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Basic Knowledge Of Compressed Air

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1. Drain in the air pressure pipeworks
2. Why does drain appear?

The atmosphere contains moisture(vapor), according to the weather conditoin.
Since compressor inhales this atmosphere and compresses it into small valume, 1 m3of the compressed air has a lot of vapor.
For example, 8 m3atmosphere is necessary to get 1 m3of 7kg/cm2compressed air, therefore 1 m3of 7kg/cm2compressed air has eight times as much vapor as the atmosphere. The compressed air heated by adiabatic pressure by the atmosphere is cooled through the after cooler and over-saturated vapor is condensed and then drainappears.For example, the temperature of the air by the adiabatic pressure by the compressor: the temperature of the atmosphere +150¡æ The cooled temperature through the after cooler: 40¡æ
When the temperature of the compressed air is different from that of the surroundings, the compressed air in the air lines radiates heat, and inside the air equipments (air motor, air diver, nozzel, electromagnetic valve, etc.) the temperature is lowered by the adiabatic expansion and the temperature of compressed air also goes down. That is, the temperature is always lowered in the air lines and drain appears each time.
Drain hastens corrosion of the pipes, washes out the lubricating oil of the electromagnetic valves, cylinders, ect. Causes slow operation, low efficiency,shorter life, and freezing problem in winter. In case of the air for paint work, it lowers the quality of the surface work and as to the air for the blow work, it causes rust on the machine parts and products.

1. Examples of calculation for the amount of drain

(1) The inhaling amount of vapor of the compressor
the temperature of the air: 32¡æ(saturated vapor 33.8g/m3)

1. Reference fugure 4 ¨C

Relative humidity: 80%
Inhaling amount: 10 m3/min
The amount of vapor in 1 m3of the air is 33.8¡Á80/100 = 27g/m3and
the inhaling amount of vapor of the compressor per minute is ?27¡Á10 = 270g/min.
(2) The amount of drain through the after cooler
EX. The cooled temperature: 40¡æ (the amount of saturated vapor 51 g/m3)
Entrance pressure: 7kg/cm2
Converted by atmospheric pressure, the amount of the saturated vapor is 51/(7+1.033) = 6.4 g/m3
It is condensed with more than 6.4 g/m3vapor.
The inhaled air by the compressor has 27 g/m3vapor, so when it is compressed into 7kg/cm2, 27-6.4 = 20.6 g/m3
20.6 g/m3vapor is condensed and drain appears.
The amount of vapor which the compressor inhales (in case of relative humidity 100%) Unit: g/min

-Figure 3-
(3) The amount of drain which appears in the air pressure lines.
EX. The surrounding temperature: 32¡æ (the amount of the saturated vapor 33.8 g/m3)
Pressure in the lines: 7kg/m3
In converting the guage pressure 7kg/m3air by atmospheric pressure, the amount of the saturated vapor at 32¡æ is 33.8/(7+1.033) = 4.3 g/m3As the amount of vapor is 6.4 g/m3through the after cooler, it is 6.4-4.3 = 2.1 g/m3
2.1g/m3vapor is condensed and drain appears.
Therefore, when the air at 32¡æ and relative humidity 80% is compressed by the compressor, 2.1 g/m3drain appears in the lines as this example shows.
For example, if the amount of drain is calculated with the amount of recommended treatment air for each model of NEUTEK Extractor/Dryer, it will be;

-Figure 1-
Even though about 5% is evaporated, a lot of drain is eliminated.

1. Dew point

At certain temperature the amount of vapor in the atmosphere exceeds the amount of the saturated vapor and water drops result. And when the amount of vapor in air is less than the saturate amount, water drops result at certain temperature by the fall the temperature. The temperature which results water drops is called dew point and shown in ¡æ. Dew point of oridinary air is atmospheric dew point. For example, when the amount of vapor in air is 12.1 g/m3, atmospheric dew point is 14¡æ, according to the figure of the amount of the saturated vapor.
Compressed air has small volume compared with atmospheric condition because air is pressurized. The temperature which results water drops in that condition is called pressurized dew point.
When the amount of the saturated vapor at certain temperature in pressurized condition is returnted to atmospheric condition, the volume of air is expanded and the content of vapor per 1 m3air gets smaller as the part of expansion. For example, as the amount of the saturated vapor of air which pressurized to 7kg/cm3is 30.3 g/m3 at 30¡æ (pressurized dew point), converted by atmospheric pressure, the volume becomes about eight times (7+1.033) and the follwing calculating is used.
30.3 / (7 + 1.033) = 3.8 g/N m3
N m3shows the unit of air volume in the atmospheric condition and N is pronounced (norumaru).
Conversion of pressurized dew point and atmospheric dew point is done by using the conversion figutre (the attached paper).