110
CAPACITY CONTROL ALGORITHMS — Three types of
capacity control are available for the 38AP units.
Supply Air Temperature Control — This control algorithm is
applicable to Control Types
Configuration
o
OPT2
o
C.TYP=
1
(VAV),
3
(Tstat Multi),
5
(SPT Multi), or
9
(VAV
Setpoint).
Capacity control is determined by the difference between
the supply air temperature and the Control Point (
Run Sta-
tus
o
VIEW
o
CTPT
) and its rate of change. The capacity con-
trol routine runs every 30 seconds. The algorithm attempts to
maintain the Control Point at the desired set point. Additional-
ly, the control calculates a rise per stage knowing which com-
pressor is on, its capacity and the temperature difference across
the evaporator coil (return or mixed air temperature minus sup-
ply air temperature) to determine the best time to turn on or off
the next compressor, institute Minimum Load Control, or
change the digital response, if equipped. Supply and return air
temperatures can be monitored at the unit’s interface device
Return Air Temperature Temperatures
o
UNIT
o
RAT
and
Supply Air Temperature Temperatures
o
UNIT
o
SAT
. With
this information, a capacity ratio is calculated to determine
whether to make any changes to the current stage of capacity.
This ratio, Capacity Load/Unload Factor (
Run Sta-
tus
o
VIEW
o
LOD.F
) value ranges from –100% to +100%
times Deadband Multiplier (
Configuration
o
SLCT
o
Z.GN
).
See Deadband Multiplier on page 115 for more information. If
the next stage of capacity is a compressor, the control starts
(stops) a compressor when the ratio r100% (–100%)
times Deadband Multiplier (
Z.GN
). Once a change in capacity
occurs, a 90-second time delay is initiated and the capacity
stage is held during this time delay.
When the unit is at stage zero (Requested Stage
Run Sta-
tus
o
VIEW
o
STGE
=
0
) as part of the capacity control routine,
the control adds a 1.2 factor on adding the first stage to reduce
cycling.
0
10
20
30
40
50
60
70
80
90
0
5
10
15
20
25
CL.MA (4-20 mA COOLING SETPOINT INPUT), mA
SETP (A
CTIVE SET POINT), °F
a38-7285
Fig. 142 — Active Setpoint (English)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
0
5
10
15
20
25
SETP (A
CTIVE SET POINT), °C
CL.MA (4-20 mA COOLING SETPOINT INPUT), mA
a38-7286
Fig. 143 — Active Setpoint (SI)
