Filter
Top Products
47
Table 62 — Run Status Cool Display
Slow Change Cap Override (
SLOW) — With a rooftop unit,
the design rise at 100% total unit capacity is generally around
30 F. For a unit with 4 stages, each stage represents about
7.5F of change to EDT. If stages could reliably be cycled at
very fast rates, the set point could be maintained very precisely.
Since it is not desirable to cycle compressors more than 6 cy-
cles per hour, slow change override takes care of keeping the
PID under control when “relatively” close to set point.
SumZ Operation
— The SumZ algorithm is an adaptive PID
style of control. The PID is programmed within the control and
the relative speed of staging can only be influenced by the user
through the adjustment of the Z.GN configuration. The capaci-
ty control algorithm uses a modified PID algorithm, with a self
adjusting gain which compensates for varying conditions, in-
cluding changing flow rates across the evaporator coil.
Previous implementations of SumZ made static assump-
tions about the actual size of the next capacity jump up or
down. This control uses a “rise per percent capacity” technique
in the calculation 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”
NOTE: “Error” is limited to between –50 and +50 and “Error
rate” is limited to between –20 and +20.
This “SUM” will be compared against the “Z” calculations
in determining 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 mechanical cooling, and then determines whether to
add or subtract a stage of cooling. 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 calcula-
tion of SumZ are:
1) the rise per percent capacity (R.PCT)
2) the amount of expected rise for the next cooling stage
addition
3) 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
COOL
Z.GN)
ADD.R = R.PCT * (C.CAP – capacity after adding a cooling
stage)
SUB.R = R.PCT * (C.CAP – capacity after subtracting a cool-
ing stage)
Both of these terms, Z.PLU and Z.MIN, represent a thresh-
old both positive 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 com-
pared 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)
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 economiz-
er position. Additionally there are some calculations in the con-
trol which can zero in over time on the relationship of return
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 RANGE UNITS CCN POINT WRITE STATUS
COOL COOLING INFORMATION
C.CAP Current Running Capacity % CAPTOTAL
CUR.S Current Cool Stage COOL_STG
REQ.S Requested Cool Stage CL_STAGE
MAX.S Maximum Cool Stages CLMAXSTG
DEM.L Active Demand Limit % DEM_LIM forcible
SUMZ COOL CAP. STAGE CONTROL
SMZ Capacity Load Factor -100 – +100 SMZ
ADD.R Next Stage EDT Decrease ^F ADDRISE
SUB.R Next Stage EDT Increase ^F SUBRISE
R.PCT Rise Per Percent Capacity RISE_PCT
Y.MIN Cap Deadband Subtracting Y_MINUS
Y.PLU Cap Deadband Adding Y_PLUS
Z.MIN Cap Threshold Subtracting Z_MINUS
Z.PLU Cap Threshold Adding Z_PLUS
H.TMP High Temp Cap Override HI_TEMP
L.TMP Low Temp Cap Override LOW_TEMP
PULL Pull Down Cap Override PULLDOWN
SLOW Slow Change Cap Override SLO_CHNG
ITEM EXPANSION RANGE
CCN
POINT
DEFAULTS
UNIT UNIT CONFIGURATION
MAT.S MAT Calc Config 0 - 2 MAT_SEL 1
MAT.R Reset MAT Table
Entries?
Yes/No MATRESET No