Thermodynamic Systems and Surroundings
Thermodynamics involves the study of various systems. A system in thermodynamics is nothing more than the collection of matter that is being studied. A system could be the water within one side of a heat exchanger, the fluid inside a length of pipe, or the entire lubricating oil system for a diesel engine.
Determining the boundary to solve a thermodynamic problem for a system will depend on what information is known about the system and what question is asked about the system. Everything external to the system is called the thermodynamic surroundings, and the system is separated from the surroundings by the system boundaries.
These boundaries may either be fixed or movable. In many cases, a thermodynamic analysis must be made of a device, such as a heat exchanger, that involves a flow of mass into and/or out of the device. The procedure that is followed in such an analysis is to specify a control surface, such as the heat exchanger tube walls. Mass, as well as heat and work (and momentum), may flow across the control surface.