Unit Functions
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IOM 1242-8 • PATHFINDER
®
MODEL AWV CHILLERS
Freeze Protection
To protect the evaporator from freezing, the evaporator pump
will start if the manual reset flow loss alarm is not active and
either of the following are true:
• LWT equal to or less than the Evap Freeze set point for at
least three seconds and LWT sensor fault is not active
• EWT equal to or less than the Evap Freeze set point for
at least three seconds and EWT sensor fault is not active
Freeze protection will end when manual reset flow loss alarm
is active or all of the following are true:
• LWT is at least 1.11°C (2°F) above the Evap Freeze set
point or LWT sensor fault is active
• EWT is at least 1.11°C (2°F) above the Evap Freeze set
point or EWT sensor fault is active
• Pump has been running for at least 15 minutes
Pump Selection
The pump output used when evaporator state is Run will be
determined by Evap Pump Control set point. This setting
allows the following configurations:
•
#1 only – Pump 1 will always be used
•
#2 only – Pump 2 will always be used
•
Auto – The primary pump is the one with the least run
hours, the other is used as a backup
•
#1 Primary – Pump 1 is used normally, with pump 2 as a
backup
•
#2 Primary – Pump 2 is used normally, with pump 1 as a
backup
Primary/Standby Pump Staging
The pump designated as primary will start first. If the
evaporator state is Start for a time greater than the recirculate
timeout set point and there is no flow, then the primary
pump will shut off and the standby pump will start. When the
evaporator is in the run state, if flow is lost for more than half of
the Flow Loss Delay set point value, the primary pump will shut
off and the standby pump will start. Once the standby pump
is started, the flow loss alarm logic will apply if flow cannot be
established in the evaporator start state, or if flow is lost in the
evaporator run state.
Auto Control
If auto pump control is selected, the primary/standby logic
above is still used. When the evaporator is not in the run state,
the run hours of the pumps will be compared. The pump with
the least hours will be designated as the primary at this time.
Leaving Water Temp (LWT) Reset
LWT Target
The evaporator LWT Target varies based on settings and
inputs and is selected as follows:
Table 51: LWT Targets Control Source
Available
Modes
Set Point
Control
Source
Set Point
Mode
Switch
BAS
Mode
Command
BAS LWT Target
Cool
Local
-
-
Cool LWT Set Point
Cool
Network
-
-
BAS Cool Set Point
Cool/Ice
Local
Off
-
Cool LWT Set Point
Cool/Ice
Local
On
-
Ice LWT Set Point
Cool/Ice
Network
-
Cool
BAS Cool Set Point
Cool/Ice
Network
-
Ice
BAS Ice Set Point
Ice
Local
-
Ice LWT Set Point
Ice
Network
-
-
BAS Ice Set Point
The base LWT target may be reset to a higher value if the
unit is in Cool Mode and it is configured for a reset. The type
of reset to be used is determined by the LWT Reset Type set
point.
When the active reset increases, the Active LWT Target is
changed at a rate of 0.1°C (0.18°F) every 10 seconds. When
the active reset decreases, the Active LWT Target is changed
all at once.
After resets are applied, the LWT target can never exceed a
value of 21.12°C (70°F). Since the Cool LWT set point can be
set as high as 21.12°C, the amount of reset may be limited in
order to meet this requirement to clamp the active LWT target
to 21.12°C.
Reset Type – None
The Active LWT target is set equal to the base LWT target.
4-20 mA External Signal Reset
The Active LWT target is adjusted by the 4-20 mA reset analog
input. Parameters used:
1. Max Reset set point
2. LWT Reset signal
Reset is 0 if the reset signal is less than or equal to 4 mA.
Reset is equal to the Max Reset Delta T set point if the reset
signal equals or exceeds 20 mA. The amount of reset will vary
linearly between these extremes if the reset signal is between
4 mA and 20 mA. An example of the operation of 4-20 mA reset
in Cool Mode follows.
Figure 69: 4-20 mA Reset - Cool Mode
Max Reset
Base LWT
Max
Reset
Base LWT
Target
20
0
Reset Signal (mA)
Active
LWT
Target
4
4
-
20 mA Reset
