Compressed Air Dryer in Textile Industry

MOISTURE

      Any average atmosphere contains as much as 8000 ppm of water particles at 45°C and at 7kg/cm² pressure. In other words, at 75% humidity a 100cfm compressor takes in 18 Gallon of water vapour when it runs for a full day. Even a very efficient after cooler will be able to only remove 60% of moisture. The remaining moisture is carried along the line to causes scales, rust and all kinds of problem in the pneumatic equipments and also contaminates with anything to which this air gets in contact.

      As an example, if we assume air is compressed from 10 ft³ to 1 ft³ the pressure is increased to 132 psig (9.10bars) (P₁V₁ = P₂V₂). Here, P₁V₁, equals original pressure and volume, and P₁ and P₂ must be stated as absolute pressure for calculation purposes. At 132 psig (9.10 bars) and 80°f (27°C), the 1 ft³ volume of air can hold only 0.00002 gal (0.00008 litre). Since there were 0.0061 litre in the air, but it can now hold only 0.00002 gal (0.00008 litre, the excess moisture will condense. If the excess water is not removed and the pressure is reduced to atmospheric, the excess water will gradually evaporate back into the air until an equilibrium is established. This will happen because the air under this condition can again hold 0.0016 gal (0.0061 litre) of water vapor. If the condensed water is removed as shown in Fig.A and the pressure again reduced as shown in Fig.A. The excess water will not be available for evaporation back into the air. The water vapor content of the 10 ft³ will than be 0.00002 gal (0.00008 litre), which was the maximum vapor content that 1 ft³ of air can hold at 80°F (27°C) and 132 psig (9.10 bars). The 0.00002 gal (0.00008 litre) per 10 ft³ thus determined is 0.002 gal per 1000 ft³. The 0.002 gal per 1000 ft³ is less than any quantity listed for 80°F (27°C). Therefore, the relative humidity is less than 5%.

DEW POINT

      A more useful term than relative humidity for indicating the condition of water vapor in a compressed air system is dew point. The dew point is the temperature at which condensate will begin to form if the air is cooled at constant pressure. At this point, the relative humidity is 100 per cent. In Fig. It will be noted that the dew point is equal to the saturated air temperature and follows the air temperature until it reaches its lowest point of 40°F (4.4° C). At this point, the air temperature was increased but the dew point remained at 40°F since the excess moisture had been removed. No further condensation would occur unless the temperature dropped below 40°F. This is further illustrated in Fig.C. If a refrigerant air dryer set at +35°F (+1.7°C) were installed at point A, Fig.B, it would cool the air to 35°C, and the moisture in the air would condense and be removed at this point. No further condensation would occur unless the temperature somewhere in the system were dropped below 35°F.

It should be noted that as air leaves a compressor it is under both an elevated pressure and elevated temperature. A delicate balance exists under this condition since air under pressure has less capacity for water vapor, where as air at elevated temperatures has a greater capacity for water vapor. The air leaving the compressor, however, is generally saturated, and any reduction in air temperature will cause water to begin to condense inside the downstream piping.

PRESSURE DEW POINT

      Refrigerant driers are rated at pressure dew points whereas desiccant driers have been traditionally rated at atmospheric dew points. However, there is a trend toward rating desiccant driers at the pressure dew point as well. A pressure dew point temperature is the more meaningful, since it indicates the temperature at which water vapor will begin to condense inside a pipeline at a given pressure. For reference purposes, pressure dew points can be converted to atmospheric dew points by use of the graph shown in Fig.C. To convert a 100 psig (6.9 bars), 35°F (1.7°C) pressure dew point to atmospheric dew point, one draws a vertical line midway between 30 and 40°F (-1 and +4°C) on the bottom scale until it intersects with the 100 psig line: then move to the left to the vertical coordinate. The atmospheric dew point will be -12°F (-24°C). For a 100 psig (6.9 bar), 50°F (10°C) dew point, the atmospheric dew point will be approximately 80°F (-18°C).