Is your compressed air all wet? If you answered yes, you are not alone. Moisture, either liquid or vapor, is present in compressed air as it exits the compressor system. If this moisture is not properly removed, your compressed air system can lose efficiency and require dramatically increased maintenance, which can result in costly downtime. To avoid these problems, compressed air system designers have a number of purification devices available to remove the remaining water vapor and other contaminants. The proper selection of these devices is critical as pneumatic applications and compressed air systems become increasingly sophisticated. This article offers an overview and several strategies for selecting downstream purification equipment that effectively removes water and lubricant from compressed air systems.

Where Does the Water Come From?

Ambient air, which includes atmospheric humidity (water vapor), is drawn into the compressed air system where it is compressed to a desired discharge pressure. Once the compressed air is discharged, its temperature is elevated and the moisture content is high. And, if a lubricated compressor is used, a small quantity of compressor lubricant, in both liquid and vapor form, is discharged with the compressed air. Since the majority of pneumatic instruments and processes can not tolerate hot compressed air, compressors are normally supplied with aftercoolers and moisture separators.

Aftercoolers are heat exchangers that utilize either water or ambient air to cool the compressed air. As the water and lubricant vapors within the compressed air cool, a significant amount condenses into liquid. The amount of condensation is dependent upon the temperature of the air when it leaves the aftercooler. Often this temperature is referred to as the aftercooler "approach" or "CTD" and typically ranges from 10°F to 15°F above the temperature of the cooling air or water. Following this stage, the condensed water is collected and removed by the moisture separator, and discharged through a drain valve. However, it is important to remember that the compressed air is still saturated with water vapor at the discharge of the aftercooler/moisture separator. Additional condensation is generated downstream when the compressed air cools further.

Points to Review When Removing Water

Plant maintenance personnel and system designers must determine the air quality requirements for their specific compressed air applications. Here are three areas that should be addressed: · Review the air quality requirements of instrumentation, tools, and other air powered equipment, which is available from the manufacturer;
· Determine the air quality required for use in processes using compressed air, which can be obtained from the process designer; and
· Estimate the expected ambient conditions for all pneumatic equipment, processes and piping. For instance, outdoor locations during the winter months require compressed air to be dried to a lower dew point than indoor, heated locations. An oversight within any of these issues can result in misapplied purification equipment, inefficient system operation, high operating and maintenance costs and even unnecessary capital expenditures.

Selecting Purification Systems

Condensed water, lubricant, water and lubricant vapors and other contaminants are removed with a variety of purification equipment used in combination. Coalescing filters are the most common form of compressed air purification. These filters remove liquid water and lubricants from compressed air and are installed downstream in a refrigerated air dryer system or upstream in a desiccant dryer system.

Most manufacturers claim a one psi "clean and dry" pressure drop, with the normal operating (wetted) pressure drop between three and six psi. Manufacturers typically require filter changes when the pressure drop reaches 10 psi, which is approximately six to 12 months of operation. Coalescing filters will also remove particulate contamination; however, this will increase the pressure drop across the filter and shorten the filter element life. These filters are rated according to liquid particle retention size (micron) and efficiency, such as 0.50 micron and 99.99% D.O.P. efficient, or 0.01 micron and 99.9999% D.O.P efficient. High efficiency coalescing filters that feature low pressure drop (less than one psi) and long element life (five years minimum) should be specified when very low operating and maintenance costs are a requirement. These designs normally have a higher initial purchase price; yet, the resulting operating cost savings usually permit payback in under one year. When specifying a coalescing filter, be sure to confirm that the filter and element are compatible with the compressor lubricant. If these elements are not compatible, the filter system can fail and allow for contamination downstream. Coalescing filters can only remove previously condensed liquids; they do not remove water or lubricant vapors from the compressed air.

Any condensation produced from subsequent compressed air cooling will have to be eliminated. When seeking to remove water and lubricant vapors from compressed air, specify air dryers. There are three styles of air dryers that are commonly specified: deliquescent, desiccant and refrigerated dryers. Deliquescent air dryers utilize an absorptive type chemical, called a desiccant, to provide a 20°F to 25°F dew point suppression below the temperature of the compressed air entering the dryer. The moisture in the compressed air reacts with the absorptive material to produce a liquid effluent which is then drained from the dryer. Keep in mind that this effluent is typically corrosive and must be disposed of in accordance with local regulations. While deliquescent dryers are typically used in applications such as sandblasting and logging operations, they are not recommended for industrial applications since the dried compressed air exiting the dryer may contain small amounts of the effluent which may be corrosive to downstream equipment. Refrigerated air dryers remove moisture from the compressed air through a mechanical refrigeration system to cool the compressed air and condense water and lubricant vapor. Most refrigerated dryers cool the compressed air to a temperature of approximately 35°F, resulting in a pressure dew point range of 33°F - 39°F. Keep in mind that this range is also the lowest achievable with a refrigerated design since the condensate begins to freeze at 32°F. Refrigerated dryers are available in two basic configurations: Direct Expansion (non-cycling) and cycling dryers. Direct expansion dryers cool the compressed air in an air-to-refrigerant heat exchanger, called an evaporator. The warm compressed air flows into one side of the evaporator while low pressure, liquid refrigerant is metered into another side. The heat from the compressed air "boils" the refrigerant, reducing the temperature of the compressed air. Operation of the refrigeration compressor is continuous and therefore requires a combination of control valves to regulate refrigerant flow as the heat load from the compressed air changes. Thermal mass dryers cool the compressed air through an intermediate fluid. Two heat exchangers, a compressed air chiller and refrigerant evaporator are fitted inside a tank which is filled with a thermal conducting fluid, which is usually a water and propylene glycol mixture.

The refrigeration system removes heat from the fluid, which in turn, removes heat from the compressed air. Since the refrigeration system is used to only cool the fluid, the refrigeration compressor is "cycled off" once the fluid temperature is chilled to the required point. This cycling of the refrigeration compressor results in significant energy savings on most compressed air systems. On average, cycling dryers provide energy savings of 50 percent when compared to equally sized non-cycling designs. Cycling dryers offer a simplified refrigeration circuit, a reduction of 60 percent or more in the required refrigerant, an elimination of dryer freeze-up potential and an increased energy savings since the dryer dew point can be raised to as high as 60°F. Ingersoll-Rand's Thermal Mass dryers also offer microprocessor controls that permit automatic dew point suppression below ambient temperature for additional energy savings. While the initial purchase price of a cycling dryer can be 25 percent or more above an equally sized non-cycling unit, the energy savings potential of cycling designs usually provide a payback period of less than one year. Desiccant dryers utilize chemicals beads, called desiccant, to adsorb water vapor from compressed air. Silica gel, activated alumina and molecular sieve are the most common desiccants used. (Silica gel or activated alumina are the preferred desiccants for compressed air dryers.) The desiccant provides an average -40°F pressure dew point performance. Molecular sieve is usually only used in combination with silica gel or activated alumina on -100°F pressure dew point applications. Desiccant dryers are configured with two pressure vessels, filled with desiccant, switching valves to direct the compressed air flow and controls for proper switching of the dryer vessels.

Basic operation of a desiccant dryer consists of one drying cycle and one regeneration cycle commonly referred to as the NEMA cycle, which is continuously repeated. For example, a 10 minute NEMA cycle consists of a five minute drying cycle and a five minute regeneration cycle. During the drying cycle, compressed air, at full pressure, flows through one desiccant vessel. As the air flows through the desiccant bed, microscopic pores on the surface of the desiccant beads "strips" the water vapor and lubricant molecules from the air, thereby reducing the relative humidity of the air. The relative humidity of the dried air is equivalent to a pressure dew point of -40°F or lower. Desiccant dryers are available in two basic designs: heatless and heated. Since the drying cycle on all desiccant dryers is similar, the difference between heatless and heated designs is found in the regeneration methods.

Heatless dryers utilize a combination of dry purge air (approximately 14 percent of the compressed air leaving the dryer at 100 psig), depressurization and the "heat of adsorption" for desiccant regeneration. Heatless dryer cycles are usually 10 minutes (five min. drying, five min. regenerating). Heatless dryers are the most popular desiccant dryers used in industry and offer several advantages, including: · Consistent -40°F (or -100°F) pressure dew point performance;

· Three to five year desiccant life, provided prefilters are properly maintained;
· Simple, long life switching valves requiring minimal maintenance;
· Simple and reliable operation; and
· Lowest purchase price of all desiccant dryers. The single disadvantage of the heatless design is the relatively high purge air consumption which results in the highest operating costs and reduces the amount of compressed air available for use in the plant.

Microprocessor controls are available to match purge consumption to actual compressed air demand, which can actually reduce operating costs. When compressed air is not available for purge consumption or when utility costs are very high, heated dryers become the preferred alternative to heatless designs. Heated desiccant dryers are available in three configurations: internally heated, externally heated and heat of compression. All three configurations regenerate the desiccant bed with a combination of heat to desorb the water vapor molecules from the desiccant beads and purge air which delivers the heat to the desiccant bed and carries the moisture out of the bed. Benefits will vary for each of the three configurations depending on applications, so consult the supplier to determine the best format for specific applications.

Maintenance of desiccant dryers vary depending on the dryer style. Heatless dryers will require desiccant replacement every three to five years while desiccant is replaced every one to two years on heated dryers. In addition, switching valves require inspection and possible rebuild annually. Blower and venturi intake filters must be cleaned or replaced and the blower motor bearings lubricated per the manufacturers instructions. In short, compressed air systems can produce dry air, provided a comprehensive plan is developed to establish the air quality requirements. To devise an appropriate plan, identify the source of the moisture and contaminants and analyze the dryer construction features and system layout before selecting a specific system (see side bar). Development of this plan can be simplified with the selection of a qualified compressed air system supplier. Qualified suppliers should be capable of understanding individual compressed air requirements, be an expert on the application and function compressed air system components and provide sound direction on the total system installation. Since all systems require maintenance and occasional repairs, the system supplier also should have a qualified service organization available to service systems regularly.

System ComparisonSystem A

The designer of system A selected a non-cycling refrigerated dryer. The heat exchangers of this dryer are constructed with a finned tube design. The dryer manufacturer required that prefilters, to remove dirt and oil, must be installed at the dryer inlet to prevent fouling of the dryer heat exchangers. A coalescing filter is also required downstream of the dryer to protect the air system in the event of a drain valve failure or plugged drain line. As a result, each filter has an initial pressure drop of three psi and requires element replacement when the pressure drop reaches 10 psi. The dryer pressure drop is five psi. Total pressure drop is 14 psi (new filter elements) and 35 psi (dirty).

System B

The designer of system B selected a cycling refrigerated dryer. The heat exchangers of this dryer are constructed with a smooth bore tube design. The dryer manufacturer does not require prefilters. A long life, low pressure drop coalescer selected for downstream from the dryer. As a result, the filter pressure drop is less than 1 psi over a life of five years. The dryer pressure drop is five psi. Total pressure drop is only six psi.

Conclusion

Assuming that both systems require a plant operating pressure of 90 psig, the air compressor in System A must run at 125 psig to overcome the 35 psi system pressure drop, while the compressor in System B will only run at 96 psig. To overcome the higher pressure drop, the compressor in System A will consume significantly more electrical power. The cost comparison shows the cost of one pound of pressure drop on a 100 hp compressor to be $247 per year.

While both systems would meet the air quality requirements, System B significantly reduces both operating and maintenance costs

Reference :
Website http://air.irco.com

Back