IOM 1242-8 • PATHFINDER
®
MODEL AWV CHILLERS
70 www.DaikinApplied.com
Circuit Functions
If the unit is configured with liquid line solenoid valves, the EXV
position should be 0% when the EXV initially enters the closed
state, while it is re-initializing to the zero position. After the
EXV position command has been 0% for one minute, the EXV
should be moved to 5% to prevent excessive pressure buildup
between the EXV and liquid line solenoid valve.
Preopen Operation
When the EXV state is Preopen, the EXV position command
will be 30%.
Pressure Control State
In Start Pressure Control, the EXV is positioned to control the
evaporator pressure to a target.
Pressure Target
The pressure target is calculated based on evaporator LWT,
then limits are applied to keep the target in an acceptable
range.
The base target value varies from 65 kPa to 420 kPa as LWT
varies from -5°C to 30°C.
Figure 74: Evaporator EXV Pressure Target
This base target is then limited to a range as defined by the
Minimum Limit and Maximum Limit shown below:
• Minimum Limit = Low Pressure Hold set point + 25 kPa
•
Maximum Limit = Min {(Cond Pr + 101.325) / 1.7 –
101.325, Cond Pr – 180}
• If Maximum Limit < Minimum Limit, then Pressure Target
= Minimum Limit
MOP Control State
In MOP (Maximum Operating Pressure) Control, the EXV is
positioned to control the evaporator pressure to a target.
MOP Target
The MOP Target is Min {450, (Cond Pr + 101.325) / 1.7 –
101.325, Cond Pr – 180}.
Superheat Control State
In superheat control, the EXV is positioned to control suction
superheat (SSH) to the SSH Target.
SSH Target
The SSH target varies linearly based on discharge superheat
(DSH). This target is updated every 10 seconds and averaged
over a 100 second period (with sampling every 10 seconds).
Figure 75: Evaporator EXV Suction Superheat Target
When the EXV transitions to the superheat control state, the
target will initially be set to the SSH value. The target will then
ramp up or down until it meets the calculated SSH target.
Limits for the SSH target are:
•
If SSH Target Min ≤ current SSH ≤ SSH Target Max, the
target will start at the current SSH value and increase
or decrease by 0.1°C every 10 seconds until it meets
calculated SSH target.
• If current SSH > SSH Target Max, the target value will
start ramping from SSH Target Max. Then the target
will decrease by 0.1°C every 10 seconds until it meets
calculated SSH target.
• If current SSH < SSH Target Min, the target value will
start ramping from SSH Target Min. Then the target
will increase by 0.1°C every 10 seconds until it meets
calculated SSH target.
Economizer EXV Control
The economizer circuit is activated and modulated using an
EXV and, for some configurations, a solenoid valve. This is
an optional feature that will only be enabled if the Economizer
Configuration set point is set to Yes. If the configuration set
point is set to No, then the economizer position should remain
0% and the economizer solenoid valve output should remain
off.
Control Concept
• EXV will be positioned using control algorithm
• Maintain DSH above requirements for compressor
• Maximize circuit performance at the operating conditions
EXV States
There are three economizer EXV states:
• Closed: EXV is closing or closed
• Purge: EXV is opening to release condensed liquid in a
controlled manner
• Superheat: EXV is controlling economizer gas superheat
30
-5
65
420
LWT (°C)
Base Target (kPa)
DSH (°C)
DSH Range Max
SSH Target Min
SSH target (°C)
DSH Range Min
SSH Target Max
