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 A/C
INFORMATIONS
by Auto Egypt
Today,
as we drive our automobiles, a great many of us,
can enjoy the same comfort levels that we are
accustomed to at home and at work. With the push
of a button or the slide of a lever, we make the
seamless transition from heating to cooling and
back again without ever wondering how this
change occurs. That is, unless something goes
awry.
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Continues below)
Since
the advent of the automotive air conditioning
system in the 1940's, many things have undergone
extensive change. Improvements, such as
computerized automatic temperature control (which
allow you to set the desired temperature and
have the system adjust automatically) and
improvements to overall durability, have added
complexity to today's modern air conditioning
system. Unfortunately, the days of "do-it-yourself"
repair to these systems, is almost a thing of
the past.
To
add to the complications, we now have tough
environmental regulations that govern the very
simplest of tasks, such as recharging the system
with refrigerant R12 commonly referred to as
Freon® (Freon is the trade name for the
refrigerant R-12, that was manufactured by
DuPont). Extensive scientific studies have
proven the damaging effects of this refrigerant
to our ozone layer, and its manufacture has been
banned by the U.S. and many other countries that
have joined together to sign the Montreal
Protocol, a landmark agreement that was
introduced in the 1980's to limit the production
and use of chemicals known to deplete the ozone
layer.
Now
more than ever, your auto mechanic is at the
mercy of this new environmental legislation. Not
only is he required to be certified to purchase
refrigerant and repair your air conditioner, his
shop must also incur the cost of purchasing
expensive dedicated equipment that insures the
capture of these ozone depleting chemicals,
should the system be opened up for repair.
Simply put, if your mechanic has to spend more
to repair your vehicle - he will have to charge
you more. Basic knowledge of your air
conditioning system is important, as this will
allow you to make a more informed decision on
your repair options.
Should
a major problem arise from your air conditioner,
you may encounter new terminology. Words like "retrofit"
and "alternative refrigerant" are now in your
mechanics glossary. You may be given an option
of "retrofitting", as opposed to merely
repairing and recharging with Freon.
Retrofitting involves making the necessary
changes to your system, which will allow it to
use the new industry accepted, "environmentally
friendly" refrigerant, R-134a. This new
refrigerant has a higher operating pressure,
therefore, your system, dependant on age, may
require larger or more robust parts to counter
its inherent high pressure characteristics. This,
in some cases, will add significantly to the
final cost of the repair. And if not performed
properly, may reduce cooling efficiency which
equates to higher operating costs and reduced
comfort.
 Vehicles
are found to have primarily three different
types of air conditioning systems. While each of
the three types differ, the concept and design
are very similar to one another. The most common
components which make up these automotive
systems are the following:
COMPRESSOR, CONDENSER, EVAPORATOR, ORIFICE TUBE,
THERMAL EXPANSION VALVE , RECEIVER-DRIER,
ACCUMULATOR.
Note: if your
car has an Orifice tube, it will not have a
Thermal Expansion Valve as these two devices
serve the same purpose. Also, you will either
have a Receiver-Dryer or an Accumulator, but not
both.
For more
information on Air Conditioning, check out The
Automotive Air Conditioning Information Server
Commonly referred
to as the heart of the system, the compressor is
a belt driven pump that is fastened to the
engine. It is responsible for compressing and
transferring refrigerant gas.
The A/C system is
split into two sides, a high pressure side and a
low pressure side; defined as discharge and
suction. Since the compressor is basically a
pump, it must have an intake side and a
discharge side. The intake, or suction side,
draws in refrigerant gas from the outlet of the
evaporator. In some cases it does this via the
accumulator.
Once the
refrigerant is drawn into the suction side, it
is compressed and sent to the condenser,
where it can then transfer the heat that is
absorbed from the inside of the vehicle.
This is the area
in which heat dissipation occurs. The condenser,
in many cases, will have much the same
appearance as the radiator in you car as the two
have very similar functions. The condenser is
designed to radiate heat. Its location is
usually in front of the radiator, but in some
cases, due to aerodynamic improvements to the
body of a vehicle, its location may differ.
Condensers must have good air flow anytime the
system is in operation. On rear wheel drive
vehicles, this is usually accomplished by taking
advantage of your existing engine's cooling fan.
On front wheel drive vehicles, condenser air
flow is supplemented with one or more electric
cooling fan(s).
As hot compressed
gasses are introduced into the top of the
condenser, they are cooled off. As the gas cools,
it condenses and exits the bottom of the
condenser as a high pressure liquid.
.
Located inside
the vehicle, the evaporator serves as the heat
absorption component. The evaporator provides
several functions. Its primary duty is to remove
heat from the inside of your vehicle. A
secondary benefit is dehumidification. As warmer
air travels through the aluminum fins of the
cooler evaporator coil, the moisture contained
in the air condenses on its surface. Dust and
pollen passing through stick to its wet surfaces
and drain off to the outside. On humid days you
may have seen this as water dripping from the
bottom of your vehicle. Rest assured this is
perfectly normal.
The ideal
temperature of the evaporator is 32° Fahrenheit
or 0° Celsius. Refrigerant enters the bottom of
the evaporator as a low pressure liquid. The
warm air passing through the evaporator fins
causes the refrigerant to boil (refrigerants
have very low boiling points). As the
refrigerant begins to boil, it can absorb large
amounts of heat. This heat is then carried off
with the refrigerant to the outside of the
vehicle. Several other components work in
conjunction with the evaporator. As mentioned
above, the ideal temperature for an evaporator
coil is 32° F. Temperature and pressure
regulating devices must be used to control its
temperature. While there are many variations of
devices used, their main functions are the same;
keeping pressure in the evaporator low and
keeping the evaporator from freezing; A frozen
evaporator coil will not absorb as much heat.
Controlling the
evaporator temperature can be accomplished by
controlling refrigerant pressure and flow into
the evaporator. Many variations of pressure
regulators have been introduced since the
1940's. Listed below, are the most commonly
found.
The orifice tube,
probably the most commonly used, can be found in
most GM and Ford models. It is located in the
inlet tube of the evaporator, or in the liquid
line, somewhere between the outlet of the
condenser and the inlet of the evaporator. This
point can be found in a properly functioning
system by locating the area between the outlet
of the condenser and the inlet of the evaporator
that suddenly makes the change from hot to cold.
You should then see small dimples placed in the
line that keep the orifice tube from moving.
Most of the orifice tubes in use today measure
approximately three inches in length and consist
of a small brass tube, surrounded by plastic,
and covered with a filter screen at each end. It
is not uncommon for these tubes to become
clogged with small debris. While inexpensive,
usually between three to five dollars, the labor
to replace one involves recovering the
refrigerant, opening the system up, replacing
the orifice tube, evacuating and then recharging.
With this in mind, it might make sense to
install a larger pre filter in front of the
orifice tube to minimize the risk of of this
problem reoccurring. Some Ford models have a
permanently affixed orifice tube in the liquid
line. These can be cut out and replaced with a
combination filter/orifice assembly.
Another common
refrigerant regulator is the thermal expansion
valve, or TXV. Commonly used on import and
aftermarket systems. This type of valve can
sense both temperature and pressure, and is very
efficient at regulating refrigerant flow to the
evaporator. Several variations of this valve are
commonly found. Another example of a thermal
expansion valve is Chrysler's "H block" type.
This type of valve is usually located at the
firewall, between the evaporator inlet and
outlet tubes and the liquid and suction lines.
These types of valves, although efficient, have
some disadvantages over orifice tube systems.
Like orifice tubes these valves can become
clogged with debris, but also have small moving
parts that may stick and malfunction due to
corrosion.
The receiver-drier
is used on the high side of systems that use a
thermal expansion valve. This type of metering
valve requires liquid refrigerant. To ensure
that the valve gets liquid refrigerant, a
receiver is used. The primary function of the
receiver-drier is to separate gas and liquid.
The secondary purpose is to remove moisture and
filter out dirt. The receiver-drier usually has
a sight glass in the top. This sight glass is
often used to charge the system. Under normal
operating conditions, vapor bubbles should not
be visible in the sight glass. The use of the
sight glass to charge the system is not
recommended in R-134a systems as cloudiness and
oil that has separated from the refrigerant can
be mistaken for bubbles. This type of mistake
can lead to a dangerous overcharged condition.
There are variations of receiver-driers and
several different desiccant materials are in use.
Some of the moisture removing desiccants found
within are not compatible with R-134a. The
desiccant type is usually identified on a
sticker that is affixed to the receiver-drier.
Newer receiver-driers use desiccant type XH-7
and are compatible with both R-12 and R-134a
refrigerants.
Accumulators are
used on systems that accommodate an orifice tube
to meter refrigerants into the evaporator. It is
connected directly to the evaporator outlet and
stores excess liquid refrigerant. Introduction
of liquid refrigerant into a compressor can do
serious damage. Compressors are designed to
compress gas not liquid. The chief role of the
accumulator is to isolate the compressor from
any damaging liquid refrigerant. Accumulators,
like receiver-driers, also remove debris and
moisture from a system. It is a good idea to
replace the accumulator each time the system is
opened up for major repair and anytime moisture
and/or debris is of concern. Moisture is enemy
number one for your A/C system. Moisture in a
system mixes with refrigerant and forms a
corrosive acid. When in doubt, it may be to your
advantage to change the Accumulator or receiver
in your system. While this may be a temporary
discomfort for your wallet, it is of long term
benefit to your air conditioning system.
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