Is your compressed air system all wet?
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 system.
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 system. If this moisture is not properly removed, a compressed air system can lose efficiency and require dramatically increased maintenance, which can result in costly downtime.
To avoid these problems, a number of purification devices are available to remove water vapor and other contaminants. The proper selection of these devices is critical as pneumatic applications and compressed air systems become increasingly sophisticated.
Where does water come from?
Ambient air, which includes atmospheric humidity as water vapor, is drawn into the compressed air system where it is compressed to a desired discharge pressure. When the compressed air is discharged, its temperature is elevated and the moisture content is high. Since the majority of pneumatic instruments and processes cannot tolerate hot compressed air, compressors are normally supplied with aftercoolers and moisture separators.
Aftercoolers are heat exchangers that use water or ambient air to cool the compressed air. As the water vapor within the compressed air cools, a significant amount condenses into liquid. Condensed water is collected and removed by the moisture separator, then it is discharged through a drain valve.
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.
Selecting purification systems
Several methods are used to remove moisture.
Liquid water removal
Coalescing filters (Fig. 1) are the most common piece of equipment used for compressed air purification. These filters remove liquid water, not vapor, from compressed air and are installed downstream in a refrigerated air dryer system or upstream in a desiccant dryer system.
Most manufacturers claim a 1-psi "clean and dry" pressure drop, with a normal operating pressure drop between 3 and 6 psi. Typically, filter changes are required when the pressure drop reaches 10 psi, which is after approximately 6-12 mo of operation. Coalescing filters also remove particulate contamination; however, this action increases the pressure drop across the filter and shortens filter element life.
High-efficiency coalescing filters that feature a pressure drop of less than 1 psi and 5-yr minimum element life should be specified when very low operating and maintenance costs are a requirement. These designs normally have a higher initial cost, but the operating cost savings usually provide a payback in less than 1 yr.
When specifying a coalescing filter, confirm that the filter and element are compatible with the compressor lubricant. If not, the filter system can fail and cause contamination downstream.
Water vapor removal
Specify air dryers to remove water vapor from compressed air. There are three styles of air dryers commonly specified: deliquescent, refrigerated, and desiccant.
Deliquescent air dryers use an absorptive chemical, called a desiccant, to provide a 20-25 deg F dew point suppression below the temperature of the compressed air entering the dryer. Moisture in the compressed air reacts with the absorptive material to produce a liquid effluent, which is then drained from the dryer. This effluent is usually corrosive and must be disposed of in accordance with local regulations. The desiccant must be replenished regularly.
While deliquescent dryers are typically used in applications such as sandblasting, they are not recommended for industrial jobs since the dried compressed air exiting the dryer may contain small amounts of effluent which can be corrosive to downstream equipment.
Refrigerated air dryers remove moisture from compressed air through a mechanical refrigeration system that cools the compressed air and condenses water vapor. Most refrigerated dryers cool compressed air to approximately 35 F, resulting in a pressure dew point range of 33-39 F. This range is also the lowest achievable with a refrigerated design since condensate begins to freeze at 32 F.
Refrigerated dryers are available in two basic configurations: direct expansion (noncycling) and cycling.
Direct expansion dryers cool compressed air in an air-to-refrigerant heat exchanger called an evaporator. Operation of the refrigeration compressor is continuous and requires a combination of control valves to regulate refrigerant flow as the heat load from the compressed air changes.
Cycling dryers shut off the refrigeration compressor once the fluid temperature is chilled to the required point. This cycling of the refrigeration compressor can result in energy savings of up to 50% when compared to equally sized, noncycling designs.
While the initial purchase price of a cycling dryer can be 25% above an equally sized noncycling unit, the energy savings potential of cycling designs usually provides a payback period of less than 1 yr.
Desiccant dryers use chemical beads 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. Molecular sieves are used in combination with silica gel or activated alumina for -100-F pressure dew point applications.
Desiccant dryers are available in two 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 method of regenerating the desiccant.
Heatless dryers utilize a combination of dry purge air, approximately 14% 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 min.
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 designs: internally heated, externally heated, and heat of compression (Fig. 2). All three regenerate the desiccant bed with a combination of heat and purge air. Benefits vary for each type, depending on the application. Consult the supplier to determine the best format for specific applications.
Maintenance of desiccant dryers varies, depending on the dryer design. Heatless dryers require desiccant replacement every 3-5 yr while desiccant is replaced every 1-2 yr on heated dryers.
Switching valves require inspection and maintenance annually. Blower and venturi intake filters must be cleaned or replaced and blower motor bearings lubricated regularly.-- Edited by Joseph L. Foszcz, Senior Editor , 630-320-7135, firstname.lastname@example.org
Aftercoolers remove liquid moisture but the compressed air is still warm and saturated with water vapor.
Coalescing filters eliminate water that has condensed; they do not remove water vapor.
Water vapor is removed by a deliquescent, refrigerated, or desiccant-type air dryer.
How much water should be removed?
- Review the air quality requirements of instrumentation, tools, and other air-powered equipment.
- Determine the air quality required for use in processes.
- Estimate the expected ambient conditions for all pneumatic equipment, processes, and piping. 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 unnecessary capital expenditures.
Advantages of heatless desiccant dryers
- Consistent -40 F or -100 F pressure dew point performance
- A 3-5 yr desiccant life, provided prefilters are properly maintained
- Long-life switching valves requiring minimal maintenance
- Simple and reliable operation
- Lowest purchase price of all desiccant dryers
The disadvantage of the heatless design is relatively high purge-air consumption resulting in high operating costs and reduced amounts of compressed air available for use in the plant. Microprocessor controls are available to match purge consumption to actual compressed air demand, which can reduce operating costs.
The designer selected a noncycling refrigerated dryer. The heat exchangers in this dryer have a finned tube design that requires prefilters to remove dirt and oil. The prefilters must be installed at the dryer inlet to prevent fouling of the heat exchangers.
A coalescing filter is required downstream of the dryer to protect the air system if a drain valve fails or a drain line gets plugged. As a result, each filter has an initial pressure drop of 3 psi and requires element replacement when the pressure drop reaches 10 psi.
The designer of this system selected a cycling refrigerated dryer. The heat exchangers are constructed with a smooth-bore tube design. The dryer does not require prefilters. A long-life, low-pressure-drop coalescer was installed downstream from the dryer. As a result, the filter pressure drop is less than 1 psi over a life of 5 yr.
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 runs at 96 psig. To overcome the higher pressure drop, the compressor in System A will consume significantly more electrical power. While both systems meet air quality requirements, System B significantly reduces energy as well as operating and maintenance costs.
Component System A System B
400-scfm noncycling dryer, Kw 4.84 --
400-scfm cycling dryer, Kw -- 2.7
Load factor, % 100 50
Total input, Kw 4.84 1.35
Annual operating cost, $ 2710 756
(based on 8000 hr/yr and $0.07/Kwh)
Cost of filtration System A System B
Total pressure drop, psi 26 6
Annual cost/psi, $ 247 247
Total pressure drop cost, $ 6422 1482
Total annual cost, $ 9132 2238
System B savings, $ -- 6894
System A pressure drop, psi
Component New Dirty Average
Prefilter 3 10 7
Coalescer 1 3 10 7
Air dryer 5 5 5
Coalescer 2 3 10 7
Total 14 35 26
System B pressure drop, psi
Component New Dirty Average
Prefilter 0 0 0
Coalescer 1 1 5 1
Air dryer 5 5 5
Coalescer 2 0 0 0
Total 6 10 6
Cost of pressure drop
(rated hp) (service factor) (Kw/hp)
P = --------------------------------
(motor eff.) (power factor)
P = compressor power consumption, Kw
Rated hp = 100
Service factor = 1.10
Kw/hp = 0.746
Motor eff. = 93%
Power factor = 1.0
(100) (1.10) (0.746)
P = ---------------------- = 88.24 Kw
C = (P) (D) (annual run time, hr) (cost/Kwhr, $)
D = 1-psi pressure drop requires 0.5% additional power by the compressor
C = annual cost of 1-psi pressure drop, $
C = (88.24) (0.005) (8000) (0.07) = $247
The author is available to answer questions about removing moisture from compressed air. He can be reached at 704-896-4594.
Two recent articles discussed moisture and its removal from compressed air systems: "Choosing the Right Compressed Air Dryer" (PE, April 1997, p 80, File 4030), and "Importance of Dew Point in Compressed Air Systems" (PE, July 1996, p 72, File 4030).
Texts of articles are available on the Plant Engineering web site: www.plantengineering. com. Complete articles may be purchased by calling 630-320-7134.
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