate of hot water and cold water than the second experiment. Equation (1) shows that the total heat transfer coefficient is proportional to the total heat transfer rate. Therefore, the total heat transfer coefficient increases as the amount of the hot water flow increases. In addition, the heat transfer rate of the hot stream is higher than the cold stream in the first experiment. The heat transfer rates of these two streams are expected to be same or close, but the results showed that they weren't close. The heat transfer rates of the hot stream and cold stream in the second experiment are close. The Reynold numbers show that both experiments have laminar flow.
Conclusion
The objective of this experiment was to determine the overall heat transfer coefficient of the heat exchanger in room EMCS 120. The overall heat transfer coefficients were found to be 1100 W/m2K and 520 W/m2K for the first and second run respectively. The first and second experiments have laminar flow. The overall heat transfer coefficient increased as the mass flow rates of hot and cold water increased.
References
1). Incropera, P. Frank and David P. DeWitt. Fundamentals of Heat and Mass Transfer. 5th Edition. pp 641-659
Appendix
1). 2003-11-18-13-18-47-25-thumb.htm.
2). 2003-11-18-14-53-13-69-thumb.htm.
7
Phuong Mai 12/3/03