heat exchanger

Heat Exchanger
Dr. G. Kumaresan
Institute for Energy Studies
Anna University, Chennai
gkumaresan@annauniv.edu

Heat Exchanger - Definition
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A heat exchanger is a device built for efficient heat
transfer from one fluid to another, whether the fluids are
separated by a solid wall so that they never mix, or the fluids are
directly contacted.
Application:
o Heating, refrigeration and air conditioning system
o Petroleum refineries
o Chemical plants
o Power plants
o Cryogenic
o Heat recovery
o Manufacturing Industries
o Space heating, etc..

Classification of Heat Exchanger
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The objective of codes and standards described by ASME
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Code rules and standards is to achieve minimum
requirements for safe construction, in other words, to provide
public protection by defining those materials, design, fabrication
and inspection requirements; whose omission may radically
increase operating hazards.
 TEMA standards (Tubular Exchanger Manufacturer Association)
www.tema.org
 HEI standards (Heat Exchanger Institute)
www.heatexchange.org
 API (American Petroleum Institute)
www.api.org

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Concurrent flow
(or Co-current
or Parallel flow)
Counter current
flow (or Contra or
Counter flow)

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Cross flow

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Cross flow (both unmixed)
Temperature profile

Condenser & Evaporator
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Condenser
Evaporator

Gasketed Plate Heat Exchanger
Consists of a series of plates with corrugated flat flow passages. The hot and
cold fluids flow in alternate passages, and thus each cold fluid stream is
surrounded by two hot fluid streams, resulting in very effective heat transfer.
Plate heat exchangers are well suited for liquid-to-liquid applications.

Plate Heat Exchanger
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Spiral type

Plate Heat Exchanger
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Lamella type

Simplest type has one tube inside another - inner tube may have
longitudinal fins on the outside
However, most have a number of tubes in
the outer tube - can have very many tubes thus
becoming a shell-and-tube
Double Pipe Heat Exchanger
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Rotary storage type Heat Exchanger
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In regenerator, Hot and cold
fluid area passage remain
same whereas in
recuparator it is different

Application
• Rotary regenerators are used extensively in electrical power
generating stations for air preheating.
• They are also used in vehicular gas turbine power plants.
• In cryogenic refrigeration units, and in the food dehydration
industry.
• Fixed bed or fixed matrix regenerators are used extensively in
the metallurgical, glassmaking and chemical processing
industries
Rotary storage type Heat Exchanger
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Run around coil heat recovery system
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Where it can be used?
Recuperative HX’s located far apart
Risk of cross contamination between the
primary fluids.
Primary fluid

Compact Heat Exchanger
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o Large heat transfer surface area per unit volume – Compact HX
o The ratio of the heat transfer surface area of a HX to its volume is called
the area density (β)
β > 700 m2/m3 – Compact
Car radiator – β ~ 1000 m2/m3
Human lung – β = 20,000 m2/m3
Shell – tube HX = Tube dia 5 mm
o Mostly preferred for gas-to-gas, liquid-to-gas HX.
o Used in Aircraft and space application, oil cooler, R&Ac industry, Cryogenics,
electronic equipment's.

• Tubular heat exchanger
• Fin-plate heat exchanger
• Tube-fin heat exchanger
• Plate-frame heat exchanger
• Regenerative heat exchanger
Compact Heat Exchanger - Types
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Compact Heat Exchanger
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Extended surface Heat Exchanger (Compact category)
Plate Fin Different Fin arrangement

Round tube Fin
Flat Tube Fin
Extended surface Heat Exchanger (Compact category)

• Mass transfer in addition to heat transfer, both are exist in this
category (eg: evaporative cooling)
• The enthalpy of phase change in such an exchanger generally
represents a significant portion of the total energy transfer.
• The phase change generally enhances the heat transfer rate.
• The exchanger construction is relatively inexpensive, and the
fouling problem is generally nonexistent, due to the absence of a
heat transfer surface (wall) between the two fluids.
• However, the applications are limited to those cases where a
direct contact of two fluid streams is permissible.
Direct contact Heat Exchangers
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• Large shell with packing at the bottom over which water is
sprayed
• Cooling by air flow and evaporation
• Air flow driven by forced or natural convection
• Need to continuously make up the cooling water lost by
evaporation
Cooling Tower
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Cooling Tower cont..
It is a Gas-Liquid type HX. Here 90% of heat exchange takes place by mass
transfer, remaining 10% heat exchange achieved by heat transfer.
Cooling tower
Natural draft
Dry typeWet type
Mechanical draft
Forced draft Induced draft
Counter flow Cross flow
Direct Indirect

Heat Exchanger – Design Methodology
Thermal Design of HX
Sizing or design problem Rating or performance analysis problem
Input data To be determined
• Flow rates
• Inlet
temperatures
• One outlet
temperature
• Stream
properties
• Pressure drop
limitation
• Surface area
• HX dimensions
Input data To be determined
• Surface
geometry and
dimensions
• Flow rates
• Inlet
temperatures
• Stream
properties
• Pressure drop
limitation
• Fluid outlet
temperature
• Pr. drop for
both streams
• Total heat
transfered
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Heat Exchanger – Design Methodology
o HX design is more of an art
than a science
o Problem of HX design is
very intricate
o No two engineers will come
up with the same HX design
for a given problem

Heat Exchanger – Shell, Front and rear end types - TEMA

Shell-and-tube heat exchanger (one pass both sides)
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Shell-and-tube heat exchanger: The most common type of heat exchanger in
industrial applications.
They contain a large number of tubes (sometimes several hundred) packed in a
shell with their axes parallel to that of the shell. Heat transfer takes place as one
fluid flows inside the tubes while the other fluid flows outside the tubes through the
shell.
Shell-and-tube heat exchangers are further classified according to the number of
shell and tube passes involved.

Shell-and-tube heat exchanger (multi pass)
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Tube assembly – Shell-and-Tube Heat Exchanger
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Baffle cut
It is expressed as the percentage of the segment height to the shell inside
diameter.
o It can vary between 15% to 45% of the shell inside diameter.
o Small baffle cut – Generating large eddies of recirculating fluid in the regions
near the baffle tips.
o Large baffle cut – Major part of the shellside stream bypasses the greater
part of the bundle as well eddies created.

Baffle cut
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o Recommended baffle cut - 20% to 35% of the shell inside diameter.
o Keep Window Flow same as Cross Flow.

Conventional Baffle - Negatives
o Leads to more leakage
o Formation of many dead zones on eiether side of baffle plate, where fouling
will pronounced
o Greater pressure drop in shell side, which leads to reduction in heat transfer
In order to avoid above problems, helical baffles (helixchanger) are suggested
in the place of conventional baffles.

Problem solving method
 Get unknown temperature from energy balance
 Get LMTD
 Get Re. number
 Nusselt number
 Heat transfer Coefficient, hi
 Get Re. number
 Nusselt number
 Heat transfer coefficient, ho
 Overall heat transfer coefficient, U
 Dimension of HX (length / area), no of tubes
Tube side
Shell side

    33.08.0
PrRe023.0Nu
Do it for shell
and Tube side
#126, Eq.2.3.1 data book

Overall heat transfer Coefficient
Various thermal resistances in the path of heat flow from the hot to the cold
fluid are combined into an overall heat transfer coefficient (U)
Total thermal resistance = (thermal resistance of inside flow)+ (thermal
resistance of tube material)+ (thermal
resistance of outside flow)

LMTDdlNULMTDUAQ T )()( 
U is the overall heat
transfer coefficient,
W/m2C.
If
The overall heat transfer coefficient U is dominated by the smaller convection
coefficient. When one of the convection coefficients is much smaller than the other
(say, hi << ho), we have 1/hi >> 1/ho, and thus U  hi. This situation arises frequently
when one of the fluids is a gas and the other is a liquid. In such cases, fins are
commonly used on the gas side to enhance the product UA and thus the heat
transfer on that side.

43
Variation of
fluid
temperatures
in a heat
exchanger
when one of
the fluids
condenses or
boils.
is the rate of evaporation or condensation of the fluid, .
hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure.
The heat capacity rate of a fluid during a phase-change process must approach
infinity since the temperature change is practically zero.
Tm is an appropriate mean (average)
temperature difference between the two fluids.

Problem solving method – NTU

)(
)(
)(
)(
11
12
11
21
minmin hch
ccc
hch
hhh
ttC
ttC
ttC
ttC






where
min
max
; C=0 if any phase change in HX
C=1 if m mh h c c
C
C
C
c c


 1 exp (1 )
1
NTU C
C
  


#153, data book
#152, data book

#153, data book
i.e. for condenser & evaporator

Heat Exchanger Fouling – effects & cost
Effects of fouling:
o Lower heat transfer
o Increased pressure drop
o Decrease in effectiveness of HX
Cost of fouling:
Fouling of heat transfer equipment introduces an additional cost to the industrial
sector. The added cost is in the form of
o Increased capital expenditure
o Increased maintenance cost
o Loss of production and
o Energy losses (due to reduction in heat transfer and increase in pumping power)

Techniques to control Fouling
Control of fouling:
o Surface cleaning techniques
i. Sponge ball
ii brush systems
iii high pressure water/jet
iv Chemical cleaning
o Additives

Heat Transfer Fouling - Cleaning
#157, data book

Heat Transfer Fouling - Cleaning
CF – Cleanliness factor (Typical value0.85)
Percent Over Surface (OS)
 Additional surface can be provided either by increasing the
length of tubes or by increasing the number of tubes (hence
shell diameter)

HX – Pressure drop
o In addition to thermal design, fluid friction effects are equally important since
they determine the pressure drop of the fluids flowing in the system, and
consequently, the pumping power (or fan work) input necessary to maintain the
flow.
o Provision of pumps or fans adds to the capital cost and is a major part of the
operating cost of the HX

Pressure drop
In a HX Pr. Drop due to
• Bends
• Fittings
• Abrupt contraction/Expansion
• Change in momentum of streams
Pump or Fan efficiency ~ 0.80-0.85
Water Power
Shaft Power Shaft Power
Pr.
Shaft Power
overall
overall
P Q
Q


 
 



Problem – Over all heat transfer coefficient
Hot oil is to be cooled in a double-tube counter-flow 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 45°C and 80°C,
respectively, determine the overall heat transfer coefficient of this heat
exchanger.
Solution:
Assumptions : 1 The thermal resistance of the inner tube is negligible since the
tube material is highly conductive and its thickness is negligible. 2 Both the oil and
water flow are fully developed. 3 Properties of the oil and water are constant.

Problem – Over all heat transfer coefficient cont...
i o
1 1 1
U h h
 
Tube side:

which is greater than 2300. Therefore, the flow of water is turbulent. Assuming
the flow to be fully developed, the Nusselt number can be determined from
Annulus side:

which is less than 2300. Therefore, the flow of oil is laminar. Assuming fully developed
flow, the Nusselt number on the tube side of the annular space Nui corresponding to
Di /Do = 0.02/0.03 = 0.667 can be determined from the table by interpolation to be
Nu = 5.45
Ref data book #129, 2.6 & 2.6.1

Problem – Effectiveness-NTU method
Solution:

Problem – Effectiveness-NTU method cont....

Problem – LMTD & ε-NTU method
A one ton split Ac removes 3.5 kW from a room and in the process rejects 4.2 kW in the
air-cooled condenser. The ambient temperature is 30oC whereas condensing
temperature of the refrigerant is 45oC. Using LMTD (Take UA=350 W/K) method,
calculate the temperature rise of the air as it flows over the condenser tubes.
Use NTU (Take NTU=0.2 )method to find the temperature rise of the air.
Solution:
In condenser,  m mh fg c c c
h c T 
Hot fluid - Refrigerant, Cold fluid –
Ambient Air
Th1=Th2=45oC
tc1 =30oC
tc2
A/c
Win
Condenser
Room
4.2 kJ/s
3.5 kJ/s

Problem – LMTD & ε-NTU method cont....
 
   
 
 
2
2
2
2
2
2
2
45 30 45
45 30
ln
45
30
4200 350
15
ln
45
30
12
15
ln
45
Using trial-and-error method, 35
Temp. rise of air = 35-30=5
c
c
c
c
c
c
o
c
o
t
Q UA LMTD UA
t
t
t
t
t
t C
C
  
 
 
   

 
 
   


 
   

 
2 1
1 1
2
2
For phase change HX,
1 exp
0.343
30
0.343
45 30
35.15
Temp. rise of air = 35.15-30
=5.15
c c
h c
c
o
c
o
NTU
T T
T T
T
T C
C



  








LMTD Method ε-NTU Method

Compact Heat HX
Tube-fin heat exchanger

Compact Heat HX
Plate-fin heat exchanger

Compact Heat HX
Core friction accounts 90% of total pressure drop.
where,
min
A Total h.t. area 4
A Min. flow area h
L
D
 

Compact Heat HX - Problem
#34 data book

Compact Heat HX – Problem cont...

heat exchanger

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