Heat Exchanger Lab Experiment Chemical Engineering PDF

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1 SCHOOL OF CIVIL, ENVIRONMENTAL & CHEMICAL ENGINEERING Concentric Pipe (Tubular) Heat Exchanger Experiment PROC 2082: Heat and Mass Transfer Farihin Afini Abdul Muthalib s3526491 Shuhada Atika Idrus Saidi s3521375 Wan Lily Aisyah Mazlan s3521196 Group 8 9 September, 2015 Dr. Nicky Eshtiaghi Dem...

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SCHOOL OF CIVIL, ENVIRONMENTAL & CHEMICAL ENGINEERING

Concentric Pipe (Tubular) Heat Exchanger Experiment PROC 2082: Heat and Mass Transfer Farihin Afini Abdul Muthalib Shuhada Atika Idrus Saidi Wan Lily Aisyah Mazlan

s3526491 s3521375 s3521196

Group 8

9 September, 2015

Dr. Nicky Eshtiaghi Demonstrator: Cameron Crombie

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1 SUMMARY A heat exchanger’s role to the industrial world is vital and highly demanded but it is difficult to find the most effective heat exchangers in transferring heat. Due to this, it becomes an obligation for young future engineers to conduct a research on heat exchangers to gain understanding on how it works and have insights on its potentials for improvement in transferring heat. The scope of this experiment is to study both co-current and counter current flow in heat exchanger in order to determine the most effective fluid motion direction in transferring heat. It is important to conduct this experiment since heat exchangers are used by most people in everyday appliances to ensure a greater technology, good quality of life and environment. Heat exchanger is built to transfer heat from one medium to another. The types of fluid motion direction inside the heat exchanger used in this experiment include co-current flow and counter current flow. Both articles from Isaza, Warnica and Bussmann (2015) and Journal of Technology and Science (2015) discuss the topic of most advantage fluid motion direction inside the heat exchanger which is counter current flow. Major manufacturers publish online brochures on types of heat exchangers that are manufactured alongside with the suggested fluid motion direction, as well as the best conditions in which these exchangers work. An experiment was conducted by connecting counter current, followed by co-current operations to a Tubular Heat Exchanger with different combinations of flow rate by following a series of steps which are setting up the heat exchanger, operation in counter current flow and lastly co-current operation. This experiment is conducted to demonstrate the differences between same flow rates and different flow rates for both counter current and co-current flows and the effect on heat transferred, temperature efficiencies and temperature profiles. Also to determine the overall heat transfer coefficient and the effect of changes in hot and cold fluid flow rate on the temperature efficiencies and overall heat transfer coefficient. From this experiment, the co-current flow with hot and cold water flow with same flow rate shows a higher efficiency in transferring heat as the overall heat transfer coefficient, change in temperature, mean temperature efficiency and heat flux are the highest compared to co-current flow with different flow rate and counter current flow in both different and same flow rates. The results from this experiment is contradict with the theory from Isaza, Warnica and Bussmann (2015) and Journal of Technology and Science (2015) stated that counter current flow is more efficient than co-current flow due to several issues discussed in the results and discussion part of the report. Lastly, a few recommendations are thought and given on how the heat exchanger or experiment process can be designed or implemented to maximize its potentials. By allocating more time for this experiment and also by using materials with good insulation for the pipe would help in better performance for the heat exchanger.

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2 TABLE OF CONTENTS 1

Summary ............................................................................................................... 2

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Table of Contents ................................................................................................... 3

3

List of Figures and Tables ...................................................................................... 4

4

Introduction ............................................................................................................ 5

5

Literature Review and Theory ................................................................................ 6

6

Experimental .......................................................................................................... 8

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6.1 Procedure and Methodology ....................................................................... 8 6.2 Risk Assessment ........................................................................................ 9 Results and Discussion ........................................................................................ 10

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7.1 Countercurrent flow .................................................................................. 11 7.2 Co-current Flow / Parallel Flow ................................................................. 15 7.3 Discussion ................................................................................................ 19 Conclusions and Recommendations .................................................................... 28

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References........................................................................................................... 29

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Appendix .............................................................................................................. 30

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3 LIST OF FIGURES AND TABLES Figure/Table

Page

Figure 1: Co-current flow

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Figure 2: Counter Current Flow

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Figure 3: Countercurrent operation

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Figure 4: Co-current operation

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Table 1: Countercurrent flow: Same flowrate

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Table 2: Countercurrent flow: Different flowrates

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Table 3: Co-current flow: Same flowrates

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Table 4: Co-current flow: Different flowrates

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Figure 5: Temperature profile for countercurrent flow with similar flowrates

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Figure 6: Temperature profile for countercurrent flow with different flowrates

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Figure 7: Temperature profile for co-current flow with similar flowrate

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Figure 8: Temperature profile for co-current flow with different flowrate

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Table 5: Change in fluid temperature for each flowtypes and flowrates

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Table 6: Calculation of Q for different flowrates using cold fluid variables

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Table 7: Calculation of Q for same flowrates using cold fluid variables

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Figure 9: Hot fluid and cold fluid temperature efficiencies in respect to flowtypes and flowrates

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Figure 10: Mean temperature efficiencies for different flowtypes and flowrates

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Figure 11: Heat transfer rate against mean temperature efficiency in both flow types

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Figure 12: U against ∆𝑇𝐿𝑀 in both flow types

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Table 8: Overall heat transfer coefficient for different flow types and flowrates

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Table 9: Heat flux for different flow types and flowrates

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Table 10: Q emitted from hot fluid and cold fluid and error between the two values

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Figure 13: Basic flow of cold fluid in both flow types

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Table 1: Calculation of U using cold water variables and the error

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4 INTRODUCTION A heat exchanger is a device built for efficient heat transfer from one medium to another which enter and exit at different temperature. Three types of fluid motion direction inside the heat exchanger are parallel flow, counter flow and cross flow. However, in this experiment the focus is only on parallel flow and counter flow inside the concentric pipe; tubular heat exchanger. The purposes of doing this experiment is to demonstrate the different between the co-current and counter current flows and the effect on the heat transferred temperature efficiencies and temperature profiles through a tubular heat exchanger. Other than that, this experiment is performed in order to determine the overall heat transfer coefficient for the tubular heat exchanger using the logarithmic mean temperature difference for calculations in both co-current and counter flow. Another purpose of this experiment is to investigate the effect of changes in hot and cold fluid flow rate on the temperature efficiencies and overall heat transfer coefficient. Thus, from the experiment, the co-current and counter current flow may be determined and the effectiveness of both co-current and counter current flows can be compared. Heat exchangers are used widely in everyday life such as refrigerator, air conditioning, power plants, sewage treatment, food and beverages industry, dairy processing and others. Those heat exchangers are designed to transfer heat and from this experiment, the basic concept of heat exchangers’ efficiency can be established and implemented in everyday applications. Normally, some aspect in the production of heat exchangers is a task undertaken by chemical engineers since it deals with temperature, pressure, heat transfer, resistance, thermal conductivity, density, viscosity, specific heat, material of construction and others. Most heat exchangers do have problem with their effectiveness in transferring heat. The main scope of this experiment is to study both co-current and counter current flows based on their overall heat transfer coefficient and logarithmic mean temperature difference calculations in order to determine the efficiency of the heat exchanger by determining the overall heat transfer coefficient. This experiment motivates the team to analyse both co-current and counter current flows in the heat exchanger on how their temperature profiles affect the heat transfer coefficient and logarithmic mean temperature difference to increase the effectiveness in transferring heat. The importance of conducting this experiment is due to its various applications outside the laboratory, as heat exchangers are used in everyday appliances by most people. Besides, effective heat exchangers might lead to a better technology, environment and good quality of life. The theory behind this experiment is briefly outlined in the next section of this report. Various methodology and procedures are adopted in conducting the experiment. Also risk assessments while doing this experiment are being considered and from these assessment limitations and modifications are identified. The results from the experiment are graphically presented and analysed with a set of calculations including some implications for the current technological society. Finally, from further readings and brainstorming, a number of possible improvements of performance are listed.

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5 LITERATURE REVIEW AND THEORY Heat exchanger is built to transfer heat from one medium to another and the medium can be air to air, air to liquid, or liquid to liquid. The major role of heat exchanger is to remove heat from a hot fluid or to add heat to the cold fluid. For the parallel flow or also known as co-current flow, the hot and cold fluid streams both flow in the same direction in which both fluids enter and exit the heat exchanger at the same ends.

Figure 1: Co-current Flow

Unlike counter current flow, both hot and cold fluid streams flow in the opposite direction which both the fluids enter and exit the heat exchanger at the opposite ends.

Figure 2: Counter Current Flow

The temperature of fluids varies over the entire length since one fluid is convectively transfers heat to the tube wall where conduction occur to the opposite wall and the heat is then convectively transferred to the second fluid. The rate of heat transfer differs along the length of the exchanger tubes since it depends on the temperature difference between the fluids. Convection is a mode of heat transfer involving motion of fluids either absorbs heat from the source or gives out heat to the surrounding while conduction is a mode of heat transfer in which heat is transfer through a stationary fluid. Mechanical, potential and kinetic energies in heat exchangers are small compared to other terms in energy-balance equation. Also no shaft work is consumed in heat exchangers. Thus, if heat is

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transferred between two objects at different temperature and constant specific heat are consumed, the enthalpy balance for a heat exchanger becomes 𝑞 = 𝑚̇𝐶𝑝 ∆𝑇 Where

q

=

amount of heat energy gained or loss by substance (W)

ṁ

=

flow rate of stream (kg/s)

𝐶𝑝

=

specific heat of fluid (J/kg K)

∆T

=

temperature difference (K)

The overall heat transfer coefficient depends on several variables including the physical properties of the fluids and the solid wall, the flow rates and the exchanger dimensions. This overall coefficient of heat transfer is calculated using the log mean temperature difference based on the co-current or counter current flow by using the formula given 𝑄 = 𝑈𝐴∆𝑇𝐿𝑀 Where

Q

=

Rate of heat transfer (W)

U

=

Overall heat transfer coefficient (W/m2K)

A

=

Surface area through which heat flows (m2)

∆𝑇𝐿𝑀

=

Log mean temperature difference (K)

Several references have been utilized to determine the effectiveness for both co-current and counter current flow inside the heat exchanger. A newspaper article from Journal of Technology and Science (2015) is used as a reference in determining the most effective fluid motion direction inside the heat exchanger either co-current flow or counter current flow. It is noted by the article that counter current flow heat exchanger provide thermal and economic advantages over co-current flow. From an article by Isaza et.al.(2015), the team gained proper insights through brief description of heat exchanger’s fluid motion direction using analytical expressions as a proof. Through both articles, the team acknowledge several factors in effectiveness of heat exchanger, for instance the overall heat transfer coefficient and log mean temperature difference. In putting the information together, several comparisons can be made with the conducted experiment to discuss a more detailed analysis based on the results obtained.

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6 EXPERIMENTAL To demonstrate the differences between countercurrent and co-current flows and the effect on the heat transferred, temperature efficiencies and temperature profiles, an experiment was conducted by connecting countercurrent, followed by co-current operations to a Tubular Heat Exchanger with different combinations of flowrate.

6.1 PROCEDURE AND METHODOLOGY 6.1.1

Setting up heat exchanger Firstly, the main switch of heat exchanger was switch on. All switches at the rear of the RCD and circuit breakers at the rear of support plinth got to be checked and up if the temperature controller did not illuminated. Then, the temperature controller was set to a point approximately 40oC above the cold water inlet temperature (T4). As the T4 was 12 oC, the controller was set to 52oC.

6.1.2

Countercurrent operation

Figure 3: Countercurrent operation

The connection of the cold and hot water was checked. As the cold water connections was reversed to the annulus of the heat exchanger, it allowed the heat exchanger to operate in countercurrent operation first as the hot and cold fluid streams flowed in the opposite directions. Firstly, the experiment was conducted with the hot and cold water have the same flowrate.The hot water regulator was switched on. The cold water flow control valve Vcold was opened and adjusted to give 3 litres/min. The step was repeated for the Vhot to give 3 litres/min. The temperatures was allowed to stabilized and recorded when stable. Since the heat exchanger was connected to the software in the computer, the T1, T2, T3, T4, T5, T6, Fhot and Fcold would automatically recorded when the save button was clicked. The experiment was repeated by using different combinations of hot and cold fluid flowrate by adjusting the Vcold with constant Vhot. After all the results was recorded, the cold water flow control valve Vcold was closed and changed the cold and hot water connections so that the streams flowed in the same directions.

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6.1.3

Co-current operation

Figure 4: Co-current operation

The last step mentioned above allowed the heat exchanger to operate in co-current operation. Next, the hot water regulator was switched on. The flowrate of cold water was set to 3 litre/min by adjusted the cold water control valve Vcold after the flow indicator was switched to Fcold. Then, the hot water control valve Vhot was adjusted, after the flow indicator was switched to Fhot, to give 3 litres/min. The temperatures was monitored using the temperature meters. The temperatures was allowed to stabilized and recorded when stable. The T1, T2, T3, T4, T5, T6, Fhot and Fcold would automatically recorded when the save button was clicked. The experiment was repeated by using different combinations of hot and cold fluid flowrate by adjusting the Vcold with constant Vhot.

6.2 RISK ASSESSMENT From the risk assessment, the experiment is quite safe to be conducted as the fluid flow in the heat exchanger was water which meant no hazardous chemical was involved that would initiate any explosion or corrosion. However there is one aspect of safety that have to be focused on which is electrical. The heat exchanger experiment was comprised of electrical equipment, thus it has to be handled with care even though it has been insulated. For personal protection, lab coat and safety goggles has to be worn all the time while in the laboratory.

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7 RESULTS AND DISCUSSION In this section, tabulated result and calculations from each run is presented first before the analysis. The first two runs are both countercurrent flow, one at which hot fluid and cold fluid have the same flowrate, and another at which these two fluid flows at different rates. The last two runs are co-current or parallel flow, also one at which hot fluid and cold fluid have the same flowrate, and another at which these two fluid flows at different rates. Technical data is as follows: Inner tube inside diameter, ID Inner tube outside diameter, OD Heat transfer length, L (Total) Heat transfer area, A

0.00515 0.00635 1.00800 0.01900

m m m m

A = π dm L

There are six readings for each table, and only one is selected for the calculation. This selection is random among the the final three readings before the operation for each run is stopped. The data selected is bolded. Logarithmic mean diameter is calculated by the formula: 𝑑𝑚 =

𝑂𝐷 − 𝐼𝐷 𝑂𝐷 ln 𝐼𝐷

0.00635 − 0.00515 = 0.005729 ≈ 0.006 0.00635 ln 0.00515

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7.1 COUNTERCURRENT FLOW Countercurrent flow is the flow when two fluids enter at different ends of the exchanger in opposite directions through the unit.

7.1.1

Same flowrates

Table 1 shows the temperature profile for countercurrent flow with the flowrate of hot fluid and cold fluid is at the same value, which is set to 3.00 L/min which is equal to 0.00005 m3/s.

HOT FLUID

COLD FLUID

Logarithmic mean diameter

Heat transfer area

qvhot flowrate (m3/s)

T1 Inlet (°C)

T2 Mid (°C)

T3 Outlet (°C)