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.
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.
COMPRESSOR
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.
CONDENSER
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.
EVAPORATOR
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.
PRESSURE
REGULATING DEVICES
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.
ORIFICE TUBE
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.
THERMAL
EXPANSION VALVE
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.
RECEIVER-DRIER
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.
ACCUMULATOR
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.
