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Occupied Fan Operation
When all alarms are cleared, the unit is occupied, and
the local auto stop is in auto, all fans that are available
will start up and operate at 30% speed for 45 seconds.
The fans will then release to control to the duct static
pressure setpoint. The fans will modulate to maintain
the Duct Static Pressure at the Duct Static Pressure
Setpoint. The fans are commanded off during the
unoccupied mode.
Heat/Cool Mode
The unit is in Cooling mode by default. The front end
Building Automation System will switch the Heat/Cool
Mode request multistate value between the Heating
and Cooling modes to get the desired control.
Cooling/Waterside Economizer
During occupied cooling mode, the waterside
economizer option and mechanical cooling are used to
control the supply air temperature. The supply air
temperature setpoint is user defined at the human
interface panel.
After the fans run for 5 minutes, and a call for cooling is
calculated, the Cooling Tower/Pump Command is
turned on. After the Condenser Water flow switch
closes proving flow, the Compressors and/or
Economizer will be allowed to start.
Waterside economizing enables when the unit's
entering water temperature is less than the entering
mixed air temperature. This is set at Waterside
Economizer Enable Offset setpoint. The factory default
is 7°F, but can be adjusted by the user at the TD7 or via
BACnet.
The economizer acts as the first stage of cooling. If the
economizer is unable to maintain the supply air
setpoint, the compressor module will bring on the
compressors as required to meet the setpoint. If the
unit does not include an economizer, only mechanical
cooling will satisfy cooling requirements.
Thermostatic Expansion Valve
N N O O T T I I C C E E
C C o o m m p p r r e e s s s s o o r r D D a a m m a a g g e e ! !
D D o o n n o o t t o o p p e e r r a a t t e e w w i i t t h h w w a a t t e e r r l l o o o o p p s s w w i i t t h h l l e e s s s s t t h h a a n n f f i i v v e e
m m i i n n u u t t e e s s c c i i r r c c u u l l a a t t i i o o n n t t i i m m e e a a s s i i t t c c o o u u l l d d r r e e s s u u l l t t i i n n p p o o o o r r
s s u u p p e e r r h h e e a a t t c c o o n n t t r r o o l l a a n n d d c c o o m m p p r r e e s s s s o o r r d d a a m m a a g g e e . .
Refrigerant system reliability and performance is
heavily dependent upon proper superheat. The
importance of maintaining the proper superheat
cannot be overemphasized. Accurate measurements of
superheat will provide the following information:
•
How well the expansion valve is controlling the
refrigerant flow.
•
The efficiency of the evaporator coil.
PKG-SVX027C-EN
•
The amount of protection the compressor is
receiving against flooding.
The expected range for superheat is 11-15°F at full load
conditions. At part load, expect a properly adjusted
expansion valve to control to 8-12°F superheat.
Systems operating with lower superheat could cause
serious compressor damage due to refrigerant
floodback.
Fan Operation
Multiple, identical fans are arranged in the fan cabinet
to meet the customer's airflow and static requirements.
Each fan's speed, start/stop command, and fault status
are controlled and monitored individually by the unit
controller. However, all fans receive the same fan
speed signal to meet airflow and static demand, and all
fans are commanded On upon Unit Controller status
transition to Occupied Mode.
Each fan assembly has its own motor with integrated
variable speed drive (VSD). Each fan VSD varies the
speed of the fan proportional to a 0-10 Vdc signal
supplied by the XM70.1 module (and XM30.5 on 70-80
ton units).
The VSD starts and stops the fan according to the
Enable/Disable Binary Output from the XM32.2 module
(and XM32.4 on 70-80 ton units). If the VSD is faulted it
annunciates this via a dry contact that is sensed by the
Unit Controller on the XM70.1 module (XM30.5 on 70-
80 ton units).
Supply Air Static Pressure High Limit
During normal operation, the Supply Air Static is
monitored. If at any time the static pressure exceeds
the software high static limit (defaulted at 2.0 inches
from the factory) the unit will immediately shut down
and generate a diagnostic.
In order for the unit to resume operation, the Alarm
Reset must be toggled from the TD7. The software high
static limit can be user adjusted at the TD7.
Compressors
The MSC uses a combination of one variable speed
compressor (VS) and one, two or three fixed speed
compressors (FS) for mechanical cooling.
The VS compressor is always the first compressor on
and last compressor off, except in the event of a VS
Compressor or Circuit lockout. A balanced starts
routine is used by the Unit Controller when starting FS
compressors.
Compressor Oil
All compressors have an Oil Sight Glass. Always
ensure oil is visible in the sight glass when the
compressor is off or running.
O O p p e e r r a a t t i i n n g g P P r r i i n n c c i i p p l l e e s s
49
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