Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Catalog CIC-2003-1/US
Thermostatic and Constant Pressure (Automatic) Expansion Valves
168
Superheat & Hunting Thermostatic Expansion Valves
A common problem facing refrigeration and air condi-
tioning service technicians and contractors is that of
superheat hunting by thermostatic expansion valves
(TXVs). Here is a better understanding of a commonly
overlooked cause of superheat hunting and how the
problem might be corrected.
Defining Superheat “ Hunting”
Superheat hunting is a cyclical fluctuation in suction
superheat due to varying refrigerant flow rate in the
system. Superheat hunting is the result of the expan-
sion valve (see the illustration below) excessively
opening and closing in an attempt to maintain a
constant operating condition. Hunting can be observed
as regular fluctuations in suction temperature, and in
extremes, suction pressure. Excessive hunting can
reduce the capacity and efficiency of the system result-
ing in uncomfortable conditions, loss of product, and
wasted energy.
Tips for Understanding and Preventing Superheat Hunting in TXVs
• Incorrect charge selection – The charge selected
does not have the necessary control characteristics
and / or dampening ability to stabilize operation.
• Undercharged system – Intermittent loss of
subcooling is causing loss of expansion valve
capacity and resulting intermittent high superheat.
• Poor bulb contact – Loss or delay of temperature
signal to bulb causes erratic and unpredictable
operation.
• An imbalanced heat exchanger (multi-circuit
coil) – An imbalance in the heat load on each circuit
creates a false temperature signal to the expansion
valve bulb and results in erratic operation. Since this
problem is commonly overlooked in the field, a
closer examination and a possible solution are in
order.
Balanced or Unbalanced Circuits?
TXVs on Multi-Circuit Heat Exchangers
TXVs respond, in part, to the temperature of the suc-
tion line. At the expansion valve outlet, flow is divided
into 2 or more paths (circuits) at the inlet of the evapo-
rator by the distributor. These paths recombine as they
exit the evaporator into the suction manifold. (See the
illustration below.)
Ideally, each circuit is equally loaded and absorbs an
equivalent amount of heat. If one assumes the refriger-
ant flow rate and heat load through each circuit is
equal, then the superheat condition exiting each circuit
will be equal and when all of the flow streams recom-
bine, the result is a “true” average condition of the
evaporator suction gas. When one or more circuits has
a lighter heat load, some refrigerant from that circuit
remains unevaporated when it exits the coil. When this
unevaporated liquid refrigerant combines with the other
superheated flow streams, the recombined suction flow
Expansion Valve Flow
Common Reasons for TXV Hunting
• Oversized valve – The expansion valve may be
oversized for the application or operating condition
of the system. If the valve capacity significantly
exceeds the requirements of the system, when the
valve attempts to adjust to system load it overcom-
pensates because it is oversized.
A conventional balanced port thermostatic expansion
valve and the three forces it responds to:
Force F1 – Thermal bulb pressure times the diaphragm
effective area. This force acts on the top of the diaphragm
which tends to open the valve.
Force F2 – Evaporator pressure times the diaphragm
effective area. This force acts on the underside of the
diaphragm. It tends to close the valve. This force is
transmitted to the diaphragm through the valve body with
internal equalized valves and through the external connection
in external equalized valves.
Force F3 – Superheat spring force which assists in closing the
valve.