Engineering Thermodynamics
In order that a cycle be reversible, following must be satisfied

Free expansion or friction resisted expansion/compression process should not be encountered
When beat is being rejected, temperature of cold source and working substance should be same
When heat is being absorbed, temperature of hot source and working substance should be same
All of these

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Engineering Thermodynamics
Zeroth law of thermodynamics

States that if two systems are both in equilibrium with a third system, they are in thermal equilibrium with each other
Deals with conversion of mass and energy
Deals with heat engines
Deals with reversibility and irreversibility of process

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