47
2
Basic Considerations
in Design
The important terms that arise in the design and optimization of thermal systems
have been dened and discussed in the preceding chapter. We are concerned with
thermal systems that are governed by considerations of uid ow, thermodynam-
ics, and heat and mass transfer. The interaction between the various components
and subsystems that constitute a given system is an important element in the
design because the emphasis is on the overall system. Additional considerations,
that may not have a thermal or even a technical basis, also have to be included in
most cases for a realistic and successful design. Though selection of components
or devices may be employed as part of system design, the focus is on design and
not on selection. Similarly, analysis is used only as a means for obtaining the
inputs needed for design and for evaluating different designs, not for providing
detailed information and understanding of thermal processes and systems. The
synthesis of information from a variety of sources plays an important part in the
development of an acceptable design. With this background and understanding,
we can now proceed to the basic considerations that arise in the design process.
2.1 FORMULATION OF THE DESIGN PROBLEM
A very important aspect in design, as in other engineering activities, is the formu-
lation of the problem. We must determine what is required of the system, what is
given or xed, and what may be varied to obtain a satisfactory design. The nal
design obtained must meet all the requirements, while satisfying any constraints
or limitations due to safety, environmental, economic, material, and other consid-
erations. The design process depends on the problem statement, as does the evalu-
ation of the design. In addition, the formulation of the problem allows us to focus
our attention on the quantities and parameters that may be varied in the system.
This gives the scope of the design problem, ranging from relatively simple cases
where only a few quantities can be varied to more complicated cases where most
of the parameters are variable.
T
o
ΔT a T a T
o
ΔT (2.2)
where o ΔT is the acceptable variation in the outow temperature.
In the design of thermal systems, common requirements concern tempera-
ture distributions and variations with time, heat transfer rates, temperature lev-
els, and ow rates. Total pressure rise, time needed for a given process, total
energy transfer, power delivered, rotational speed generated, etc., may also be
the desired outputs from a thermal system, depending on the particular applica-
tion under consideration. Consider the thermal annealing process for materials
such as steel and aluminum. The material is heated to a given elevated tempera-
ture, known as the annealing temperature; held at this temperature level for
a specied time, as obtained from metallurgical considerations of the chosen
material; and then cooled very gradually, as shown in Figure 2.1. By heating
Time
Envelope of acceptable
temperature variation
Desired temperature
variation
CoolingSoakingHeating
Annealing temperature
Temperature
FIGURE 2.1 Required temperature variation, with an envelope of acceptable variation,
for the thermal process of annealing of a given material.
Basic Considerations in Design 49
the material beyond a particular temperature T
o
, known as its recrystallization
o
, T
o
, and B are specied constants, obtained from the basic charac-
teristics of the given material. The acceptable variations in these constants,
often given as percentages of the desired values, may also be included in these
equations. Then, a thermal system is to be designed so that the given mate-
rial or body is subjected to the required temperature cycle, with the allowable
tolerance.
Similarly, the requirements for other thermal systems outlined in Chapter 1
may be considered. For instance, the mass ow rate, as well as the tempera-
ture and pressure at the inlet to the die in the plastic extrusion process, shown
in Figure 1.10(b), are the requirements for a screw extruder. The rate of heat
removal and the lowest temperature that can be obtained in the freezer could
be taken as the requirements for a refrigeration system. The maximum power
delivered and speed attained could be the requirements for a transportation sys-
tem. The energy removal rate and the maximum allowable temperature of the
electronic devices may be the requirements for a cooling system for electronic
equipment.
It is critical to determine the main requirements of the system and to focus
our efforts on satisfying these. Since it is often difcult to meet all the desired
features of the system, requirements that are not particularly important for the
chosen application may have to be ignored. It is best to rst satisfy the most essen-
tial requirements and then attempt to satisfy other less important ones by varying
the design within the specied constraints and limitations. For instance, after a
refrigeration system has been designed to provide the specied temperature and
heat removal rate, effort may be exerted to nd a substitute for the refrigerants
R-11 and R-12, both of which are chlorouorocarbons, or CFCs; to replace the
compressor with one that is more efcient; to vary the dimensions of the freezer;
or to improve the temperature control arrangement. Thus, it is important to rec-
uid ow considerations (Tadmor and Gogos, 1979). It is an extensively used
manufacturing process for a variety of parts ranging from plastic cups and toys
to bathtubs, car bumpers, and molded parts made of composite materials. As
shown here, the polymer is melted and injected into a mold cavity by applying
force on the melt by means of a plunger or a rotating screw. As the polymer starts
to solidify, additional amounts of melt may be injected to ll the gaps left due
to shrinkage during solidication. The mold is held together by a clamping unit,
which opens and closes the mold and also ejects the nal solidied product.
For system design, the mold and the injected material may be kept xed, while
the melting and injection processes are varied. Similarly, the mold, as well as the
material, may be varied while keeping the rest xed. The system is a complicated
one, but it can be considerably simplied by keeping several components and fea-
tures xed while a few components, such as the injection mechanism, are varied
during design. In addition, the basic concept may be kept unchanged, using, for
instance, either of the two schemes shown in the gure. Other approaches to melt
Basic Considerations in Design 51
and inject the mold, as well as to clamp and open the mold, are also possible. All
these considerations substantially inuence the design process.
Similarly, in the design of an electronic system, consisting of electronic com-
ponents located on circuit boards, the electronic component size, the geometry
and dimensions of the board, the number of electronic components on each board,
and the distance between two boards may be given. The design then focuses on
the cooling system, such as a fan and duct arrangement. A two-stroke engine may
be chosen for the design of a transportation system, thus xing the basic concept.
In a solar energy system, sensible heat storage in water may be chosen as the
concept, with the dimensions, geometry, and material of the tank being varied
for the design. In the design of a cooling pond for a power plant, the location of
the pond, which determines the local ambient conditions, is xed. In all of these
cases, some of which are considered in later chapters, the given quantities are kept
unchanged during the design process.
leads to a domain of acceptable designs. An appropriate design may be chosen
based on additional considerations such as cost, power requirements, size, etc.
If the other components of the system, such as geometry and dimensions of the
melting and injection section, are to be varied as well, the design becomes much
more involved and the domain of acceptable designs is much larger.
The design variables are usually taken to represent the hardware of the
system such as the plunger, heating arrangement, mold, clamping unit, cooling
channels, and so on, in the above example. However, the system performance
is also affected by the operating conditions, which can be adjusted over ranges
determined by the hardware. Therefore, the variables in the design problem
may be classied as:
Hardware
This includes the components of the system, dimensions, materials, geometrical
conguration, and other quantities that constitute the hardware of the system.
Varying these parameters generally entails changes in the fabrication and assem-
bly of the system. As such, changes in the hardware are not easy to implement
if existing systems are to be modied for a new design, for a new product, or for
optimization.
Operating Conditions
These refer to quantities that can often be varied relatively easily, over specied
ranges, without changing the hardware of the given system, such as the settings
for temperature, ow rate, pressure, speed, power input, etc. The design process
would generally yield the ranges for such parameters, with optimization indicat-
ing the values at which the performance is optimal.
The design of a thermal system must include both types of variables and the
nal design obtained must indicate the materials, dimensions, and congurations
of the various components, as well as the ranges over which the operating condi-
tions such as pressure, temperature, and ow rate may be varied. These ranges
Basic Considerations in Design 53
are xed by the hardware design; for instance, the temperature range may be
All of these operating conditions can be varied over ranges that are determined
by the hardware design of the system. In addition, in actual practice these may not
be varied completely independent of each other. For instance, the screw geometry
and dimensions, along with the speed, will determine the maximum ow rate in the
extruder. The heating/cooling arrangement determines the range of temperature
variation. The plastic or polymer used may limit the speed or the temperature level,
and so on.
2.1.4 CONSTRAINTS OR LIMITATIONS
The design must also satisfy various constraints or limitations in order to be
acceptable. These constraints generally arise due to material, weight, cost, avail-
ability, and space limitations. The maximum pressure and temperature to which
54 Design and Optimization of Thermal Systems
a given component may be subjected are limited by the properties of its material.
For instance, a plastic or metal component may be damaged if the temperature
exceeds the melting point. The performance of semiconductor devices is very
sensitive to the temperature and, therefore, the temperatures in electronic equip-
ment are constrained to values less than 100nC. The pressure rise in a thermal
system is constrained by the strength of the materials at the operating temperature
levels. Such constraints may be written for temperature T, pressure P, and volume
ow rate R as
T a T
max
, P a P
max
, R a R
max
(2.4)
Generally, the maximum values, indicated here by the subscript max, would be
considerably less than levels at which permanent damage to the component or sys-
tem might occur. Therefore, T
2
across a set
of rollers, as shown in Figure1.10(d), mass conservation leads to the equation
U
1
D
1
U
2
D
2
, where U
1
is the speed before the rollers and U
2
after, if the density
of the material remains unchanged. Then this equation serves as a constraint on
the speed after the rollers if the remaining quantities are specied.
Similarly, the energy rejected Q
rejected
from a power plant to a cooling pond is
mC
p
ΔT, where
m is the mass ow rate of the cooling water, ΔT is its temperature
rise in going through the condensers, and C
p
is the specic heat. This energy must
well as on the total ow rate (Moore and Jaluria, 1972).
2.1.5 ADDITIONAL CONSIDERATIONS
Several additional considerations have to be taken into account for obtaining an
acceptable or workable design. These considerations may arise from safety and
environmental concerns, procurement of supplies needed, availability of raw
materials, national interests, import and export concerns, waste disposal problems,
nancial aspects, existing technology, and so on. Many of these aspects affect
the overall engineering enterprise, as discussed earlier in Chapter 1. However,
the design itself may be strongly inuenced by these considerations, particularly
those pertaining to the environmental and safety issues. For instance, even though
nuclear energy is one of the cheapest and cleanest methods of generating electric-
ity, concerns on radioactive releases have strongly curbed the growth of nuclear
power systems. Systems are designed in the steel industry to use the hot combus-
tion products from the blast furnace in order to reduce the discharge of pollutants
and thermal energy into the environment, while also decreasing the overall energy
input. Thermal pollution concerns could make it undesirable to depend only on a
lake or river for discharge of thermal energy from a power plant, making it neces-
sary to design additional systems such as cooling towers for heat disposal.
Disposal of solid waste, particularly hazardous waste from chemical plants and
radioactive waste from nuclear facilities, is another very important consideration that
could substantially affect the design of the system. The energy source is chosen in
order to meet the federal or state guidelines for solid waste disposal. Adequate arrange-
ments have to be included in the design to satisfy waste disposal requirements.
Safety concerns, particularly with nuclear facilities, demand that adequate
safety features be built into the system. For instance, if the temperature or heat
ux levels exceed safe values, the system must shut down. If the uid level were
too low in a boiler, a safety feature would not allow it to be turned on, thus
avoiding damage to the heaters and keeping the operation safe. Similarly, the
energy source may be changed from gas to electricity because of safety concerns
in an industrial system. An oil furnace may be developed instead of a gas furnace
(22 – 5nC to 22 5nC), which is to be maintained in the house. No constraints are
given in the problem. However, typical constraints will involve limitations on the
size and volume of the system, on the ow rate of air circulating in the building,
and on the total cost. Use of chlorouorocarbons (CFCs) as refrigerants may be
unacceptable due to environmental considerations.
The thermal load due to heat transfer to the house from the ambient must be
determined. This load will involve absorbed solar ux, back radiation to the envi-
ronment, convective transport from ambient air, evaporation or condensation of
moisture, and conductive energy loss to the ground. The ambient thermal load is
a function of ambient conditions, geometry of the building, its geographical loca-
tion, and dimensions. It can often be modeled as hAΔT, where h is the overall heat
transfer coefcient, A is the total surface area, and ΔT is the temperature difference
between the ambient and the house. The total thermal load Q is then the ambient
load plus the rate of energy dissipated in the building. The rate of heat removal Q
r
by the thermal system shown in Figure 2.3 must be greater than this total load.
Basic Considerations in Design 57
The transient cooling of the building is also an important consideration. If the
total thermal capacity of the building (mass X specic heat) is estimated as S, then
its average temperature T is governed by the energy balance equation
S
dT
dT
Q – Q
r
From this equation, the time T
r
needed to cool the building to 1/e of its initial tem-
perature difference from the ambient, i.e., the characteristic response time, may be
calculated, as discussed in Chapter 3. If this time is posed as a requirement, the heat
FIGURE 2.3 A thermal system for air conditioning a house.
58 Design and Optimization of Thermal Systems
2.2 CONCEPTUAL DESIGN
At the very core of any design activity lies the basic concept for the process or the
system. The design effort starts with the selection of a conceptual design, which is
initially expressed in vague terms as a method that might satisfy the given require-
ments and constraints. As the design proceeds, the concept becomes better dened.
Conceptual design is a creative process, though it may range from something inno-
vative, representing an invention or a new approach not employed before, to modi-
cations in existing systems. Inventions may lead to patents, as discussed later.
Creativity, originality, experience, knowledge of existing systems, and information
on current technology play a large part in coming up with the conceptual design.
For instance, microprocessors, laser-Doppler velocimeters, ultrasonic probes,
composite materials, iPod, digital cameras, and liquid crystals represent some of
the innovative ideas introduced in recent years. Solutions based on existing and
developing technology can also lead to valuable conceptual designs such as those
of interest in computer workstations, automobile fuel injection systems, hybrid
cars, and solar power stations. Changes can be made in existing systems to meet
the given need or opportunity. In fact, much of the present design and development
effort is based on improvements in current processes and systems.
so on. But the concepts that appear to have promise must be considered further to
determine if it is possible to develop a successful design based on them.
It is not easy to teach someone how to be creative and innovative. In most
cases, creativity is a natural talent and some people tend to be more original than
others. There are no set rules that one might follow to become creative. However,
experience with current technology and knowledge of systems being used for
applications similar to the one under consideration are a big help in the search for
a suitable conceptual design. In addition, it is necessary to provide an environ-
ment that is open to new ideas. Creative problem solving requires imaginative
thinking, persistence, acceptance of all ideas from different sources, and con-
structive criticism. Several such methods that may help to develop creative think-
ing are discussed by Alger and Hays (1964) and by Lumsdaine and Lumsdaine
(1995). Techniques such as brainstorming, where a group of people collectively
try to generate a variety of ideas to solve a given problem, design contests, and
awards to employees with the best ideas also promote the generation of innovative
solutions. Many impressive designs, such as the Vietnam Veterans Memorial in
Washington, D.C., have arisen from design competitions.
An Example
In the manufacture of electronic systems, a classical process that is frequently
used is that of soldering a pin to a board. Solid solder is placed around the pin in
the form of a doughnut, as shown in Figure 2.4, and heated to beyond its melt-
ing point. The molten solder is driven by surface tension forces to form a joint,
which solidies on cooling to give the desired connection between the pin and
the copper plated through hole in the board. The heating had traditionally been
done by radiation or by convection, using air or a liquid for immersion. Excessive
and nonuniform heating of the boards was a common problem with radiation.
Cleaning of the uid and low heat transfer rates were the concerns with convec-
tion. In response to the need for an improved technique for this problem, a new
and innovative method based on condensation of a vapor was proposed to yield a
rapid heat transfer rate, while ensuring a clean environment with no overheating
Condensing coils
Condensation
interface
Trough
Valve
Condensate
(b)(a)
Vapor
Boiling sump
Heater
Condensed
fluorocarbon
FIGURE 2.4 (a) Solder ow for forming a bond between a pin, or terminal, and a plated
through hole. (b) Schematic of a condensation soldering facility for electronic circuitry
manufacture.
Basic Considerations in Design 61
another area that has beneted from many new and original ideas that arose in
the last three decades in response to the many challenging problems encoun-
tered due to, for example, high temperature, pressure, and velocity during rocket
launching and re-entry. The space program has led to many signicant advances
Surge tank
Moisture condensing
coils
Secondary condensing
coils
Secondary vapor
zone
Primary vapor zone
Boiling fluid
Filtration system
been tried. In a given industry, the ideas that have been tried in the past to solve
problems similar to the one under consideration are well known. Existing litera-
ture can also be used to generate additional information on various concepts and
solutions that have been previously employed. The conceptual design for a given
problem may then be selected from the list of earlier concepts or developed on
the basis of this information. In this case, only the basic concept is similar to the
earlier concepts; the system design may be quite different.
Let us consider the problem of cooling of electronic equipment. If forced convec-
tive cooling is to be employed for a given electronic circuitry, the extensive information
available in the literature on these cooling systems may be used to select or develop
the conceptual design. Figure 2.7 shows the schematics of some of the arrangements
Ventilation
port
Soldered
PCB
Product
output and
cooling
Condensing
coil
Condensing
surfaces
Heating
elements
Stainless
steel tank
Boiling liquid
fluorocarbon
Ventilation
port
Tu be a x ia l
Vane axial
Centrifugal
(squirrel
cage)
Centraxial
Blower
Air
Forced convection
cooling
Positive
displacement
Propeller
Radial wheel
Fan
Centrifugal
Backward-
curved
blades
Forward-
curved
blades
Radial
blades
Backward-
curved
blades
Forward-
curved
blades
because the effort involved is relatively small and because changes in the design
of current systems can often lead to the desired result. Many thermal systems in
use today have evolved through such modications through the years.
Let us consider a few examples where modications in the design of exist-
ing systems may lead to viable conceptual designs. The Rankine cycle is the
basic thermodynamic cycle used for steam power plants. However, the desire to
improve the overall thermal efciency of the system has led to many modica-
tions. Some of the variations in the conceptual design that may be mentioned are
those related to superheating of the vapor leaving the boiler, reheating the steam
passing through the boiler, and regenerative heating of the working uid using
stored energy from an earlier process in the system (Cengel and Boles, 2002). All
of these are different conceptual designs based on an existing system design.
Another example is provided by the plastic screw extrusion process, shown
schematically earlier in Figure 1.10(b). Though electric heaters are generally used,
water or steam circulating in jackets, as shown in Figure 2.8, may also be used to
avoid possible overheating and for better temperature control and higher thermal
efciency. Different jackets may be used to impose a temperature variation along
the axis of the extruder. In a screw extruder, considerable variation in the product
Basic Considerations in Design 65
FIGURE 2.8
Schematic of a single screw extruder heated or cooled by the ow of steam or water in jackets
at the extruder barrel.
satisfactory solution, different available concepts would be considered to develop a
FIGURE 2.9 A practical plastics/food extrusion system. (From Center of Advanced Food
Technology, Rutgers University, New Jersey. With permission.)
Basic Considerations in Design 67
conceptual design for the given problem. If even this does not work, new approaches
and techniques will have to be considered. This may lead to new and original con-
ceptual designs. The conceptual design is then subjected to the detailed, quantita-
tive design process, as outlined in the next section, in order to obtain an acceptable
design that satises the given requirements and constraints. Obviously, there are
circumstances where a satisfactory solution to the given problem is not obtained.
Then the problem statement may be examined again or the project terminated.
Example 2.3
For the soldering problem sketched in Figure 2.4, consider different heating strate-
gies to obtain a conceptual design for the condensation process.
Solution
The basic problem under consideration involves heating the solid solder preform so
that it melts and ows under the action of surface tension, gravitational and viscous
forces to yield the solder llet that joins the pin or terminal with the copper-plated
hole and thus with the printed circuit board. The llet solidies on cooling to yield
the desired bond. Figure 2.10 shows the typical variation of the solder temperature
8. Aging
7. Further cooling to room temperature
6. Solidification of solder
5. Initial cooldown
4. Solder flow and approach to equilibrium
3. Further heating and flux action
1. Initial heating
123
Temperature
4 56 7
transfer coefcient for natural convection in air or gases is extremely small. This
is undesirable unless the uid temperature is taken very large to obtain high heat
transfer rates. If this is done, the materials may overheat and be damaged. Forced
convection has higher heat transfer coefcients than natural convection. However,
forced ow is strongly geometry-dependent and can lead to uneven heating due
to separation and wakes, as shown in Figure 2.11(a). In addition, it will affect the
shape of the solder llet by exerting drag on the molten solder.
Natural convection using a liquid is attractive because it has reasonably high heat
transfer coefcients and provides uniform heating. However, immersion in a liquid
has the problem of accumulation of impurities, dust particles, and other undesirable
deposits. Therefore, cleaning is a major concern in this case. Radiation provides a
clean environment, but the heat ux absorbed is a strong function of the geometry and
(b)(a)
Board
Solder
preform
Pin
Flow
ermal
radiation
Mask
Board
Solder preform
Pins
FIGURE 2.11 Heating of the solid solder preform by (a) forced convection and (b) ther-
mal radiation.
Basic Considerations in Design 69
the surface properties of the material. Therefore, overheating is commonly encoun-
tered when radiation is used to heat the preform. Radiation masks, as shown in
Figure 2.11(b), are generally needed to avoid overheating. Different masks are required
Condensate
Condensation
region
Electronic
part
Boiler
Vapor
Boiling liquid
Opening
Vapor
FIGURE 2.12 A possible conceptual design for a condensation soldering facility.
70 Design and Optimization of Thermal Systems
The systems shown in Figure 2.4(b) and Figure 2.6 are other conceptual designs.
In these cases, the boiling liquid sump and the condensing vapor region are located
in the same container. Condensing coils, which are cooled by circulating cold water,
condense the vapor and generate a vapor region. If a part is immersed in this region,
the vapor condenses on it and thus heats it at the desirable high heat transfer rates.
Though the vapor region is physically contained in Figure 2.6, it is not contained
in Figure 2.4(a), resulting in greater uid loss in this design. The part to be heated
passes through the top as well. However, the interface generated at the top reduces
the uid loss. Additional mechanisms to minimize uid loss can also be devised
because the uid is generally quite expensive. Again, the conceptual design is not
unique and several other solutions and systems are possible.
2.3 STEPS IN THE DESIGN PROCESS
The conceptual design yields the basic approach and the general features of the sys-
tem. These form the basis of the subsequent quantitative design process. The start-
ing or initial design is then specied in terms of the conguration of the system,
the given quantities from the problem statement, and an appropriate selection of the
design variables. This initial selection of the design variables is based on informa-
tion available from other similar designs, on current engineering practice, and on
7. Automation and control
8. Communicating the nal design
Figure 2.14 shows a schematic of these different steps in the design and opti-
mization of a system. The iterative process to obtain an acceptable design by
varying the design variables is indicated by the feedback loop connecting simu-
lation, design evaluation, and acceptable design. There is a feedback between
simulation and modeling as well in order to improve the model representation of
the physical system based on observed behavior and characteristics of the sys-
tem, as obtained from simulation. Optimization of the system is undertaken after
acceptable designs have been obtained. Automation and control are important
for the satisfactory and safe performance of the given system. The results from
the detailed design and optimization process are nally communicated to groups
involved with the fabrication, sales, and marketing. The basic considerations
Conceptual
design
Initial design
Modeling
and simulation
Evaluation
No
Yes
Acceptable?
Solution
Iterative redesign
FIGURE 2.13 Iterative process to obtain an acceptable design.
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