Page 181 - Boiler_Operators_Handbook,_Second_Edition

166 Boiler Operator’s Handbook

those two. The difference between the two forces on the
diaphragm is opposed by a spring. When the difference
increases, due to an increase in superheat, the spring is
compressed to open the valve further and increase the
flow of liquid into the evaporator. Conversely, a de-
crease in superheat will result in the valve closing down
to reduce liquid flow.
In some applications where the pressure drop of
the liquid and gas flowing through the evaporator is
low the evaporator pressure is sensed only at the valve
outlet. For most installations however, the lower dia-
phragm chamber is connected to the evaporator outlet
with an external sensing line, commonly referred to as
an equalizing tube, which is shown in Figure 5-25. The
TXV outlet pressure sensing connection is a small orifice
in the valve body and the flow through it is dumped off
so fast through the sensing line to the evaporator outlet
that the pressure on the bottom of the diaphragm is very Figure 5-25. TXV installation
close to the evaporator outlet pressure. This installation
change the setting of the superheat. This should be a “set
ensures the TXV is controlling by the actual superheat at
and forget” adjustment. If you have reason to believe
the outlet of the evaporator.
that the valve is not working properly there are many
The TXV has a strainer at the inlet that is accessible
things to check before adjusting the superheat setting.
for cleaning. It cannot be removed for cleaning without
Understanding the operation of the valve should aid
removing the refrigerant from the system. Periodic clean-
you in determining what the problems might be. A com-
ing is not required; the principal purpose of the strainer is
mon problem is vibration loosening the mounting of the
to catch droplets of solder and contaminants in the liquid
thermal bulb.
line upon startup of the system or products carried down
Proper mounting of the thermal bulb is shown in
the liquid line from breakdown of the filter dryer.
Figure 5-26. The temperature of the bulb is maintained
The TXV also has a cap that can be removed to
by heat transfer through contact of the bulb and tube
access an adjustment of the spring pressure that will
because heat transfer through the metal is faster than

Figure 5-26. Mounting of thermal bulb