MACS Monthly Newsletter - 2020 - SEP6

Dianetti/Milliman

Dianetti/Milliman

Figure 9: In this picture the refrigerant is shown as droplets
on the diaphragm inside the power element. The state of the
valve is represented by the arrows in the middle and effects
of this are shown. The result is an amount of stroke from the
closed state to the open state.

MOP

Dianetti/Milliman

Note: When discussing expansion valves, refrigerant flow, evaporator temperatures and pressures, the
term "MOP" will often come up. MOP means "maximum operating pressure" but to the expansion valve
it is more like a pressure limit. This pressure limit is
a design feature in the valve that is directly related to
the charge in the power element.
Since the underside of the power element diaphragm is exposed to the pressure returning from the
evaporator (Figure 8), this is the pressure the system
needs to reach before the expansion valve starts to meter and control refrigerant flow. This pressure limit is
also known as MOP and if the pressure returning from
the evaporator exceeds this designed threshold, the
ball seat shown in Figure 6 will close. This limits the
pressure of the refrigerant supplied to the compressor.

Figure 10: Compressor damage caused by use of an incorrect TXV.
This MOP or pressure limit in the expansion valve is
created by the stroke of the ball off the seat. See Figure
9. At the superheat condition, the refrigerant in the
power element has less pressure in it and the stroke
(the distance the ball is off the seat) is very small. In
the MOP / pressure limit condition, the refrigerant in
the power element has a greater pressure in it, and the
stroke (the distance the ball is off the seat) is greater.
This stroke does have a limitation and is different for
all valves but should not exceed other design parameters such as overall system tonnage for risk of damaging other components like the one pictured in Figure 10.
Stroke of the valve is an important
topic because it translates almost directly
to the mass flow of refrigerant. Figure
11 shows the test results of an R-134a
charged power element in a 2-ton valve
as the stroke is increased from 0 to full
open. The results of which will change
slightly for different refrigerant charges,
but basic principle will remain the same.

Are there different types of TXVs?

When talking about different types of
TXVs, there are conventional ones that
use all of the principals we have been
talking about in this MACS Service Report,
and there are electronic TXVs. The
Figure 11: Test results of an R-134a charged power element in a 2-ton valve
electronic
TXVs are still relatively exas the stroke is increased.
pensive and require feedback from the

September 2020

6

MACS Service Reports



MACS Monthly Newsletter - 2020

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