User Guide

and subtracting the saturation temperature from the vapor
temperature measured at the sensing bulb location. For
example, the superheat of R-22 vapor at 50°F and 68.5 psig
at the sensing bulb location is calculated as follows:
saturation temperature of R-22 vapor at 68.5 psig = 40°F
superheat = 50°F - 40°F = 10°F
Another method of measuring superheat the TEV is control-
ling is the two temperature method. With this method,
saturation temperature is measured directly by placing a
temperature probe on the evaporator surface, normally at
a location one-half to two-thirds the distance through the
evaporator coil. Since this method can only approximate true
saturation temperature, it is not as reliable as the pressure-
temperature method, and it should be avoided whenever
possible.
The TEV is designed to control superheat at a constant
value at the location of its sensing bulb. The level of super-
heat determines to what extent the valve is open. A TEV
controlling at a high superheat will be further open than
a TEV controlling at a low superheat. Refer to the section,
How the Thermostatic Expansion Valve Works, on Page 3
for additional information. Figure 13 shows a plot of valve
capacity versus superheat for a typical TEV, illustrating
the effect superheat has on valve capacity. For the purpose
of understanding the relationship between superheat and
valve capacity, superheat may be described as follows:
Static Superheat — static superheat is the amount of
superheat necessary to overcome the spring and equalizer
pressures so that any additional superheat will cause the
valve to open.
Opening Superheat — opening superheat is the amount of
superheat required to move the valve pin away from the seat
after the spring and equalizer pressures have been overcome
to permit refrigerant flow.
Operating Superheat — operating superheat is the super-
heat at which the TEV operates on a refrigeration system.
Operating superheat is the sum of static and opening super-
heats. The valve capacity versus operating superheat curve
is referred to as the valve gradient.
The most desirable operating superheat for a particular sys-
tem largely depends on the temperature difference (TD)
between the refrigerant and the medium being cooled. The
basic definition of TD is the difference between evaporator
temperature and the entering temperature of the medium
being cooled, i.e., air or water. Systems with a high TD, such
as air conditioning and heat pump systems, can tolerate
higher superheats without appreciable loss in system capac-
ity. Refrigeration and low temperature systems require low
superheats due to their lower TDs. The table below provides
general recommendations for superheat settings for differ-
ent evaporator temperature ranges. These settings are
only estimates for typical system designs and should
only be used if setting guidelines are unavailable
from the system manufacturer:
Page 14 / BULLETIN 10-9
serve as a secondary orifice to reduce pressure drop across
the valve port.
Thermostatic Charges for Ammonia Valves
Thermostatic charges C, Z, and L are available for the Type
D thermostatic expansion valve. The Type L thermostatic
charge is the only charge available for the Type A valve.
The Types C and Z thermostatic charges provide operating
advantages for systems that cycle in response to a suction
pressure switch or thermostat. These charges are also rec-
ommended for systems using a small capacity compressor.
The table below lists the recommended temperature range
for each charge.
Cold storage plants will often have large centralized ammo-
nia systems. These systems will consist of many evaporators
connected to one or more large compressors. With many
thermostatic expansion valves operating at a common evapo-
rator pressure, a change in flow rate made by one valve will
not have a significant effect on the evaporator pressure.
This operating characteristic makes it more desirable for
the thermostatic expansion valve to be more responsive to
changes in bulb temperature. This is the feature of the Type
L charge. Therefore, for large ammonia systems consisting
of multiple evaporators, the Type L charge is recommended.
FACTORS AFFECTING TEV OPERATION AND
PERFORMANCE
Many factors exist which influence TEV operation and per-
formance. The following discussion lists the major factors:
Superheat
Superheat is defined as the difference between the refrig-
erant vapor temperature and its saturation temperature.
To properly measure the superheat the TEV is controlling,
the pressure-temperature method is used. This method
consists of measuring the suction pressure at the sensing
bulb location, converting this pressure to its saturation
temperature by using a pressure temperature (P-T) chart,
sgnitteStaehrepuSrofsenilediuGlareneG
noitacilppA 
riA
gninoitidnoC
pmuPtaeH&
laicremmoC
noitaregirfeR
woL
erutarepmeT
noitaregirfeR
rotaropavE
erutarepmeT °F
50° to 40° 40° to 0° 0° to –40°
taehrepuSdetsegguS
gnitteS °F
°8 °21ot °6 °8ot
4° to 6°
egrahCcitatsomrehT erutarepmeTrotaropavE F°
C 40° to 0°
Z 0° to –30°
For applications at evaporator temperatures below minus 30°F, consult Parker
Refrigerating Specialties Division.
Rated
Capacity
Full Open
Capacity
Reserve
Capacity
Superheat
Valve Capacity
0
A B
C
A = Static Superheat
B = Operating Superheat
C = Operating Superheat
Figure 13
Table 6

Table 7