York J Series, Manual

Page: 21 / 92

FORM 100.50-NOM7 (808)
21
JOHNSON CONTROLS
3
When the space temperature error is at or less than the
“space satisfied” value, destaging compressors is ac -
complished by temporarily requesting an SAT setpoint
of 90°F. This setpoint assures that there is no need for
compressors even at the highest outdoor air temperature
and/or enthalpies.
Note that the destaging process still occurs in cooling
state, so there is no possibility of heating being activated
due to the temporarily high SAT setpoint, or due to
space temperature dropping below its cooling setpoint
and towards space heating setpoint.
Staging of Compressors When Exiting
Economizer Mode
This situation occurs when one or more compressors
are running in the economizer mode and free cooling
becomes unavailable. The sensor control mode changes
– sensor algorithm is no longer selecting the active
economizer SAT setpoints, but now is used to directly
control the compressors. The control mode changeover
process involves turning off the compressors that were
running during the economizer operation, and then turn-
ing back on the compressors needed during mechanical
cooling only. This staging process will be acceptable,
providing that it completes within 10 minutes.
Cooling Operation with a Space Sensor
For cooling operation, the space temperature will be
controlled to either the:
• Unoccupied Cooling Setpoint or,
• Occupied Cooling Setpoint based on the Occupied/
Unoccupied state of the unit (i.e. Occ/Unocc Input,
or Internal Scheduling).
Control Algorithm For Space Sensor Cooling
– Economizer Not Suitable
The Space Sensor Cooling control algorithm is as illus-
trated in Figure 4.
• A direct acting, proportional – integral space tem -
perature controller generates a 0 – 100% output
signal that responds to space cooling demand.
• The output of the controller is used to calculate on
and off time of the compressor most recently started
based on the calculation method in Figure 4.
• PI gain values are to be determined by such that
all compressors are on within 20 minutes with a
10°F space temperature error; or 30 minutes for a
6-compressor system with a 10°F space temp. er-
ror.
• When the duty cycle reaches 100% (the PI algorithm
reaches a “High Saturation state” and a saturation
time delay expires), the modulated compressor stops
cycling and remains on.
• An additional compressor is staged every time the
space control PI algorithm reaches a High Satu-
ration state and a programmed saturation time delay
expires.
• A compressor is destaged when the PI algorithm
reaches a Low Saturation state and a programmed
saturation time delay expires.
FIG. 4 – CV SPACE SENSOR COOLING ALGORITHM
- ECONOMIZER NOT SUITABLE
Compressor
Stays OFF
Compressor
Stays ON
OFF -Time
O N-Time
O n-Time
(m inutes)
M in.
OFF -Time
(6 minutes)
100%
High Saturation
O utput of Cooling Zone
Temp. Control (PI)
Low Saturation
0%
M in.
O N-Time
(6 minutes)
OFF -Time
(m inutes)
LD06553
FIG. 5 – CV SPACE SENSOR COOLING
ALGORITHM - ECONOMIZER SUITABLE
Economizer 2nd
Stage SAT
Setpoint
Economizer
1st Stage SAT
Setpoint
Space Satisfied
(Destaging SAT
Setpoint) -1.5˚F
+0.5˚F +0.5˚F
Space Temperatur e
Setpoint For Cooling
Space Temperatur e Deviation From Setpoint ( ˚F)
LD06554
Control Algorithm For Space Sensor Cooling -
Economizer Suitable
In CV cooling operation with a space sensor and when
free cooling is available (“economizer suitable”), the
SAT is controlled by the economizer control algorithm
(see “Economizer Operation” in this section of the
IOM). The control algorithm is shown in Figure 5 (see
section on Economizer Operation for a more specific
explanation on economizer operation).