Contaminants in compressed air adversely affect all components of the air distribution system Wet, dirty compressed air costs money by: Robbing the system of useful power Air lines rust and leak The power and efficiency of air tools decreases Increasing maintenance and repair expenses: Lubricants wash out of air tools Solid particles abrade wear surfaces Contributing to product rejects: The color/adherence of paint spraying is affected Product spoilage in food processing is promoted With the proper compressed air treatment equipment, system power is maintained, operating expenses are reduced, and production quality is improved. The cost of using wet air far exceeds the cost of keeping it dry! Where does moisture come from? Atmospheric air always contains moisture,or water vapor. The amount of moisture air can retain is determined by its temperature and, to a lesser extent, its pressure. Any sufficient drop in temperature or increase in pressure will cause moisture to condense out of the air. Relative humidity is the amount of moisture that is present in the air, compared to the total amount the air could hold at a certain temperature. For example: One cubic foot of air at 80°F, 27°C can hold about 12 grains of moisture. If it is holding all 12 grains, it is said to be at 100% relative humidity or saturated. If that same cubic foot of air is holding only 6 grains of moisture, it is at 50% relative humidity. Saturation point is the point at which air is holding all the water vapor it can (100% relative humidity). If we added more water vapor to saturated air, the water vapor would condense into a liquid. Remember that a drop in temperature or increase in pressure will cause moisture to condense out of air that is saturated. Condensation is the changing of water vapor into a liquid. Dew point is the temperature at which water vapor in the air condenses into a liquid. Pressure dew point is more meaningful because it indicates, at a given pressure, the temperature at which condensate forms in the compressed air lines. How do these concepts relate to a compressed air system? Contaminants in compressed air adversely affect all components of the air distribution system Water vapor and other contaminants in the ambient air enter the compressor intake. One cubic foot of air at atmospheric pressure contains: Liquids – Water droplets Solids – Dust and pipe scale Gases – Oil and water vapors Once atmospheric air passes through a compressor operating at 100 psig; 7 bar, the air is compressed to 1/8th its previous volume, yet still contains the same amount of contaminants. The increase in pressure would normally cause moisture to condense out of the air, however, during the compression process, the temperature of the air rises due to frictional heat, increasing its ability to hold water vapor. As air leaves the compressor and travels throughout the system, it begins to cool. Once the air temperature drops below the pressure dew point temperature, water droplets begin to condense. Condensation is the changing of water vapor into a liquid. It is necessary to remove the moisture and contaminants from the air system in order to lower the dew point and avoid operating problems, costly maintenance, and repair expenses. Important Guideline: For every 20°F, 11°C drop in compressed air temperature, the moisture holding capacity of air is reduced by 50%. How are moisture and contaminants removed? Aftercoolers are a good first step. They lower the compressed air temperature to safe, usable levels and remove up to 70% of the water vapor; however, the air is still saturated. A further drop in temperature will cause additional condensation to occur in downstream air lines. Water vapor and other contaminants in the ambient air enter the compressor intake. Air dryers remove the water vapor and lower the dew point temperature of compressed air. This prevents liquid water from forming downstream, but does not eliminate all the entrained contaminants. Separators and filters remove the liquid water, and solid and gaseous contaminants that adversely affect the air system. Drains discharge the accumulated water and liquid contaminants from various points throughout the air system. Condensation is the changing of water vapor into a liquid. Why and when should I clean the condensing unit on my refrigerated air dryer? Dust and dirt that accumulate on the condenser coil of an air-cooled refrigerated dryer will hamper the dryer’s ability to produce a consistent outlet dew point, and ultimately to function at all. The condenser coil should be cleaned a minimum of once a month to prevent this from happening. How do I select the right air dryer? When selecting a compressed air dryer, consider these factors and then determine the most cost-effective system suitable for the application. Compressed air dryers vary in relation to their dew point, initial cost, and ongoing maintenance requirements. Choose a dew point temperature below the lowest ambient temperature to which your compressed air system will be exposed. Consider air lines: Located in front of open doors or windows Throughout air conditioned or unheated areas Running underground or between buildings Determine which dryers will produce the required dew point. Consider initial and operating costs. The lower the dew point, the more expensive the dryer is to purchase and operate How do I calculate the correct size dryer? Once the appropriate drying system has been selected, determine the actual conditions under which the dryer will be operating. Flow capacity (scfm, Nm3/hr, Nm3/min, l/sec) Pressure dew point (°F, °C) Inlet air pressure (psig, bar, kg/cm2) Inlet air temperature (°F, °C) Ambient or cooling water temperature (°F,°C) Maximum allowable pressure drop (psi, bar, kg/cm2) In North America, in accordance with CAGI ADF100, conditions for rating compressed air dryers are: 100 psig (7 bar), 100°F (38°C) saturated inlet, 85°F (25°C) cooling water, 100°F (38°C) ambient air and a maximum 5 psi (0.35 bar) pressure drop. In Europe, and much of the rest of the world, the ISO 7183 standard for rating compressed air dryers is used: 7 bar (100 psi), 35°C (95°F) inlet, 25°C (77°F) ambient air and a maximum 0.35 bar (5 psi) pressure drop. When operating at other than rated conditions, adjustments to the dryer’s rated flow capacity must be made.
