Design and Optimization of Thermal Systems Episode 1 Part 4 pps

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2 Basic Considerations

in Design

The important terms that arise in the design and optimization of thermal systems

have been defined and discussed in the preceding chapter. We are concerned with

thermal systems that are governed by considerations of fluid flow, 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 fixed, and what may be varied to obtain a satisfactory design. The final

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.

2.1.1 REQUIREMENTS AND SPECIFICATIONS

Certainly the most important consideration in any design is the desired function

or task to be performed by the system. This may be given in terms of require￾ments to be met by the system. A successful, feasible, or acceptable design must

satisfy these. The requirements form the basis for the design and for the evalu￾ation of different designs. Therefore, it is necessary to express the requirements

48 Design and Optimization of Thermal Systems

quantitatively and to determine the permitted variation, or tolerance level. Sup￾pose a water flow system is needed to obtain a specified volume flow rate Ro.

Since there may be variations in the operating conditions that may result in

changes in the flow rate R, it is essential to determine the possible increase or

decrease in the flow rate that can be tolerated. Then the system is designed to

deliver the desired flow rate Ro with a possible maximum variation of o ΔR. This

may be expressed quantitatively as

Ro ΔR a R a Ro  ΔR (2.1)

If a water cooler is being designed, the flow rate Ro and the desired temperature

To at the outflow become the requirements. The former is expressed as given in

Equation (2.1) and the latter as

To ΔT a T a To  ΔT (2.2)

where o ΔT is the acceptable variation in the outflow temperature.

In the design of thermal systems, common requirements concern tempera￾ture distributions and variations with time, heat transfer rates, temperature lev￾els, and flow 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 specified 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

Heating Soaking Cooling

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.