www.DaikinApplied.com 61 IOM 1322 • WATER-COOLED SCROLL COMPRESSOR
Reset is accomplished by changing the Active Heating Leaving
Water set point from the Heat LWT set point to the value of
[Heat LWT (-) Heating Maximum Reset set points] as the
condenser delta temperature (leaving minus entering) varies
from the Heating Start Reset Delta T set point towards 0ºF
delta temperature.
Active LWT Set Point
The active LWT set point represents the current control set
point based on unit mode and reset. If unit mode is ice, then
the active set point is equal to the ice set point. If the unit mode
is cool, the active set point is the cool set point plus the leaving
water reset value. If the unit mode is heat, the active set point
is the heat set point minus the leaving water reset value.
LWT Error
LWT error compares the actual LWT to the active LWT set
point.
The equation for cool mode is:
LWT error = LWT – active LWT set point
The equation for heat mode is:
LWT error = active LWT set point – LWT
LWT Slope
LWT slope is calculated such that the slope represents a time
frame of one minute.
Every 12 seconds, the current LWT is subtracted from the
value 12 seconds back. This value is added to a buer
containing values calculated at the last ve intervals. The nal
result is a slope value that is an average over the past 60
seconds.
Pull Down Rate
The slope value calculated above will be a negative value as
the water temperature is dropping. For use in some control
functions, the negative slope is converted to a positive value by
multiplying by –1.
Evaporator Saturated Temperature
Evaporator saturated temperature is calculated from the
evaporator pressure for each circuit.
R410A Evaporator Saturated Temperatures
When R410A refrigerant is selected the refrigerant pressure
will be tted to a curve made up of 24 straight-line segments.
The accuracy of calculated saturated temperatures are less
than +/- 0.5°F when compared to standard look up tables for
R410A.
When R134a refrigerant is selected the refrigerant pressure
will be tted to a curve made up of 12 straight-line segments.
The accuracy of calculated saturated temperatures is +/-
0.5º F when compared to standard look up tables for R134a
refrigerant.
Condenser Saturated Temperature
Condenser saturated temperature shall be calculated from the
condenser pressure for each circuit.
R410A Evaporator Saturated Temperatures
When R410A refrigerant is selected the refrigerant pressure
will be tted to a curve made up of 24 straight-line segments.
The accuracy of calculated saturated temperatures are less
than +/- 0.5°F when compared to standard look up tables for
R410A.
When R134a refrigerant is selected the refrigerant pressure
will be tted to a curve made up of 12 straight-line segments.
The accuracy of calculated saturated temperatures is +/-
0.5º F when compared to standard look up tables for R134a
refrigerant.
Evaporator Approach
The evaporator approach is calculated for each circuit.
For R134a and R410A refrigerant the equation is:
Evaporator Approach = LWT – Evaporator Saturated
Temperature
Suction Superheat
Suction superheat is calculated for each circuit using the
following equation:
Suction superheat = Suction Temperature – Evaporator
Saturated Temperature
Pump Down Pressure
The pressure to which a circuit will pump down is based on the
Low Evaporator Pressure Unload set point. The equation is as
follows:
Pumpdown pressure = Low evap pressure unload – 15 psi
The low limit for the calculated Pumpdown Pressure set point
is 10.0 psi
Unit Enable
The Unit Enable Set Point controls enabling and disabling the
unit. The Unit Enable Set Point has options of OFF and ON.
The Unit OFF input, Remote input, keypad entry, and BAS
request can alter this set point. The Control Source Set Point
determines which sources can change the Unit Enable Set
Point with options of SWITCHES, KEYPAD or NETWORK.
Changing the Unit Enable Set Point is accomplished according
to the following table.
NOTE: An “x” indicates that the value is ignored.
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