A Thermodynamic Efficiency Concept for Heat Exchange Devices

1983 ◽  
Vol 105 (1) ◽  
pp. 199-203 ◽  
Author(s):  
L. C. Witte ◽  
N. Shamsundar
Keyword(s):  
Heat Exchanger ◽  
Heat Exchange ◽  
Energy Use ◽  
Second Law Of Thermodynamics ◽  
Thermodynamic Efficiency ◽  
Second Law ◽  
Two Fluids ◽  
Environment Temperature ◽  
Efficiency And Effectiveness ◽  
The Mean

A thermodynamic efficiency based on the second law of thermodynamics is defined for heat exchange devices. The efficiency can be simply written in terms of the mean absolute temperatures of the two fluids exchanging heat, and the appropriate environment temperature. It is also shown that for a given ratio of hot to cold inlet temperatures, the efficiency and effectiveness for particular heat exchange configurations are related. This efficiency is compared to second-law efficiencies proposed by other authors, and is shown to be superior in its ability to predict the effect of heat exchanger parameter changes upon the efficiency of energy use. The concept is applied to typical heat exchange cases to demonstrate its usefulness and sensitivity.

scholarly journals From Entropy Generation to Exergy Efficiency at Varying Reference Environment Temperature: Case Study of an Air Handling Unit

Entropy ◽  
2019 ◽  
Vol 21 (4) ◽  
pp. 361 ◽  
Author(s):  
Giedrė Streckienė ◽  
Vytautas Martinaitis ◽  
Juozas Bielskus
Keyword(s):  
Heat Pump ◽  
Entropy Generation ◽  
Energy Demand ◽  
Dynamic Modelling ◽  
Second Law Of Thermodynamics ◽  
Exergy Efficiency ◽  
Second Law ◽  
Air Handling Unit ◽  
Individual Character ◽  
Environment Temperature

The continuous energy transformation processes in heating, ventilation, and air conditioning systems of buildings are responsible for 36% of global final energy consumption. Tighter thermal insulation requirements for buildings have significantly reduced heat transfer losses. Unfortunately, this has little effect on energy demand for ventilation. On the basis of the First and the Second Law of Thermodynamics, the concepts of entropy and exergy are applied to the analysis of ventilation air handling unit (AHU) with a heat pump, in this paper. This study aims to develop a consistent approach for this purpose, taking into account the variations of reference temperature and temperatures of working fluids. An analytical investigation on entropy generation and exergy analysis are used, when exergy is determined by calculating coenthalpies and evaluating exergy flows and their directions. The results show that each component of the AHU has its individual character of generated entropy, destroyed exergy, and exergy efficiency variation. However, the evaporator of the heat pump and fans have unabated quantities of exergy destruction. The exergy efficiency of AHU decreases from 45–55% to 12–15% when outdoor air temperature is within the range of −30 to +10 °C, respectively. This helps to determine the conditions and components of improving the exergy efficiency of the AHU at variable real-world local climate conditions. The presented methodological approach could be used in the dynamic modelling software and contribute to a wider application of the Second Law of Thermodynamics in practice.

The Use of the Second Law of Thermodynamics in Process Design

1995 ◽  
Vol 117 (3) ◽  
pp. 179-185 ◽  
Author(s):  
D. A. Sama
Keyword(s):  
Heat Exchanger ◽  
Design Procedure ◽  
Second Law Of Thermodynamics ◽  
Global Optimum ◽  
Second Law ◽  
Complex Processes ◽  
Cost Of Energy ◽  
Heat Exchanger Networks ◽  
Improved Design ◽  
Optimum Solution

The importance of using the second law of thermodynamics in the design of heat exchangers, heat exchanger networks, and processes in general, is discussed. The optimal ΔT at a refrigerated heat exchanger is considered from a second law viewpoint. It is shown that the use of minimum total annualized cost as the single optimizing factor is unsatisfactory. Total annualized costs are based on predicted costs of fuel, equipment, and capital, which are uncertain at best. Instead of a singular or “global optimum” ΔT, there is a range of optimal ΔTs, over which the total annualized cost is essentially the same, but within which the distribution between cost of capital and cost of energy is significantly different. In selecting a design ΔT, this distribution of costs should also be considered. The possibility of only one singular, or global optimum, solution for complex processes is also considered from a philosophical viewpoint, and is again rejected. The existence and identification of design decisions which unnecessarily waste thermodynamic availability (physical exergy) are discussed and identified as “second law errors.” Elimination of a second law error from a design guarantees an improved design. An optimal design, which may be any one of a numerous set of optimal designs, will result when all second law errors are eliminated. A design procedure to develop optimal process designs, using such thermodynamic insights, is proposed.

scholarly journals Thermal Performance in Heat Exchangers by the Irreversibility, Effectiveness, and Efficiency Concepts Using Nanofluids

2020 ◽  
Vol 7 (2) ◽  
pp. F1-F7
Author(s):  
E. Nogueira
Keyword(s):  
Heat Exchanger ◽  
Ethylene Glycol ◽  
Heat Exchangers ◽  
Volume Fraction ◽  
Second Law Of Thermodynamics ◽  
Second Law ◽  
Output Data ◽  
Tube Heat Exchanger ◽  
Shell And Tube ◽  
Effectiveness And Efficiency

The objective of the work is to obtain the outlet temperatures of the fluids in a shell and tube heat exchanger. The second law of thermodynamics is applied through the concepts of efficiency, effectiveness, and irreversibility to analyze the results. Water flows in the shell, and a mixture of water-ethylene glycol is associated with fractions of nanoparticles flows in the tube. Water enters the shell at 27 °C, and the mixture comes to the tube at 90 °C. The mass flow is kept fixed in the shell, equal to 0.23 kg/s, and varies between 0.01 kg/s to 0.50 kg/s. Volume fractions equal to 0.01, 0.10, and 0.25 were considered for analysis, for both nanoparticles from Ag and Al2O3. Results for Reynolds number, heat transfer rate, efficiency, effectiveness, and irreversibility are presented for critique, discussion, and justification of the output data found. It is shown that the flow regime has a significant effect on the performance of the analyzed heat exchanger. Keywords: thermodynamics, second law, ethylene glycol, volume fraction.

scholarly journals From Entropy Generation to Exergy Efficiency at Varying Reference Environment Temperature: Case Study of an Air Handling Unit

2019 ◽  
Author(s):  
Giedrė Streckienė ◽  
Vytautas Martinaitis ◽  
Juozas Bielskus
Keyword(s):  
Heat Pump ◽  
Entropy Generation ◽  
Energy Demand ◽  
Dynamic Modelling ◽  
Second Law Of Thermodynamics ◽  
Exergy Efficiency ◽  
Second Law ◽  
Air Handling Unit ◽  
Individual Character ◽  
Environment Temperature

The continuous energy transformation processes in heating, ventilation and air conditioning systems of buildings are responsible for 36% of global final energy consumption. Tighter thermal insulation requirements for buildings have significantly reduced heat transfer losses. Unfortunately, this has little effect on energy demand for ventilation. On the basis of the First and the Second Law of Thermodynamics, the concepts of entropy and exergy are applied to the analysis of ventilation air handling unit (AHU) with a heat pump in this paper. This study aims to develop a consistent approach for this purpose, taking into account the variations of reference temperature and temperatures of working fluids. An analytical investigation on entropy generation and exergy analysis are used, when exergy is determined by calculating coenthalpies and evaluating exergy flows and their directions. The results show that each component of the AHU has its individual character of generated entropy, destroyed exergy and exergy efficiency variation. However, the evaporator of heat pump and fans have unabated quantities of exergy destruction. The exergy efficiency of AHU decreases from 45-55% to 12-15% when outdoor air temperature is within the range of –30°C…+10°C, respectively. This helps to determine conditions and components of improving the exergy efficiency of the AHU at variable real-world local climate conditions. The presented methodological approach could be used in the dynamic modelling software and contribute to a wider application of the Second Law of Thermodynamics in practice.

The Second Law Quality of Energy Transformation in a Heat Exchanger

1990 ◽  
Vol 112 (2) ◽  
pp. 295-300 ◽  
Author(s):  
D. P. Sekulic
Keyword(s):  
Heat Exchanger ◽  
Heat Exchange ◽  
Entropy Generation ◽  
Exchange Process ◽  
Quantitative Measure ◽  
Inlet Temperature ◽  
Energy Transformation ◽  
Two Fluids ◽  
Heat Exchange Process

This paper presents the entropy generation (irreversibility) concept as a convenient method for estimating the quality of the heat exchange process in heat exchanger analysis. The entropy generation caused by finite temperature differences, scaled by the maximum possible entropy generation that can exist in an open system with two fluids, is used as the quantitative measure of the quality of energy transformation (the heat exchange process). This measure is applied to a two-fluid heat exchanger of arbitrary flow arrangement. The influence of different parameters (inlet temperature ratio, fluid flow heat capacity rate ratio, flow arrangements) and the heat exchanger thermal size (number of heat transfer units) on the quality of energy transformation for different types of heat exchangers is discussed. In this analysis it is assumed that the contribution of fluid friction to entropy generation is negligible.

Optimization of the Thermodynamic Efficiency of a Recuperative Heat Exchanger

2000 ◽  
Author(s):  
George A. Adebiyi
Keyword(s):  
Heat Exchanger ◽  
Thermal Capacity ◽  
Thermodynamic Efficiency ◽  
Second Law ◽  
Inlet Temperature ◽  
Counter Flow ◽  
Cold Stream ◽  
Prandtl Numbers ◽  
Recuperative Heat Exchanger

Abstract A heat exchanger is strictly speaking a thermal exergy transfer device, and the proper measure of its efficiency is the second law efficiency. This article considers the efficiency of a single-pass, recuperative heat exchanger in which a given stream of hot fluid is available for heating a cold stream of fluid in a specified manner. The analysis takes cognizance of the required exergy input to overcome fluid friction in the flow passages, as well as the thermal exergy flow rates for the fluid streams, in the determination of the second-law efficiency. Maximization of the second-law efficiency is found to provide a basis for sizing the heat exchanger for optimum thermodynamic efficiency of operation. The key parameters that determine this optimum include the number of transfer units (NTU), the ratio of thermal capacity rates (Cr), a dissipation parameter which involves the Eckert and Prandtl numbers, and the flow configuration (whether parallel-flow, or counter-flow). Other parameters relevant to the performance of the heat exchanger are the tare capacity (εtare), and the ratio of the inlet temperature of the hot fluid to the ambient temperature.

Irreproducibility in Biomedical Science

2018 ◽  
pp. 186-194
Author(s):  
Erwin B. Montgomery
Keyword(s):  
Second Law Of Thermodynamics ◽  
Biomedical Science ◽  
Second Law ◽  
Central Tendency ◽  
Loss Of Information ◽  
Root Cause ◽  
Journal Editors ◽  
The Mean ◽  
Logical Fallacies ◽  
The Individual

Widespread irreproducibility of biomedical research has raised concerns. Journal editors and grant administrators are calling for greater safeguards. The causes go far beyond fraud, lack of transparency, and poor statistical analyses, as commonly thought. The root cause may stem from the same epistemic issues that confront medical reasoning, the necessary use of logical fallacies. However, the use of these fallacies increases the risk of uncertainty and subsequent irreproducibility. Furthermore, many procedures in data analysis actually result in an irretrievable loss of information by the Second Law of Thermodynamics as Applied to Information, thereby increasing the risk for irreproducibility. The second law holds that any irreversible process, such as operating only from the central tendency, as in the mean of a sample, results in a loss of information about the actual sample. The loss of that information makes the study less informative about the management of the individual patient.

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