Daikin MicroTech III Operation Manuals page 109

If the Discharge Air Temperature exceeds Discharge Cooling
Setpoint, the difference is added to the Degree Time Above
value . If the Discharge Air Temperature is below the Discharge
Cooling Setpoint, the difference is added to the Degree Time
Below value . These values are limited to a maximum value of
250 to prevent remaining too long in one stage because one
value or the other became very large .
When the unit enters the Cooling state the first compressor
is turned on immediately . When the unit is equipped with
evaporative condensing, the sump pump must be turned on
before any compressor is turned on . If there is a sump pump
fail condition, cooling will stay in stage 0 .
With DAT staging control, there are four possible staging
transitions; Stage up after stage up, stage up after stage down,
stage down after stage down, and stage down after stage up .
These are described in the following paragraphs:
Stage Up After Stage Up:
If the time since the last stage change exceeds the cooling
stage timer, the discharge air temperature is greater than the
Discharge Cooling Setpoint by more than half the deadband,
the last stage change was a stage up, and dehumidification is
not active; cooling capacity is increased by one stage
Stage Up After Stage Down:
If the time since the last stage change exceeds the cooling
stage timer, the discharge air temperature is greater than the
Discharge Cooling Setpoint by more than half the deadband,
the last stage change was a stage down, the Degree Time
Above value is greater than or equal to the Degree Time Below
value, and the dehumidification is not active; cooling capacity
is increased one stage .
Stage Down After Stage Down:
If the time since the last stage change exceeds the cooling
stage timer, the discharge air temperature is less than the
Discharge Cooling Setpoint by more than half the deadband,
the last stage change was a stage down, and dehumidification
is not active; cooling capacity is decreased one stage .
Stage Down After Stage Up:
If the time since the last stage change exceeds the cooling
stage timer, discharge air temperature is less than the
Discharge Cooling Setpoint by more than half the deadband,
the last stage change was a stage up, the Degree Time Below
value is greater than or equal to the Degree Time Above
value, and dehumidification is not active; cooling capacity is
decreased one stage .
The Degree Time Below and Degree Time Above values
change whenever a stage change occurs . If the previous stage
change was a stage up and the number of stages increases
again, both Degree Time Above and Degree Time Below are
set to zero .
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If the last stage change was a stage up and the stage
decreases due to the Degree Time Below exceeding the
Degree Time Above, the Degree Time Below is reduced by
an amount equal to Degree Time Above and then the Degree
Time Above is set to zero .
If the last stage change was a stage down and the stage
increases due to the Degree Time Above exceeding the
Degree Time Below, the Degree Time Above is reduced by an
amount equal to Degree Time Below and then the Degree Time
Below is set to zero .
Degree Time logic is not used when dehumidification is
active. When dehumidification is active, cooling capacity is
increased if the time since the last stage change exceeds the
cooling stage timer and the Leaving Coil Temperature (LCT)
is greater than the Maximum Leaving Coil Setpoint . When
dehumidification is active, cooling capacity is decreased if
the time since the last stage change exceeds the cooling
stage timer amd the leaving coil temperature is less than the
minimum leaving coil setpoint .
Average Discharge Control Method Illustration
Figure 16: Average Discharge Control Method on page 110
an illustration of the "Degree Time" compressor staging control
method and is meant to show a variety of staging possibilities
not normal unit operation .
graph of the discharge air temperature changing with time . The
Cooling Interstage Timer setting is 5 minutes .
Point 1 Assume that the controller has just staged up and
that DTA and DTB are zero . As a result, the discharge air
temperature drops and the Cooling Interstage Timer is reset .
Point 2 DTA (Area A) equals DTB (Area B) . The discharge air
temperature is below the Effective Discharge Cooling Set Point
by more than half the Discharge Cooling Dead Band . However,
since the Cooling Interstage Timer has not yet expired, no
staging action occurs .
Point 3 The Cooling Interstage Timer has expired . DTB (Area
B + Area C) is greater than DTA (Area A) and the discharge
air temperature is below the Effective Discharge Cooling Set
Point by more than half the Discharge Cooling Dead Band .
Therefore, cooling is staged down . As a result, the discharge
air temperature rises, the Cooling Interstage Timer is reset, and
DTA is subtracted from both DTA and DTB . This zeros DTA and
leaves DTB equal to Area C .
Point 4 The Cooling Interstage Timer has expired . The
discharge air temperature is above the Effective Discharge
Cooling Set Point by more than half the Discharge Cooling
Dead Band . However, since DTA (Area E) is not yet equal
to DTB (Area C + Area D), no staging action occurs and the
discharge air temperature continues to rise .
109 OM 920-6 • MICROTECH UNIT CONTROLLER
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Figure 16 shows nine points on a
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