User Guide

valve’s diaphragm. The sensing bulb, capillary tubing, and
diaphragm assembly is referred to as the thermostatic
element. The thermostatic element on all standard Sporlan
TEVs is replaceable.
The diaphragm is the actuating member of the valve. Its
motion is transmitted to the pin and pin carrier assem-
bly by means of one or two pushrods, allowing the pin to
move in and out of the valve port. The superheat spring
is located under the pin carrier, and a spring guide sets it
in place. On externally adjustable valves, an external valve
adjustment permits the spring pressure to be altered.
There are three fundamental pressures acting on the valve’s
diaphragm which affect its operation: sensing bulb pressure
P1, equalizer pressure P2, and equivalent spring pressure
P3 (see Figure 1). The sensing bulb pressure is a function of
the temperature of the thermostatic charge, i.e., the sub-
stance within the bulb. This pressure acts on the top of the
valve diaphragm causing the valve to move to a more open
position. The equalizer and spring pressures act together
underneath the diaphragm causing the valve to move to a
more closed position. During normal valve operation, the
sensing bulb pressure must equal the equalizer pressure
plus the spring pressure, i.e.:
P1 = P2 + P3
Equivalent spring pressure is defined as the spring force divided
by the effective area of the diaphragm. The effective area of the
diaphragm is simply the portion of the total diaphragm area
which is effectively used by the bulb and equalizer pressures to
provide their respective opening and closing forces. Equivalent
spring pressure is essentially constant once the valve has been
adjusted to the desired superheat. As a result, the TEV functions
by controlling the difference between bulb and equalizer pres-
sures by the amount of the spring pressure.
The function of the sensing bulb is to sense the temperature
of the refrigerant vapor as it leaves the evaporator. Ideally,
the bulb temperature will exactly match the refrigerant
vapor temperature. As the bulb temperature increases, bulb
pressure also increases causing the valve pin to move away
from the valve port, allowing more refrigerant to flow into
the evaporator. The valve continues in this opening direc-
tion until the equalizer pressure increases sufficiently that
the sum of the equalizer and spring pressures balance with
the bulb pressure. Conversely, as the bulb temperature
decreases, the bulb pressure decreases causing the valve pin
to move toward the valve port, allowing less refrigerant to
flow into the evaporator. The valve continues to close until
the equalizer pressure decreases sufficiently that the sum
of the equalizer and spring pressures balance with the bulb
pressure.
A change in refrigerant vapor temperature at the outlet of
the evaporator is caused by one of two events: (1) the spring
pressure is altered by means of the valve adjustment, and
(2) the heat load on the evaporator changes. When spring
pressure is increased by turning the valve adjustment
clockwise, refrigerant flow into the evaporator is decreased.
Vapor temperature at the evaporator outlet increases since
the point where the refrigerant completely vaporizes moves
further back within the evaporator, leaving more evapora-
tor surface area to heat the refrigerant in its vapor form.
The actual refrigerant vapor and bulb temperature will be
controlled at the point where bulb pressure balances with
the sum of the equalizer and spring pressures. Conversely,
decreasing spring pressure by turning the valve adjustment
counterclockwise increases refrigerant flow into the evapora-
tor and decreases refrigerant vapor and bulb temperature.
Spring pressure determines the superheat at which the
valve controls. Increasing spring pressure increases super-
heat, decreasing spring pressure decreases superheat.
An increase in the heat load on the evaporator causes refrig-
erant to evaporate at a faster rate. As a result, the point of
complete vaporization of the refrigerant flow is moved fur-
ther back within the evaporator. Refrigerant vapor and bulb
temperature increase, causing bulb pressure to rise and the
valve to move in the opening direction until the three pres-
sures are in balance. Conversely, a reduction in the heat load
on the evaporator will cause the vapor and bulb temperature
to fall and the valve to move in a closed direction until the
three pressures are in balance. Unlike a change in the spring
pressure due to valve adjustment, a change in the heat load
on the evaporator does not appreciably affect the superheat
at which the thermostatic expansion valve controls. This
is due to the fact that the TEV is designed to maintain an
essentially constant difference between bulb and equalizer
pressures, thus controlling superheat regardless of the heat
load.
Effect of Pressure Drop Across the Valve Port
An additional pressure affecting valve operation, which is
not considered fundamental, arises from the actual pressure
drop across the valve port. This pressure P4 can be related to
the three fundamental pressures as the product of pressure
drop across the valve port and the ratio of the port area to
the effective area of the diaphragm, i.e.:
P4 = Pressure Drop x (Port Area / Effective Diaphragm Area)
With Sporlan’s conventional TEV design, this pressure is an
opening influence since refrigerant flow tends to move the
valve in an opening direction. As a result, our original equa-
tion is modified as follows:
P1 + P4 = P2 + P3
Page 4 / BULLETIN 10-9
Superheat
Figure 1
Evaporator Temperature
Bulb Temperature
Closing Force
2
3
2
Closing Force
2
+
3
2
1
Bulb
Pressure
Spring
Pressure
Evaporator
Pressure
Refrig.
Curve
1
O
p
e
n
i
n
g
F
o
r
c
e
Figure 1