# Introduction: Entropy Balances, Open Systems

Entropy change due to mass flow in and out of the system
So, we must add another term to the right-hand side of the

#### closed system entropy balance equation

:
+
=
Entropy generation within the system
Entropy change due to heat transfer
Change in the entropy of the system
In Lesson 8A we wrote the

#### entropy balance equation

for closed systems.
=
+
Closed System
Open
System
In this lesson, we will develop

#### entropy balance equations

for open systems.
Mass flowing into and out of the system carries entropy with it, just as it carries energy with it.
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### Ch 8, Lesson B, Page 2 - Introduction: Entropy Balances, Open Systems

• In the previous lesson, we found that the change in the entropy of a closed system can be attributed to either heat transfer or entropy generation.
• Of course, in an open system, we have the added complication of mass flowing into the system and mass flowing out of the system.
• This affects our entropy balance because the flowing mass carries entropy along with it.
• Adding to the challenge is the fact that all the flowing mass may not have the same specific entropy because we know that specific entropy is a property or state variable.
• So, if some of the flowing mass is at a different temperature or pressure or perhaps is in the vapor phase instead of the liquid phase, it is going to have a different specific entropy.
• How are we going to deal with this problem ?
• Well, we are going to get a few clues by looking at how we were able to simplify the mass and energy balance equations that we developed back in chapters 4 and 5.