96
RT-SVX24J-EN
Unit Startup
Sequence of Operation
Cooling Sequence of Operation
Time delays are built into the controls to increase
reliability and performance by protecting the compressors
and maximizing unit efficiency.
SZVAV Cooling Sequence of Operation
Single Zone VAV units will be equipped with a VFD
controlled supply fan which will be controlled via the 0-
10VDC RTM VFD output and the RTM Supply Fan output.
With the RTM Supply Fan output energized and the RTM
VFD output at 0Vdc the fan speed output is 37% (22Hz)
from the VFD motor, by default, and at 10VDC the Fan
Speed output is 100% (60Hz). The control scales the 0-
10Vdc VFD output from the RTM linearly to control
between the 37%-100% controllable range.
If the RTM determines that there is a need for active
cooling capacity in order to meet the calculated
Temperature Setpoint (Tset), the unit will begin to stage
compressors accordingly once supply fan proving has
been made. Note that the compressor staging order will be
based on unit configuration and compressor lead/lag
status.
Once theTset calculation has reached its bottom limit (Tset
Lower Limit Setpoint) and compressors are being utilized
to meet the demand, theTset value continues to calculate
below theTset Lower Limit Setpoint and the algorithm will
begin to ramp the Supply Fan Speed up toward 100%.
Note that the supply fan speed will remain at the
compressor stage’s associated minimum value (as
described below) until the Tset value is calculated below
the Tset Lower Limit Setpoint.
As the cooling load in the zone decreases the zone cooling
algorithm will reduce the speed of the fan down to
minimum per compressor stage and control the
compressor outputs accordingly. As the compressors
begin to de-energize, the Supply Fan speed will fall back to
the Cooling Stage’s associated minimum fan speed, but
not below. As the load in the zone continues to drop
cooling capacity will be reduced in order to maintain the
discharge air within the ± ½ Tset deadband.
Cooling Stages Minimum Fan Speed
As the unit begins to stage compressors to meet the
cooling demand, the following minimum Supply Fan
Speeds will be utilized for each corresponding Cooling
Stage. Note that the Supply Fan Speed will be allowed to
ramp up beyond 37% as determined by the active Tset
calculation; the speeds below are only the minimum
speeds per cooling stage. Note that when transitioning
between active cooling stages, compressors may energize
prior to the supply fan reaching the minimum speed for the
associated step.
1.
2-Stage DX Cooling -The minimum fan speed for units
with 2 stages of DX Cooling will be 37% of the unit’s full
airflow capacity. At Stage 1 of DX Cooling the
minimum Fan Speed will be 37% and at Stage 2 of DX
Cooling the Fan Speed will be at a minimum of 67%.
2. 3-Stage DX Cooling - There are no IntelliPak
applications with 3 stages of DX Cooling.
3. 4-Stage DX Cooling -The minimum fan speed for units
with 4 stages of DX Cooling will be 37% of the unit’s
total airflow. At Stage 1 the minimum Supply Fan
Speed will be 37%, at Stage 2 the minimum Supply Fan
Speed will be 58%, and at Stages 3 & 4 the minimum
Supply Fan Speed will be 67%.
Rapid Restart
This feature will occur after every power cycle. Once
power is restored (e.g., via a backup generator), the RTM
will maximize cooling capacity within 3-5 minutes. Once
the space has returned to its ZoneTemperature Setpoint,
the RTM controls the load using normal capacity control
algorithms.The supply fan will be turned on immediately
after a power cycle, module initialization, or after the Unit
Start Delay has timed out. The supply fan proving switch
input must be closed prior to continuing with Rapid
Restart.
Once the supply fan proving switch input has closed, the
unit will consider the outside air temperature to determine
whether economizing or DX mechanical cooling will be
utilized to provide the necessary cooling. If the outside air
temperature is less than 50°F and economizing is enabled,
the outside air damper will be utilized. If the outside air
temperature is above 50°F, the outside air damper will
remain closed and DX mechanical cooling will occur for
the duration of Rapid Restart.
Compressor Sequence of Operation
Each compressor is equipped with a crankcase heater and
is controlled by a 600 volt auxiliary switch on the
compressor contactor. The proper operation of the
crankcase heater is important to maintain an elevated
compressor oil temperature during the “Off” cycle to
reduce oil foaming during compressor starts.
When the compressor starts, the sudden reduction in
crankcase pressure causes the liquid refrigerant to boil
rapidly causing the oil to foam. This condition could
damage compressor bearings due to reduced lubrication
and could cause compressor mechanical failures.
NOTICE:
Compressors Failure!
Unit must be powered and crankcase heaters energized
at least 8 hours BEFORE compressors are started. This
will protect the compressors from premature failure.
