Page 861 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

816 Appendix D: Fluid Mechanics—Reviews of Selected Topics

Irreversible work: If the system and the surroundings cannot depiction of pressures in a pipeline, however, its use
be restored to their initial states, once a process is has utility similar to that of the hydraulic-grade-line.
started, the process is irreversible. All real processes Pneumatic: A term that relates to gas, either static or flowing.
are irreversible. Three causes of irreversibility are Pneumatics: That branch of engineering describing the flow
viscosity, heat conduction, and diffusion and are of gases in terms of the associated dependent and
molecular level phenomena (Shapiro, 1958, p. 30). independent variables or that describes the state rela-
For an irreversible process, it must always be true tionships of a static gas, such as in a pressurized
that dS dq=T. tank.
Isentropic: Adiabatic and frictionless change of state, mean- Polytropic: Change of state in which both temperature and
ing a reversible path is followed in the compression pressure change.
or expansion. Pressure head: Pressure due to a column of fluid above a
Isothermal: Constant temperature process. given elevation, expressed as ‘‘meters of water’’ with
k: Ratio of specific heats, i.e., c p =c v 1.4. dimension of length, as abbreviation of energy per
Manifold: A pipe or chamber having multiple apertures for unit of fluid mass; expressed mathematically as p=g.
making connections (Oxford American Dictionary, Process: A change of ‘‘state’’ of a system and may be
1980). described by the series of states passed through by
Manometer: An instrument filled with a liquid, e.g., gly- the system (Shapiro, 1958, p. 24).
cerin, mercury, that measures pressure by means of Reversible work: In a reversible expansion of a gas, the
a U-tube. If, for example, the pressure is to be classic example of a reversible process, the internal
measured within a tank or pipe, one end to the tube pressure is just infinitesimally higher than the exter-
is attached to the respective wall. The other end nal pressure, and in the limit, Dp ! dp (see Pitzer
is exposed to the atmosphere, as a rule. The height and Brewer, 1961, p. 35). In reversible work, there is
of rise of the liquid, e.g., mercury, in the manometer no entropy production (p. 81). A reversible process
is the basis for calculating the pressure in the pipe or is a standard against which real processes may be
tank. The specific weight of the fluid on each side of evaluated (Shapiro, 1958, p. 30). For a ‘‘cycle,’’ the
the measuring fluid, e.g., mercury, must be included entropy change for a reversible process is zero.
in the calculation of pressure. State: The ‘‘state’’ of a gas is defined by T, P, V, which are
MW(air): Determined to be 28.9 g=mol ¼ 0.028964 kg=mol. ‘‘state’’ variables. In general, a state is the configur-
Nozzle: A device shaped to accelerate a fluid (Munson et al., ation of a system described in sufficient detail such
1998, p. 121). Usually a nozzle is a short pipe length that one state may be distinguished from another (the
that has varying diameter with length, generally latter is from Shapiro, 1958, p. 24).
reducing to a ‘‘throat’’ section, intended to cause a STP: Acronym for ‘‘standard temperature and pressure,’’
high velocity at its outlet. sometimes called, NTP for ‘‘normal temperature
Orifice: A circular opening in a plate or pipe for controlled and pressure.’’ Standard values may vary, depending
flow of a fluid. upon the application. For chemists, STP means 08C
Piezometer: A tube attached to a point at which pressure is to and 1.00 atm pressure (Silberberg, 1996, p. 186). In
be measured. The height of rise of the liquid (usually, engineering, a standard temperature of 208C is com-
water is the liquid) in the tube is the basis for calcu- mon. A standard temperature of 258C may be used
lating the pressure at the point of attachment of the for some purposes.
tube. For example, if the water rises 11.3 m (33.9 ft) Venturi: Refers to the Venturi meter. Giovanni Batista Ven-
above the point of attachment, the pressure is 11.3 m turi (1746–1822) was an Italian physicist who con-
water head, which is 101.325 kPa (14.7 psi), or ducted experiments and showed that a reduction of
1.00 atm. eddies was brought about by a gradual transition of
Pneumatic grade line: (1) Locus of points defined by (zg þ p) a boundary rather than an abrupt transition and that
along a pipeline that flows with a gas. The pneumatic the throat pressure was the minimum. The Venturi
grade line is to the flow of gases as the hydraulic grade meter was, however, an invention of Clemens
line is to the flow of liquids. But in lieu of using Herschel (1842–1930), described in an 1899 paper,
hydraulic head (z þ p=g), pressure is used instead. ‘‘The Venturi Water Meter,’’ with the name coming
The reason is that the ‘‘head’’ for a gas would be from the practice in his laboratory of calling the
inordinately large, since its density, r, is much smal- throat pressure the ‘‘Venturi.’’ Hershel combined
ler than that of a liquid. (2) The idea of a pneumatic- the already existing ideas into a meter for flow
grade-line was not found in the literature, but was measurement (foregoing from Rouse and Ince,
based on discussions with Professor Robert Meroney, 1957).
Colorado State University (Meroney, 1998) and may Work: The effect produced by a system on its surroundings
or may not be unique to this text. As a graphical or by the surroundings on the system.