|
Back to Family Car's Classroom on the Web
by Chris Bede
www.aircondition.com
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.
(Article 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.
This article Copyright © 1983-2005, SmartTrac
Computer Systems, Inc.
All Rights Reserved.
|
Family Car Parts Store
