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Orifice
tubes and expansion valves (commonly called Thermostatic Expansion Valves
or TXV's) both perform the same function in auto air conditioning systems,
they just look a little different. These components provide a restriction
in the flow of refrigerant. That restriction causes the refrigerant to change
from high pressure liquid freon into low pressure liquid freon before it
enters the evaporator. Have a closer look at the different designs and how
they function.
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| Location: |
| Typically,
expansion valves (TXV's) will be located at the inlet of the evaporator. |
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| Block
type valves will have both the inlet and outlet (liquid and vapor) of the
evaporator passing through the valve. The conventional design expansion valve
will be mounted on the inlet (liquid line) only.
On
the other hand, orifice tubes (also called fixed orifice tubes or
CCOT- Cycling Clutch Orifice Tube Systems) could have the orifice
tube located in a variety of positions. As shown on the right, the orifice
tube could be located at the EVAPORATOR INLET, mid way between the EVAPORATOR
and the CONDENSER (in-line) or at the CONDENSER OUTLET.
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Typical
locations of expansion valves and fixed orifice tubes. |
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See
photos of orifice tube location
here |
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a general rule, Expansion Valves and Orifice Tubes are serviced by replacement.
In some cases, where an expansion valve is manufactured to include an inlet
screen, the inlet screen may be removed and cleaned to restore proper flow
of refrigerant to the system. Orifice tubes likewise fail because of debris
and corrosion being caught up on the inlet screen. However, as they are usually
a low cost item, it is generally accepted that they be replaced rather than
cleaned, therefore assuring that debris is eliminated from the A/C system.
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| A
Close up look at the Orifice Tube: |
| Originally
introduced on GM applications in the 1977 vehicle model year, orifice tubes
have become far more popular and are used by several manufacturers. |
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A
typical orifice tube. The inlet (red side) includes a fine screen to protect
the actual orifice (illustrated in yellow). The outlet portion
(blue) also includes a filter screen. O-Ring seal(s) separate the
high pressure side from the low pressure side of the system. |
The
design appears simple because there are no moving parts. The internal brass
orifice (illustrated in yellow on the left) is the actual opening that the
refrigerant flows through. The inlet screen is in place to protect the orifice
tube from getting plugged up with any debris that may be in the system. Is
the design fail safe? You be the judge of that!
See
up close photo of actual orifice tube
here! |
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concept of the orifice tube is great. By contrast, expansion valves rely
on moving parts to open and close the valve opening, thus metering the flow
of refrigerant into the expansion valve. Additionally, expansion valves may
have external 'temperature sensing' devices know as capillary tubes. Just
another part of the valve exposed and subject to damage.
With
an orifice tube, the flow of refrigerant through the valve (tube) is constant.
Overall flow throughout the A/C system is controlled by cycling the A/C system
compressor on and off as required. The end result remains the same; there
is a controlled amount of refrigerant entering into the evaporator.
Historically,
the biggest problem with orifice tubes is that they tend to easily get plugged
with debris that may be traveling throughout the A/C system. As soon as the
flow of refrigerant through an orifice tube is restricted, the flow of oil
(which travels throughout the A/C system in the refrigerant) is also restricted.
That becomes the start of catastrophic compressor failure. |
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What's
wrong with these orifice tubes?
How can you help prevent this? |
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As
stated above, orifice tubes can easily become restricted with debris that
may travel through the system. Restricting the orifice tube immediately restricts
the flow of refrigerant. It also limits the flow of refrigerant oil. The
oil is required to maintain compressor lubrication. Therefore, orifice tube
restrictions can easily cause a catastrophic compressor failure. |
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What
causes the debris and contamination? In short, moisture which enters the
system mixes with the refrigerant to form acids. Those acids corrode aluminum
components. That corrosion moves throughout the system and get caught by
the filter screens on the orifice tube. As shown on the left, if takes less
than 1/10 of a teaspoon of debris to plug the orifice tube. |
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How
can you prevent plugged orifice tubes like this?
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Connection!™ |
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This was probably one of the best books on recharging
air conditioning that I have seen...
