SUMMARY OF ENTROPY

SUMMARY

The second law of thermodynamics leads to the definition of a new property called entropy, which is a quantitative measure of microscopic disorder for a system. The definition of entropy is based on the Clausius inequality, given by

ENTROPY-0284where the equality holds for internally or totally reversible processes and the inequality for irreversible processes. Any quantity whose cyclic integral is zero is a property, and entropy is defined as

ENTROPY-0285

where Sgen is the entropy generated during the process. Entropy change is caused by heat transfer, mass flow, and irreversibilities. Heat transfer to a system increases the entropy, and heat transfer from a system decreases it. The effect of irreversibili- ties is always to increase the entropy.

Entropy is a property, and it can be expressed in terms of more familiar properties through the T ds relations, expressed as

ENTROPY-0286

These two relations have many uses in thermodynamics and serve as the starting point in developing entropy-change relations for processes. The successful use of T ds relations de- pends on the availability of property relations. Such relations do not exist for a general pure substance but are available for incompressible substances (solids, liquids) and ideal gases.

The entropy-change and isentropic relations for a process can be summarized as follows:

ENTROPY-0289

ENTROPY-0290

The work done during a steady-flow process is proportional to the specific volume. Therefore, u should be kept as small as possible during a compression process to minimize the work input and as large as possible during an expansion process to maximize the work output.

The reversible work inputs to a compressor compressing an ideal gas from T1, P1 to P2 in an isentropic (Puk = constant), polytropic (Pun = constant), or isothermal (Pu = constant) manner, are determined by integration for each case with the following results:

ENTROPY-0291

The work input to a compressor can be reduced by using multistage compression with intercooling. For maximum savings from the work input, the pressure ratio across each stage of the compressor must be the same.

ENTROPY-0292

Related posts:

Selection process retrofitting to existing equipment:Flow charts
Variable speed drives:Variable speed drives for other AC motors
Applications on pneumatic:Compressed air in marine applications.
Gas–solid separation devices:Separation devices and Gravity settling chambers.
Health and safety:Conveying operations
Optimizing and up-rating of existing systems:The influence of changing pipeline diameter
Material property influences:Material grade influences and Alumina.
ACTUATORS:MULTIPLE CYLINDER CIRCUITS AND CYLINDER AIR CONSUMPTION
ROUBLESHOOTING HYDRAULIC SYSTEMS:FAILURE MODES OF HYDRAULIC COMPONENTS
MAINTENANCE OF HYDRAULIC SYSTEMS:PREVENTIVE MAINTENANCE.
PNEUMATIC BASICS
Hydraulic and Pneumatic Accessories:Hydraulic and Pneumatic fuses
INTRODUCTION AND OVERVIE:HEAT TRANSFER
FORCED CONVECTION:GENERAL CONSIDERATIONS FOR PIPE FLOW
RADIATION HEAT TRANSFER:THE VIEW FACTOR

Leave a comment

Your email address will not be published. Required fields are marked *