System ? Surrounding ? Boundary ?
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            The first thing we should know as process engineer is what is the system ?
           
So, the system is part of the universe that we will study or we interested to study about it, and the part of universe outside the system is the surrounding. The real or imaginary surface separates the system from the surrounding is the boundary. The boundary of system can be moveable or fixed. (Note : the boundary is the contact surface shared by both the system and the boundary.)

The system may be considered closed or isolated or open system.

The closed system or control mass consists of fixed amount of mass, and no mass can cross its boundary, but the energy can cross its boundary.

Different from closed system, the isolated system is special case that no mass and energy can cross its boundary.

The open system or control volume is a selected region of the universe, either mass or energy can cross its boundary.

Ok, you got it ? Maybe you’re little bit confuse right ? Its ok

When you choose the closed system, you try learning the behaviour or propertie of every single molecules that you choose as your system that totally give a new effect or a new propertie to your system.

But if you choose the open system, you try learning what is going on inside your selected region.

            The last is the isolated system, this sytem is just imaginary system that you never meet

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