Communication Air Dryers


Ingolf Plath


C July 14,1996


Canadian Puregas Equipment Limited


Table of Contents


1. Introduction 3
2. Moisture 4
3. Dryers 5
3.1. Types of Dryers 5
3.2. Membrane Drying 5
4. Comparison of Dryers 6
4.1. Refrigeration 6
4.2. Reactivated Desiccant 6
4.3. Membrane 7
5. Summary 8


1. Introduction


There are many applications of air dryers in the world today. Different air
drying principles are used to obtain specific results for the application
involved. We are concerned only with telecommunications applications and
the comparisons made herein are specific to the same. General industrial
air drying is not considered as the flows and pressures required are different
and these comparisons are not valid for high flows and pressures. 

Pressurization of cable was first instituted in the 1940’s to protect
telephone cables from moisture intrusion and subsequent problems due to sheath
and/or splice integrity faults. A dry pure gas such as air was forced, under
pressure, into the cables. This pure gas under pressure within the cable
prevents moisture from entering the cable and purges the cable of any inherent
moisture. 


Over the years, pressurization use has been expanded to include other types
of communication carriers such as F.O. cable, coax and waveguide. Air dryers
are also used extensively in other fields such as instrumentation. Air dryers
have changed from the original refrigeration type to heatless regenerative
desiccant to the newest technology of permeable membrane dryers.
This paper will provide information on air drying and compare the different
drying principles, specifically permeable membrane technology.


2. Moisture

Air always contains moisture or water vapor. This moisture present in air
can be a minimal amount or it can be enough to be at saturation where water
droplets form (condensation). To preclude condensation we must remove the
moisture from (dry) the air so that when the air is used in our application
condensation cannot occur or so that the equipment cannot discern the moisture.
The quantity of moisture or humidity can be expressed in different ways according
to our requirements:

relative humidity - a ratio between the moisture in the air to the moisture
saturation capacity of the air, expressed as a percentage and dependent upon
the temperature and pressure.

Dew point - the temperature at which the air will become saturated, expressed
as a temperature and dependent upon pressure.


Generally we use dew point expressed at atmospheric pressure (zero psig)
as a constant for comparisons. 


Pressurization of equipment is done to preclude moisture from entering or
to purge moisture in the form of natural humidity and water.

3. Dryers


3.1. Types of Dryers


Originally bottled nitrogen was used until refrigerated dryers were introduced.
These dryers supplied a constant flow of dry air without the risks and costs
associated with pressurized bottles. Further developments led to oilless
compressor, heatless desiccant dryers using the swing adsorption and reactivation
principle. These units are the mainstay of low pressure air dryers in operation
today. There are many variants of these dryers available, including heat
reactivated and liquid ring compressor versions for different capacities
and applications. 


Another method of drying air is by the deliquescent method. The deliquescent
must be replaced and therefore this type of dryer has very limited in
applications. The three main principles of drying for communications that
concern us for comparison are: refrigeration, reactivated desiccant and permeable
membrane. 

The air dryer must provide the drying capacity at the air flow necessary
to achieve a constant pressure at its normal capacity. If the dryer must
provide higher flows than its normal rated capacity the efficiency and drying
of the air will be affected.


3.2. Membrane Drying


Membrane dryers use the process of selective permeation of the components
of a gas mixture (air). The dryer is a cylinder which houses thousands of
tiny hollow fibers constructed of polysulphone semipermeable, polymeric
membranes. These fibers have a selective permeation for the removal of water
vapor (moisture). This selective permeation rate allows water vapor to permeate
the membrane wall while not allowing nitrogen and oxygen to pass through
the wall. The permeated water and a sufficient amount of air to retain the
water as vapor is vented to the atmosphere outside of the cylinder. The dry
air continues to the end of the fibre at almost the same pressure as the
entering air. The permeation or separation is caused by the difference between
the partial pressure of a gas on the inside and the outside of the hollow
fiber. 


4. Comparison of Dryers


4.1. Refrigeration


Refrigeration air dryers of course use refrigeration or cooling of the air
to condense the moisture in the air. They have a very low purge rate of less
than 2% of air compressor delivered volume which is energy efficient for
the air compressor but also require another compressor for the refrigerant.
Their drawback is due to the possibility of ice forming in the heat exchanger
at temperatures below the freezing point of water. They operate on high pressure
of approximately 150 psig to produce a maximum dewpoint level.
 
The general output dewpoint is -20 degrees Celsius which equates to 3% R.H.
at 20 degrees Celsius. Maintenance and power consumption costs are high due
to the refrigeration drying unit and the high pressure air system required.
For communication purposes it is not feasible to use this type of dryer for
lower dewpoints. Environmental concerns of the refrigerant and oil contamination
from the air compressor must also be considered.


4.2. Reactivated Desiccant


Reactivated desiccant air dryers are the most common type in use today. These
are low pressure dryers using oilless compressors. They have mostly replaced
the original refrigeration type dryers due to their ease of maintenance and
lower operating costs. Dessicant dryers also supply air at a much lower dewpoint
than refrigeration dryers. The standard dewpoint is -40 degrees Celsius which
equates to ½ % R.H. at 20 degrees Celsius and are capable of dewpoints
to -100 degrees Fahrenheit. 


The purge rate for desiccant dryers is in the range of 2% to 25% ( the amount
of air required to purge the moisture to atmosphere) depending upon the capacity
required. Due to the swing adsorption method of the dehydrator some electricity
is used for the timer and solenoid actuation. The maintenance required is
a simple semiannual filter change and an annual compressor kit installation.
Every two years a dryer (dehydrator) maintenance kit is required. Output
air flow capacities range from 500 to 10,000 SCFD (standard cubic feet per
day). With liquid ring compressors (which only require maintenance every
5 years) and heat reactivation of the desiccant capacities increase to 30,000
SCFD. 


4.3. Membrane


Membrane air dryers are the newest technology air dryers and are used in
selective low capacity applications. Since there are no moving parts on the
membrane itself maintenance on the membrane is not required. A prefilter
is required to protect the membrane from contaminants and liquid moisture
(water). This prefilter must be changed annually and requires an automatic
water drain for unattended operation. The oilless compressor requires an
annual maintenance kit installation.


The simple operation of the membrane dryer and minimal maintenance makes
it ideal for remote site applications such as microwave communication towers.
Low power requirements enable the dryer to be powered by many sources including
direct current battery power. To obtain the normal communications standard
of -40 degrees dewpoint a purge rate of approximately 45% is required. This
high purge rate limits the use of membrane dryers to low output capacities,
usually not more than 400 SCFD. It is not cost efficient in the larger capacities
as desiccant dryers are more efficient and less costly to operate at higher
capacities.


5. Summary 


Each type of air dryer has its applications. Although refrigerated air dryers
are used in many industrial applications they are no longer cost efficient
in the communications industry and cannot achieve the industry standard of
a -40 degree dewpoint. Desiccant dryers are the most common type of dryers
in use today and can supply a complete range of air outflow capacities. The
membrane dryer is the newest technology drier available and is comparable
to desiccant in drying capability. Due to the high purge rate membrane dryers
are restricted by operating costs to low capacity applications. They are
ideal for applications requiring less than 400 SCFD and remote sites where
minimal power is available and low maintenance is a requirement. Membrane
dryers are generally specified for remote microwave tower application

Back to Canadian Puregas home page | E-mail Canadian Puregas

90406