Why
add a dessicant air dryer to the compressed air
system?
Browse our selection of Compressed Air
Dryers online. If you have questions or can't find
what you are looking for online, call our knowledgeable customer
service team toll-free: 866-650-1937.
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:
Application |
Pressure Dew Point (ºF) |
Pressure Dew Point (ºC) |
Air Motors (high efficiency) |
-40 to 38 |
-40 to 3.3 |
Air Motors (low efficiency) |
-20 |
-28.8 |
Breathing Air (subsequently humidified) |
-40 to 38 |
-40 to 3.3 |
Chemical Processing |
-40 |
-40 |
Control Air (industrial services) |
-40 |
-40 |
Cryogenic Systems |
-100 |
-73.3 |
Drying Processes |
-40 to 4 |
-40 to -15.5 |
General Services (indoors) |
-40 to 50 |
-40 to 10 |
General Services (outdoors) |
-100 to 38 |
-73.3 to 3.3 |
Instrumentation (industrial services) |
-40 |
-40 |
Instrumentation (laboratory services) |
-60 |
-51.1 |
Microelectronic Service |
-100 to -40 |
-73.3 to -40 |
Paint Spray Systems |
-40 to 38 |
-40 to 3.3 |
Pharmaceutical Services |
-100 to -40 |
-73.3 to -40 |
Figure AD1-1 |
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.
- 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.
- 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.
- Browse our selection of all the types of Dessicant
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
eCompressedair 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
recomendations
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.
Condition |
System Condition |
Correction Factor** |
Inlet Air Pressure |
150 psig (10 bar) |
.86 |
Inlet Air Temperature |
120ºF (48.8ºC) |
1.51 |
Ambient Temperature* |
110ºF (43.3ºC) |
1.16 |
*It is a best practice to use 100ºF (38ºC) or higher for the
ambient temperature to avoid accidentally under-sizing a compressed
air dryer.
**Correction factors are standard. See tables below for correction
factors for other temperatures. Figure AD1-8 |
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
Inlet Air Pressure |
Psig (bar) |
Correction Factor |
50 (3.4) |
1.29 |
75 (5.2) |
1.10 |
100 (7) |
1.00 |
150 (10.3) |
.86 |
250 (17.2) |
.79 |
|
Inlet Air Temperature |
ºF |
ºC |
Correction Factor |
80 |
26.6 |
.61 |
90 |
32.2 |
.79 |
100 |
37.7 |
1.00 |
110 |
43.3 |
1.23 |
120 |
48.8 |
1.51 |
|
Ambient Air Temperature |
ºF |
ºC |
Correction Factor |
80 |
26.6 |
.80 |
90 |
32.2 |
.89 |
100 |
37.7 |
1.00 |
110 |
43.3 |
1.16 |
120 |
48.8 |
1.30 |
Figure AD1-9 |
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
Type of Desiccant Air Dryer |
Initial Purchase |
Utilities |
Maintenance |
Pressure-swing |
Low |
Moderate |
Low |
Internally Heated |
Moderate |
Low - Moderate |
Moderate |
Externally Heated |
Moderate |
Moderate |
Moderate |
Blower Purge |
High |
Moderate |
Moderate |
Heat of Compression* |
High |
Low |
High |
*Only operates with an oil-free compressor. Figure AD1-10 |
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.
Heatless Desiccant Air Dryer |
6-10 minute cycle (5 minutes online and 3-5 minutes
regenerating) |
Externally Heated Desiccant Air Dryers |
4 hour cycle (2 hours online and 2 hours offline |
Internally Heated Dryer Blower Purge |
8 hour cycle (4 hours online and 4 hours offline) |
Figure AD1-11 |
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
Operating Pressure (psig)(bar) |
Correction Factor |
60 (4.1) |
.65 |
70 (4.8) |
.74 |
80 (5.5) |
.83 |
90 (6.2) |
.91 |
100 (6.9) |
1.00 |
110 (7.6) |
1.04 |
120 (8.3) |
1.08 |
130 (9) |
1.12 |
140 (9.7) |
1.16 |
150 (10.3) |
1.20 |
175 (12.1) |
1.29 |
200 (13.8) |
1.37 |
225 (15.5) |
1.45 |
250 (17.2) |
1.52 |
Figure AD1-13 |
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
Inlet Pressure (psig)(bar) |
Correction Factor |
50 (3.4) |
.56 |
60 (4.1) |
.65 |
70 (4.8) |
.74 |
80 (5.5) |
.83 |
90 (6.2) |
.91 |
100 (6.9) |
1 |
110 (7.6) |
1.09 |
120 (8.3) |
1.18 |
130 (9) |
1.27 |
140 (9.7) |
1.37 |
150 (10.3) |
1.43 |
Figure AD1-18 |
Heated Desiccant Air Dryer
Inlet Temperature Correction Factors
Temperature (ºF) |
Temperature (ºC) |
Correction Factors |
90 |
32.2 |
1.35 |
95 |
35 |
1.16 |
100 |
37.7 |
1 |
105 |
40.5 |
.85 |
110 |
43.3 |
.74 |
115 |
46.1 |
.64 |
120 |
48.8 |
.56 |
Figure AD1-19 |
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
Recomendations

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 Recomendations
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
Browse our selection of Compressed Air Dryers online. If
you have questions or can't find what you are looking for online,
call our knowledgeable customer service team toll-free:
866-650-1937.