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Document: GFH-NT 1500-4000 IM 14 DEC 2016
Gas Fired Heater Instructions
©2016 NOVATEC, Inc. All Rights Reserved
8.0 PROCESS HIGH TEMPERATURE LIMIT
A process high temperature limit switch (PHTL) with display has been supplied with the gas heater.
It requires a manual reset if tripped. The process high temperature limit thermocouple for this
device is mounted in the process air outlet duct on the gas heater. The high temperature limit
switch will shut down the gas heater on an over-temperature condition. In normal operation, the
PHTL indicates “
SAFE
”. If the temperature exceeds the high temp limit, the display will indicate the
actual temperature and “
OP1
” on the limit controller will be illuminated. Press the “
RESET
” button
on the high temp limit switch to reset the circuit. If the temperature is below the limit value, the
“
SAFE
” display will return. “
OP1
” is displayed until the “
RESET
” has been pressed.
CAUTION: The gas heater should be turned off if the process blower motor is off for a
period of time exceeding two minutes. If no process air is flowing through the gas heater,
excessive combustion temperatures made occur even when the gas heater is in "low heat
mode" (LF). This may cause the combustion high temperature limit switch to trip, and/or
could result in over temperature process air being delivered to the hopper when the process
blower is re-started, which may cause melting of the material in the hopper. Extreme
caution should be used to prevent this condition.
Optional controls have been added to
automatically shut down the gas heater if the conditions described above are present in lieu of
manually turning the gas heater off. If the process blower maximum off delay time elapses, the
operator manually resets the circuit by toggling the gas heater burner off-on switch.
Shutting down of the gas heater is necessary as described above because the heat exchanger is
very efficient. With no process air flow, the pilot alone will transfer a large amount of heat to the
process side, so that when the process air is restored this built up heat would be released into the
process air, resulting in the over temperature condition. The process blower maximum off delay
prevents this from happening.
Photograph, Process and Combustion High Temperature Limit Switch (PHTL & CHTL)
