EGN 3033 Heat Transfer
TD360: Bench-Top Heat Exchangers
EXPERIMENT 5: The Effect of Varying Temperature (Driving Force)
REPORT SUBMITTED BY:
Khadeja Tamrouq – H00271232
Aisha Saeed – H00249519
Badryah Rashed – H00252704
Jawaher Sohail – H00283718
We certify that the narrative, diagrams, figures, tables, calculations and analysis in this report are our own work.
DATE EXPERIMENT PERFORMED:05/02/2018
DATE REPORT DUE: 05/07/2018
DATE REPORT SUBMITTED:05/06/2018
Heat exchange is an important unit operation that contributes to efficiency and safety of many processes. In this report you will evaluate performance of heat exchanger (shell & tube). This heat exchanger can be operated in both parallel- and counter-flow configurations. The heat exchange is performed between hot and cold water.
Description of TD360 Bench-Top Heat Exchangers
Concentric Tube Heat Exchanger (TD360a)
This heat exchanger is a simple shell and tube heat exchanger. It has two tubes, one inside the other. The outer tube is the shell. The inner tube carries the water from the hot circuit of the service module, the outer tube carries the water from the cold circuit. Heat transfer between the two tubes. You may connect the water circuit to give counter-flow or parallel flow experiments.
Figure 1 The Concentric Tube Heat Exchanger (TD360a)
Shell and Tube Heat Exchanger (TD360c)
This is the most common heat exchanger used in many industries, especially oil refineries and chemical plants. It is compact and can work at high temperatures. It is a larger tube (shell) that surrounds several smaller tubes (a bundle). Cold water circuit passes through shell and the hot water circuit passes through the bundle of tubes. Heat transfers between them. The bundle has baffles to help create a turbulent (mixed) flow in the shell.
Figure 2 The Shell and Tube Heat Exchanger (TD360c)
Versatile Data Acquisition System (VDAS)
VDAS is an optional extra device for the Bench-top Heat Exchanger. It is a two-part equipment (Hardware and Software) that will:
Automatically log data from your experiments
Automatically calculate data for you
Save you time
Create charts and tables of your data
Export your data for processing in other software
Figure 3 The VDAS display when Shell and Tube Heat Exchanger (TD360c) is selected
Average Temperature in Heat Exchanger
For most heat transfer equations, and calculations of specific heat capacity and density of water, you must find the average temperature for the hot and cold circuits in your heat exchanger. This is the calculated value of the temperature at a mid-point between the inlet and outlet of the circuit.
To find the average temperature of the cold circuit:
TC=TC1+TC22To find the average temperature of the hot circuit:
TH=TH1+TH22Heat Transfer, Energy Balance and Efficiencies
In heat exchangers, heat transfers or flows from the hot water circuit to the cold water circuit.
The heat transfer rate is a function of the fluid mass flow rate, the temperature change and the specific heat capacity of the fluid (at mean temperature).
Q=m×Cp×?TIn an ideal exchanger, that does not lose or absorb heat from its surroundings, the cool fluid absorbs all the heat from the hot fluid. So the heat transfer rate is:
Q=Qe=Qa=mH×CpH×?TH=mC×CpC×?TCRearranged for volumetric flow gives:
Q=Qe=Qa=VH×?H×CpH×?TH=VC×?C×CpC×?TCLogarithmic Mean Temperature Difference (LMTD)
This is a measure of the heat driving force that creates the heat transfer. It is a logarithmic average of the temperature difference between the hot and cold circuits at each end of the heat exchanger.
LMTD=TH2-TC2-TH1-TC1lnTH2-TC2TH1-TC1Heat Transfer Coefficient (U)
This is the overall heat transfer coefficient for the wall and boundary layers. It is a measure of how well the heat exchanger works. A good heat exchanger will give a high coefficient; therefore, this value is important to engineers.
U=QeA×LMTDFitting the Heat Exchanger
Switch off the pump and heater switches.
Put the heat exchanger (TD360a or TD360c) onto the front of the Service Module. Use the thumbscrews to hold it in position.
Connect the hot and cold circuits to the heat exchanger for parallel or counter flow, as shown on the diagram on the bedplate of each heat exchanger.
Connect the thermocouples to their sockets as shown in the diagram on the bedplate of each heat exchanger.
Switch on the electrical power and the cold water supply to the Service Module. Fully open the hand operated hot and cold water circuit flow control valves.
Make sure the heater tank is full and switch on the pump and the heater, use the buttons on the controller to set heater set point (SP) to the temperature shown in the experiment procedure (see Figure 4).
Figure 4 How to use the controller
Make sure any large air bubbles have moved out from the heat exchanger. You may need to tilt or gently rock the heat exchanger to do this.
Now you are ready to do your experiment.
Objective of the Experiment:
To show how different hot water supply temperatures affect the performance of the heat exchanger in both parallel flow and counter flow connection (flow rates are fixed).
Procedure 1 – Parallel Flow:
For the heat exchanger attached in the frame, Connect the water circuits for parallel flow and set the heater tank temperature to 40 oC.Start the VDAS and in the top left of the layout, select the correct heat exchanger. The software will record the data when you start taking readings.
Use an accurate thermometer to check the local ambient air temperature for reference.
For the first test, use the hand operated flow control valves to set the hot and cold flow rates: keep the hot water flow rate at 3 L/min and the cold water flow rate at 2 L/min. Allow at least 5 minutes for the heat exchanger temperatures to stabilize.
Record the hot and cold circuit temperatures.
Repeat the procedure for the heated tank temperatures of 50 and 60 oC.Procedure 2 – Counter Flow:
Reconnect the cold water flow circuit for counter flow and repeat procedure 1.
Tc1Tc2TH1TH2?Tc?THTc,avgTH, avg31.3 32.3 39.0 38.3 1.0 0.7 31.8 38.6
31.4 33.9 49.3 47.6 2.5 1.7 32.6 48.4
31.5 35.4 59.4 56.4 3.9 3.0 33.4 57.9
Tc1Tc2TH1TH2?Tc?THTc,avgTH, avg31.3 32.4 40.2 39.4 1.1 0.8 31.8 39.8
31.5 34.3 50.4 48.6 2.8 1.8 32.9 49.5
31.5 35.7 60.8 58.0 4.2 2.8 33.6 59.4
Find the change in temperature (?T) for each circuit and the average temperature for each circuit:
TC=TC1+TC22?counter flow; TC=31.3+32.42=31.85??T=TC2-TC1=32.4-31.3=1.1?TH=TH1+TH22TH=40.2+39.42=39.8??T=TH1-TH2=40.2-39.4=0.8??Parallel flow; TC=31.3+32.32=31.8??T=TC1-TC2=32.3-31.3=1?TH=TH1+TH22TH=39.0+38.32=38.65??T=TH1-TH2=39-38.3=0.7?Convert your flow rates from L/min to m3/s and calculate the heat emitted, heat absorbed, mean temperature efficiencies and energy balance coefficient of the parallel-flow and counter-flow systems for all flow rates.
flow rate?V=1.99 L/min1.99Lmin×100060=33.1667 m3/sHeat emitted:
counter flow ?Q=VH×?H×CpH×?TH=33.1667×992.74×1007×0.8=26.47kJ/kgQ=VC×?C×CpC×?TC=33.1667×995.09×1007×1.1=36.48kJ/kgparallel flow ? Q=VH×?H×CpH×?TH=33.1667×992.29×1007×0.7=23.15kJ/kgQ=VC×?C×CpC×?TC=33.1667×992.29×1.005×1=33.07kJ/kgCreate charts of energy balance coefficient (vertical axis) against heater temperature (horizontal axis).
Find the LMTD and use this to calculate the heat transfer coefficient (U) for each heater temperature. From your results, comment on how the heater temperature (driving force) affects the heat exchanger performance.
LMTD=TH2-TC2-TH1-TC1lnTH2-TC2TH1-TC1=38.3-32.3-(39.0-31.3)ln(38.3-32.339.0-31.3)=6.868If you conduct the test in multiple heat exchangers, compare the heat transfer coefficient of your heat exchangers for any given flow rate.
The heat transfer coefficient, U, may change because of variations in flow conditions and fluid properties. However, in many applications, it is reasonable to work with an average value of U.
If you conduct the test in multiple heat exchangers, compare the heat transfer coefficient of your heat exchangers for any given flow rate.
Discussion & Conclusion:
The driving force for heat transfer is Temperature difference… same as concentration gradient in mass transfer. For enhancement, there could be many factors viz. body geometry, area, fluid and solid properties, emissivity, fluid velocity etc.