DRYING COMPRESSED AIR
All atmospheric air contains water vapour, more at high temperatures and less at lower temperatures. When the air is compressed the water concentration increases.
Example, a compressor with a working pressure of 7 bar and a capacity of 200 l/s that draws in air at 20°C with a relative humidity of 80% will give off 80 litres of water in the compressed air line during an eight hour working day.
The term pressure dew point (PDP) is used to describe the water content in the compressed air. It is the temperature at which water vapour transforms into water at the current working pressure.
Low PDP values indicate small amounts of water vapour in the compressed air. It is important to remember that atmospheric dew point can not be compared with PDP when comparing different dryers.
Example, a PDP of +2°C at 7 bar is equivalent to -23°C at atmospheric pressure.
To use a filter to remove moisture (lower the dew point) does not work. The reason is because further cooling means continued precipitation of condensation water.
You can select 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 costs for air drying.
Refrigerant drying means that the compressed air is cooled, whereby a large amount of the water condenses and can be separated. After cooling and condensing the compressed air is reheated to around room temperature so that condensation does not form on the outside of the pipe system. Cooling of the compressed air takes place via a closed coolant system. By cooling the incoming compressed air with the cooled air in the heat exchanger the energy consumption of the refrigerant dryer is reduced. Refrigerant dryers are used with dew points between +2°C to +10°C and are limited downwards by the freezing point of the condensed water.
Over-compression is perhaps the easiest method to dry compressed air. Air is first compressed to a 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.
Adsorption drying is a chemical process, where water vapour is bound to the adsorption material. The absorption material can either be a solid or liquid. Sodium chloride and sulphuric acid are frequently used, which means the possibility of corrosion must be taken into consideration. This method is unusual and has a high consumption of adsorption material. The dew point is only lowered to a limited degree.
This method of drying produces compressed air with very low dew point (-20 °C to 70 °C). Drying by adsorption is a physical process. The water vapour in the compressed air is attracted to the surface of solid adsorption materials (desiccants) by molecular forces. Different types of desiccants are used in adsorption dryers like: activated alumina (standard), silicagel and molecular sieves. When the drying capacity of the desiccant has been used it must be regenerated, where after it is again active for drying. To enable continuous drying and regeneration to go on, the adsorption dryers have two drying towers that alternate functions: drying regeneration by means of an automatic valve system. Adsorption dryers can be classified depending on the type of regeneration used: Heatless, Heat reactivated, Rotating drum Adsorption dryers only handle water vapour. Since water and oil droplets can impair their efficiency, a filter is necessary to remove such droplets before they reach the dryer. Dust filters need to be installed down stream the adsorption dryers to stop any desiccant dust which might leave the dryer.
HOW IT WORKS
Water molecules are transported into the pores trough diffusion. Molecules are accumulated on the pore surface due to: physical binding chemical binding capillary-condensation
Water vapour transfers to the desiccant because. desiccant has a greater attraction for water than air. but The attraction slowly decreases as the desiccant gets wet meaning that the wet layer slowly rises up the tower, as dry desiccant is more attractive than wet desiccant. After 3 minutes (cycle time), the wet layer is almost at the top of the dryer. This means: That, the amount of water being taken out of the air is stable, and therefore a dew point -40°C is constant from the beginning to the end of the cycle. Also: If the dryer vessel continues to accept air, the PDP would slowly decay. After about 45 minutes it would be around -20°C. If the PDP becomes positive, the desiccant is ruined Finally: For a -70°C PDP, more water attractive desiccant is needed.
THINGS TO REMEMBER
The more desiccant used, the longer the vessel can keep the desire dew point. Dryer give the most optimum performance when the desiccant is suited to the PDP required In order that the dryer performance to reliable and stable, we add slightly more desiccant than the minimum required Crucial Parameters If the air temperature increases, the wet layer reaches the top more quickly. Pressure does NOT directly effect the dryer performance. BUT A desiccant bed is sized for a fixed volume of inlet gas, meaning the higher the pressure the more FAD can be dried The lower the nominal PDP, the less desiccant is needed for a fixed flow Why Do We Use Different Desiccant ? Different desiccant have different levels of water attraction Most desiccant cannot provide a -70°C PDP. Desiccant that can provide a -70°C PDP is more expensive What Desiccant Do We Use ? For PDPs of -20 to -40°C PDP Activated Alumina For PDPs of -70°C PDP Molecular Sieve
ADSORPTION DRYING TYPES
Heat less adsorption dryers:
The regeneration is carried out by cold dried air, it is expanded to just over atmospheric pressure, which makes it extremely dry. For this type of regeneration, approximately 15 % of the dried air is used. These dryers are very simple in design, as no heating equipment is required. This results in very low electric power consumption. The size of the drying towers can be kept very small because the drying/regeneration cycle is very short (1-2 minute).
Aftercoolers An aftercooler is a heat exchanger, which cools the hot compressed air to precipitate the water that otherwise would condensate in the pipe system. It is water or air cooled, generally equipped with a water separator with automatic drainage and should be placed next to the compressor. 80-90% of the precipitated condensation water is collected in the aftercooler's water separator. A common value for the temperature of the compressed air after the aftercooler is approx. 10°C above the coolant temperature, but can vary depending on the type of cooler. An aftercooler is used in virtually all stationary installations. In most cases an aftercooler is built into modern compressors.
Particles in an air stream that pass a filter can be removed in several different ways. If the particles are larger than the opening in the filter material they are separated mechanically. This usually applies for particles greater than 1 mm. The filter's efficiency in this regard increases with a tighter filter material, consisting of finer fibres. Particles between 0.1 µm and 1 µm can be separated by the air stream going around the filter material's fibres, while the particles through their inertia continue straight on. These then hit the filter material's fibres and adhere to the surface. The efficiency of the filter in this regard increases with an increased flow velocity and a tighter filter material consisting of finer fibres. Very small particles (<0.1 µm) move randomly in the air stream influenced by collisions with air molecules. They "hover" in the air flow changing direction the whole time, which is why they easily collide with the filter material's fibres and adhere there. The efficiency of the filter in this regard increases with a reduction in the stream velocity and a tighter filter material consisting of finer fibres. The separating capacity of a filter is a result of the different sub-capacities as set out above. In reality, each filter is a compromise, as no filter is efficient across the entire particle scale, even the effect of the stream velocity on the separating capacity for different particle sizes is not a decisive factor. For this reason particles between 0.2 µm and 0.4 µm are the most difficult to separate. The separating efficiency for a filter is specified for a specific particle size.
A separation efficiency of 90-95% is frequently stated, which means that 5-10% of all particles in the air go straight through the filter. Furthermore, a filter with a stated 95% separation efficiency for the particle size 10 µm can let through particles that are 30-100 µm in size. Oil and water in aerosol form behave as other particles and can also be separated using a filter. Drops that form on the filter mater-ial's fibres sink to the bottom of the filter due to gravitational forces. The filter can only separate oil in aerosol form. If oil in vapour form is to be separated the filter must contain a suitable adsorption mate-rial, usually active carbon. All filtering results in a pressure drop, that is to say, an energy loss in the compressed air system. Finer filters with a tighter structure cause a greater pressure drop and become blocked more quickly, which demands more frequent filter replacement resulting in higher costs. Accordingly, filters must be dimensioned so that they not only handle the nominal flow, but also have a greater capacity threshold so they can manage a pressure drop due to a degree of blockage.