Operating Principles; Control Sequences Of Operation; Occupied/Unoccupied Switching; Compressor Lead/Lag - Trane SCWM Series Installation, Operation And Maintenance Manual

Operating Principles

Control Sequences of Operation

Occupied/Unoccupied Switching

There are three ways to switch occupied/unoccupied:
1. Field-supplied contact closure (hardwired binary input
to field wiring terminal block (2TB3)) — This input
accepts a field supplied switch or contacts closure such
as a time clock.
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2. Tracer — The Trane Tracer
occupied/unoccupied status of the self- contained unit.
3. Through the TD7 Display.

Compressor Lead/Lag

The UC600 selects fixed speed compressors (B, C, and D)
to start using a Lead \ Lag function. Variable speed
compressor A is always the first and last stage of
mechanical cooling. Balanced starts and hours are
achieved by changing the Fixed Speed compressors
sequence number once a week.

Emergency Stop Input

A binary input is provided on the unit's field wiring
terminal block (2TB3) for installation of a field-provided
switch or contacts to immediately shutdown all unit
functions.

Water Flow Control

With compatible piping configurations, the unit can be
configured to provide variable water flow with the water
regulating valve option only, which maximizes energy
saving by unloading the water pumping system.

Head Pressure Control

Units that are set up for variable water flow will modulate
a water valve to maintain a user-defined condensing
temperature setpoint and compressor refrigerant
pressure differential. Condensing temperature and
pressure differential will be referenced utilizing factory
installed suction and discharge pressure sensors located
at each condenser.

Water Purge

This user-definable feature allows the user to select a
purge schedule to automatically circulate water through
the economizer and condensers periodically during non-
operational times. This allows fresh chemicals to circulate
in waterside heat exchangers.

Supply Air Static Pressure Limit

The opening of the VAV boxes are coordinated during unit
start up and transition to/from occupied/unoccupied
modes to prevent over-pressurization of the supply air
duct-work. However, if for any reason the supply air
PKG-SVX027A-EN
®
system can control the
pressure exceeds the supply air static pressure limit, the
supply fan/VFD is shut down and a manual reset
diagnostic sets and displays at the human interface panel.
Supply Air Temperature Control
Unit Sequence of Operation
Occupied Supply Air Temperature Control
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.

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.
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.
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