Back to Basics
Heat Exchanger
Overview
Arnold B. Kleijn
HRS Heat Exchangers
Understand the basics of heat exchanger
operation and design to ensure effective
equipment sourcing and efficient operation.
H
eat exchangers are used to thermally process materials, and can be employed to heat or cool a range of
products in applications such as food processing
(refrigeration, pasteurization, and sterilization), chemical
processing, energy production, and waste treatment. The
thermodynamic considerations involved in the design and
construction of heat exchangers are complex.
Designing a heat exchanger begins with two primary concerns. The first is selecting the right type of heat
exchanger: plate, shell-and-tube (Figure 1), scraped-surface,
etc. Various factors influence this choice, including the nature
of the material(s) to be heated or cooled, the objective of the
process (e.g., refrigeration or pasteurization), and any restrictions of the environment where the heat exchanger is to be
p Figure 1. Shell-and-tube heat exchangers are common in the chemical
process industries (CPI).
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used. The second consideration is size, which is critical to the
effectiveness of the equipment in the application.
This article provides an introduction for new engineers,
plant managers and operators, or anyone else who needs
a refresher on heat exchanger design. As is true with any
equipment specification and purchasing, seek the opinion of
an expert familiar with your process, but use the information
presented here as guidance in this effort.
Types of heat exchangers
Original equipment manufacturers produce many different models of heat exchangers, but most fall into one of three
categories — plate, shell-and-tube, and scraped-surface.
Plate heat exchangers. These consist of thin, corrugated
plates packed inside a frame, with the process fluid and
heat-transfer (or service) fluid (HTF) in alternating channels
(Figure 2). They are ideal for applications in which the fluids
have relatively low viscosities and contain no particles.
They are also effective when the product outlet temperature
is close to the temperature of the HTF. However, plate heat
exchangers can be prone to blockages and fouling, and in
some cases, irregular flows through the exchanger can produce uneven heat distribution or additional fouling.
Performance can be improved through clever design,
such as using herringbone-patterned heat-transfer plates
(Figure 3). These are assembled with two sets of parallel
channels for each liquid. The herringbone pattern points in
opposite directions, which increases the number of points of
support and creates a lattice in each channel. This arrangement produces a high level of turbulence, increasing the
heat-transfer rate.
Copyright © 2020 American Institute of Chemical Engineers (AIChE).
Not for distribution without prior written permission.
Shell-and-tube heat exchangers. Shell-and-tube heat
exchangers consist of one or more tubes filled with either
the process fluid or the HTF and a shell filled with the other
fluid (Figure 4). The tubes may be corrugated (Figure 5) to
increase both the heat-transfer rate and efficiency, minimize
the potential for fouling, and enable more-compact and economical heat exchangers. Shell-and-tube exchangers may be
made of stainless steel for use in the food, pharmaceutical,
and chemical industries.
Scraped-surface heat exchangers. These heat exchangers
are used in applications with low heat-transfer rates caused
by fouling or viscous fluids. As with shell-and-tube heat
exchangers, they are composed of a tube or tubes within
a vessel. However, the tubes include a physical method of
scraping the inside surface to prevent and remove fouling
layers (Figure 6).
Fouling occurs when fluids degrade near the tube wall
and layers of solids deposit onto the wall, acting as an insulator and preventing effective heat transfer. Another form
of fouling is crystallization, whereby cooling or increasing
concentration of solids causes components in the fluid to
accumulate on the heat exchanger surface. Scraping the heattransfer surface to remove these layers of fouling maintains
high heat-transfer rates.
Typically, the more viscous a fluid, the lower the heat-
transfer rate. Very viscous fluids, such as thick syrups,
nut butter, or creams, etc., require very large heat-transfer
areas. Scraped-surface heat exchangers mix the fluid vigorously, which increases the volume of fluid contacting the
heat exchanger surface. This increases the heat-transfer rate
and reduces the necessary surface area.
Heat exchange calculations
Once the type of exchanger has been chosen, the supplier’s engineering staff ensures that the model chosen is
correctly sized for the job. A correctly sized heat exchanger
Tube
Outlet
Shell
Inlet
Baffles
Shell
Outlet
Tube
Inlet
p Figure 4. Shell-and-tube heat exchangers contain a hot or cold fluid in
parallel tubes surround by a shell that contains the other fluid.
t Figure 5. Corrugated
tubes can enhance the
operation of shell-andtube heat exchangers and
enable more-compact
designs.
Cold Fluid Out
Hot Fluid In
Cold Fluid In
Hot Fluid Out
p Figure 2. A plate heat exchanger consists of a series of parallel plates
packed together to allow hot and cold fluid to flow in alternating channels
between the plates.
q Figure 6. Similar to
a shell-and-tube heat
exchanger, scrapedsurface exchangers are
composed of a tube or
tubes within a vessel. The
tubes include a mechanism
to scrape their interior
surface.
t Figure 3. A
herring­bone pattern
on the heat-transfer
plates increases fluid
turbulence and, thus,
the heat-transfer rate.
Vessel Outlet
Scraper Blades
Blade Fixation
Tube Inlet
Tube Outlet
Rotor
Vessel Inlet
Copyright © 2020 American Institute of Chemical Engineers (AIChE).
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offers the appropriate amount of heat transfer for the fluid(s)
being treated at the required throughput rate.
Given inlet and outlet temperatures for specified fluids,
sizing calculations determine the necessary heat-transfer
area. Most calculations also factor in variables such as
whether the heat exchanger operates using a countercurrent
or parallel flow arrangement.
The common heat exchanger sizing equation is:
where Q is the rate of heat transfer between the two fluids (Btu/hr); U is the overall heat-transfer coefficient
(Btu/ft2-hr-°F), which depends on the conductive properties
of the fluids and the heat exchanger material; A is the heattransfer surface area (ft2); and ∆Tlm is the log mean temperature difference (°F) calculated from the inlet and outlet
temperatures of both fluids.
The log mean temperature difference calculation depends
on the heat exchanger flow pattern. For countercurrent flow:
and for parallel flow:
where T denotes temperature, the subscripts h and c indicate
the hot and cold streams, and the subscripts o and i indicate
the inlet and outlet of the heat exchanger.
The value of U is harder to calculate, requiring
the individual heat-transfer coefficients hh and hc for
each fluid (W/m2-K), the thermal resistance of the tube
wall Rw (m2-K/W), and the fouling factors Rf for each
fluid (m2-K/W):
in commercial operation. Some fluids, such as viscous and
non-Newtonian fluids and materials containing particles,
have complex properties — thus the need for advanced heat
exchanger technologies (e.g., corrugated-tube and scrapedsurface heat exchangers).
The resistance to heat flow of the barrier between the
two fluids is an important factor that controls heat transfer.
The barrier, in effect, consists of five layers, each of which
contributes to the overall resistance. The resistance of the
inside and outside boundary layers, formed by the fluid flowing in close contact with the inside and outside surface of the
tube, can typically be estimated based on experience. The
thickness of the tube wall and the material used affect the
resistance to heat transfer, and the heat exchanger designer
will select the tube size, thickness, and materials to suit the
application. Fouling layers on the inside and outside surfaces
of the tube (which may or may not be present) depend on
both the nature of the fluids and the geometry of the heattransfer surfaces.
Increasing the speed at which fluids pass through the
exchanger creates turbulence so that the boundary layer
breaks away from the surface of the tube, creating turbulent
flow. During turbulent flow, fluid does not flow in smooth
layers but is mixed and agitated. The velocity at which turbulence occurs is influenced by many factors. The Reynolds
number is useful in quantifying flow when specifying a
heat exchanger.
Reynolds number. The Reynolds number (Re)
is the ratio of the inertial force to the viscous force:
The values of Rf for the hot and cold fluids are usually
specified by the user, while h and Rw values can be influenced directly by the designer through choice of tube size,
tube thickness, and material of construction. The values of h
depend greatly on the nature of the fluids and, crucially, the
geometry of the heat-transfer surfaces that the fluids contact,
as boundary layer interactions heavily influence the value.
where 𝜌 is the density of the fluid, V is the velocity of the
fluid, L is the length of the fluid path (e.g., the tube diameter), and µ is the viscosity of the fluid.
Reynolds numbers of less than 2,000 denote laminar
flow, while numbers above 10,000 indicate turbulent flows.
Between those two values is an area of uncertainty called the
transition zone, where turbulence may or may not be generated depending on some unpredictable factors. For example,
deforming the tube, as in the case of corrugated-tube heat
exchangers, introduces turbulent flow at Reynolds numbers
below 10,000. In practice, heat exchanger designs should
avoid transition flow regimes.
Nusselt number. The Nusselt number (Nu) is the ratio
of convective heat transfer to conductive heat transfer at a
surface or boundary:
Laminar and turbulent flow
The driving force for heat transfer in a heat exchanger
is the difference between the temperatures of the hot and
cold fluids. This simplification, however, has limited use
where k is the thermal conductivity of the fluid.
The convective heat transfer depends on the fluid characteristics and the geometry of the heat exchanger, while the
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Copyright © 2020 American Institute of Chemical Engineers (AIChE).
Not for distribution without prior written permission.
conductive heat transfer depends on the fluid properties. The
Nusselt number and Reynolds number are related to each
other, as the flow regime dictates the rate of heat transfer.
Figure 7 illustrates the relationship between Re and Nu.
As the flow regime becomes more turbulent, Nu increases.
Designs that achieve a higher Nu value enable greater heat
transfer, requiring less area to meet the heat load requirements and reducing the overall cost of the heat exchanger.
Designs that attempt to reduce the value of Re at which
turbulent flow occurs mostly increase the resistance to fluid
flow and cannot be used with fluids that contain solids or
very viscous fluids. Designs that deform the tube, such as
with a continuous shallow spiral indentation (Figure 5), can
reduce the value of Re at which turbulence occurs, and they
(a) Laminar
Corrugated Tube
Smooth Tube
Nusselt
1,000
Outlet Inlet
100
10
100
1
(b) Transition
10,000
100,000
Outlet Inlet
Corrugated Tube
Smooth Tube
Nusselt
1,000
1,000
Reynolds
100
10
100
1
(c) Turbulent
1,000
Reynolds
100,000
Outlet Inlet
Corrugated Tube
Smooth Tube
Nusselt
1,000
10,000
100
10
1
100
1,000
Reynolds
10,000
100,000
p Figure 7. (a) In a laminar flow regime, corrugated tubes have no effect on
the value of Nu. (b) While engineers attempt to avoid transition flow regimes,
corrugated tubes improve the convective heat transfer in this flow region.
(c) At Reynolds numbers above 10,000, corrugated tubes provide a marked
benefit, increasing the heat transfer greatly over that of smooth tubes.
Copyright © 2020 American Institute of Chemical Engineers (AIChE).
Not for distribution without prior written permission.
do not have these disadvantages. Adjusting the depth, angle,
and width of the indentation can decrease the Reynolds
number at which turbulent flow is produced significantly
below 10,000. When balanced correctly with other factors,
tube deformation reduces the surface area requirement and
the cost of the heat exchanger.
Fouling factors
Fouling factors account for the resistance to heat flow due
to the buildup of a layer of dirt or other fouling substance on
the tube surfaces. Fouling factors are normally specified by
the user based on experience running the plant or process, or
calculated based on an assessment of the materials involved.
Controlling fouling is critical because it can negate any
benefits of careful heat exchanger design, but fouling factors
are often overstated in an attempt to minimize the need for
cleaning. However, if the wrong fouling factor is used, cleaning may actually be required more frequently.
Fouling mechanisms vary and depend on the application,
but they can be broadly classified into four common and
readily identifiable types.
Chemical fouling occurs when chemical changes within
the fluid cause a fouling layer to deposit onto the tube surface. A common example is scaling in a kettle or boiler —
as the solubility of the salts decreases with increasing temperature, the salts deposit onto the heating element. Careful control of the tube wall temperature in contact with the
fluid can minimize scaling. When chemical fouling occurs,
it must be removed by chemical treatment or mechanical
descaling processes (e.g., wire brushes, drills, high-pressure
water jets).
Biological fouling occurs when biological organisms
in the fluid grow and deposit onto the surface of the heat
exchanger. While design has little influence on this type of
fouling, biological growth can be affected by the choice of
materials, as certain materials such as nonferrous brasses
are poisonous to some organisms. When biological fouling
occurs, it can be removed by either chemical treatment or
mechanical brushing processes.
Deposition fouling occurs when particles in the fluid
settle onto the surface because the fluid velocity is below
a critical value. This is largely within the control of the
heat exchanger designer, as the critical velocity for any
fluid and particle combination can be calculated to inform
a design that maintains a sufficient fluid velocity. Mounting a heat exchanger vertically can also minimize the effect
of deposition fouling, as gravity tends to pull the particles
away from the heat-transfer surface, even at low velocities.
Deposition fouling on surfaces can be removed by mechanical brushing.
Corrosion fouling occurs when a layer of corrosion
products builds up on the surfaces of a tube, forming an
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extra layer of material that often has a high thermal resistance. Careful choice of construction materials can minimize
corrosion. Many corrosion-resistant materials made of stainless steel and nickel-based alloys are now available.
The design process
The next step in heat exchanger design is to assess the
application to define the heat exchanger. Designing a heat
exchanger requires considering the process energy balance,
determining the heat exchanger geometry, and completing
the necessary calculations.
Energy balance. Evaluating the energy balance using
the known data for the fluids or gases to be heated or cooled
helps to define the system. Users typically define the process
fluid flowrate and the desired entry and exit temperatures,
and indicate the type of HTF to be used, as well as its
flowrate, entry temperature, and/or exit temperature. (If two
of these HTF values are known, the third parameter can
be calculated.)
Heat exchanger geometry. Design engineers define the
geometry of the heat exchanger. For a shell-and-tube heat
exchanger, they choose the shell diameter and define the
tube bundle that is placed inside the heat exchanger, including the number of tubes, the tube diameter, thickness, and
length. Next, they define the dimensions of the shellside
and tubeside fluid connections. During this phase, designers
determine the materials of construction, as this choice affects
the next step.
Thermal calculations. For a shell-and-tube exchanger, the
Nomenclature
A
= heat-transfer surface area
h
= heat-transfer coefficient
k
= fluid thermal conductivity
L
= length of the fluid path
Nu
= Nusselt number
Q
= heat-transfer rate between the hot and cold fluids
= fouling factor
Rf
= tube-wall thermal resistance
Rw
Re
= Reynolds number
T
= temperature
U
= overall heat-transfer coefficient
V
= fluid velocity
Subscripts
c
= cold fluid
h
= hot fluid
i
= inlet
o
= outlet
Greek Letters
= log mean temperature difference
DTlm
m
= fluid viscosity
r
= fluid density
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objective of this stage is to obtain the shellside and tubeside
heat-transfer coefficients, which depend on the fluid properties and the velocity of the fluid. The relationship between the
fluid properties and the heat-transfer coefficients is specific to
the heat exchanger geometry. Heat exchanger manufacturers
have their own specific formulas for use with their designs.
Once the shellside and tubeside coefficients are known, the
overall heat-transfer coefficient can be calculated by Eq. 4
and, subsequently, the total heat-transfer area by Eq. 1.
The pressure drop across the heat exchanger is another
important parameter that is calculated for the shellside and
tubeside fluids. The pressure drop is a function of Re, the
type of flow (turbulent or laminar flow), and the roughness of the shell and tubes. If the calculated pressure drop
exceeds a design maximum, then a new geometry must be
selected to reduce the pressure drop.
Mechanical calculations. The mechanical design calculations ensure that the heat exchanger design is compatible
with the design pressure and conditions. Typical calculations
for a shell-and-tube exchanger include:
• shell wall thickness
• nozzle wall thickness
• tube wall thickness
• expansion joint dimensions (to compensate for shellside and tubeside differential expansion due to temperature differences)
• tubesheet thickness.
Mechanical design calculations may produce wall thicknesses or other parameters that do not adhere to the original
geometrical design. In that case, a new geometry must
be proposed.
Drawings. The drawing package for the heat exchanger
contains the details of the various components, including the
shell, tubes, expansion joints, connections, etc.
Design advice
Many of the commonly used references for calculating and modeling heat exchanger performance are up to
80 years old and do not always reflect the most recent
science. In addition, while scientific literature provides data
for the behavior of fluids in smooth and corrugated tubes,
little published data on scraped-surface heat exchangers is
available. Heat exchanger manufacturers are one of the best
resources for the latest data to ensure correct heat exchanger
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specification and design.
ARNOLD B. KLEIJN is the sales and product development director at HRS
Heat Exchangers, where he is responsible for sales and equipment
development. His proficiency in Dutch, Spanish, English, German, and
French help him to engage with customers around the world. Kleijn
holds a degree in chemical engineering from the Univ. of Twente (Netherlands) and a degree in engineering from the IFP School in Paris.
Copyright © 2020 American Institute of Chemical Engineers (AIChE).
Not for distribution without prior written permission.