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compensates for varying conditions, including changing flow rates
across the evaporator coil.
Previous implementations of SumZ made static assumptions
about the actual size of the next capacity jump up or down. This
control uses a "rise per percent capacity" technique in the calcu-
lation of SumZ, instead of the previous "rise per stage" method.
For each jump up or down in capacity, the control will know
beforehand the exact capacity change brought on. Better overall
staging control can be realized with this technique.
SUM Calculation
The PID calculation of the "SUM" is evaluated once every
80 seconds.
SUM = Error + "SUM last time through" + (3 * Error Rate)
Where:
SUM = the PID calculation
Error = EDT – Cooling Control Point
Error Rate = Error – "Error last time through"
"Error" is clamped between –10 and +50 and "Error rate" is
clamped between –5 and +5.
This "SUM" will be compared against the "Z" calculations in de-
termining whether cooling stages should be added or subtracted.
Z Calculation
For the "Z" calculation, the control attempts to determine the
entering and the leaving-air temperature of the evaporator coil
and, based upon the difference between the two during mechani-
cal cooling, determines whether to add or subtract a stage of cool-
ing. This is the adaptive element.
The entering-air temperature is referred to as MAT (mixed-air
temperature) and the leaving-air temperature of the evaporator coil
is referred to as EDT (evaporator discharge temperature). They
are found at the local display under the Temperatures
CTRL sub-menu.
The main elements to be calculated and used in the calculation of
SumZ are:
•
the rise per percent capacity (R.PCT)
•
the amount of expected rise for the next cooling stage
addition
•
the amount of expected rise for the next cooling stage
subtraction
The calculation of "Z" requires two variables: Z.PLU, used when
adding a stage and Z.MIN, used when subtracting a stage.
They are calculated with the following formulas:
Z.PLU = Z.GN * (10 + (4*(–ADD.R))) * 0.6
Z.MIN = Z.GN * (–10 + (4*(–SUB.R))) * 0.6
Where:
Z.GN = configuration used to modify the threshold levels
used for staging (Configuration
ADD.R = R.PCT * (C.CAP – capacity after adding a cooling
stage)
SUB.R = R.PCT * (C.CAP – capacity after subtracting a
cooling stage)
Both terms, Z.PLU and Z.MIN, represent a threshold both posi-
tive and negative upon which the "SUM" calculation must build
up to in order to cause the compressor to stage up or down.
Comparing SUM and Z
The "SUM" calculation is compared against Z.PLU and Z.MIN.
•
If "SUM" rises above Z.PLU, a cooling stage is added.
•
If "SUM" falls below Z.MIN, a cooling stage is subtracted.
There is a variable called SMZ which is described in the reference
section and which can simplify the task of watching the demand
build up or down over time. It is calculated as follows:
•
If SUM is positive: SMZ = 100*(SUM/Z.PLU)
AIR.T
COOL
Z.GN)
•
If SUM is negative: SMZ = 100*(SUM/Z.MIN)
Mixed Air Temperature Calculation (MAT)
The mixed-air temperature is calculated and is a function of the
economizer position. Additionally, there are some calculations in
the control which can zero in over time on the relationship of re-
turn and outside air as a function of economizer position. There
are two configurations which relate to the calculation of "MAT."
These configurations can be located at the local display under
Configuration
UNIT.
ITEM
EXPANSION
UNIT
UNIT CONFIGURATION
MAT.S MAT Calc Config
MAT.R Reset MAT Table Entries?
MAT Calc Config (MAT.S)
This configuration gives the user three options in the processing of
the mixed-air temperature (MAT) calculation:
•
MAT.S
There will be no MAT calculation.
•
MAT.S
The control will attempt to learn MAT over time. Any time
the system is in a vent mode and the economizer stays at a
particular position for long enough, MAT = EDT. Using
this, the control has an internal table whereby it can more
closely determine the true MAT value.
•
MAT.S
The control will stop learning and use whatever the control
has already learned. Using this setting infers that the con-
trol has spent some time set to MAT.S = 1.
First, set MAT.S = 1. Then go into the Service Test mode, turn on
the fan, and open the economizer to a static position for 5 minutes.
Move to several positions (20%, 40%, 60%, 80%). It is important
that the difference between return and outside temperature be
greater than 5 degrees. (The greater the delta, the better.) When
done, set MAT.S = 2 and the system has been commissioned.
Reset MAT Table Entries? (MAT.R)
This configuration allows the user to reset the internally stored
MAT learned configuration data back to the default values. The
defaults are set to a linear relationship between the economizer
damper position and OAT and RAT in the calculation of MAT.
SumZ Overrides
There are a number of overrides to the SumZ algorithm which
may add or subtract stages of cooling.
•
High Temp Cap Override (H.TMP)
•
Low Temp Cap Override (L.TMP)
•
Pull Down Cap Override (PULL)
•
Slow Change Cap Override (SLOW)
Economizer Trim Override
The unit may drop stages of cooling when the economizer is per-
forming free cooling and the configuration Configuration
ECON
E.TRM is set to Yes. The economizer controls to the
same supply air set point as mechanical cooling does for SumZ
when E.TRM = Yes. This allows for much tighter temperature
control as well as cutting down on the cycling of compressors.
For a long cooling session where outside-air temperature may
drop over time, there may be a point at which the economizer has
closed down far enough for the unit to remove a cooling stage and
open up the economizer further to make up the difference.
Mechanical Cooling Lockout (Configuration
MC.LO)
This configuration allows a configurable outside-air temperature
set point below which mechanical cooling will be completely
locked out.
57
RANGE CCN POINT DEFAULTS
0 - 2
MAT_SEL
1
Yes/No
MATRESET No
=
=
=
COOL
0
1
2
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Troubleshooting
