Johnson Controls
5513350-JTG-1018
76
the variable speed drive compressor. The controller moni-
tors each of those requirements to provide the appropriate
minimum and maximum speeds to the compressor.
Condensate Overflow Alarm
When the condensate overflow switch alarm is ON, the unit
disables mechanical cooling. Supply fan operation remains
enabled during this period. The alarm must be manually
reset at the controller.
Condenser System
The rooftop unit condenser fan control is designed to con-
trol staged and modulating condenser fans based on the
saturated liquid line temperature. Constant speed con-
denser fans are staged up or down based on the saturated
liquid line temperature. Condenser fan start count equal-
ization is used for fixed speed fans.
Saturated Liquid Line Temperature Control
After a stabilization period of 60 seconds during saturated
liquid temperature control and after the first compressor
is energized, condenser fans are staged ON or OFF to
achieve stage up or down setpoint. If saturated liquid line
temperature is above the stage up setpoint, an additional
condenser fan is energized. If saturated liquid line temper-
ature falls below the stage down setpoint, a condenser fan
).
The variable speed fan is the first ON and the last OFF
when optionally equipped on the unit. The variable speed
fan modulates condenser fan speed between 80.0–110.0°F.
If saturated liquid line temperature becomes unreliable, the
unit reverts to fail-safe mode.
Critical High
Setpoint
Stage Up
Setpoint
Stage Down
Setpoint
Critical Low
Setpoint
Normal Operation
(deadband region)
Saturated Liquid Line
Temperature
LD26894
Figure 10:
Saturated Liquid Line Temperature Control
Controls (Continued)
Cooling System
The controller supports both staged and variable speed di-
rect expansion (DX) cooling.
DX Cooling
Capacity Control
Constant speed compressors stage up or down to control
the DAT to setpoint. During dehumidification control, the
compressors switch to evaporator temperature control.
When equipped with a variable speed drive compressor,
the capacity modulates via vernier control. The first stage
on refrigeration circuit one is variable speed, while all other
stages are constant speed. Start count equalization does
not apply to the variable speed compressor. The variable
speed compressor is always the first compressor ON and
last compressor OFF.
Compressor Staging Algorithm
The controller employs patented algorithms to sequence the
compressor stages as a method of quickly meeting cooling
demand while reducing overshoot as temperature reaches
setpoint.
The controller excludes staging combinations that result in
one refrigeration circuit having two or more compressors en
-
ergized than the other refrigeration circuit.
Rapid Start
To shorten the time it takes to get compressors running,
the optimum stage up algorithm is used. It estimates the
number of compressor steps required to meet the initial
cooling demand and quickly starts the appropriate number
of compressors. When the unit first starts up or after an
unoccupied to occupied transition, if there are no compres-
sors running and the economizer is not suitable, rapid start
is active for the first 5 minutes of cooling operation. During
rapid start, the inter-stage delay time between compres
-
sors reduces to 15 seconds.
Variable Speed Drive Compressors
Envelope control keeps the variable speed drive compres
-
sor within the allowable range of operating conditions for
the compressor at the superheat and sub-cooling values
defined on the envelope. Oil management algorithms are
designed to keep the oil level above the minimum on vari
-
able speed drive compressors. Overall cooling demand, oil
management, saturated DAT, and saturated suction tem-
perature all influence the allowable operational speed of
