THERMODYNAMIC SYSTEMS AND PROCESSES
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.
Types of Thermodynamic Systems
Systems in thermodynamics are classified as isolated, closed, or open based on the possible
transfer of mass and energy across the system boundaries. An isolated system is one that is not
influenced in any way by the surroundings. This means that no energy in the form of heat or
work may cross the boundary of the system. In addition, no mass may cross the boundary of the
A thermodynamic system is defined as a quantity of matter of fixed mass and identity upon
which attention is focused for study. A closed system has no transfer of mass with its
surroundings, but may have a transfer of energy (either heat or work) with its surroundings.
An open system is one that may have a transfer of both mass and energy with its surroundings.
When a system is in equilibrium with regard to all possible changes in state, the system is in
thermodynamic equilibrium. For example, if the gas that comprises a system is in thermal
equilibrium, the temperature will be the same throughout the entire system.
A control volume is a fixed region in space chosen for the thermodynamic study of mass and
energy balances for flowing systems. The boundary of the control volume may be a real or
imaginary envelope. The control surface is the boundary of the control volume.
Steady state is that circumstance in which there is no accumulation of mass or energy within the
control volume, and the properties at any point within the system are independent of time.