Brochure

Catalog CIC-2003-1/US
Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
163
Service
& Check
Valves
Thermostatic and Constant Pressure (Automatic) Expansion Valves
TXVs & AXVs
General Information Thermostatic Expansion Valves
Internally Equalized Valves
The outlet pressure of an internally equalized valve is
transmitted to the underside of the diaphragm through
an equalizer hole inside the valve body. Internally
equalized valves are used with evaporators that have
a pressure drop of less than 3 psi.
The equalizer passageway is the communication link
from the evaporator to the underside of the diaphragm
(F2). Internally equalized valves incorporate an internal
communication passage from the outlet valve cavity to
the underside of the diaphragm. In applications where
the pressure drop between the valve outlet and the
evaporator outlet is negligible, internal equalizers
are effective to communicate the actual evaporator
pressure to the underside of the diaphragm. In the
schematic below, the F-1 force corresponding to refrig-
erant R-22 at 37°F is 64 psig. The evaporator pressure
F-2 is 52 psig at 28°F and the superheat spring force
F-3 is set for the balancing pressure of 12 psig. The
valve is now in balance with 64 psig above and below
the diaphragm and the superheat setting is 9°F.
Externally Equalized Valves
Employment of an externally equalized valve is re-
quired to control the evaporator at the proper superheat
when the pressure drop of the evaporator is high, i.e.
greater than 3 psig. The externally equalized valve will
sense the pressure at the outlet of the evaporator. In
the schematic below, the pressure under the diaphragm
now totals 64 psig (12 plus 52 psig). The thermal bulb
pressure above the diaphragm force, F-1, also equals
64 psig while the corresponding saturation temperature
is 37°F.
The superheat at the outlet of the evaporator is 9°F
(37°F-28°F). The use of a valve with an external
equalizer has decreased the superheat from 16°F to
9°F and restored the superheat to the original value of
9°F with the same spring force of 12 psig.
The following schematic shows the application of an
internally equalized valve with a pressure drop of 10 psi
across the evaporator. The evaporator saturated inlet
pressure is 62 psig at 35°F. The superheat spring force
(F-3) is set for an equivalent of 12 psig. The pressure
under the diaphragm for an internally equalized valve
would total 74 psig (12 + 62 psig). The remote thermal
bulb force F-1 is 74 psig, for balanced conditions. This
bulb pressure corresponds to a saturation temperature
of 44°F. The pressure at the outlet of the evaporator is
only 52 psig, 10 psig below the inlet pressure. The satu-
ration temperature at 52 psig is 28°F. Use of an inter-
nally equalized valve will result in a superheat of 16°F
(44°F- 28°F) at the evaporator outlet. Accordingly, the
internally equalized valve used with a high pressure
drop evaporator will cause excessive superheat and
corresponding capacity loss.
Note: Never cap an external equalizer service port.
Refer to the evaporator manufacturer’s installation bul-
letin or look for a service port near the outlet of the
evaporator for external equalizer installation.
Internally Equalized Valve with 0 PSI Drop in Evaporator
R-22 Internally Equalized Valve with 10 PSI Drop in
Evaporator
Externally Equalized Valve with 10 PSI Drop in Evaporator