The Technical and Economic Optimization, a Fundamental Requirement of an Engineering Course

IONITA, Ion¹ & AMARFI, Rodica²

47 Domneasca Street, 6200 Galati Romania, Lower Danube University, Fax 4036-461353
¹ iionita@tmt.ugal.ro
² ramarfi@tmt.ugal.ro

Abstract: The purpose of this paper is to argue that when a teacher is presenting to his students a constructive variant of a machine part, an energetic product or a service, he can characterize completely that product or service expressing quantitatively the value of the cost/quality ratio. The authors consider that this is a fundamental engineering optimization criterion that has to be used in engineering education.

Keywords: optimization, cost, quality, improving, education

1 Introduction

Many of the courses now taught in the engineering departments of material or energetic production are treating the technical calculus of sizing of the machine parts, the thermodynamics calculus of the energetic or mass balance, the mathematical calculus of some functions variation relate to certain parameters as well the optimization of that functions in relation to the determinant parameters.

The technical optimizations can be made in relation to more parameters: minimum weight, minimum volume, minimum or maximum area, maximum efficiency, minimum fuel consumption, maximum quality etc.

In the last time, due to management of quality development, it is used with certain difficulty that's right, a very comprehensive criterion of engineering optimization: the cost/quality ratio (CQR):

(1)

This criterion expresses the cost C of the achievement of a certain quality product Q. For a constant value, the criterion CQR shows that when a beneficiary wants to purchase a higher quality product, he must be ready to pay more for that, or conversely.

For the same guaranteed quality, is more competitive the lower cost product that is having less CQR.

The aim of this paper is to show that any engineering course must be structured on idea to follow and to obtain a minimum CQR value for a material or energetic product of certain quality.

Let us say something more about the two parts of the CQR, cost and quality.

2 The cost of a product or of a service

The cost of a product or of a service is obtained by well-known procedures of economics. It has three main components:

• the raw material cost, which is the fuel in the case of energetic product;
• the invested capital;
• operation and maintenance cost.

When a teacher is comparing in front of the students two or more variants of product or service, he must to expose as well the relative weight of those three components in the total cost.

To estimate a cost of a product or of a service we have at our disposal many very good works in the economics field. Among them, we prefer to use the scheme made by Rodwey D. Stewart (Mobile Data Services, Huntsville, Alabama USA) [2] and showed here in the figure 1.

Figure 1. Anatomy of an estimate

According him, the total cost can be estimated as follows:

	(2)
	(3)

where L 1, L 2, ... L N are various labor rate categories

Symbols

Subscripts

3 The quality of a product or of a service

All the human society needs are results of the activity of its members, results that can be classify in products and services.

The products are obtained as result of some processes of manufacturing and transforming of the raw materials, of soil remaking and animals growing, of information processing, as results of talents and creative skills. By their nature they can be classified in the following categories of products: agricultural, industrial, scientific, artistic and cultural.

The services are activities useful for society, but they have not as result the products obtaining.

The services field covers very large area of activities as: trade, tourism, education, public health, the popularisation of science, art and culture, country defence, state administration, maintenance and repairing of the private and public goods etc.

The quality is an essential feature of the products and services. According to STAS ISO 8402 from 1991, the quality represents the aggregate of properties and characteristics of a product or of a service that confers to it the aptitude to satisfy the expressed or implicit needs.

Therefore the quality of a product or service is not induced only by the characteristics and properties of it, but also by the measure that it is satisfying the needs expressed by the user or by the beneficiary, as well as other wants that are not stipulated but must be carried out.

The available space here is allowing us only to present the list of the 30 quality characteristics of a product, material or energetic (Tab.1)

Tab.1. Quality characteristics of a product

and the list of the 15 quality characteristics of a service (Tab.2)

Tab.2. Quality characteristics of a service

In the material production and in the area of the services it is no so easy to express numerically the quality, that is defined by some of the characteristics listed in Tab.1 or Tab.2.

However, there is a field in which the product quality one can express numerically and directly: in thermodynamics one expresses the energy quality by its exergy [kWh], the available part of the energy [1].

To define analytically the exergy, we will use the definition and then the notions provided by [1].

"When one of the two systems is a suitable idealised system called an environment and the other is some system of interest, exergy is the maximum theoretical useful work (shaft work or electrical work) obtainable as the systems interact to equilibrium, heat transfer occurring with the environment only. Alternatively, exergy is the minimum theoretical useful work required to form a quality of matter from substances present in the environment and to bring the matter to a specified state".

The first definition allows to optimise the thermal systems working, their aim being the heat or work producing.

The second one allows approaching the material producing of the all goods in a thermodynamically manner. Using the thermodynamics as analytical fundamentals of the material production, the exergetic - based approaching allows to optimise the manufacturing process. Now this way records the first starts [5].

It is the time to say some analytical notions about the exergy [1]. This extensive property expressed on a unit-of-mass or molar basis has four components: physical exergy e^ph, kinetic exergy e^c, potential exergy e^p and chemical exergy e^ch:

(4)

The kinetic and potential energies of a system are fully convertible to work, as:

	(5)
	(6)

For a closed system, the physical exergy at a specified state is given by:

(7)

The properties u, v, s denote the internal specific energy, the specific volume and the entropy of the system at a specified state. The values u_o, v_o, s_o, p_o and T_o are for the system at restricted dead state (e.g. environment).

For the total exergy transfer associated with a substance flow we can write:

(8)

For the chemical exergy e^ch determination there are several proposals [6]. With the equations (5), (6), (7) and (8), the equation (4) becomes:

- for a closed system

(9)

- for the total exergy transfer associated with a substance flow:

(10)

Let us notice that the two last terms and gz exist in the Bernoulli's equation from fluid mechanics, as a confirmation of the exergy consideration in the first law of thermodynamics.

The exergy of the heat Q is:

(11)

The anergy of the heat Q is:

(12)

Thus

(13)

Symbols:

4 Closure

Using the notions above mentioned in section 3, the size of the cost/quality ratio (CQR) in energetics is [$/kWh] for electricity and [$/kJ] for thermal energy. We can easy to recognise the procedures now frequently used in the payment of the electric or thermal energy.

By authors' opinion, any engineering course have to characterize a product, material or energetic one, by CQR value, a general optimization criteria, obtained by combination of both technical and economic analysis.

When a professor is telling to his students about a constructive variant of a machine part, of a whole machine or about an energetic product, he can characterize completely that product, expressing quantitatively the value of the CQR, this fundamental engineering optimization criterion.

References

1. Bejan, A., Tsatsaronis,G., Moran,M. Thermal Design and Optimization. John Wiley & Sons, Inc.New York, Chichester, Brisbane, Toronto, Singapore.

2. Mechanical Engineers'Handbook. John Wiley & Sons, Inc.New York, Chichester, Weinheim, Brisbane, Toronto, Singapore, (1998). ISBN 0-471-13007-9.

3. IONITA, I.C. Romanian university project of exergoeconomic analysis development. Efficiency, Cost, Optimisation, Simulation and Environmental Aspects of Energy Systems and Processes Congress ECOS'98, Nancy, France, Proceedings,vol.II, pp.1133-1141 ISBN 2-905-267-29-1.

4. Gonzales Barajas, A., Cananau, N., Dima, O. & Gurau, Gh. Sisteme de asigurare a calitatii (Quality Assurance Systems) Junimea (1998), Iasi, Romania, ISBN 973-37-0347-8.

5. Creyts, J.C., Carey, V.P. Use of Extended Exergy Analysis as a Tool for Assessment of the Environmental Impact of Industrial Processes. AES vol.37, Proceedings of the ASME Advanced Energy Systems Division ASME 1997.

6. ION V.I., IONITA, C.I. & UZUNEANU, K. Some ways to estimate the chemical exergy of the organic fuels. Power Engineering 1998 Maribor, Slovenia, Proceedings, vol.B, pp.237-245.