Daikin McQuay AGZ-D Operating And Maintenance Manual page 33

Condenser Target
A condenser target should be selected from the
setpoints based on the number of compressors on
the unit and the number of compressors running.
Each stage of capacity on a circuit will use a
different condensing target set point.
A minimum condenser target should be enforced.
This minimum will be calculated based on the
evaporator LWT. As the LWT varies from 7.2°C
(45°F) to 32.2°C (90°F), the minimum condenser
target will vary from 23.9°C (75°F) to 48.9°C
(120°F).
Staging Up
The first fan should turn on when the saturated
condenser temperature exceeds the condenser
target. After this, the four stage up dead bands
shall be used. Stages one through four should use
their respective dead bands. Stages five through
six should all use the Stage Up Dead Band 4.
When the saturated condenser temperature is
above the target + the active deadband, a stage up
error is accumulated.
Stage Up Error Step
Temperature – (Target + Stage Up dead band)
The Stage Up Error Step is added to Stage Up
Accumulator once every 5 seconds, only if the
Saturated Condenser Refrigerant Temperature is
not falling. When Stage Up Error Accumulator is
greater than 11°C (19.8°F) another stage is added.
When a stage up occurs or the saturated condenser
temperature falls back within the stage up dead
band the Stage Up Accumulator is reset to zero.
Staging Down
Four stage down dead bands shall be used. Stages
one through four should use their respective dead
Stage Up Compensation
In order to create a smoother transition when another fan is staged on, the VFD compensates by slowing down
initially. This is accomplished by adding the new fan stage up deadband to the VFD target. The higher target
causes the VFD logic to decrease fan speed. Then, every 2 seconds, 0.1°C (0.18°F) is subtracted from the
VFD target until it is equal to the saturated condenser temperature target set point.
OMM 1087
= Saturated Condenser
AGZ 025D through 190D
bands. Stages five through seven should all use
Stage Down Dead Band 4.
When the saturated
temperature is below the target – the active
deadband, a stage down error is accumulated.
Stage Down Error Step = (Target - Stage Down
dead band) - Saturated Condenser Temperature
The Stage Down Error Step is added to Stage
Down Accumulator once every 5 seconds. When
the Stage Down Error Accumulator is greater than
2.8°C (5°F) another stage of condenser fans is
removed.
When a stage down occurs or the saturated
temperature rises back within the Stage Down
dead band the Stage Down Error Accumulator is
reset to zero.
VFD
Condenser pressure trim control is accomplished
using an optional VFD on the first fan. This VFD
control should vary the fan speed to drive the
saturated condenser temperature to a target value.
The target value is normally the same as the
saturated condenser temperature target.
The speed should be controlled between the
minimum and maximum speed setpoints.
VFD State
The VFD speed signal should always be 0 when
the fan stage is 0.
When the fan stage is greater than 0, the VFD
speed signal should be enabled and control the
speed as needed.
condenser refrigerant
33