83
(18) High Exhaust Gas
Temperature Limit and
Cutout
The purpose of this limit and cutout is to protect the direct-
fired generator from excessive temperatures that could
result from a loss of solution flow. The unusual
temperature increase results from the loss of heat transfer
to the lithium bromide solution surrounding the direct fired
chamber and flue gas tubes, when the solution flow is
restricted.
The high exhaust gas temperature probe is located in the
stack above the top row of fire tubes and senses the
temperature approximately 12” from the side of the shell.
There are two conductor-jacketed cables mounted in the
sensor probe. They are the exhaust gas temperature RTD
temperature sensor and the HTC thermal couple. A twisted
cable containing two stranded wires connects the RTD to
the burner module. The flat-jacketed cable connects the
HTC sensor to the high temperature generator cutout
switch.
High Exhaust Gas Temperature Limit
This limit function reduces the burner fire rate or turns off
the burner lowering the overall generator temperature
when the temperature zone is violated. Figure 33 illustrates
the associated temperatures.
Several events could potentially cause a high exhaust gas
temperature.
•
extremely high tower temperature is one possible
cause.
•
solution flow to the direct fired generator being
interrupted because of a failed high temperature
solution pump,
•
a restricted solution flow thru the HTG.
The limit responds to the Exhaust Gas Temperature sensor
located in the exhaust gas stream. The limit is based on a
schedule (shown below).
Figure 33. Exhaust stack limit during automatic operation
Burner Operation
Machine Shutdown
above 425°F MMR
Burner Off
Hold at Minimum Fire
10% fire maximum at
400.1°F and Ramp to
minimum fire at 410°F
Exhaust Gas Temperature