Introduction
SS-SVX002B-EN
13
Four Stage Compressor Rotation
Compressors 1A and 1B rotate the lead role after each run
cycle in the cooling mode. That is, after compressor 1A has
completed an operating cycle as the primary compressor
for stage 1, the next time cooling stage 1 is called for, the
compressors switch roles and compressor 1B becomes
the primary compressor for stage 1. When compressor 1B
is the primary for stage 1 cooling, compressor 1A’s role is
switched to operate with compressor 2 for the third stage
of cooling. Compressor 2 is not involved in the cooling
mode rotation so it will always be used for cooling stages
2, 3 and 4.
For the dehumidification mode, compressor circuits 1 and
2 (see
) rotate the lead role after each run
cycle. The tandem compressors rotate as a single, dual
compressor circuit because a single small compressor
from the tandem set does not have enough capacity to
satisfy the dehumidification load alone. In the
dehumidification mode, after circuit 1 (compressors 1A
and 1B) has completed an operating cycle as the lead, the
next time dehumidification stage 1 is called for the
compressor circuits switch roles and circuit 2 (compressor
2) becomes the primary compressor circuit for
dehumidification.
Free-Cooling Operation
The free-cooling configuration is available to minimize the
use of compressor operation during low ambient
conditions for system energy savings. A FC (free-cooling)
system utilizes a remote drycooler to provide water/glycol
coolant to a free-cooling coil positioned within a DX
refrigerant system. If outdoor air temperatures permit
free-cooling operation (adjustable user set point), the free-
cooling mode is enabled to take advantage of the low
ambient conditions to provide cooling with partial use or
without the use of the system compressor(s). Free-cooling
provides an excellent opportunity for reduced operational
cost by reducing the compressor operating hours.
The free-cooling sequence is enabled if the entering fluid
temperature is below the user adjustable free-cooling
enable set point and the return air temperature rises to the
free- cooling set point plus dead band.
The drycooler pump activates and the 3-way control valve
directs chilled water/glycol coolant to the FC coil. The
outdoor fluid cooler is controlled by first switching the
leaving fluid control set point from typical DX heat
rejection to free-cooling control (adjustable set point,
ambient air) and by controlling the leaving fluid to its user
adjustable set point. The free-cooling control valve opens
proportionally to the demand for cooling based on the
return air temperature’s deviation from set point.
If the return air temperature continues to rise, the free-
cooling valve position eventually reaches 100% open,
maximizing the flow of coolant through the free- cooling
coil. Continued operation in this position indicates the A/
C unit is unable to lower the air temperature to the desired
set point in the free- cooling mode.
The compressor activates if the DX cooling stage enable
temperature set point has been reached or if the control
valve position reaches 100% open for 20 seconds (default).
The free-cooling circuit and the compressor operate in
series to provide maximum cooling. The 3-way control
valve continually modulates the flow of coolant in
response to temperature with the compressor running.
The compressor cycles off based on the normal
compressor temperature cut-out settings once the set
point is maintained.
