Search

For drying and moistening processes in batch fluidization and fluidization in a horizontal exchanger, the overall mathematical model describing simultaneous heat and mass exchange is given. On this basis, the problem of finding the optimal temperatures of inlet gas is formulated and investigated for the case when inlet gas humidity is constant. It is explained that when unit economical values of gas and solid phases are linear with regard to unit available energies (exergies), the economical problem of minimum costs is equivalent to the thermodynamical problem of minimum available energy (exergy) dissipation. Therefore, the performance index expression in exergy terms is formulated as a functional of the Bolza form. The dynamic programming method is used to obtain results of optimal trajectories and decisions. The dependence of optimal process time on apparatus price, which is characterized here by a Lagrangian multiplier λ, is pointed out. The results indicate that the constant gas inlet temperature policy commonly applied to the processes considered here prevents attainment of the process optimum. In fact, increasing inlet temperature until an admissible gas temperature is attained is the optimal policy.

A method of circumventing the turbine inlet temperature limitation of present-day gas turbines is presented. This method is based on a direct fluid-to-fluid energy exchanger whereby the available energy of expansion of the hot combustion gas in a gas turbine cycle is transferred directly to a colder gas. The aerodynamic wave processes in several possible modes of operation are examined to determine the inherent limitations in efficiency of direct fluid-to-fluid energy exchange processes. In particular, it is demonstrated that, by using a system of isentropic compression waves to avoid shock losses and by carefully choosing the molecular weights of the fluids utilized in the energy exchanger, perfect energy exchange is possible in principle. When allowances are made for losses due to mixing, leakage, and viscous effects, an energy exchanger utilizing heated combustion air at 3240 deg F to drive steam at 1500 deg F with a potential energy exchange efficiency of 85 percent is feasible. Applications of the air-steam energy exchanger operating in gas turbine cycles utilizing a conservative choice of component efficiencies indicate that thermal efficiencies of gas turbine power cycles of 50–60 percent may be possible.

A thermodynamic explanation of the operation of a flash evaporator by the principle of exergy losses is of considerable importance, especially in the study of the desalination process. In a flash evaporator an amount of circulating heat to which additional energy is supplied is subjected to the irreversibilities of the installation. A calculation of the losses of exergy and measures of these irreversibilities, give information about the process details which are mainly responsible for the energy consumption. Assuming some simplifications, the numerical values of the loss of exergy are calculated for the special case of a ten stage once-through flash evaporator. It is found that the dominant losses are caused by the irreversibilities of the heat transfer from steam to brine and by the isenthalpic expansion of the brine between the flash chambers.

The idea of exergy—a notion which nowadays is becoming increasingly widespread—was recently introduced into the field of radiation. In this paper formulas for the computation of exergy of heat radiation are set out. The ratio of exergy to the radiation energy has been considered and a discussion is presented of the dependence of substance exergy and radiation on temperature. In addition, the possible applications of radiation energy are mentioned as well as numerical examples using the relations derived in this paper.

The claim of this communication is to present an outline of a program of research and development of thermodynamics. The scope of this and subsequent discussions will be “materialistic,” by which the author means a combination of both formal and experimental aspects.

A simple connection is pointed out between the theory of heat formulated in Sadi Carnot's: “Reflexions sur la puissance motrice du feu” (1824) and the later Kelvin-Clausius thermodynamics. In both theories two well-defined quantities, a heat function and a work function, exist and can be calculated by integrating along a reversible path. In thermodynamics the work function (energy) is conserved, whereas the heat function (entropy) increases by irreversible processes. In Carnot's theory the heat function is conserved, whereas the work function decreases, so that in this theory the irreversible process is characterized by a loss of work.

The exergy of a substance is the theoretical amount of mechanical work which can be derived from its heat content at given initial and end conditions. At given end conditions, the exergy is a state variable. The exergy per unit mass of pure water has been tabulated for the most important initial and end temperatures. An example showing the exergy flux in geothermal power plants is given.

Synopsis.---This paper outlines a symmetric form of exposition of the principles of thermodynamics made possible through recognizing on a parity from the outset the two kinds of conservation naturally called after Black and Carnot, each of which is physically valid under proper conditions, and which lead to the energy and entropy scales of measurement of heat; thus exhibiting both of these, together with the thermodynamic scale of temperature, as calorimetric concepts. In the two parts of the paper are given respectively an experimental and a deductive treatment.In part I. the experimental meaning of conservation is considered in terms of certain conditions of consistency; first in connection with single-temperature transfers of heat, where those conditions underlie the uniqueness of meaning of an arbitrary scale of heat for various temperatures; second in connection with passages of heat between bodies at different temperatures, which reveal the dual nature of a quantity of heat, here illustrated by some analogies.In part II. are investigated the deductive consequences of Carnot's hypothesis of the universal ratio of efficiency. With only the single-temperature conservation of heat assumed to be known experimentally (probably the mildest basis on which this principle is intelligible), it is shown that there follows practically the entire theory of reversible thermodynamics, in particular the existence of thermal energy and entropy and the equivalence of thermal energy and work.

A cryogenic air separation process that generates nitrogen has different process configurations depending on the utilization of either a cold heat supply method or a distillation method. Each method presents different initial investment costs, operating costs and energy efficiencies. Therefore, an accurate estimation of the energy efficiency as well as the cost is an important factor in selecting an appropriate process and in designing the most efficient equipment. Accordingly, the current study formulated criteria that can be applied to select the most efficient process. Processes using liquid nitrogen and an expansion turbine representing different cold heat supply methods were analyzed and evaluated to compare the exergy efficiency. In addition, the exergy efficiency of structure packing and sieve tray distillation method was also compared based on the experimental results.

An examination is made of two methods of determining the value of mass flow exergy upon variation of the parameters of the surrounding medium. The first method consists in using an exergetic diagram in dimensionless quantities, and the second in constructing correction scales on the existing diagram.

La Thermodynamique et le Theoreme de l'Energie Utilisable Par R. Marchal. Pp. xxx + 208 + 7 planches. (Paris 1 Dunod, 1956.) 1580 francs.