PKG-SVX027D-EN
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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
NOTICE
Compressor Damage!
Do not operate with water loops with less than five
minutes circulation time as it could result in poor
superheat control and compressor damage.
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.
• 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.
High Pressure Cutout Switch (HPC)
Each VS and FS compressor has an HPC installed in its
discharge line. The switch is set to open at 553 psi +/- 15
and re-close at 424 +/-30.
Operating Principles
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