Thermodynamics THERMODYNAMIC SYSTEMS AND PROCESSESEverything 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.TypesofThermodynamicSystemsSystems in thermodynamics are classified as isolated, closed, or open based on the possibletransfer of mass and energy across the system boundaries. An isolated system is one that is notinfluenced in any way by the surroundings. This means that no energy in the form of heat orwork may cross the boundary of the system. In addition, no mass may cross the boundary of thesystem.A thermodynamic system is defined as a quantity of matter of fixed mass and identity uponwhich attention is focused for study. A closed system has no transfer of mass with itssurroundings, but may have a transfer of energy (either heat or work) with its surroundings.An open systemis one that may have a transfer of both mass and energy with its surroundings.ThermodynamicEquilibriumWhen a system is in equilibrium with regard to all possible changes in state, the system is inthermodynamic equilibrium. For example, if the gas that comprises a system is in thermalequilibrium, the temperature will be the same throughout the entire system.ControlVolumeA control volume is a fixed region in space chosen for the thermodynamic study of mass andenergy balances for flowing systems. The boundary of the control volume may be a real orimaginary envelope. The control surface is the boundary of the control volume.SteadyStateSteady state is that circumstance in which there is no accumulation of mass or energy within thecontrol volume, and the properties at any point within the system are independent of time.Rev. 0 Page 27 HT-01
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