Giuseppe Rubatto

Thermodynamics applied to oxygen enrichment of combustion air

Abstract In order to reduce overall fuel consumption, or to partially substitute a “valuable” fuel with a poor one, in industrial heating, oxygen enrichment of combustion air can be very effective. For the second option, a general criterion is stated in this paper for examining the suitability of oxygen enrichment in single cases. The topic is particularly interesting, as for the first time, it is now feasible to produce oxygen enriched air using permeable membranes on a commercial scale and with costs that are remarkably lower than those of other existing techniques. In this paper, the subject is investigated after some remarks about the definition of the “usable exergy” parameter, which was already proposed in previous papers by one of the authors and is here utilized for the above criterion.

Heat transfer, energy saving and pollution control in UHP electric-arc furnaces

The use of hot-water and evaporative cooling enables the residual energy potential of iron and steel works to be utilized. In particular, water-traversed wall elements are used at present in the top zone of an electric-arc furnace. Indeed, the remarkable increase in the electrical power has made the cooling of the wall of the furnace compulsory. Thus, it seems suitable to use the energy recovered in the cooling elements, instead of transferring it to the atmosphere via closed cooling towers. Nucleate boiling leads to a substantially more intensive cooling, but even a limited increase in the input heat flow can be dangerous. If the slag accretions, temperature and thermal power are examined in cases of steady state furnace operation and completely slag-covered arcs, it is found that the thermal losses decrease remarkably as the melting temperature of the slag adhering to the furnace shell increases. The aim of this paper is, first of all, an analysis of heat transfer phenomena in UHP (ultra high power) electric-arc furnaces. Then, energy and exergy analyses are dealt with after some remarks about exergy efficiency and chemical exergy of elements. In addition, means to reduce noise, air and water pollution are examined.

Energy saving and some environment improvements in coke-oven plants

The enthalpy of inlet coal and fuel gas is discharged from a coke-oven plant in the following forms: chemical and thermal enthalpy of incandescent coke, chemical and thermal enthalpy of coke-oven gas, thermal enthalpy of combustion exhaust gas, and waste heat from the body of the coke oven. In recent years the recovery of several kinds of waste energy from coke ovens has been promoted mainly for energy saving purposes, but also for the improvement of environmental conditions. Among the various devices yet realized, the substitution of the conventional wet quenching method with a coke dry cooling is the most technically and economically convenient. The aim of this paper is mainly a review of the main types of coke dry cooling plants and a detailed examination of the influence of some parameters, particularly of temperature and pressure of the produced steam, and on the exergy efficiency of these plants.

Magnetic systems depending on three or two variables; thermodynamic analysis and some existing and possible applications

Until quite recently, the theory of continuous media dealt mainly with diamagnetic and paramagnetic fluids. The researchers did not consider strongly magnetizable fluids, probably because such media had not yet been discovered in natural conditions and did not exist until recently as manufactured products. However, in the early 1960 s, magnetic fluids (MF) could be synthetically prepared, and then, technological interest in MF grew rapidly. Besides, magnetic refrigeration has been used for over 60 years as a technology to achieve temperatures below 1 K, employing the magnetocaloric effect of some solid materials. However, in the last 20 years, the technology has been developing for refrigeration applications above 1 K up to and including heat pumps above room temperature. The work has been multinational in scope and has focussed on the analysis of magnetic thermodynamic systems, investigation of magnetic materials suitable for refrigerants and development of prototype refrigerators. After a synthetic review of the basic properties of MF, of magnetic refrigerators and heat pumps and of magnetic properties of living matter, the aim of this paper is a thermodynamic examination of these systems in comparison with the relations usually applied to fluids. Three independent variables are generally considered, whereas usually only two variables have been considered until now for what is known to the authors. Also, in the case of only two independent variables, some results are obtained which are original. Furthermore, some applications of MF are taken into account, in particular, possibilities of converting thermal into mechanical energy are examined.

Process improvements in iron and steel industry by analysis of heat and mass transfer

Abstract The purpose of this paper is, firstly, a critical review of the recent literature about heat and fluid flow and mixing phenomena for some main processes in the iron and steel industry with the aim of their improvement and, thus, cost reduction. The criteria for bubbling and jetting during gas injection are examined, including interaction of the powder load on flow regimes. It is important to know the domains of bubbling and jetting behavior in order to minimize the effect of back attack, which is responsible for additional wear of bottom lines and tuyere pipes. It is shown that two types of models can describe the mixing process: the turbulent recirculation model and the model of partial volumes. The comparison between calculated time dependencies and plant results confirmed the practicability and usefulness of the model of partial volumes. It was also shown that the water model results for ladles might be transferred to plant conditions. Subsequently, some remarks are made about “generalized thermodynamics” and synergetics and their applications to mass transfer in the steel industry. As an example, the existence of considerable oxidation gradients and diffusion streams of oxygen can cause pumping of impurities in the absence of convection, i.e. under conditions of pure diffusion. The above concepts have been applied in various cases of the iron and steel industry, in particular for optimization of ingot solidification at continuous casting.

Thermodynamics of dielectric systems depending upon three variables

Abstract In a previous paper, a thermodynamic examination of magnetic fluids was conducted and the obtained relations were compared with the relations usually applied to fluids. Three independent variables were considered in the general formulation. The aim of this paper is to examine, in a similar way, dielectric systems (gases, liquids and solids) considering three independent variables in the general case. The independent variables can be assumed as only two, when the volume is practically constant. In some ranges of temperature, various dielectrics present a dependence of the electric field strength upon temperature, similar to that of magnetization (Curie’s law); this case also is studied. Quasi-static irreversible processes with uniform intensive properties are considered.

Work and entropy production aspects of irreversible processes in closed and steady-state open systems

Abstract This paper systematically deals with the influence of different sources of irreversibilities in work transfer expressions for closed and steady-state open systems. In the various cases, the hypotheses necessary to an analytical formulation and therefore the limits of the latter are specified. The influence of the non-uniformity of temperature and pressure on the irreversibilities in the system is particularly examined. The main aim is that of deducing a criterion with the conditions sufficient to separate in distinct relations the different phenomena (mechanical, electrical, chemical, etc.) simultaneously present in the same system.

Thermodynamic analysis of elastic systems

Abstract The purpose of this paper is firstly a general analysis of piezoelectric elastic systems with constant volume, to which one can apply an equation of the kind f ( L , Φ , q , T )=0 with three independent variables. Subsequently, one deals with a more detailed analysis of elastic systems without piezoelectric effects with two independent variables in an equation of the kind f ( L , Φ , T )=0 analogous to the relation f ( p , v , T )=0 that is valid for fluid systems. In addition, a system with variable volume and, thus, depending on four independent state functions is synthetically examined.

Thermodynamic analysis of heat transformers employing thermal sources and sinks of finite capacity

The problem of using low-level energies is a general one, in the light of the amount of thermal waste in numerous sectors. It is important both in the utilization of new energy sources available in thermal form (e.g., low enthalpy geothermal and solar energies) and in large-size works. Upgrading low-level energy can be achieved either by means of absorption and resorption cycles or by heat transformers or by their combinations. In this paper, after a definition of heat transformers, all the types of existing or thermodynamically possible heat transformers are considered by means of a suitable diagram; besides, the relative entropy production and the exergy efficiency, defined in another paper, are determined for various of the above devices operating with thermal sources and sinks of finite capacity. In addition, a new absorption heat transformer cycle using a partially miscible mixture is synthetically described.

Comparing hydraulic and polytropic efficiencies with exergy efficiency

Abstract This paper uses the hydraulic and polytropic efficiency concepts in a domain wider than that of their original definition. A generalized Bernoulli equation taken from previous papers is also considered. The aim is to find a general statement yielding hydraulic efficiency values ranging from one to zero, and a relative dissipative term, suitably chosen, that varies between zero and one. This will be following an approach similar to that outlined in previous papers for exergy efficiency. The paper also clarifies the physical meaning of polytropic efficiency as compared with exergy and isentropic efficiency. Two simple numerical examples are also presented to illustrate the different values of the various efficiencies in the same process.