Thermodynamics involves the study of various systems. A system in thermodynamics is nothingmore than the collection of matter that is being studied. A system could be the water within oneside of a heat exchanger, the fluid inside a length of pipe, or the entire lubricating oil system fora diesel engine. Determining the boundary to solve a thermodynamic problem for a system willdepend on what information is known about the system and what question is asked about thesystem.Everything external to the system is called the thermodynamic surroundings, and the system isseparated from the surroundings by the system boundaries. These boundaries may either be fixedor movable. In many cases, a thermodynamic analysis must be made of a device, such as a heatexchanger, that involves a flow of mass into and/or out of the device. The procedure that isfollowed in such an analysis is to specify a control surface, such as the heat exchanger tubewalls. Mass, as well as heat and work (and momentum), may flow across the control surface.

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