72
The SDR uses temperature sensors to detect reverse flow
from the high temperature generator. Under normal
operation solution from the high temperature generator
mixes with solution from the low temperature generator
before passing through the low temperature heat
exchanger. A temperature sensor placed above the mixing
point will normally measure a temperature equal to the
solution temperature leaving the low temperature
generator. When the mixed lithium bromide solution flow
through the low temperature heat exchanger is restricted
with crystals, the normal path of solution flow is
interrupted. Solution from the high temperature generator
flows back wards into the low temperature generator
outlet. A temperature sensor above the mixing point will
see the hotter solution from the high temperature
generator. The temperature sensor is designated
Crystallization Sensing Detection sensor. Figure 22
illustrates the solution flows.
C) RECOVERY SEQUENCE
When reverse flow is sensed the microprocessor system
will perform the crystallization recovery sequence. This
sequence dilutes the lithium bromide solution breaking up
the “Blockage” that caused the reverse flow. Each SDR
cycle performed will be recorded by the control system.
Each of the first two SDR detections will cause the active
margin to be increased by 5 degrees. The minimum
solution pump flow command is increased 5%, and the
ON/OFF Pump sequence will be performed. On the third
SDR occurrence reverse flow detection, an MMR will be
issued, and a Dilution Cycle will be initiated.
System Response to a SDR is as follows:
1. Energy Inputs Commanded Off
All solution pumps stop,
RDV1 opens to dilute the absorber .
3 minutes elapses
2. High and Low Temperature Solution Pumps turn ON.
The display indicates a 5-min Pump On SDR timer.
5 minutes elapses
3. High and Low Temperature Solution Pumps turn OFF.
The display indicates 3 min Pump Off SDR timer.
3 minutes elapses
4. System will restart normal chiller operation (unless the
system has been commanded to Stop or there are
pending diagnostics).
Total time elapsed is 11 minutes
If a Stop has been commanded or there is an active MMR
or MAR diagnostic the unit will enter a Dilution Cycle.
The active Low Temperature Heat Exchanger (LTHX)
Margin is increased 5°F and the minimum solution pump
speed increased 10%, to improve ability to stay on line.
After completion of the recovery operation:
•
If SDR temperature is less than or equal to the Low
Temperature Generator (LTG) solution tempe3
deg, then control returns to normal operating status. .
If SDR Temperature<= [Low Temperature Generator
(LTG) sol +3°F] then OK
•
If SDR temperature is greater than (Low Temperature
Generator (LTG) tempe temperature leaving
float valve) / 2 ) AND there is 10°F or greater difference
between Leavingg Float Valve and Leaving LTG
Temperatures, then control returns to SDR mode.
If SDR Temperature> [(Low Temperature Generator (LTG)
sol + High Temperature Generator (HTG) sol) / 2 ] and [High
Temperature Generator (HTG) sol >= (Low Temperature
Generator (LTG) sol + 10°F)] then returns SDR.
The active Low Temperature Heat Exchanger (LTHX)
Crystallization Temperature Margin Setpoint will return to
the service settings value and active minimum solution
pump flow command at the next Reset. The front panel
Low Temperature Heat Exchanger (LTHX) Crystallization
Temperature Margin Setpoint does not reflect the increased
value of the 5°F increase.
Reset, Stop, or Panic stop during SDR cycle
If a Reset is issued, the unit will remain in a SDR Mode until
the ON/OFF Pump sequence is complete. The Reset will
then be performed and the unit will start if the unit mode is
Auto and there are no latching diagnostics. Otherwise, a
Dilution Cycle will be initiated.
If a Stop is issued, the unit will remain in a SDR Mode until
the ON/OFF Pump sequence is complete. A Dilution Cycle
will then be initiated.
If a Panic Stop is issued, a Panic Stop is initiated without
delay.