All atmospheric air contains water vapors, more at high temperatures and less at lower temperatures. When the air is compressed the water concentration increases.
Pressure Dew point
The term of pressure Dew Point (PDP) is use to describe the water content in the compressed air it is the temperature at which water vapors transforms into water at the current working pressure. Low Pressure Dew Point values indicate small amount of water vapors in the compressed air.
It important to remember that the atmospheric dew point can not be compared with PDP when comparing different dryers, After selecting the main type of drying equipment based on the Pressure Dew Point. Seen from a cost point of view, the lower the dew point required the higher the acquisition and operating cost for air drying. In the principle, there are four method to remove cooling from compressed air, cooling, over-compression adsorption and adsorption. There is equipment available, based on these methods for different types of compressed air systems.
An after-cooler is a heat exchanger, which cools the hot compressed air to precipitate the water that otherwise could condensate in the pipe system. It is water of air cooled, generally equipped with a water separator with automatic drainage and should be placed next to the compressors. 80 to 90% of the precipitated condensation water is collected in the after-cooler’s water separator. A common value for the temperature of the compressed air after the after-cooler is approx. 50°F above the cooling temperature, but can vary depending of the type of cooler. An after-cooler it’s built into modern compressors.
Refrigerated Air Dryer
Refrigerant drying means that the compressed air is cooled, where by a large amount of water condensate and can be separated. After cooling and condensing the compressed air is reheated to around the room temperature so that condensation does not form on the outside of the pipe system. Cooling of the compressed air takes place via a close coolant system. By cooling the incoming compressed air with the cooled air in the heat exchanger the energy consumption of the refrigerant dryer is reduce. Refrigerant dryers are used with dew-point between 35 to 39°F and are limited downward by the freezing point of the condensate water.
Over-compression is perhaps the easiest method to dry compressed air. Air is first compressed to higher pressure than the intended working pressure, which means the concentration of water vapour increases. Thereafter the air is cooled, whereby the water is separated. Finally the air is allowed to expand to the working pressure, whereby a lower PDP is attained. However, this method is only suitable for very small air flow rates, due to the high energy consumption.
Absorption drying is a chemical process where the water vapor is bound to the absorption material can either be a solid or liquid. Sodium chloride and sulphuric acid are frequently used, which means a possibility of corrosion must be taken into consideration. This method is unusual and has a high consumption of absorption material. The dew point is only lowered to a limited degree.
There are two types of adsorption dryer, cold regenerative and hot regenerative. Cold regenerative dryers are best suited to smaller air flow rates. The regeneration process takes places with the help of compressed air and requires approximately from 15 to 20% of the dryer nominal capacity @ 100 PSIG working pressure. Hot regenerative adsorption drying regenerative the desiccant by means of electrical of compressor heat, which is more economical than cold regeneration. Very low dew point can be obtained.
Guaranteed separation and drainage of the condensation water shall always be arrangement before adsorption drying. If the compressed air has been produced using oil lubricated compressors, an oil separating filter should also be fitted before the drying equipment. In most case a particle filter after absorption drying.
Heat-of- Compression dryer
There are adsorption dryers for oil free screw compressors that use the heat from the compressor to regenerate the desiccant. These types of dryers are generally fitted with a rotating drum with desiccant of which one sector (a quarter) is regenerated by means of a partial flow of hot compressed air (250 to 390°F) from the compressor stage. Regenerated air is then cooled, the condensation drained and the air returned via the ejector to the main air flow. The rest of the drying drum’s surface (three-quarters) is used to dry the compressed air from the compressor’s aftercooler. The system gives no compressed air losses. The power requirements for such a dryer is limited to that requirements for such a dryer is limited to the required for powering the drum, for example, a dryer with a capacity of 1000 l/s only requires 120 W. In addition, no compressed air is lost and neither oil nor particle filters are not required.