Thermon TraceNet TCM 18 - Manual (Page 50)

8
Heat Trace Control and
Monitoring
The TraceNet system allows a variety of control options for heat trace operation. The
most energy efficient control mode is to use one or more process sensing RTD’s for
each heat trace circuit. When configured with two RTD sensors, TraceNet will con-
trol off of the lowest reading and alarm off of the highest reading encountered. In
the case of process sensing control, however, one must be aware of the normal flow
directions within the process piping and only group process piping having a com-
mon flow condition with the control sensors. A failure to do so can result in non
flowing areas cooling and freezing where the flowing portions have appropriately
turned the heat trace circuit off. Process sensing control is also a necessity where
steam outs and high exposure temperature process conditions are expected and
where the heat trace (due to its inherent characteristics) cannot be operated during
such events. When using this control mode, the TraceNet TC Series panel will have
RTD’s hard wired directly back to the panel.
As an alternate control mode which is a bit less energy conservative, the TraceNet
TC Series panel may be configured for Ambient Proportional Control (APC). In this
case, one or two RTD’s may be used to sense ambient temperatures in the process
area. The heat trace will be set to operate at 100% power at the maintenance tem-
perature (which is the minimum ambient condition) and then ramp down to a 20%
power level at the maintenance temperature plus the control band. If the ambient
rises above this value, the heat trace will then turn off. For example, to freeze protect
a process unit in a minimum ambient of -40
◦ F (-40 ◦ C), one would set the circuit to
operate on APC. The APC MAINTAIN TEMP (at or below which power is on 100%)
would be programmed to be a value of -40
◦ F (-40 ◦ C). The Control Band would be
set to 90 ◦ F (50 ◦ C) and thus the heat trace circuit would turn off above 50 ◦ F (= -40
+ 90) or 10 ◦ C (= -40 + 50) ambient conditions. Obviously, this type of control mode
will reduce RTD requirements but still achieve a good measure of temperature con-
trol. In addition, due to the amount of power cycling it is important to realize that
this should only be utilized when using solid state relay switching of the heat trac-
ing circuits. APC control should not be used where steam outs and high exposure
temperature process conditions are expected and where the heat trace due to its
inherent characteristics cannot be operated during such events.
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