JOHNSON CONTROLS
30
FORM 100.50-NOM7 (808)
Unit Sequence of Operation
Control of Compressors with Economizer
In Economizer Operation, compressors are started only
when the specied SAT setpoint can not be achieved
through outside air and return air mixing. Compressors
will be turned ON / OFF individually.
Turning On Compressor #1:
If no compressors are ON, and the economizer con-
troller is saturated High (i.e. the economizer is 100%
open and can no longer maintain the SAT setpoint by
just free cooling),
• Calculate a temporary SAT setpoint as (SAT set-
point) T = (SAT setpoint + 5°F);
• When the SAT is 0.5°F < (SAT Setpoint) T, turn ON
compressor #1;
• Resume SAT control at SAT setpoint.
This is done in order to read and store the SAT
temperature dierential (drop) due to turning on
compressor #1, while assuring that turning it on will
not drop the SAT below compressor #1 trip point
(where the Excessive SAT control would turn it o
again). The same staging sequence is used for the
remaining compressors.
Turning On Compressors #2 through #4:
If the economizer controller is saturated High (i.e. the
economizer is 100% open while one or more compres-
sors are running and the control can no longer maintain
the SAT setpoint requested by the space control),
• Calculate a temporary SAT setpoint as (SAT set-
point) T = (SAT setpoint + 5°F);
• When the SAT is 0.5°F < (SAT Setpoint) T, turn ON
compressor #N;
• Resume SAT control at SAT setpoint.
Turning Compressors O
Compressors are turned OFF as described in the re-
spective sections for CV and VAV cooling operation
with economizer suitable.
Compressor Control When Exiting Cooling Lock-
out on OAT
A situation may arise when in Economizer Mode and
one or more compressors are required in addition to
full available free cooling in order to maintain the SAT
setpoint, but Cooling Lockout on OAT prevents the
compressors’ use. This situation may arise when the
SAT setpoint is set very close to, or even below, the
temperature set for Cooling Lockout on OAT – a rela-
tively unusual case. If the OAT then increases above the
Cooling Lockout on OAT parameter value while the call
for several compressors exists, the compressors in this
case (and only in this case) will turn ON simultaneously.
This is considered acceptable, as this situation is not
expected to occur frequently.
VENTILATION OPTION OPERATION
The rooftop units will support several dierent ventila-
tion options including true Outside Airow measurement
and control. Each option will be controlled dierently,
depending upon what equipment is installed and how the
unit is congured. For any Ventilation option operation,
the unit must be congured for Economizer Operation.
The ventilation options are described as follows:
Fixed Minimum OA Damper Position
This is the most common approach to ventilation con-
trol used with air handling equipment today. With this
approach, the OA and RA dampers are mechanically
linked together such that when the OA damper is open,
the RA damper is closed, and vice versa. The sequence
of operation for a xed minimum position system simply
controls the OA damper open to a xed minimum posi-
tion during the occupied mode of operation.
Simply opening the OA damper to a xed minimum
position can be quite eective for bringing in a xed
volume of outside air with Constant Volume (CV)
equipment. However, with Variable Air Volume (VAV)
equipment, the amount of outside air brought into the
air handler varies because the Supply Fan speed and
total system air volume varies.
To compensate for VAV equipment, a minimum position
reset scheme will be implemented as follows:
The two Unit Setup parameters OAD Min Position #1
and OAD Min Position #2 will be used to indicate two
minimum positions of the OA damper. Both OAD Min
Position settings will be used to linearly reset the min-
imum position of the OA Damper based on the output
to the VFD. OAD Min Position #1 will indicate the
Minimum Position of the OA Damper when the output
to the VFD is at 100% (10 VDC). OAD Min Position #2
will indicate the Minimum Position of the OA Damper
when the output to the VFD is at 50% (5VDC).
Figure 13 indicates how this algorithm will function:
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