Specifications

38
Table 44 — Run Status Cool Display
Next Stage EDT Increase (
SUB.R) — This variable repre-
sents (if subtracting a stage of cooling) how much the tempera-
ture should rise in degrees depending on the R.PCT calculation
and exactly how much capacity is to be subtracted.
SUB.R = R.PCT * (C.CAP — capacity after subtracting a
cooling stage)
For Example: If R.PCT = 0.2 and the control would be sub-
tracting 30% capacity by taking the next step down, 0.2 times
–30 = –6 F (SUB.R)
Rise Per Percent Capacity (
R.PCT) — This is a real time cal-
culation that represents the amount of degrees of drop/rise
across the evaporator coil versus percent of current running
capacity.
R.PCT =(MAT – EDT)/ C.CAP
Cap Deadband Subtracting (
Y.MIN) — This is a control vari-
able used for Low Temp Override (L.TMP) and Slow Change
Override (SLOW).
Y.MIN = -SUB.R*0.4375
Cap Deadband Adding (
Y.PLU) — This is a control variable
used for High Temp Override (H.TMP) and Slow Change
Override (SLOW).
Y.PLU = -ADD.R*0.4375
Cap Threshold Subtracting (
Z.MIN) — This parameter is
used in the calculation of SumZ and is calculated as follows:
Z.MIN = Configuration
→ COOL → Z.GN * (–10 + (4*
(–SUB.R))) * 0.6
Cap Threshold Adding (
Z.PLU) — This parameter is used in
the calculation of SumZ and is calculated as follows:
Z.PLU = Configuration
→ COOL → Z.GN * (10 + (4*
(–ADD.R))) * 0.6
High Temp Cap Override (
H.TMP) — If stages of mechani-
cal cooling are on and the error is greater than twice Y.PLU,
and the rate of change of error is greater than 0.5° F per minute,
then a stage of mechanical cooling will be added every 30 sec-
onds. This override is intended to react to situations where the
load rapidly increases.
Low Temp Cap Override (
L.TMP) —Iftheerrorislessthan
twice Y.MIN, and the rate of change of error is less than
–0.5° F per minute, then a mechanical stage will be removed
every 30 seconds. This override is intended to quickly react to
situations where the load is rapidly reduced.
Pull Down Cap Override (
PULL) — If the error from set
point is above 4° F, and the rate of change is less than –1° Fper
minute, then pulldown is in effect, and “SUM” is set to 0. This
keeps mechanical cooling stages from being added when the
error is very large, but there is no load in the space. Pulldown
for units is expected to rarely occur, but is included for the rare
situation when it is needed. Most likely pulldown will occur
when mechanical cooling first becomes available shortly after
the control goes into an occupied mode (after a warm unoccu-
pied mode).
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
7.5° F 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, described in
the reference section. The capacity control algorithm uses a
modified PID algorithm, with a self adjusting gain which
compensates for varying conditions, including 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 clamped between –50 and +50 and “Error
rate” is clamped 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, 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.
ITEM EXPANSION RANGE UNITS CCN POINT WRITE STATUS
COOL COOLING INFORMATION
C.CAP Current Running Capacity % CAPTOTAL
CUR.S Current Cool Stage COOL_STG
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 ^FSUBRISE
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