Brochure

Catalog CIC-2003-1/US
Filter Dryers
Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
19
Filter
Dryers
Introduction to Dryers
Introduction to Dryers
A dryer in a refrigeration system has two functions;
one, to adsorb system contaminants such as water
and acid, and two, to provide physical filtration.
Selecting a filter dryer for a particular application
requires various technical factors to be considered.
These factors include the type of system, connecting
line size, water capacity, flow capacity (size of system),
filtration capability, material of construction (steel
versus copper), and safe working pressures. Evaluation
of each factor is necessary to ensure proper and
economical dryer design.
Parker Hannifin has developed filter dryer recommen-
dations based on current technical data, as well as
many years of actual field experience. They have been
tested for flow and water capacity using the American
Society of Heating, Refrigeration and Air Conditioning
Engineers (ASHRAE) Standard 63.1 and are rated for
use based on the Air Conditioning and Refrigeration
Institute’s guideline ARI 710. Data obtained from this
testing is shown in the Capacities “(Ratings at ARI
Standard Conditions)” tables for liquid line filter dryers
and can be used as a comparison for filter dryers.
However, other factors must be considered for various
types of equipment.
Adsorbing Moisture and Preventing Acids
The ability to remove moisture from a refrigeration
system is the most important function of a dryer.
Moisture can come from many sources such as
trapped air from improper evacuation, system leaks,
and motor windings to name a few. Another source
is due to improper handling of polyolester (POE)
lubricants, which are hygroscopic; that is, they readily
absorb moisture. POEs can pick up more moisture
from its surroundings and hold it much tighter than the
previously used mineral oils. This moisture can cause
freeze-ups and corrosion of metallic components.
Moisture in the system can also cause a reaction with
POEs called hydrolysis, forming organic acids.
To prevent the formation of these acids, the moisture
within the system must be minimized. This is accom-
plished by the use of desiccants within the filter dryer.
The three most commonly used desiccants are
molecular sieve, activated alumina and silica gel.
Molecular sieves are crystalline sodium alumino-
silicates (synthetic zeolites) having cubic crystals,
which selectively adsorb molecules based on
molecular size and polarity. The crystal structure
is honeycombed with regularly spaced cavities or
pores. Each of these cavities or pores are uniform in
size. This uniformity eliminates the co-adsorption of
molecules varying in size. This permits molecules, such
as water, to be adsorbed, while allowing other larger
molecules, such as the refrigerant, lubricant and acids,
to pass by. The surface of this desiccant is charged
positively with cations, which act as a magnet and will
therefore adsorb polarized molecules, such as water,
first and hold them tightly. The water molecules are
physically separated from the lubricant, minimizing the
potential for POE hydrolysis.
Activated alumina is formed from aluminum oxide
(Al
2
O
3
) and is not a highly crystalline material. Both
alumina and silica gel show a wide range of pore sizes
and neither exhibit any selectivity based on molecular
size. Due to the varying pore sizes, they can co-adsorb
the much larger refrigerant, lubricant and acid mol-
ecules, eliminating the surface area available to adsorb
water. Alumina can also aid in the hydrolysis of the
POE lubricants creating organic acids since both water
and lubricant are adsorbed into the pore openings of
the alumina.
Silica gel is a non-crystalline material with a molecular
structure formed by bundles of polymerized silica
(SiO
2
). Gel-type desiccants are indicative of the weaker
bond formed between water and the desiccant. Silica
gel is the old type of desiccant and is not widely used in
today’s filter dryers.
Selecting Desiccant Material
There are many factors involved when selecting which
desiccant material is best for which application. Water
capacity, refrigerant and lubricant compatibility, physical
strength, and acid capacity are important characteris-
tics of desiccants that should be considered.
Dryers are often placed on the liquid side of the system
to remove moisture and some solids before the refriger-
ant travels through the expansion valve. Acid levels,
both organic and inorganic, are not typically high on the
liquid side, but some does exist and a dryer should be
able to remove some of it. However, acid levels will
remain low if the dryer can remove most of the mois-
ture in the system. To accomplish this, the dryer must
have an adequate water capacity level. Water capacity
is the amount of water or moisture the desiccant can
hold while maintaining low levels within the refrigeration
system.
Filter Dryers