Desiccants
& Driers
By Norm Christopherson
Filter-driers
play a pivotal role in the operation of hvac systems. At the heart of the drier is the desiccant held in the
drier’s cylindrical metal container. As
important as the filter-drier is few actually understand how they work.
Here are some details.
The word “desiccate” means to dry out completely and a desiccant is a material or substance that accomplishes the moisture removal. Moisture in the mechanical refrigeration cycle is detrimental to the operation and life of the system. The filter-drier is an accessory that performs the functions of filtering out particles and removing and holding moisture to prevent it from circulating through the system.
Moisture In A System
Consider
a chemist working with chemical elements to create new substances.
The chemist combines atoms of selected elements to cause them to bond or
link together to form new combinations of molecular structures. These new
molecular structures are called compounds.
Chemists perform such creations in the process of developing new
synthetic oils, refrigerants, glues, rubbers, metal allows and a host of other
products that are useful in many ways.
Some combinations of atomic elements create molecular structures that can be either useful or harmful. Acids are formed when the right combination of elements are linked together chemically. If we have a use for the acid and use it for its intended purpose, all is well. However, in some cases unwanted chemical combinations occur where we least want them and where they cause serious harm. Under certain circumstances hydrochloric and hydrofluoric acids chemically form in the mechanical refrigerant system. This, of course is what we want to prevent.
Again, let us consider how the chemist facilitates the chemical bonding process. The chemist wants certain chemical reactions to take place in an effort to create new substances that hopefully have special properties that are useful. Perhaps the chemist is attempting to create a new refrigerant to replace another that is being phased-out. The chemist combines particular elements to form bonds or links that when complete meet all the qualities of a great refrigerant. A catalyst is anything that hastens, encourages or helps bring about a result. Heat is one of chemistry’s most active catalysts. A chemist may purposely add heat to a beaker of chemicals to cause them to combine to form a new substance.
The HVAC System As A Chemistry Set
That’s
right, the mechanical refrigeration system consisting of a compressor,
condenser, metering device, evaporator, copper lines, oil and refrigerant are a
complete chemistry set including several powerful catalysts!
The system
contains components which consist of a number of metals such as the iron casting
of the compressor, copper lines, steel condenser, aluminum evaporator, brass
valves and fittings and perhaps still other metals in smaller quantities.
The components are assembled using still other metals and chemicals
during the brazing process. Flux is
applied to facilitate the chemical process of brazing and heat is applied with a
torch as the catalyst.
Still other
materials (chemicals) are contained in the system. Compressor motor winding insulation and varnishes, epoxy
glues and perhaps rubber and gasket materials are applied. Of course, two of the major chemical materials that
constantly circulate through the system are the refrigerant and oil.
The system contains a vast number of chemicals. (Everything physical is
chemical and consists of atoms capable of bonding with atoms of other elements
under the right circumstances) Now,
if yet additional elements are introduced to the existing combination of
elements making up the system the chemical bonding possibilities become still
greater. During the installation or servicing of the system air
consisting of hydrogen, nitrogen and oxygen may be introduced. Moisture may be introduced and flux or even powder from the
inside of Armaflex insulation may get into the system.
The moisture and oxygen are very active components that act as catalysts
themselves.
Additional catalysts in the form of heat of compression as well as latent
heat in the condenser and pressure are present.
Imagine the possibilities! The
chemicals present are compressed, heated and liquefied.
Then they are evaporated and cooled as the pressure is released.
Then, the process is continually repeated for hours, days and months
until a chemical reaction takes place. On
hot days the high temperature and pressure on the high side of the system
reaches still higher levels. The
catalysts of heat and pressure could almost make a chemist jealous.
When a chemical reaction occurs the typical chemical bonding creates
hydrochloric and hydrofluoric acids. These
acids then go to work breaking down the metals and other materials of
construction adding soluble metal to the chemical reaction.
A number of other chemical reactions may take place and the circulating
refrigerant and oil carry the entire mix throughout the system where it can
continue the process.
One authority on acids informs us that for every 18 degrees Fahrenheit an
acid is heated, its activity level doubles.
Eventually, the motor winding insulation may be destroyed and the motor
windings begin to pass electrical current between each other.
As the motor begins to burnout smoldering products from the burning motor
are pumped throughout the system. Remember,
the motor will be cooking and burning while the compressor is pumping these
products through the system. Liquid
refrigerant and oil are fairly good cleaning agents so the piping where liquid
refrigerant is located may remain fairly clean of the resulting debris. However, in the evaporator a distillation process is taking
place, the refrigerant is changing from a liquid to a gas so the debris becomes
separated from the refrigerant and begins getting deposited in the evaporator
and suction line. This is why the
low side of a system that has experienced a compressor burnout is where the
majority of the debris is located.
Keep
It Clean!
The case has been made as to how important it is to prevent chemical
reactions from taking place in a system. It
almost seems from what we have described up to this point that it could be
difficult to prevent chemical breakdown from occurring.
Fortunately, the installation crew and service technician can prevent
system failure due to a chemical reaction.
It is imperative that installation and service technicians prevent
foreign materials, air, moisture, brazing flux, carbon created during brazing
and Armaflex insulation powder from entering or remaining in a system.
Good piping practice includes bleeding a small amount of dry nitrogen
through the system while brazing. Pipe
ends need to be sealed prior to sliding pipe insulation over the piping.
A good 500 micron evacuation should be reached to remove air and moisture
before charging with refrigerant. And, the addition of a properly sized
filter-drier is important on both new systems as well as anytime a system is
opened for service. The filter-drier is designed to both remove any particulates
that may circulate as well as collect and hold any moisture that may remain in
the system. The use of a
filter-drier containing a good desiccant has become even more important with the
advent of R-410A systems, which utilize the highly hygroscopic synthetic Poly
Ester oils.
How A Desiccant Works
Modern filter-driers contain desiccants that function on the principle of
adsorption. Adsorption is not the
same as absorption. The term absorption is commonly misused in the technical
sense of the term. When we say that
a sponge or paper towel absorbs a liquid spill we are using the term in its
non-technical sense. Actually,
absorption is the attraction and holding power through chemical action. Modern desiccants do not function on the basis of a chemical
attraction. The desiccants commonly
used in filter-driers utilize the process of adsorption.
Adsorption is a physical process and is simpler and easier to understand
then the more complex chemical process of absorption.
The modern desiccant of choice is a material called zeolite.
Zeolite has gained in popularity over the older desiccants activated
alumina, silica gel, calcium chloride and calcium oxide.
Zeolite is a mineral that occurs in nature or can be manufactured.
Zeolite is an inorganic tan or gray porous solid consisting of a
structure of pores and tiny chamber capable of collecting and holding moisture
through capillary action. Adsorption
is the physical trait of capillary action whereby moisture is drawn into small
pores much like a sponge or paper towel collects liquid spills.
There are hundreds of different zeolites each with its own micro sized
shape, lattice structure and size. Zeolites can be selected to collect and hold
many different substances according to the molecular size and structure of the
specific molecule one wishes to collect. The
zeolite selected for use in a filter-drier is selected to adsorb moisture while
allowing refrigerant to pass through. One
example of a zeolite is a very light and porous volcanic rock. Zeolite filters are used as desiccants and filters for
refrigerant, acids, specific chemicals and to remove ammonia in fish tanks.
Zeolite desiccants are formed into a porous solid core, which is placed
in the filter-drier container. Older
loose fill desiccants like silica gel occasionally broke down into particles or
dust that sometimes left the filter-drier and circulated through the system
often creating a restriction especially on capillary tube systems. This was
avoided by positioning the filter-drier vertically so pressure pulsations in the
system did not shift the loose fill back and forth physically breaking down the
loose fill. Solid core zeolite
desiccant filter-driers may be installed in any position. Most solid core
desiccants are molded into a cylindrical block with a tapered axial hole down
the center to allow for the uniform flow of the refrigerant through the entire
bed of desiccant. This is why
filter-driers are directional with the direction of flow indicated on the
container. Installing the
filter-drier in the wrong direction causes non-uniform refrigerant to desiccant
contact and increases pressure drop. Bi-flow
filter-driers are available for heat pump applications.
Capacity
Capacity refers to the amount of moisture the desiccant in the
filter-drier can hold. Capacity is measured in “parts per million”.
One part per million (ppm) is one part of water per million parts of
refrigerant. In practical terms
this would be approximately equal to one drop of water in a 125-pound drum of
refrigerant. Desiccant capacities are rated at 75 & 125 degrees F. The older
desiccant, activated alumina had a moisture holding capacity of 4 grams of
moisture per each 100 grams of desiccant. Silica
gel had a moisture holding capacity of 3 grams of moisture per each 100 grams of
desiccant. Modern zeolite,
molecular sieve desiccants have a capacity of approximately 16 grams of moisture
per 100 grams of desiccant.
The capacity of a desiccant is temperature dependent.
The colder the desiccant the more moisture it can hold.
Therefore, locating a filter-drier in a cooler location is an advantage.
Removing a brazed filter-drier with a torch flame causes moisture to be
driven out of the desiccant and into the system.
Generally, it is better to cut the filter-drier out with a tubing cutter.
Location
The desiccant works better at removing and holding moisture when it is
placed in a refrigerant line where the refrigerant is in the liquid state.
The filter-drier is often called a “liquid line filter-drier” for
this reason.
Suction Line Filter-Driers
The desiccant is still able to adsorb moisture when applied to the
suction line but not quite as effectively.
Special suction line filter-driers are made for cleaning up a system
after a compressor burnout. A larger shell is used to minimize pressure drop on
suction line driers. Suction line
filter-driers marked as “HH” driers contain carbon filter material in
addition to the zeolite desiccant. The
carbon and zeolite are capable of capturing and holding acids as well as
moisture. Suction line filter-driers used to clean up a system after a burn out
should be replaced until the system is known to be clean and no longer tests
positive for acids in the system. A
suction line filter-drier with an excessive pressure drop across it should not
be left in a system. An excessive
pressure drop in the suction line reduces the volumetric efficiency of the
compressor thus reducing system-operating capacity. Many suction line filter
driers have a pressure tap on the inlet end so the pressure on the inlet of the
drier can be compared to the pressure at the suction service valve at the
compressor. Still other suction line filter-driers have pressure taps on both
the inlet and outlet.
Alcohol Additives
Some technicians add alcohol-based additives to a moisture-ridden system
to prevent moisture from freezing and restricting the metering device.
Modern zeolite molecular sieve desiccants have the ability to adsorb
these additives to an even greater degree than moisture.
It is possible for a desiccant that has already captured moisture to
release some of that moisture and replace it with the alcohol additive thus
reducing the moisture capacity of the desiccant.
Reactivating Filter-Driers
In the past, some have attempted to reactivate and reuse a filter-drier
by heating and evacuating the desiccant. Heating
and evacuating does actually remove much of the moisture and allow the drier to
be used again. However, oils,
carbon and other particles are not removed during this reactivation attempt. In fact, the oil may be cooked into the desiccant creating
new contamination possibilities. The
cost of a new filter-drier is not worth the effort and is not recommended.
Don’t allow a system to become an out of control chemistry set.
Good piping practice, a nitrogen purge during brazing, a deep evacuation
and the proper installation and use of filter-driers containing modern and
effective molecular sieve desiccants will prevent many system failures. Many compressor failures are blamed on the compressor when
the actual problem was caused by a system problem.
That system problem may have been a chemical problem due to moisture.
Norm is a technical writer, seminar speaker and test proctor for EPA, 410A and ESCO & NATE certifications.
He can be contacted at nchristo@juno.com