Page 15 - Reciprocating Compressors Operation Maintenance

2. Reciprocating Compressors: Operation and Maintenance

Gases may be composed of only one specific gas maintaining its own
identity in the gas mixture. Air, for example, is a mixture of several gases,
primarily nitrogen (78% by volume), oxygen (21%), argon (about 1%),
and some water vapor. Air may also, due to local conditions, contain vary-
ing small percentages of industrial gases not normally a part of air.

The First Law of Thermodynamics

This law states that energy cannot be created or destroyed during a
process, such as compression and delivery of a gas. In other words, when-
ever a quantity of one kind of energy disappears, an exactly equivalent
total of other kinds of energy must be produced.

The Second Law of Thermodynamics

This law is more abstract, but can be stated in several ways:

1. Heat cannot, of itself, pass from a colder to a hotter body.
2. Heat can be transferred from a body at a lower temperature to one at
a higher temperature only if external work is performed.
3. The available energy of the isolated system decreases in all real
processes.
4. By itself, heat or energy (like water), will flow only downhill (i.e.,
from hot to cold).

Basically, then, these statements say that energy which exists at various
levels is available for use only if it can move from a higher to a lower level.

Ideal or Perfect Gas Laws
An ideal or perfect gas is one to which the laws of Boyle, Charles, and
Amonton apply. Such perfect gases do not really exist, but these three
laws of thermodynamics can be used if corrected by compressibility fac-
tors based on experimental data.
Boyle's Law states that at a constant temperature, the volume of an
ideal gas decreases with an increase in pressure.
For example, if a given amount of gas is compressed at a constant tem-
perature to half its volume, its pressure will be doubled.
P
V 2 = —L or P 2V 2 = P, V, = constant
V, B