# Open and Closed Systems

In Chapter 4 we studied closed systems in which no mass crossed the system boundaries.
In this chapter we will be considering

## open systems

.
For closed systems we considered a control mass.
The mass remains constant throughout the process.
Thus .
Closed
System
For open systems we consider a

## control volume

.
The mass does not necessarily remain constant.
Thus, we must keep track of the mass flow rate in, , and the mass flow rate out, , of the control volume .
Open
System
Roll your mouse over this box to close.
• Homework problem hints and answers
• Get Help from Dr. B in the LT Blog
• 120 day membership

Get it ALL for \$5 US

### Ch 5, Lesson A, Page 1 - Open and Closed Systems

• In chapter 4, we used the conservation of mass principle to write mass balance equations on closed systems.
• In this lesson we will apply the conservation of mass principle to open systems.
• Recall that mass crosses the boundary of open systems.
• In an integral mass balance, mass crossing a system boundary is expressed as an amount.
• For example during a process 32 kg of water entered a system.
• In this course, however, a differential mass balance is more common.
• A differential mass balance is used when mass flows continuously across the system boundary.
• For example, watered entered that system at a RATE of 12 kg/min during a process.
• The symbol for a mass flow rate is an m with a dot over it and I will sometimes call this symbol m-dot
• Systems can also have mass leaving the system during a process as well.
• In that case, we can use the subscripts in and out on the symbol m-dot, so we don’t get the two streams confused.
• Now, let’s write the two different kinds of mass balance equations: integral and differential.