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Compressed
air is used in industries for various applications in pneumatic
tools, pneumatic instruments, pneumatic machines and in a
variety of production processes. The consequences of using
wet air are rust and scale deposits in steel pipes, increased
resistance in flow, malfunctioning of process control instruments,
corrosion and damage to electromagnetic valves and pneumatic
system, peeling and blistering effect on spray painted surface
etc. Thus it will affect the quality of product and lead to
excessive maintenance cost.
| Ideal
compressed Air System |
Air from
the compressor has excessive moisture and contaminants. The
sketch below shows a typical Compressed air system to remove
these damaging impurities and get Clean and Dry air.When
compressed to 7Kg/cm2g the temperature of discharge
air reaches around 140oC. The after cooler and
Separator installed after the compressor will cool the air,
and removes 90% of moisture and oil. For instance, when the
compressor takes in 100 NM3/hr of ambient air at
40oC and 50% relative humidity, at 7Kg/cm2g
the after cooler will condense an average of 30 liters of
water in 24 hours. If the outlet air temperature from the
after cooler is 40oC, it still contains 8000 ppm
moisture. At 45o C compressed air contains 10,500
ppm, at 50oC, 13,500 ppm moisture which is removed
by Air Dryer. Therefore, lower the temperature of air at dryer
inlet, smaller would be the size of Air Drying Unit.
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If
compressed air temperature is higher than 400C,
it is essential to precool it to 400C temperature
before Air dryer. This cooling is achieved in a shell &
tube type heat exchanger using cooling water. Due to this
cooling some moisture condenses out which then helps in keeping
the moisture load low on Air dryer and thus better Dew-point
of air. |
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"Dew
point is the measure of moisture in Air. It is the temperature
at which moisture present in the Air, starts condensing.
Lower the Dew Point, more dry is the Air”.
Air
of (-) 400C Dew point means no moisture would
condense unless the temperature of the compressed air
goes below (-) 400C. For most industrial
applications Dew point of around (-) 100C
is satisfactory. However, as a common industrial practice,
most of Air dryers are rated for (-) 400C
Dew point. Lower the Dew point, higher would be the
operating cost. Hence, proper selection of Dew point
is very important for economical operation. |
Dew point, at Atmospheric pressure |
Moisture content, w/w |
| 0oC |
3800 ppm |
| (-) 5oC |
2500 ppm |
| (-) 10oC |
1600 ppm |
| (-) 20oC |
685 ppm |
| (-) 30oC |
234 ppm |
| (-) 40oC |
80 ppm |
| (-) 60oC |
6.5 ppm |
| (-) 80oC |
0.3 ppm |
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Oil present
in Compressed air is detrimental to the desiccant of Air Dyer.
If not removed, it gets coated on the desiccant and reduces
moisture adsorption capacity resulting in frequent replacement
of desiccant.Therefore if your compressor is oil lubricated
type, an oil filter before the air dryer is essential. This oil
filter has Activated Carbon which removes oil from the air
completely and thus enhances life of desiccant. |
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