calculation contexts. The composition is relatively constant, from seal level and up
to an altitude of 25 kilometers.
Air is always more or less contaminated with solid particles, for example, dust, sand,
soot and salt crystals.
The degree of contamination is higher in populated areas, less in the countryside and at higher altitudes.
Air is not a chemical substance, but a mechanically mixed substance. This is why it can be separated into its constituent elements, for example, by cooling.
Moist Air :
air’s humidity can vary within broad limits.
Extremities are completely dry air and air saturated with moisture.
The maximum water vapour pressure that air can hold increases with rising temperatures.
A maximum water vapour pressure corresponds to each temperature.
Air usually does not contain so much water vapour that maximum pressure is reached.
Relative vapour pressure (also known as relative humidity) is a state between the actual partial vapour pressure and the saturated pressure at the same temperature.
The dew point is the temperature when air
is saturated with water vapour. Thereafter with a fall in temperature the condensation
of water takes place. Atmospheric dew point is the temperature at which water
vapour starts to condense at atmospheric pressure. Pressure dew point is the equivalent temperature with increased pressure.
The following relation applies:
(p - ϕ x ps) x 105 x V = Ra x ma x T
ϕ x ps x 105 x V = Rv x mv x T
p = total absolute pressure (bar)
ps = saturation pressure at the actual temperature (bar)
ϕ = relative vapour pressure
V = total volume of the moist air (m3)
Ra = gas constant for dry air
= 287.1 J/Kg x K
Rv = gas constant for water vapour
= 461.3 J/Kg x K
ma= mass of the dry air (kg)
mv= mass of the water vapour (kg)
T = absolute temperature of the moist air (K)
Types of compressors
There are two basic principles for the compression of air (or gas), the displacement principal and dynamic compression.
Among displacement compressors are, for example, piston compressors and different types of rotary compressors. They are the most common compressors in most countries.
On a piston compressor for example, the air is drawn into a compression chamber, which is closed from the inlet.
Thereafter the volume of the chamber decreases and the air is compressed. When the pressure has reached the same level as the pressure in the outlet manifold, a valve is opened and the air is discharged at a constant pressure, under continued reduction of the compression chamber’s volume.
In dynamic compression air is drawn into a rapidly rotating compression impeller and
accelerates to a high speed. The gas is then discharged through a diffuser, where the kinetic energy is transformed to static pressure.
There are dynamic compressors with axial or radial flow. All are suitable for
large volume rates of flow.
Displacement compressors
A bicycle pump is the simplest form of a
displacement compressor, where air is drawn into a cylinder and is compressed
by a moving piston. The piston compressor has the same operation principle, with a piston whose forward and backward movement is accomplished by a connecting rod and a rotating crankshaft. If only one
side of the piston is used for compression
this is called single acting. If both the piston’s top and undersides are used the compressor is called double acting. The difference between the pressure on the inlet side and the pressure on the outlet side is a measurement of the compressor’s work.
The pressure ratio is the relation between absolute pressure on the inlet and outlet
sides. Accordingly, a machine that draws in
air at atmospheric pressure and compresses
it to 7 bar overpressure works with a pressure ratio of (7 + 1)/1 = 8.
See the Figure below to understand compressors types:
The chart shows the most common types of compressor, divided according to their working principles.
They can also be divided according to other principles, for example, air or liquid cooled, stationary or portable, etc..
