MACS Monthly Newsletter - 2020 - SEP5

Dianetti/Milliman

ing from the lower housing of the power element. See
Figure 5. This thin slice of metal is the very thing that
determines how much movement there is and how
long it will function. For this reason, the diaphragm is
the single most important piece of metal in the valve.
So now you understand how the expansion valve
power element creates movement, but the real trick
is controlling this movement in a way that will benefit
the system it is used in. This is where the operating
half of the valve takes over. The power element trans-

Figure 8: The spring needs the outlet pressure from the
return of the evaporator to balance the force in the power
element.
Figure 7: The sensing side will try to maintain a given superheat in the evaporator by adjusting the ﬂow oriﬁce on the
operating side.

into the operating pin. This force is considered a variable because it changes pressure when the temperature changes. The spring in the adjusting screw gland
provides a constant force up, opposing the power element. This spring force is not strong enough to overcome the force in the power element by itself. It needs
the outlet pressure from the return of the evaporator
to balance all of the forces. This refrigerant flow from
the outlet of the evaporator imparts its temperature
through thermal transfer to the power element and
then the magic happens.
As the thermal tranfer into the power element happens, the refrigerant temperature in the power element
decreases, the charge pressure decreases, the ball seat
starts to close and the mass flow starts to decrease.
When the mass flow decreases, the pressure from the
outlet of the evaporator decreases and the temperature starts to increase. As the thermal transfer into
the power element starts to decrease, the temperature
of the rerigerant in the power element increases, the
charge pressure increases, the ball seat starts to open
and the mass flow starts to increase again. This is
considered one cycle of the expansion valve. Refer
to the "pressure/enthalpy diagram" in Figure 4 for
a graphical representation of this process.

fers its movement via an operating pin to a springloaded orifice.
We will use a block valve as an example as it is easier
to understand. The operating pin through a machined
hole from the inlet to the outlet of the valve will create
a path for refrigerant to flow. See Figure 6. A ball seat
will create an infinitely variable orifice that will be
used to meter the refrigerant flow to the evaporator.
This balancing of forces from the power element to
the ball seat is a delicate dance. The net effect of the
forces (evaporator temperature & evaporator pressure) from Figure 5 on the sensing side, regulates the
operating side to the desired evaporator superheat
highlighted in Figure 7. The sensing side will try to
maintain a given superheat in the evaporator by adjusting the flow orifice on the operating side. This
is how the TXV maintains superheat and maximizes
evaporator efficiency.
Figure 8 illustrates the concept of how the expansion valve works. The charge in the power element
provides a variable force down on the diaphragm and

September 2020

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MACS Monthly Newsletter - 2020

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