LAB SESSION # 1 HEAT EXCHANGER Heat exchangers are used to transfer heat from a fluid on a side of a barrier to a fluid on the other side without allowing the fluids to mix together. Heat exchangers maximize the surface area of a wall that is used between the two fluids while minimizing any resistance to the flow of a fluid through the exchanger. Example The most well-known type of heat exchanger is a car radiator, in a radiator, a solution of water and ethylene glycol, also known as antifreeze, transfers heat from the engine to the radiator and then from the radiator to the ambient air flowing through it. This process helps to keep a car's engine from overheating. PARALLEL-FLOW HEAT EXCHANGER Parallel-flow, a major type of heat exchangers, allows two fluids to enter the exchanger at the same end. The two fluids then travel in parallel to the other side of the exchanger. The hot fluid transfers heat to the wall via convection. Parallel-flow heat exchangers are often used when two fluids must be brought to close to the same temperature.
Transcript
LAB SESSION # 1
HEAT EXCHANGERHeat exchangers are used to transfer heat from a fluid on a side of a barrier to a fluid on the other side without allowing the fluids to mix together. Heat exchangers maximize the surface area of a wall that is used between the two fluids while minimizing any resistance to the flow of a fluid through the exchanger.
Example
The most well-known type of heat exchanger is a car radiator, in a radiator, a solution of water and ethylene glycol, also known as antifreeze, transfers heat from the engine to the radiator and then from the radiator to the ambient air flowing through it. This process helps to keep a car's engine from overheating.
PARALLEL-FLOW HEAT EXCHANGER
Parallel-flow, a major type of heat exchangers, allows two fluids to enter the exchanger at the same end. The two fluids then travel in parallel to the other side of the exchanger. The hot fluid transfers heat to the wall via convection. Parallel-flow heat exchangers are often used when two fluids must be brought to close to the same temperature.
LOGARITHMIC MEAN TEMPERATURE DIFFERENCE (LMTD) METHOD
The logarithmic mean temperature difference (also known as log mean temperature difference or simply by its initialism LMTD) is used to determine the temperature driving force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred. The use of the LMTD arises straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties.
From Heat Transfer Eq. we get
Where, A is the total contact area and TΔ lm is the logarithmic mean temperature difference (LMTD)
LMTD=∆T lm=¿
Significance of the LMTD
The LMTD is the driven force for the heat exchange between the two fluids. As the LMTD value increases, the amounts of heat transfer between the two fluids also increase. The LMTD value is used for calculating the heat duty of the heat exchanger.
CONCENTRIC TUBE HEAT EXCHANGER
The purpose of this unit is to demonstrate the working principles of industrial heat exchangers in the most convenient way in the laboratory. Experiments show the practical importance of the temperature profiles, co- and counter flow, energy balances, log-mean temperature difference, and heat transfer coefficients.
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. Cold fluid is cold water, which flows through the annulus. Control valves are provided so that 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 counter flow heat exchanger. The temperatures are measured with thermometer. Thus, the heat transfer rate, heat transfer coefficient, LMTD and effectiveness of heat exchanger can be calculated for both parallel and counter flow.
EXPERIMENT # 01
DEMONSTRATE THE WORKING PRINCIPLE OF A DOUBLE PIPE CONCENTRIC TUBE HEAT EXCHANGER OPERATING UNDER
PARALLEL FLOW CONDITION.Apparatus
Double pipe heat exchanger Apparatus with both parallel and counter flow arrangement
Procedure
Start the water supply. Adjust the water supply on hot and cold sides. Keep the valves V2 & V3 closed and V1 & V4 opened so that arrangement is Parallel flow. Switch ON the geyser. Temperature of water will start rising. After temperatures become steady, note down the readings in the observation table.
Temperature Difference at inlet = ∆T 1=T hi−T ci=20C
Temperature Difference at Outlet =∆T 2=T ho−T co=7.5C
Log mean Temperature Difference = LMTD = ∆Tm= 12.74 C
Amount of heat Given out by hot liquid=
qemit=mh .Cp .(T hi−T ho)=557.6Watts
Amount of heat Absorbed by Cold Water,
q|¿|=mc .C p .(Tci−T co)=592.3Watts¿
Losses,
qemitt−q|¿|=35.3Watts¿
Efficiency of Heat Exchanger,
Ƞ=¿
Overall Heat Transfer Coefficient,
U=q|¿|/(At x ∆Tm )=694Watt /m2C ¿
Efficiency of cold water,
Ƞcold water=42.5 %
Efficiency of hot water,
Ƞhot water=20 %
Temperature Profile:
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.620
22
24
26
28
30
32
34
36
38
40
Hot Water Temperature ProfileCold Water Temperature Profile
Distance along tube length X
(m)
Tem
pera
ture
T (C
)
Comments
Never switch ON the geyser unless there is water supply through it. If the red indicator on geyser goes off during operation, increase the water supply, because it
indicates that water temperature exceeds the set limit. Ensure steady water flow rate and temperatures before noting down the readings, as fluctuating
water supply can give erratic results.
LAB SESSON#02
COUNTER-FLOW
In counter-flow heat exchangers, the fluids enter the exchanger from opposite ends. As the two flows move toward each other from opposite directions, the system is able to maintain almost a constant gradient between the two liquid flows as they travel the length of the exchanger. This enables nearly all the heat properties from one flow to be transferred to the other flow.
CONCENTRIC TUBE HEAT EXCHANGER
EXPERIMENT#02
DEMONSTRATE THE WORKING PRINCIPLE OF A DOUBLE PIPE CONCENTRIC TUBE HEAT EXCHANGER OPERATING UNDER
COUNTER FLOW CONDITION.Apparatus
Double pipe heat exchanger Apparatus with both parallel and counter flow arrangement
Procedure
Start the water supply. Adjust the water supply on hot and cold sides. Open the valves V2 & V3 and then close the valves V1 & V4. The arrangement is now counter
flow. Switch ON the geyser. Temperature of water will start rising. Wait until the steady state is reached and note down the readings. After temperatures become steady, note down the readings in the observation table.
Observations and Calculations
Length of pipe=L= 1.5 m
Tube outer diameter=do= (15x0.7) mm
Tube inner diameter=di= (22x0.9) mm
Set Temperature= 65°C
Transmission Area=A=0.067 m2
Hot water Flow rate= Qh=2 ltr/min
Cold water Flow rate= Qc =1ltr/min
Mass Flow rate of hot water=mh= pQh=0.0333 (kg/s)
Mass Flow rate of cold water =mc= pQc= 0.01667(kg/s)
Temperature Difference at inlet= ∆T1=Thi-Tco= 15 C
Temperature Difference at Outlet=∆T2=Tho-Tci= 20 C
Log mean Temperature Difference=LMTD=∆Tm= 17.4 C
Amount of heat Given out by hot liquid=qemit=mh.Cp.(Thi-Tho)= 697.8 Watts
Amount of heat Absorbed by Cold Water= qabs=mc.Cp.(Tco-Tci)= 697 Watts
Losses= qemit - qabs= 0.8 Watts
Efficiency of Heat Exchanger=Ƞ= (qabs/ qemit)x100=99.88%
Overall Heat Transfer Co-efficient=U=qabs/(Atx∆Tm)=598.6 Watt/m2C
Efficiency of cold water= Ƞcold water =40%
Efficiency of cold water= Ƞhot water =20%
Temperature Profile:
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.620
22
24
26
28
30
32
34
36
38
40
Hot Water Temperature ProfileCold Water Temperature Profile
Distance along tube length X
(m)
Tem
pera
ture
T (C
)
Comments
If heat capacity rates of the cold and hot fluids are the same and the heat exchanger is operated in the counter-flow regime then T is independent of position in the heat exchanger.Δ
Lab Sesson#03
THEORYIn general with higher flow rates the overall heat transfer coefficient will increase a bit and with lower flow rates it will decrease. This is because the change in velocity will affect the film coefficient portion of the overall heat transfer coefficient.
So with a higher flow rate you will get slightly better heat transfer but you will also have a greater mass flowing through the exchanger. The net result will be the transfer of more heat but the actual temperature of the fluid exiting the exchanger will likely be lower because of the extra mass involved.
Other things that are affected by changes in flow rate are the pressure through the exchanger (Higher flow rates leads to higher pressure drops. Lower flow rates lead to lower pressure drops.) and in some cases possible fouling of the exchanger.
Experiment#03
DETERMINE THE EFFECT OF VOLUME FLOWRATE VARIATION OF
A) HOT FLUID
B) COLD FLUID
ON THE PERFORMANCE OF A GIVEN HEAT EXCHANGER (FOR BOTH PARALLEL AND COUNTER FLOW ARRANGEMENTS)Apparatus
Double pipe heat exchanger Apparatus with both parallel and counter flow arrangement
(a) For variation in Hot fluid Flowrate
1. For Parallel-Flow Arrangement
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. Temperature of water will start rising. 4. After temperatures become steady, note down the readings in the observation table.5. Vary the hot water flowrate and repeat the same procedure.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 60°C
Transmission Area = A = 0.067 m2
Cold water Flow rate = Qc =1 ltr/min
Mass Flow rate of cold water,
mc=pQc=0.01667 kg/s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs
Flowrate of hot water
Qh
Hot water Temperature (°C) Cold Water Temperature (°C)
Scale : X-axis 1 big box=0.2 m Y-axis 1 big box=5 (C)
R1: for hot water Flow rate= Qh =1ltr/min R2: for hot water Flow rate= Qh =2ltr/min R3 :for hot water Flow rate= Qh =3ltr/min R4 :for hot water Flow rate= Qh =4ltr/min
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.620
25
30
35
40
45
50
55
60
65
29
35
42
5552
50
29
37
44
60
56 55
29
35
39
53 5249
2931
34
4644
41
R1 Hot FluidR1 Cold FluidR2 Hot FluidR2 Cold FluidR3 Hot FluidR3 Cold fluidR4 Hot FluidR4 Cold Fluid
Distance along Length X
(m)
Tem
pera
tture
T (C)
2. For Counter-flow Arrangement
Procedure
1. Start the water supply. Adjust the water supply on hot and cold sides.2. Open the valves V2 & V3 and then close the valves V1 & V4. The arrangement is now counter
flow. 3. Switch ON the geyser. Temperature of water will start rising. 4. Wait until the steady state is reached and note down the readings.5. After temperatures become steady, note down the readings in the observation table.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 60°C
Transmission Area = A = 0.067 m2
Cold water Flow rate = Qc =1 ltr/min
Mass Flow rate of cold water,
mc=pQc=0.01667 kg/s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs.
Flowrate of hot water
Qh
Hot water Temperature (°C) Cold Water Temperature (°C)
Scale: X-axis 1 big box=0.2 m Y-axis 1 big box=5 (C)
R1: for hot water Flow rate= Qh =1ltr/min R2: for hot water Flow rate= Qh =2ltr/min R3 :for hot water Flow rate= Qh =3ltr/min R4 :for hot water Flow rate= Qh =4ltr/min
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.625
30
35
40
45
50
55
40
34
29
5250.5 49.5
40
36
29
5048.5 48
42
35
29
4947 46
43
36
29
53
48 47
R1 Hot FluidR1 Cold FluidR2 Hot FluidR2 Cold FluidR3 Hot FluidR3 Cold fluidR4 Hot FluidR4 Cold Fluid
Distance along Length X
(m)
Tem
pera
tture
T (C)
(B)For variation in Cold fluid Flowrate
1. for parallel-flow Arrangement
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. Temperature of water will start rising. 4. After temperatures become steady, note down the readings in the observation table.5. Vary the hot water flowrate and repeat the same procedure.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 60°C
Transmission Area = A = 0.067 m2
Hot water Flow rate = Qh =2 ltr/min
Mass Flow rate of hot water,
mh=pQh=0.0333kg/ s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs.
Flowrate of cold
waterQc
Hot water Temperature (°C) Cold Water Temperature (°C)
Scale: X-axis 1 big box=0.2 m Y-axis 1 big box=5 (C)
R1: for cold water Flow rate= Qc =1ltr/min R2: for cold water Flow rate= Qc =1.5ltr/min R3 :for cold water Flow rate= Qc =2ltr/min R4 :for cold water Flow rate= Qc =2.5ltr/min
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.625
30
35
40
45
50
55
29 30
34
46
42.5 42
2931
35
47.5
44 43
2931.5
35.5
49
44.5 44
29
36.538
5048
45R1 Hot FluidR1 Cold FluidR2 Hot FluidR2 Cold FluidR3 Hot FluidR3 Cold fluidR4 Hot FluidR4 Cold Fluid
Distance along Length X
(m)
Tem
pera
tture
T (C)
(2) for Counter-flow Arrangement
Procedure
1. Start the water supply. Adjust the water supply on hot and cold sides.2. Open the valves V2 & V3 and then close the valves V1 & V4. The arrangement is now counter
flow. 3. Switch ON the geyser. Temperature of water will start rising. 4. Wait until the steady state is reached and note down the readings.5. After temperatures become steady, note down the readings in the observation table.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 60°C
Transmission Area = A = 0.067 m2
Hot water Flow rate = Qh =2 ltr/min
Mass Flow rate of hot water,
mh=pQh=0.0333kg/ s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs.
Flowrate of cold
waterQc
Hot water Temperature (°C) Cold Water Temperature (°C)
Scale: X-axis 1 big box=0.2 m Y-axis 1 big box=5 (C)
R1: for cold water Flow rate= Qc =1ltr/min R2: for cold water Flow rate= Qc =1.5ltr/min R3 :for cold water Flow rate= Qc =2ltr/min R4 :for cold water Flow rate= Qc =2.5ltr/min
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.625
30
35
40
45
50
55
30.528 29
44
41 40
3028.5 29
4341 40
3331.5
29
41 4038.5
40
36
29
4947 46
R1 Hot FluidR1 Cold FluidR2 Hot FluidR2 Cold FluidR3 Hot FluidR3 Cold fluidR4 Hot FluidR4 Cold Fluid
Distance along Length X
(m)
Tem
pera
tture
T (C)
Conclusion
The higher the flow rate the higher heat transfer, the higher the efficiency, and greater the temp change.
The pay back is the higher the flow rate, the greater the pressure loss, the bigger pump you need.
Lab Sesson#04
THEORYHeat exchangers are typically employed in the process industries as a means of providing heat transfer between two streams of fluid across a medium. The heat exchanger ensures the conservation of heat energy otherwise known as heat economic operations.
Effects of heat exchanger operating temperature
The heat exchanger operating temperature affects heat exchange. For example in refineries, stream temperatures can vary due to changes in the operating procedures. Any alterations in the stream temperature will create a variation in the approaches; the exchanger duty and log mean temperature difference. Low approach difference will give a corresponding log mean temperature difference, and high load vice versa. When the operating temperature limits are exceeded, the material condenses as a result of deposits and coats the internals of heat exchangers, which produces a wall temperature that is lower than the bulk limit temperature. To maintain the operating temperature, the inlet and outlet temperature must be monitored
Experiment#04
DETERMINE THE EFFECT OF TEMPERATURE VARIATION OF HOT FLUID (SET) TEMPERATURE ON THE PERFORMANCE OF A GIVEN HEAT EXCHANGER. (FOR BOTH PARALLEL AND COUNTER FLOW
ARRANGEMENTS)Apparatus
Double pipe heat exchanger Apparatus with both parallel and counter flow arrangement
1. for parallel-flow ArrangementProcedure
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. Temperature of water will start rising. 4. After temperatures become steady, note down the readings in the observation table.5. Vary the hot fluid set temperature and repeat the same procedure.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 65°C
Transmission Area = A = 0.067 m2
Hot water Flow rate = Qh =2 ltr/min
Cold water Flow rate = Qc =1 ltr/min
Mass Flow rate of hot water,
mh=pQh=0.0333kg/ s
Mass Flow rate of cold water,
mc=pQc=0.01667 kg/s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs.
Set temperature of Hot water
Hot water Temperature (°C) Cold Water Temperature (°C)
Scale: X-axis 1 big box=0.2 m Y-axis 1 big box=5 (C)
R1: for hot water set temperature= 60 C R1: for hot water set temperature= 65 C R1: for hot water set temperature= 70 C
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.625
30
35
40
45
50
55
60
65
29
37
44
60
5655
29
33.5
38
50
4746
29
35
39
5352
49
R1 Hot FluidR1 Cold FluidR2 Hot FluidR2 Cold FluidR3 Hot FluidR3 Cold fluid
Distance along Length X
(m)
Tem
pera
ttur
eT (C
)
2. for Counter-flow Arrangement
Procedure
Start the water supply. Adjust the water supply on hot and cold sides. Open the valves V2 & V3 and then close the valves V1 & V4. The arrangement is now counter
flow. Switch ON the geyser. Temperature of water will start rising. Wait until the steady state is reached and note down the readings. After temperatures become steady, note down the readings in the observation table. Change the set temperature and repeat above procedure.
Observations and Calculations
Length of pipe = L = 1.5 m
Tube outer diameter = do = (15x0.7) mm
Tube inner diameter = di = (22x0.9) mm
Set Temperature = 65°C
Transmission Area = A = 0.067 m2
Hot water Flow rate = Qh =2 ltr/min
Cold water Flow rate = Qc =1 ltr/min
Mass Flow rate of hot water,
mh=pQh=0.0333kg/ s
Mass Flow rate of cold water,
mc=pQc=0.01667 kg/s
Density of water= 1000kg/m3
Specific Heat of Water= Cp= 4.186 kJ/kg°C
No. of obs.
Set temperature of Hot water
Hot water Temperature (°C) Cold Water Temperature (°C)
