If there is no mass transfer across the boundary, the system is closed.
Problems involving a closed system - a block of solid, steam cotained in a rigid tank, a gas trapped in a piston-cylinder device, a heat engine, or
even a refrigerator involve only transfer of heat and work, but not mass,
between the system and its surroundings.
You inspect only the boundary of the system to detect the presence
or absence of mass transfer. There may be mass flow among different components
of a refrigerator, but when the boundary encloses the entire refrigerator, only heat and work (electrical work) transfer with the surroundings are possible.
Open
System
Daemons
When there is mass interaction between the system and its surroundings, the system is called an open system. Of course, heat and work transfer are still possible. Pumps, compressors, diffusers, turbines, pipes, and nozzles are
examples of open systems.
A Generic Open Unsteady System and Its Governing Balance Equations
System
A generic system, by definition, is open, unsteady, and non-uniform. This animation shows an open unsteady system having all possible interactions - mass (the two ports serve as representatives of all inlet and exit ports), heat (with atmosphere and the k-th reservoir at temperature T_k), and external work (boundary, shaft and electrical) - with its surroundings. The changing color indicates that the system need not be uniform and the local states can change with location and time. To explore the balance equations further visit the animations in VT, chapter 2.
