Exv Control - Daikin AGZ070EH Installation, Operation And Maintanance Manual

Unit Controller Operation

EXV Control

Auto Control
Any time the circuit is not in the run state, the EXV position
should be 0. The EXV control state should display that the
EXV state is closed when this is the case.
When the circuit is in the Pre-open state, the EXV should go
into superheat control.
While a circuit is in the run state, the EXV should go into
superheat control. While in superheat control, the EXV
controls suction superheat. The suction superheat target is set
by a set point. A PID function will be used to control the
superheat to the target value. The EXV response should be
faster when the SSH is lower than 1.67
the SSH Target + 1.67 C (3
o
The EXV should also prevent the evaporator pressure from
exceeding the Maximum Evaporator Pressure set point. This
is done by using another PID function to control evaporator
pressure to the maximum evaporator pressure.
The EXV position should be lesser position output from the
two PID functions.
Position Commands
In order to improve the reliability of the EXV positioning, the
position commands that are issued to the stepper driver are
limited in two ways:
First, position commands are filtered so that the minimum
change in position is 0.3%. Changes of less than this are
ignored. This avoids unnecessary movement of the EXV and
lowers the chances of losing steps as a result.
Second, the position commands are issued once per program
cycle with a maximum change of 0.7% each time. This allows
the stepper to move the valve to the commanded position
before the next position command is issued. Issuing commands
in this way may also lower the chances of losing steps.
EXV Position Range
The following table shows the EXV range based on the
number of compressors running and the total number of fans
on the unit.
EXV Min
Num Fans = 4
EXV Max
44
C (3 F) or higher than
o o
F) .
o
Compressors Running
1 2
8% 8%
50% 100%
The expansion valve maximum position may be increased if
after two minutes both the suction superheat is greater than
7.2°C (13°F) and the expansion valve has been within 5% of its
current maximum position. The maximum should increase at a
rate of 0.1% every six seconds up to a total of an additional
10%. This offset to the maximum position should be reset
when the EXV is no longer in the Superheat Control state or a
compressor on the circuit stages.
Manual Control
The EXV position can be set manually. Manual control can
only be selected when the circuit is in the run state. At any
other time, the EXV control set point is forced to auto.
When EXV control is set to manual, the EXV position is equal
to the manual EXV position setting. If set to manual when the
circuit state transitions from run to another state, the control
setting is automatically set back to auto. When in manual
control, the EXV control state displayed should reflect that it is
manual control.
Liquid Line Solenoid Valve
The liquid line solenoid output should be on when the circuit
state is either Pre-open or Run. This output should be off at all
other times.
Hot Gas Bypass Solenoid Valve
This output shall be on when circuit state is Run and one
compressor on the circuit is running. The output should be off
at all other times.
Capacity Overrides – Limits of
Operation
The following conditions shall override automatic capacity
control as described. These overrides keep the circuit from
entering a condition in which it is not designed to run.
Low Evaporator Pressure
If the Low Evaporator Pressure Hold or Low Evaporator
Pressure Unload alarms are triggered, the circuit capacity may
be limited or reduced. See the Circuit Events section for
details on triggering, reset, and actions taken.
High Condenser Pressure
3
-
If the High Condenser Pressure Unload alarm is triggered, the
-
circuit capacity may be limited or reduced. See the Circuit
Events section for details on triggering, reset, and actions
taken.
IOM 1206-1