Air Dryers

Several technologies for air drying are available, with the most common detailed here. The moisture content is typically expressed in pressure dewpoint temperature (PDP), which is the temperature at which liquid water begins to condense as the air is cooled.

Refrigerated Air Dryers

Dewpoint: Down to 38°F
Energy Source: Electric
Energy Consumption:  Approx. 3-6% of compressor power (including refrigeration system and pressure drop effects)

The majority of compressed air dryers are of refrigerated design.  These dryers use refrigerant to cool the incoming compressed air to near freezing.  As the air cools, moisture condenses to liquid water, and is drained away.  The cool exiting air then typically exchanges heat with the warm incoming air to pre-cool the inlet air (reducing the required refrigeration system), and re-heat the outgoing air (to move the air temperature away from the dewpoint and prevent pipe sweating).

These dryers reduce moisture level to around 38°F dewpoint (any lower risks ice formation, which would block flow through the dryer), and are well suited to air systems that are not exposed to freezing temperatures and simply require the absence of liquid water.

Traditionally, these dryers are controlled using a hot gas bypass valve on the refrigeration circuit to maintain refrigeration temperature, but this provides no energy savings.  Energy saving controls are becoming more common on refrigerated dryers, and can include variable speed refrigeration compressors, cycling refrigerant compressors with thermal storage (masses of sand, glycol, or excess refrigerant), and unloading scroll compressors.

Some refrigerated dryers require pre-filters to prevent heat exchanger fouling, depending upon their design, but most do not.

Desiccant Air Dryers

Desiccant dryers (also called regenerative air dryers) are comprised of two pressure vessels containing adsorbent desiccant materials that remove water from the air stream.  At any given time, one tower is drying, while the other is regenerating (having the moisture stripped back out of the adsorbent desiccant).  There are three main sub-classes of desiccant dryer, differentiated by the method of regenerating the desiccant.

Heatless Desiccant Dryers (Pressure Swing Dryers)

Dewpoint: Down to -100°F
Energy Source:  Electric (controls only), Compressed air (regeneration)
Energy Consumption: Approx. 15 to 23% of compressor power (including compressed air usage and pressure drop effects of dryer and required filtration)

Heatless desiccant dryers regenerate the offline desiccant tower by taking approximately 15% of their rated capacity of dry compressed air, and expanding it through the regenerating tower. Once expanded, this dry air is extremely dry, and then has a greater affinity for moisture than the desiccant beads. This “purge air” and moisture are then exhausted from the dryer to atmosphere.

These dryers reduce moisture level to as low as -100°F (-40°F is typically standard). This makes them well suited to systems that are exposed to the elements in colder climates, or for feeding processes that are extremely sensitive to moisture (even in vapor form).

These dryers are controlled using a timer to cycle the towers. Energy saving controls are typically optional, and are used to regenerate the offline tower, stop purging air, and leave that tower in standby until the online tower becomes saturated.  This allows the dryer to match energy consumption to load.

All desiccant dryers require oil removal pre-filters and particulate after-filters. The oil removal pre-filter protects the desiccant from oil, which cannot be stripped back out of the desiccant during the regeneration cycle. The particulate after-filter prevents abrasive desiccant dust from traveling down line and fouling machinery.

Heated Purge / Exhaust Purge / Internally Heated Dryers

Dewpoint:            Down to -40°F
Energy Source:  Electric (controls, heaters), Compressed air (regeneration)
Energy Consumption:   Approx. 13.5 to 21% of compressor power (including compressed air usage, heater power, and pressure drop effects of dryer and required filtration)

Heated purge desiccant dryers reduce the purge rate of heatless dryers to approximately 7.5% of capacity, and augment it by heating it. This combination of hot, dry air strips the moisture back out of the regenerating desiccant bed.

These dryers reduce moisture level to as low as -40°F. This makes them well suited to systems that are exposed to the elements in colder climates.

These dryers are controlled using a timer to cycle the towers and heaters.  Energy saving controls are typically optional, and control similarly to heatless dryers, by extending the drying time of the online tower until it is saturated.

The heated purge dryer requires an oil removal pre-filter and particulate after-filter, but the particulate after-filter must also be rated to withstand high temperatures at the time immediately following tower switch-over.

Heated Blower Purge Dryers

Dewpoint:            Down to -40°F
Energy Source:  Electric (controls, heaters, blower)
Energy Consumption:     Approx. 11 to 18.5% of compressor power (including blower power, heater power, and pressure drop effects of dryer and required filtration)

Heated blower purge desiccant dryers eliminate compressed air purge by installing a blower to utilize ambient air, and augment it by heating.  This hot, dry air strips the moisture back out of the regenerating desiccant bed.

These dryers reduce moisture level to as low as -40°F.  This makes them well suited to systems that are exposed to the elements in colder climates.

These dryers are controlled using a timer to cycle the towers, blower, and heaters.  Energy saving controls are typically optional, and control similarly to heatless dryers, by extending the drying time of the online tower until it is saturated.

The heated blower purge dryer requires an oil removal pre-filter and particulate after-filter, but the particulate after-filter must also be rated to withstand high temperatures at the time immediately following tower switch-over.

Deliquescent Air Dryers

Dewpoint: Approximately 20°F below inlet temperature
Energy Source:  none
Energy Consumption:  Approx. 3% of compressor power (due to pressure drop effects, not including drain waste)

Deliquescent dryers are becoming increasingly uncommon, but are still sometimes used in remote areas or with portable diesel driven compressors due to their lack of input electricity requirement.

These dryers consist of a pressure vessel filled with absorbent desiccant tablets.  As these tablets absorb water, they dissolve to form an aggressive brine condensate solution which must be eliminated from the system.  This brine solution is most often drained using wasteful motorized ball valves, as these valves remain reliable even when contaminated with brine, rust, and fragments of desiccant tablets.

The deliquescent dryer requires no pre-filters, but a particulate after-filter is highly recommended to prevent passage of aggressive desiccant down line. Due to the expense of replacing the consumable desiccant, the aggressive nature of the condensate, and the poor dewpoint performance, these dryers are rarely employed.

Membrane Air Dryers

Dewpoint:            Down to -40°F
Energy Source:  Compressed Air
Energy Consumption:     Approx. 12-60% of compressor power (due to compressed air usage and pressure drop effects)

Membrane dryers essentially filter water vapor molecules from the other molecules comprising compressed air at the molecular level.  Smaller molecules (such as oxygen, hydrogen, and nitrogen) pass through the membrane and continue downstream while the larger water molecules are vented to atmosphere.

The membrane dryer requires no additional power input, but exhausts a portion of the compressed air.  The amount of air wasted is typically dependent upon dryer size, which is determined by the required flow rate, pressure, inlet dewpoint, and required outlet dewpoint.  A dryer may be suitable for a wide flow range depending upon the required dewpoint.

For example, a given dryer may output 57 CFM with an input dewpoint of 100°F and an output dewpoint of 40°F, or it may output 8.9 CFM with an input dewpoint of 100°F and an output dewpoint of -40°F.  In either case, this dryer would vent 9.6 CFM, for a loss of between 15% and 52%.

Due to the potentially high losses, membrane dryers are frequently used only in small “point of use” applications to improve air quality to a given machine or process.

Membrane dryers require the use of a pre-filter to prevent oil from clogging the membrane.

**Membrane dryers are not for use in breathing air systems.  The membrane may vent additional oxygen with the exhausted moisture, creating a lower concentration of oxygen in the compressed air.  This is not a problem for most compressed air applications, but could create a hazard when air is used for breathing.

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