Aim: To determine,
(i) LMTD for parallel flow arrangement
(ii) LMTD for counterflow arrangement
(iii) Effectiveness (ε) for the parallel flow arrangement
(iv) Effectiveness (ε) for counterflow arrangement
Learning Objectives: To,
1. Study about various heat exchanging equipment
2. Study fundamental methods of designing heat exchanging equipment and comparing them
3. Study LMTD and its necessity
4. Know the effectiveness of the heat exchanger
5. Know the temperature profiles of parallel and counterflows
Apparatus: Experimental test rig, stopwatch, thermometers.
Theory:
A heat exchanger is a device used for the process of heat exchange between two fluids that are at different temperatures. Heat exchangers are used in many engineering processes like those in refrigerating and air conditioning systems, power systems, Food processing systems, chemical reactors and space or Aeronautical applications. Heat exchangers are of basically of three types.
(1) Transfer type, in which both fluids pass through the exchanger and heat gets transferred through the separating walls between the fluids,
(2) Storage type- in this, firstly the hot fluid passes through a medium having high heat capacity and then cold fluid is passed through the medium to collect the heat. Thus hot and cold fluids are alternately passed through the medium,
(3) Direct contact type- in this type, the fluids are not separated but they mix with each other and heat passes directly from one fluid to the other.
Heat exchangers may be classified in several ways. One classification is according to the fluid flow arrangement or the relative direction of the hot and cold fluids. The fluids may be separated by a plane wall but more commonly by a concentric tube (double pipe) arrangement shown in fig. If both the fluids move in the same direction, the arrangement is called a parallel flow type. In the counterflow arrangement, the fluids move in parallel but opposite directions. In a double pipe heat exchanger, either the hot or cold fluid occupies the annular space and the other fluid moves through the inner pipe. The method of solving the problem using logarithmic mean temperature difference is typical and more iteration must be done. So it takes more time for the problem to solve. Therefore another method is practised for solving this type of problems. This method is known as the Effectiveness and Number of Transfer Units or simply ε-NTU method.“The effectiveness of heat exchangers is defined as the actual heat transfer rate by maximum possible heat transfer rate”.The LMTD method may be applied to design problems for which the fluid flow rates and inlet temperatures, as well as the desired outlet temperature, are prescribed. If the LMTD method is used in performance calculations for which both outlet temperatures must be determined from knowledge of the inlet temperatures, the solution procedure is iterative. For both design and performance calculations, the effectiveness-NTU method may be used
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| Double-Pipe Heat Exchanger Experiment |
The experimental setup consists of two concentric tubes in which fluids pass. The hot fluid is hot water, which is obtained from an electric geyser. Hot water flows through the inner tube, in one direction. The cold fluid is cold water, which flows through the annulus. Control valves are provided so that the direction of cold water can be kept parallel or opposite to that of hot water. Thus, the heat exchanger can be operated either as paralle1 or counterflow heat exchanger. The temperatures are measured with a thermometer. Thus, the heat transfer rate, heat transfer coefficient, LMTD and effectiveness of heat exchanger can be calculated for both parallel and counter flow.
Specifications1. Heat exchanger - (a) Inner tube - 12 mm OD and11 mm ID copper tube.
(b) Outer tube - 25 mm G. I. Pipe.
(c) Length of the Heat exchanger is 1 m.
(2) Electric heater - 3 KW Capacity to supply hot water.
(3) Valves for flow and direction control- 5 No’s.
(4) Thermometers to measure temperatures - 10 to 110°C - 4 No's.
(5) Measuring flask and stop the clock for flow measurement.
Experimental Procedure:
1. Start the water supply. Adjust the water supply on hot and cold sides.
2. Keep the valves V2 & V3 closed and V1 & V4 opened so that arrangement is
parallel flow.
3. Switch ON the geyser. The temperature of the water will start rising. After temperatures
become steady, note down the readings in the observation table.
4. Repeat the experiment by changing the flow. Now open the valves V2 & V3 and
then close the valves V1 & V4. The arrangement is now counterflow. Wait until
the steady state is reached and note down the readings.
Observation Table:
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| Observation table |
1. The mass flow rate of hot water, mh = Density x Volumetric flow rate of hot water (kg/s)
2. Specific heat of hot water at bulk temperature, Cph = J/kg-0C
3. The mass flow rate of cold water, mc = Density x Volumetric flow rate of cold water (kg/s)
4. Specific heat of cold water at bulk temperature, Cpc = J/kg-0C
5. (mCp)h =
6. (mCp)c =
Precautions:
1. Never switch ON the geyser unless there is water supply through it.
2. If the red indicator on geyser goes off during operation, increase the water supply because it indicates that water temperature exceeds the set limit.
3. Ensure a steady water flow rate and temperatures before noting down the readings, as
fluctuating water supply can give erratic results.
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| LMTD |
Hence,
(i) LMTD for parallel flow arrangement = deg C
(ii) LMTD for counter flow arrangement = deg C
(iii) Effectiveness (ε) for parallel flow arrangement =
(iv) Effectiveness (ε) for counter flow arrangement =