Page 81 - Industrial Ventilation Design Guidebook

46 CHAPTER 4 PHYSICAL FUNDAMENTALS

The equation can also be expressed as

The state equation of an ideal gas such as steam is

where

v is the specific volume, v = I /p,
h is enthalpy,
a and b are constants.
This equation is seldom used, because the tables of the thermodynamic prop-
erties of fluids (steam tables) allow the values of the fluid/gas vapor to be ac-
curately obtained.

Specific Weight
Specific weight is the weight per unit volume and is equal to p • g, where g
is the acceleration due to gravity. In the case of water of density 1000 kg m~ 3
3
3
the specific weight is 9.81 x 10 N nr .
Specific Gravity
Sometimes called relative density, specific gravity is the ratio of the fluid
density with respect to a reference substance at a specified temperature.
3
Mercury has a density of 13600 kg m~ and is 13.6 times as heavy as wa-
ter, or 11333 times as heavy as the same volume of air.
Water is taken to have a specific gravity of 1.0 at 4 °C, where it has its
maximum density, with other liquids having a value either greater or less
than this.
In the case of air, the specific gravity is taken as 1.0, with all other gases
having specific gravity greater or less than this value.
Plastic Fluids
Various types of fluids, known as plastic fluids, may be encountered,
which do not start to flow until a certain minimum shear stress is reached. The
relationship between shear stress and the rate of shear strain may or may not
take a linear form.
If linear, the plastic is known as a Bingham plastic, a typical case being
sewage sludge.
Pseudo-plastic fluids
With this type of fluid the viscosity decreases as the shear strain increases,
typical cases being mud and liquid cement.
Dilatant Fluids
Quicksand is included in this category. The viscosity increases as the rate
of shear strain increases.