Heat transfer laboratory HEAT EXCHANGERS

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‫االسم‬
:
‫حا‬ ‫عدي‬
‫حمزة‬ ‫مرزة‬ ‫تم‬
A ‫الشعبة‬
:
-
‫المرحلة‬
‫ال‬
‫ثالثة‬

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• Recognize numerous types of heat exchangers, and classify them,
• Develop an awareness of fouling on surfaces, and determine the overall heat
transfer coefficient for a heat exchanger,
• Perform a general energy analysis on heat exchangers,
• Obtain a relation for the logarithmic mean temperature difference for use in the
LMTD method, and modify it for different types of heat exchangers using the
correction factor,
• Develop relations for effectiveness, and analyze heat exchangers when outlet
temperatures are not known using the effectiveness-NTU method,
• Know the primary considerations in the selection of heat exchangers.
Heat exchangers are devices that facilitate the exchange of heat between two fluids
that are at different temperatures while keeping them from mixing with each other.
Heat exchangers are commonly used in practice in a wide range of applications,
from heating and air-conditioning systems in a household, to chemical processing
and power production in large plants. Heat exchangers differ from mixing
chambers in that they do not allow the two fluids involved to mix. In a car radiator,
for example, heat is transferredfrom the hot water flowing through the radiator
tubes to the air flowing through the closely spaced thin plates outside attached to
the tubes. Heat transfer in a heat exchanger usually involves convection in each
fluid and conduction through the wall separating the two fluids. In the analysis of
heat exchangers, it is convenient to work with an overall heat transfer coefficient
U that accounts for the contribution of all these effects on heat transfer. The rate of
heat transfer between the two fluids at a location in a heat exchanger depends on
the magnitude of the temperature difference at that location, which varies along
the heat exchanger. In the analysis of heat exchangers, it is usually convenient to
work with the logarithmic mean temperature difference LMTD, which is an
equivalent mean temperature difference between the two fluids for the entire heat
exchanger. Heat exchangers are manufactured in a variety of types, and thus we
start this chapter with the classification of heat exchangers. We then discuss the
determination of the overall heat transfer coefficient in heat exchangers, and the
1.Objectives
2. Introduction

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LMTD for some configurations. We then introduce the correction factor F to
account for the deviation of the mean temperature difference from the LMTD
in complex configurations. Next we discuss the effectiveness–NTU method,
which enables us to analyze heat exchangers when the outlet temperatures of
• Different heat transfer applications require different types of hardware and
different configurations of heat transfer equipment.
3.1-Double-Pipe Heat Exchangers:-
• The simplest type of heat exchanger is called the double-pipe heat exchanger.
• One fluid flows through the smaller pipe while the other fluid flows through the
annular space between the two pipes.
• Two types of flow arrangement
– parallel flow, – counter flow.
3. Types of Heat Exchangers

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3.2-Compact Heat Exchanger:-
• Large heat transfer surface area per unit volume.
• Area density β ─ heat transfer surface area of a heat
exchanger to its volume ratio.
• Compact heat exchanger β >700 m2/m3.
• Examples:
– car radiators (β ≈1000 m2/m3),
– glass-ceramic gas turbine heat
exchangers (β ≈6000 m2/m3),
– the regenerator of a Stirling
engine (β ≈15,000 m2/m3), and
– the human lung (β ≈20,000 m2/m3).
• Compact heat exchangers are commonly used
in
– gas-to-gas and
– gas-to liquid (or liquid-to-gas) heat exchangers.
• Typically cross-flow configuration ─ the two
fluids move perpendicular to each other.
• The cross-flow is further classified as

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3.3- Shell-and-Tube Heat Exchanger
• The most common type of heat exchanger in industrial applications.
• Large number of tubes are packed in a shell with their axes parallel to that of the
shell.
• The other fluid flows outside the tubes through the shell.
• Baffles are commonly placed in the shell.
• Shell-and-tube heat exchangers are relatively large size and weight.
• Shell-and-tube heat exchangers are further classified according to the number of
shell and tube passes involved.
3.4 Plate and Frame Heat Exchanger
• Consists of a series of plates with corrugated flat flow passages.
• The hot and cold fluids flow in alternate passages
• Well suited for liquid-to-liquid heat exchange applications, provided that the hot
and cold fluid streams are at about the same pressure.

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The main function of heat exchanger is to either remove heat from a hot fluid or to
add heat to the cold fluid. The direction of fluid motion inside the heat exchanger
can normally categorized as parallel flow, counter flow and cross flow. In this
experiment, heat exchanger used is only counter-current flow. For counter-current
flow, both the hot and cold fluids flow in the opposite direction. Both the fluids
enter and exit the heat exchanger on the opposite ends. This experiment focused on
the shell and tube heat exchanger.
Hot water flow rate (Hw )
QH = mH x CpH x (t1-t2)
Hot water flow rate (Cw )
QC = mC x CpC x (T2-T1)
Where:
QH = Heat load for hot water flow rate
QC= Heat load for cold water flow rate
mH=Hot water mass flow rate
mC=Cold water mass flow rate
t1=Hot water inlet temperature
t2=Hot water outlet temperature
T1=Cold water inlet temperature
T2=Cold water outlet temperature
LMTD
Calculations of log mean temperature difference (LMTD)
4. THEORY

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)
(
)
(
ln
)
(
)
(
1
2
2
1
1
2
2
1
T
t
T
t
T
t
T
t
LMTD






Heat loss rate = QH - QC
Dirt factor, Q = 0.5 (QH + QC )
Overall heat transfer coefficient, U
Overall heat transfer coefficient at which equivalent to D
U can be calculated by
using equation below. In this case, the value of total heat transfer area A has been
given and equal to 0.05 m2
.
LMTD
A
Q
U


Where:

Q Heat rate with respect to the average head load
Reynolds Number Calculation
Re =
ρv(ds−do)
μ
At which
Tube outside diameter, m
ds = Shell diameter, m
Viscosity, taken at average fluid temperature in the shell, Pa.s
Exchange area, m2

do



As

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5.1-General Start-up Procedure :
1. A quick inspection was performed to make sure that the equipment is in proper
working condition.
2. All valves were initially closed except V1 and V12.
3. Hot tank was filled via a water supply hose connected to valve V27. Once the
tank is full, the valve was closed.
4. The cold water tank was filled up by opening valve V28 and the valve was left
opened for continuous water supply.
5. A drain hose was connected to the cold water drain point.
6. Main power was switched on. The heater for the hot water tank was switched on
and the temperature controller was set to 50o
C.
7. The water temperature in the hot water tank was allowed to reach the set point.
8. The equipment was now ready to be run.
5.2-Counter-current Shell & Tube Heat Exchanger Procedures :
1. General start-up procedures was performed.
2. The valves to counter-current Shell & Tube Heat Exchanger arrangement was
switched.
3. Pumps P1 and P2 were switched on.
4. Valves V3 and V14 were adjusted and opened to obtain the desired flowrates for
hot water and cold water streams, respectively.
5. The system was allowed to reach steady state for 10 minutes.
6. FT1, FT2, TT1, TT2, TT3 and TT4 were recorded.
7. Pressure drop measurements for shell-side and tube side were recorded for
pressure drop studies.
5.PROCEDURE

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8. Steps 4 to 7 were repeated for different combinations of flowrate FT1 and FT2.
9. Pumps P1 and P2 were switched off after the completion of experiment.
5.3-General Shutdown Procedure :
1. The heater was switched off. The hot water temperature was waited until it
dropped below 40o
C.
2. Pump P1 and pump P2 were switched off.
3. The main power was switched off.
4. All water in the process line was drained off. The water in the hot and cold water
tanks were retained for next laboratory sessions.
5. All valves were closed.
In this experiment of shell and tube heat exchanger particular apparatus, water is used as
both the hot and cold fluid. The purpose of this heat exchanger is to cool a hot stream. Cooling
water flows through the outer pipe (the shell), and hot water flows through the inner pipe on the
inside. Heat transfer occurs in both directions; the hot water is cooled, and the cooling water is
heated. This arrangement is called a “shell-and-tube” heat exchanger. There are many other
forms of heat exchangers; most notably, the double-pipe heat exchanger.
The main objectives of this experiment is to evaluate and study the overall heat transfer
coefficient, LMTD, heat transfer and heat loss for energy balance as well as to evaluate and
study the performance of shell and tube heat exchanger at various operating condition. In this
shell and tube heat exchanger, the fluids flow in counter-current flow which results in faster heat
exchange. The basic theory in this air experiment is QH=QC, which the amount of heat release by
hot water is equal to the amount of heat absorb by cold water. However, the results is different
than the basic theory where the amount of heat release by hot water is not equal to the amount of
heat absorb by cold water, QH ≠ QC. This is due to some errors during conducting this
experiment which are the presence of bubbles in tube where the hot water flows. The presence of
these bubbles can cause corrosion and disturb the process of heat transfer. Although the results
6. DISCUSSION
7. CONCLUSION

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are not followed the basic theory, this experiment can be said as successful as the objectives of
this experiment is already achieve.
1-Yunus A.Cengel (2007). Heat and Mass Transfer Fundamentals and Application.
2-Shankar.S (n.d). Shell and Tube Heat Exchanger.
3- Chris.W (2016). Heat Exchanger. Retrieved from http://www.explainthatstuff.com/how-heat-
exchangers-work.html
8. References

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