Brochure

Catalog CIC-2003-1/US
Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
169
Service
& Check
Valves
Thermostatic and Constant Pressure (Automatic) Expansion Valves
TXVs & AXVs
Superheat & Hunting Thermostatic Expansion Valves
no longer represents an average condition. The suction
temperature where the bulb is mounted will be lower
than the “true” average of the circuits if they were all
properly superheated.
Sensing a “cold” suction condition will cause the valve
to close down because it is sensing a condition which
is not superheated enough; when the valve closes
down, it restricts flow to all circuits and eventually “dries
out” the circuits which are is flooding. By this time, the
remaining circuits have become highly superheated
due to the reduced flow rate. At the point the “flooding”
circuit(s) begin to be superheated, the suction tempera-
ture rises rapidly because there is no more liquid
present to falsely reduce the suction temperature.
Sensing a now “warm” suction condition, the valve
opens to decrease superheat and the lightly loaded
circuit begins to flood into the suction manifold again.
Suction temperature drops rapidly again, the valve
closes down again, the sequence repeating in a cyclical
fashion.
Again, the ideal situation is to assume each circuit is
equally loaded and absorbs an equivalent amount of
heat; in reality, this situation does not always occur.
There are several reasons why circuits can become
unevenly loaded:
• Poor heat exchanger design – In this case, each
circuit is not of equal length and loading.
• Poor refrigerant distribution – This problem
occurs due to the wrong choice of distributor or
feeder tubes, partially blocked passageways of
feeder tubes, unequal feeder tube lengths, and/or
kinked feeder tubes.
• Uneven air flow – Air flow across the evaporator
is reduced in some areas while increased in other
areas. Dirty coils or damaged coil fins can have a
similar effect on air flow.
Diagnosing a Hunting Problem:
Is It the Heat Exchanger?
Diagnosing a hunting problem due to an imbalanced
heat exchanger requires measuring the exit tempera-
ture of each circuit upstream of the suction manifold.
To perform this process, average the temperatures of
all of the circuits upstream of the suction manifold
and compare this average temperature to the actual
temperature of the suction manifold close to where the
bulb is mounted. If the average value of the circuit exit
temperatures exceeds the actual suction temperature
value by more than 2°F, then there is likely one or more
circuit(s) which are not completely superheated (flood-
ing). A closer examination of the individual circuit
temperatures and the associated suction pressure
should reveal which circuit(s) are causing the problem.
One simple rule to remember is that the valve’s
response will favor the circuit that is flooding. Because
of this favorable response, a heat exchanger can be
operating at a reasonable exit superheat but still have a
significant loss in capacity because the expansion valve
is responding to one or more flooding circuits while the
other circuits remain highly superheated, and thus
highly inefficient.
Correcting the Problem
Correcting the problem can be a difficult task. First, the
service tech must recognize the cause of the problem.
If not, the problem can only be compensated for and
this could mean a reduction in system performance.
Here are some tips for correcting or compensating for
an imbalanced heat exchanger:
• If possible, examine and correct any problems with
air flow, coil circuitry, and distribution such that the
circuits are more evenly fed and loaded. The goal
is a more consistent circuit exit temperature on all
circuits. One lightly loaded circuit may be tolerable if
there are, for example, eight circuits. However, this is
probably not the case if there are only three.
• Adjust the superheat of the valve to a slightly higher
value. Attempting to control an evaporator near to
or lower than 5°F operating superheat can exceed
the sensing capability of most expansion valves
and result in hunting and subsequent intermittent
flooding.
• If practical, move the bulb farther downstream on
the suction line. Better mixing of the refrigerant prior
to the bulb can “smooth” out the valve response
although capacity and efficiency may not improve
significantly.