Heat exchangers
Heat exchangers
• Device that facilitate the exchange of heat
between fluids that are at different
temperatures while keeping them from
mixing with each other.
• Heat exchanger involves convection in
each fluid and conduction through wall that
separating the two fluids
• It is convenient to use an overall heat
transfer coefficient (U)
HEAT EXCHANGERS
Hot In
Hot Out
Cold Out
Cold In
Heat Exchangers
• Parallel flow
• Counterflow
• Crossflow
Ref: Incropera & Dewitt (2002)
Cross Flow Heat Exchanger
HEAT EXCHANGERS,
Shell and Tube
HEAT EXCHANGERS, U-Tube
HEAT EXCHANGERS,
Plate Design
HEAT EXCHANGERS,
Condenser
Watt Equipment, Inc., Equip. No. E-203-TR, 2690 Sq. Ft. Surface, 400 PSI Shell,
400 PSI Tubes, 3/4" Stainless Steel Tubes, Stainless Steel Shell.
HEAT EXCHANGERS
Heat exchanger stainless steel, serial no. 3410,
170 tubes 1 inch diameter x 10 foot long,
overall length of 124 inches.
APV condenser/shell and tube heat exchanger.
The shell side is 316L and the tube side is
Titanium. It is 18 foot 10 in. and has a tube
surface of 1,752 square feet. There are (2)16 in,
shell side inlets. There are (558) 3/4 in. od
tubes, and the shell is 31 in. inside diameter.
Temperature profile in double-pipe
heat exchanger
Heat Exchanger Analysis
Heat Exchanger Analysis
Counterflow


th,o  tc,i   th,i  tc,o 
tm 
 th,o  tc,i 
ln 

 th,i  tc,o 
Parallel
tm

ta  tb 

 ta 

ln
 tb 
Counterflow Heat Exchangers

t h , i  t h, o 
R


th,o  tc,i   th,i  tc,o 
tc,o  tc,i 
tm 
 th,o  tc,i 
ln 


tc, o  tc ,i 


t

t
 h,i c,o 
P
th,i  tc,i 
Counterflow Heat Exchanger
tm


tc,o  tc,i R  1

 1  P  
ln 



1

RP


• What about crossflow heat exchangers?
• Δtm= F Δtm,cf
Overall heat transfer coefficient in
heat exchanger
Example
• Hot oil is to be cooled in a double-tube counterflow heat exchanger. The copper inner tubes
have a diameter of 2 cm and negligible
thickness. The inner diameter of the outer tube
(the shell) is 3 cm. Water flows through the tube
at a rate of 0.5 kg/s, and the oil through the shell
at a rate of 0.8 kg/s. Taking the average
temperatures of the water and the oil to be 45C
and 80C, respectively, determine the overall
heat transfer coefficient of this heat exchanger.
Estimation of h
• For turbulent flow
Nu = 0.023Re0.8Pr0.4
• For laminar flow
Di/Do
0.00
0.05
1.00
0.25
0.50
1.00
Nui
17.46
11.56
7.37
5.74
4.86
Nuo
3.66
4.06
4.11
4.23
4.43
4.86
Analysis of heat exchangers
• Select a heat exchanger when specified
temperature change in a fluid stream of
known mass flow rate log mean
temperature difference method (LMTD)
• Predict the outlet temperature of hot and
cold fluid streams in a specified heat
exchanger effectiveness-NTU method
Log mean temperature difference
=
• c = cold fluid, h = hot fluid
=
Example-the condensation of
steam in a condenser
• Steam in condenser of a steam power
plant is to be condensed at a temperature
of 30C with cooling water from a nearby
lake, which enters the tube of the
condenser at 14C and leaves at 22C.
The surface area of the tubes is 45 m2 and
the overall heat transfer coefficient is 2100
W/m2.C. Determine mass flow rate of
cooling water needed and the rate of the
steam in condenser.
Example-heating water in a
counter-flow heat exchanger
• A counter-flow double-pipe heat exchanger is to
heat water from 20C to 80C at a rate of 1.2
kg/s. The heating is to be accomplished by
geothermal available at 160C at a mass flow
rate of 2 kg/s. The inner tube is thin-walled and
has a diameter of 1.5 cm. If the overall heat
transfer coefficient of the heat exchanger is
640 W/m2.C, determine the length of the heat
exchanger required to achieve the desired
heating.
The effectiveness-NTU method
• Dimensionless parameter
• Actual heat transfer rate
• Maximum possible heat transfer rate
Example-heating water in a
counter-flow heat exchanger
• A counter-flow double-pipe heat exchanger is to
heat water from 20C to 80C at a rate of 1.2
kg/s. The heating is to be accomplished by
geothermal available at 160C at a mass flow
rate of 2 kg/s. The inner tube is thin-walled and
has a diameter of 1.5 cm. If the overall heat
transfer coefficient of the heat exchanger is
640 W/m2.C, determine the length of the heat
exchanger required to achieve the desired
heating.
Example-cooling hot oil by water in
a multipass heat exchanger
• Hot oil is to be cooled by water in a 1-shell-pass and 8tubes passes heat exchanger. The tube are thin walled
and are made of copper with an internal diameter of 1.4
cm. The length of each tube pass in the heat exchanger
is 5 m, and the overall heat transfer coefficient is 310
W/m2C. Water flows through the tube at a rate of
0.2 kg/s, and the oil through the shell at a rate of
0.3 kg/s. The water and the oil enter at temperature of
20C and 150C, respectively. Determine the rate of heat
transfer in the heat exchanger and the outlet
temperature of the water and the oil.