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Compressed air dryers reduce the quantity of water vapor, liquid water, hydrocarbon, and hydrocarbon vapor in compressed air. Moisture in compressed air is harmful. Water damages a compressed air system several ways

• Erosion--Water mist erodes piping, valves and other system components
• Corrosion--Mist condenses and combines with salts and acids within the system forming highly corrosive solutions
• Microbial Contamination--Moisture supplies a growth medium for bacteria and mold, which produce acidic waste and can be a health threat
• Freezing--Water can freeze in compressed air lines shutting down the system

The result is lower productivity, increased maintenance, and higher operating costs. You can minimize the damage wet compressed air can inflict on your system by drying it.

Compressed air is dried to protect the system's piping and process equipment. Dry air also protects against lost product. Most pneumatic equipment has a recommended operating pressure, dryness level, and a maximum operating temperature. Set your compressed air's systems dryness level to exceed the requirements of the equipment it powers.

Recommended Dew Points:

Audit locations that compressed air is used in your plant and piping runs to determine the dew point requirement.

Types and functions of compressed air dryers

Refrigerated Compressed Air Dryers
Refrigerated Air Dryers utilize a mechanical refrigeration system to cool the compressed air and condense water and hydrocarbon vapor. Most refrigerated air dryers cool the compressed air to a temperature of approximately 35ºF to 50°F (1.6°C to 10°C) which results in a pressure dew point range of 33ºF - 39ºF (.5°C to 3.8ºC). This range permits the pressure dew point to fall within limits that are achievable with common refrigeration system controls. This pressure dew point range is also the lowest achievable with a refrigerated design since the condensate will begin to freeze at 32ºF (0ºC).

Refrigerated air dryers are available in two basic configurations, non-cycling and cycling.

Refrigerated Air Dryers
Refrigerated Air Dryers utilize a mechanical refrigeration system to cool the compressed air and condense water and hydrocarbon vapor. Most refrigerated air dryers cool the compressed air to a temperature of approximately 35ºF to 50°F (1.6°C to 10°C) which results in a pressure dew point range of 33ºF - 39ºF (.5°C to 3.8ºC). This range permits the pressure dew point to fall within limits that are achievable with common refrigeration system controls. This pressure dew point range is also the lowest achievable with a refrigerated design since the condensate will begin to freeze at 32ºF (0ºC).

Refrigerated air dryers are available in two basic configurations, non-cycling and cycling.

Non-Cycling Compressed Air Dryer
Non-cycling compressed air dryers (fig. AD1-2) cool the compressed air in an air-to-refrigerant heat exchanger (evaporator). The warm compressed air flows into one side of the evaporator while low pressure, liquid refrigerant is metered into another side of the evaporator. The heat from the compressed air "boils" the refrigerant thereby reducing the temperature of the compressed air. Operation of the refrigeration compressor is continuous or non-cycling, therefore requiring a combination of control valves to regulate refrigerant flow as the heat load from the compressed air changes.

Cycling Compressed Air Dryers
Cycling compressed air dryers (fig. AD1-3) cool the compressed air through an intermediate heat exchanger medium. The intermediate can be sand, metal, or a fluid. Two heat exchangers, a compressed air chiller and refrigerant evaporator are fitted inside a tank which is filled with a thermal conducting fluid, usually water with propylene glycol added as a safe guard to prevent freezing and corrosion. The refrigeration system removes heat from the water/glycol fluid. The chilled fluid 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. The compressor will "cycle on" only when the fluid temperature rises to its upper limit. This cycling of the refrigeration compressor results in significant energy savings on most compressed air systems. On average, cycling compressed air dryers provide energy savings of 50% when compared to equally sized non-cycling designs.

Additional cycling compressed air dryers benefits include:

• Simplified refrigeration circuit since hot gas bypass valves are not required
• A 60% or more reduction in the required quantity of refrigerant charge
• Elimination of compressed air dryer freeze-up potential since the refrigeration system "cycles off" before freeze-up can occur. Over sizing of the compressed air dryer is therefore not a problem
• Additional energy savings since the compressed air dryer dew point can be raised to as high as 60ºF
• Microprocessor controls permit automatic dew point suppression below ambient temperature for maximum energy savings

Montreal Protocol
To protect the ozone layer from further depletion the European Union, Switzerland, United Kingdom, United States, and other World powers developed and signed the Montreal Protocol. The protocol freezes HCFC refrigerant production levels at the 1986 level. Under the protocol developed nations will phase HCFC refrigerants out by 2020. Some HCFC production will continue until 2030. HCFC refrigerants contain chlorine, which damages the ozone layer.

There are several replacement refrigerants in production. The most common replacement refrigerant is HFC-134a. It is non-flammable and is a near equivalent to HCFC. HFC-134a does not contain chlorine and therefore does not damage the ozone layer.

Desiccant Air Dryers
Desiccant Air Dryers utilize chemicals beads, called desiccant, to adsorb water vapor from compressed air. Three different types of desiccant are commonly used:

• Silica Gel - an amorphous form of silica with very good water vapor adsorbing capacity. Silica gel provides -40ºF (-40°C) to -85°F(-65°C) pressure dew point performance. A high efficiency coalescing prefilter is required to prevent damage due to liquid water (and oil) slugging. Moisture resistant silica gel is also available, but with less capacity.
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• Activated Alumina - a porous form of aluminum oxide with silicon dioxide, activated alumina also has very good water vapor adsorbing capacity and will provide -40ºF (-40°C) to -100°F (-73.3°C) pressure dew point performance. Activated alumina provides good resistance to liquid water and is the preferred desiccant for heatless air dryers, however a high efficiency coalescing prefilter is required.
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• Molecular Sieve - a porous form of zeolites of specialized desiccants formulated to remove specific vapor or gas molecules. They have excellent water vapor adsorbing capacity with low relative humidity inlet and will provide pressure dew points of -100ºF (-73.3°C) or lower. Molecular sieve is easily damaged when slugged with liquid water or oil, therefore a high efficiency coalescing prefilter is an absolute must.
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• Browse our selection of all the types of Desiccant available online.

Silica gel or activated alumina is the preferred desiccants for compressed air dryers. Molecular sieve is usually only used in combination with silica gel or activated alumina as a polishing bed.

Desiccant air dryers (fig. AD1-4) are usually configured with two pressure vessels, filled with desiccant, switching valves to direct the compressed air flow and controls for proper switching of the desiccant air dryers vessels. Single tower versions are available.

Basic operation of a desiccant air dryer consists of one drying cycle and one regeneration cycle commonly referred to as the NEMA cycle. For example, a 10 minute NEMA cycle consists of a 5 minute drying cycle and 5 minute regeneration cycle. This cycle is continuously repeated.

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 hydrocarbon 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 -40ºF (-40ºC) or lower. Since this "stripping" action by the desiccant beads is a result of a chemical reaction, it produces a small amount of heat, called "heat of adsorption." The heat produced is minimal and increases the outlet temperature only slightly.

During the regeneration cycle, the switching valves isolate the moisture saturated desiccant vessel, this "off-line" vessel is depressurized and the desiccant regeneration process begins. Following regeneration, the vessel re-pressurized with compressed air, is again ready for the next drying cycle.

Desiccant air dryers are available in two basic designs, heatless and heated. Since the drying cycle on all desiccant air dryers is similar, the difference between heatless and heated designs is found in the regeneration methods.

Deliquescent Dryers
ISS sells deliquescent dryers. For more information on our Van Air Freedom Deliquescent Dryers click here.

Deliquescent Dryers (fig. AD1-5) utilize an absorptive type chemical (desiccant) to provide a 20ºF to 25ºF (-6.6°C to -3.8°C) dew point suppression below the compressed air temperature entering the dryer. The moisture in the compressed air reacts with the absorptive material to produce a liquid effluent that is drained out of the dryer. This effluent must be disposed of in accordance with local regulations.

Deliquescent dryers are typically used in applications such as, sandblasting and logging. They are not recommended for industrial applications since the dried compressed air exiting the dryer may contain small amounts of the effluent that may be corrosive to downstream equipment

Selection and sizing recommendations

Dryness is relative (fig. AD1-6). For example for general plant air the compressed air's dew point should be at least 18°F (-7.8°C) lower than the lowest ambient air temperature encountered to avoid condensation freezing. Other applications like compressed air used in microelectronics manufacturing can require a dew point of -40ºF to -100ºF (-40ºC to -73.8ºC). There is a big range of what is considered dry. The technology employed to dry compressed air depends largely on the dew point you require.

When selecting a dryer consider several factors:

• Dew point requirement
• Inlet air temperature
• Ambient air temperature
• Operating pressure
• Airflow
• Available utilities

Inlet air temperature is a key factor. A 20°F reduction in temperature condenses 50% of the humidity out of air.

Refrigerated Air Dryers
Both non-cycling and cycling refrigerated air dryers are available from 5 scfm to sizes in excess of 20,000 scfm. When choosing between a cycling and non-cycling compressed air dryer you have to weigh the larger investment in a cycling compressed air dryer versus the potential utility savings. In general, a cycling compressed air dryer will pay back on applications with inlet flows over 200 scfm. However, this depends on how your plant consumes compressed air. If your plant has peaks and valleys in its compressed air demand, you will generally save money with a cycling compressed air dryer because the compressed air dryer will shut off during low demand times.

Manufacturers rate their compressed air dryers in accordance with recommended Standard CAGI Standard No. ADF100 for 33ºF-39ºF (.5ºC - 3.8ºC) pressure dew point. This is based on 100 psig inlet air pressure, 100ºF (37.7ºC) inlet air temperature, 85ºF (29.4ºC) cooling water temperature (water cooled units) and 100ºF (37.7ºC) ambient air temperature (air cooled units). The maximum airside pressure drop allowed is 5 psi.

Good installation practice for refrigerated compressed air dryers include:

• Minimize inlet air temperature
• Highest air pressure possible
• Lowest year round ambient air temperature available

To select a refrigerated compressed air dryer calculate a corrected flow (scfm). The example below shows how to perform the calculation.

Multiply correction factors by compressor flow (scfm) for the corrected flow (scfm).

.86 x 1.51 x 1.16 x 500 scfm (14 m3/min.)= 753 scfm (21 m3/min.)

Refrigerated compressed air dryers are rated in flow (scfm)( m3/min.). Round the corrected flow up to choose a compressed air dryer.

Refrigerated Compressed Air Dryer Correction Factors

Desiccant Air Dryers

A wide-range of desiccant air dryers are available. There are two main categories of desiccant air dryers: heated and heatless. A general rule of thumb is applications below 2000 scfm (56 m3/min) work best with heatless and applications above 2000 scfm (56 m3/min) work best with heated.

Desiccant Air Dryer Operating Cost Comparison

All desiccant air dryers have NEMA cycles. See below chart for the most common NEMA cycles. While one desiccant tower is online the other tower is regenerating.

Selecting a Heatless Desiccant Air Dryer
Advantages of heatless desiccant air dryers are they do not require additional utilities to regenerate the desiccant, long desiccant life, and a dependable design. Desiccant is reactivated by taking approximately 15% of the compressed air and channeling it through the regenerating tower (fig. AD1-12). A disadvantage of heatless desiccant air dryers is that they consume 15% of your compressed air.

To select a heatless desiccant air dryer, obtain the inlet temperature, inlet flow, and inlet pressure. The maximum operating temperature of most heatless desiccant air dryers is 120°F (48.8ºC). To calculate a corrected flow, multiply your inlet flow (scfm)(m3/min) by the pressure correction factor.

Inlet Flow (scfm) (m3/min) x Correction Factor = Corrected Flow

For example:
Inlet Temperature: 105°F (40.5°C) (within 120°F (48.8ºC) maximum)
Inlet Flow: 325 scfm (9.1 m3/min)
Inlet Pressure: 120 psig has a correction factor of 1.08

325 (9.1) x 1.08 = 351 (9.83)
Corrected Flow is 351 scfm (9.83 m3/min)

Heatless Compressed Air Dryer Pressure Correction Factors

Heated Desiccant Air Dryers
A heated desiccant air dryer regenerates by heating the desiccant either directly or indirectly. Heated desiccant air dryers have longer NEMA cycles and work well in large scfm applications. Heated desiccant air dryers are more complicated devices and require more maintenance then heatless desiccant air dryers. Constant heating and cooling of the desiccant shortens desiccant life requiring more frequent replacement. Some heated desiccant air dryer designs do not use compressed air to regenerate the towers; therefore, conserving compressed air.

Heated desiccant air dryers have heat spikes and dew point spikes when operated on a fixed cycle. The desiccant air dryer's tower coming back online after regeneration causes these changes in air quality. The desiccant is hot from regeneration. That heat dissipates as compressed air passes through it. The desiccant does not collect as much moisture when it is hot. So, some moisture can get past the desiccant air dryer until its temperature returns to normal operating temperature.

The heat spike can cause downstream filters to catch fire. To minimize this risk, use high temperature filters downstream of the desiccant air dryer. A downstream air receiver is recommended to protect downstream operations from heat spikes and dew point spikes. The air receiver will give the compressed air time to cool.

Manufacturers rate desiccant air dryer capacity in accordance with recommended Standard NFPA/T3.27.2.3M-81 (ANSI B93, 45). This rating is based on 100 psig (7 bar) inlet air pressure, 100ºF (38ºC) inlet air temperature, and 100ºF (38ºC) ambient air temperature. The maximum air pressure drop allowed is 5 psi. Standard pressure dew point performance is -40ºF (-40ºC). Many standard models can be modified to provide -100ºF (-73.3ºC) pressure dew point.

Manufacturers recommend several methods to reduce a desiccant air dryer's dew point to -100°F (-73.3°C) including:

• Shorten the NEMA cycle
• Increase the compressed air dryer's size by 37% over the recommendation for -40ºF (-40ºC)
• Change the desiccant mixture and extend the NEMA cycle on heated compressed air dryers

Types of heated desiccant air dryers include:

Selecting a heated desiccant air dryer

To select a heated desiccant compressed air dryer determine the inlet temperature, inlet flow, and inlet pressure. To calculate a corrected flow, multiply the inlet flow (scfm)(m3/min) by the pressure correction factors.

Inlet Flow (scfm)(m3/min) x Pressure Correction Factor x Inlet Temperature Correction Factor = Corrected Flow

For example:
Inlet Flow: 800 scfm (22.4 m3/min)
Inlet Pressure: 130 psig (9 bar) has a correction factor of 1.27
Inlet Temperature: 90ºF (32ºC) has a correction factor of .91

800 x 1.27 x .91 = 925 scfm (22.4 x 1.27 x .91 = 25.9 m3/min)

The corrected flow is 925 scfm (25.9 m (25.9 m3/min). From the manufacturer's selection, you would choose the closest standard model that is rated with a larger capacity. For example a 1000 scfm (28 m3/min) heated desiccant air dryer.

Heated Desiccant Air Dryer Inlet Pressure Correction Table

Heated Desiccant Air Dryer Inlet Temperature Correction Factors

Selecting a Deliquescent Dryer
To select a deliquescent dryer you need to know the system's inlet flow (scfm)(m3/min) and inlet pressure (psig)(bar). Using these two factors, select a deliquescent dryer, which is rated to handle pressure and flow.

Installation Recommendations

Dryers are installed downstream of the aftercooler. To improve a dryer's performance, install it where: the inlet air temperature is the lowest, the air pressure is the highest, and the ambient air temperature is the lowest. Depending on the type of dryer, different filter and valve configurations are recommended.

A deliquescent dryer or desiccant dryer require an oil coalescing filter upstream of the dryer. A particulate filter is required downstream of the unit. The particulate filter downstream of a heated dryer should be a high temperature design. An oil vapor removal filter may be necessary when the air must be free of hydrocarbons and organic odors. (fig. AD1-20)

Refrigerated compressed air dryers may require a coalescing filter upstream, should have a coalescing filter downstream and may require a vapor filter downstream. (fig. AD1-21)

During an emergency or maintenance, the temporary use of the bypass valve will provide a supply of non-filtered/non-dried compressed air. Critical applications should utilize redundant filtration and drying systems.

Note: All condensate discharged from the compressed air system must be disposed of in accordance with all local, state and federal regulations.

Installation Recommendations

Refrigerated Compressed Air Dryers

• 1. Replace particulate filter and coalescer elements based on pressure drop
• 2. Check drain valve on coalescer and compressed air dryer daily
• 3. Perform a preventive maintenance service once per year.
  • 4.Blow out (clean) coils
  • 5. check for leaks
  • 6. check refrigerate charge

Desiccant Air Dryers

• 1. Replace particulate, coalescer, and filter elements as necessary based on pressure drop. Replace vapor filter elements when vapors or odors are first detected.
  • 2. Inspect drain valves daily, clean as necessary.
• 3. Replace desiccant based on manufacturer recommendations.
  • 4. Inspect desiccant once a year and change based on color and condition of beads.
• 5. Calibrate dew point analyzer regularly
• 6. Once a year inspect valves replace/repair