MASS System (Closed System)It is a system of fixed mass with a fixed identityThis type of system is USUALLY referred to as "closed system"There is no mass transfer across the system boundaryEnergy transfer may take place into or out of the systemControl Volume System (OPEN System)It is a system of fixed volumeThis type of system is usually referred to as "open system” or a "control volume"Mass transfer can take place across a control volumeEnergy transfer may also OCCUR into or out of the systemA control volume can be SEEN as a fixed region across which mass and energy transfers are studiedControl Surface- It is the boundary of a control volume across which the transfer of both mass and energy takes placeThe mass of a control volume (open system) may or may not be fixedMost of the engineering devices, in general, represent an open system or control volume.