Its a system of fixed volume.
This type of system is usually referred to as “open system” or a “control volume”
Mass transfer can take place across a control volume.
Energy transfer may also occur into or out of the system.
Control Surface- Its the boundary of a control volume across which the transfer of both mass and energy takes place.
The mass of a control volume (open system) may or may not be fixed.
When the net influx of mass across the control surface equals zero then the mass of the system is fixed and vice-versa.
The identity of mass in a control volume always changes unlike the case for a control mass system (closed system).
Most of the engineering devices, in general, represent an open system or control volume.
Heat exchanger – Fluid enters and leaves the system continuously with the transfer of heat across the system boundary.
Pump – A continuous flow of fluid takes place through the system with a transfer of mechanical energy from the surroundings to the system