Dx Cooling Operation - Trane LPC Install And Operation Instructions

DX cooling operation

This sequence applies to both space
temperature control and discharge air
temperature control.
The Tracer AH540/541 controller pro-
vides four DX cooling binary outputs to
control up to four stages of cooling. A
cascade control algorithm is used for
space temperature control. Valid dis-
charge air temperature sensor and space
temperature sensor inputs are required
for operation. As space temperature
rises above the cooling setpoint, it creates
a demand for more discharge-air cooling
capacity. Discharge air temperature
control directly controls DX cooling to
provide discharge air temperature at the
desired discharge-air cooling setpoint.
Anytime the discharge air temperature
drops below the discharge-air low-limit
setpoint (45°F , configurable) DX cooling
capacity is reduced to prevent low
discharge air temperatures.
DX cooling operation will be suspended if
the outdoor air temperature falls below
the compressor lockout temperature
setpoint (50°F default, configurable). DX
cooling will automatically resume when
the outdoor air temperature is greater
than the compressor lockout tempera-
ture plus 5°F . DX cooling can also be
suspended by communicated mechanical
cooling lockout command from a building
automation system. DX cooling will
remain disabled until the mechanical
cooling lockout is released.
DX cooling can be coordinated with
economizer operation by adjusting the
Economizer Enable Temperature (60°F ,
default). Given the default setpoints for
DX cooling and economizer operation
based on outdoor air temperature, DX
and economizer operation will be initiated
when outdoor air temperature falls below
60°F . DX cooling will be disabled below
50°F outdoor air temperature and
economizing will be disabled above 65°F
outdoor air temperature.
When outdoor air temperature is less
than 50°F DX cooling operation will be
disabled. When outdoor air temperature
rises above 65°F , economizer operation is
disabled.
82
Operation
DX cooling is normally disabled if:
• System mechanical cooling lockout is
active
• Outside air temperature is less than
compressor cooling lockout setpoint
• Defrost condition exists
• Supply fan is Off
Staged DX cooling
One to four DX cooling stages can be
directly controlled from binary outputs.
Since the controller binary outputs can
also be configured for stages of electric
heat the binary outputs are assigned as
follows where the number of DX cooling
stages plus the number of electric heat
stages cannot exceed four outputs.
• BO3: DX cooling stage 1 or Electric heat
stage 4
• BO4: DX cooling stage 2 or Electric heat
stage 3
• BO5: DX cooling stage 3 or Electric heat
stage 2
• BO6: DX cooling stage 4 or Electric heat
stage 1
The controller will enforce a minimum On
time and minimum Off time for each
compressor stage. A compressor will not
be allowed to stage On until the
compressor minimum Off time has
expired and a compressor will not be
allowed to stage Of until the compressor
minimum On time has expired.
Inter-stage delays are also enforced. A
minimum of 3 minutes will be enforced
between additional cooling stages. A
minimum of 2 minutes will be enforced
between subtracting cooling stages.
Defrost operation
Low evaporator refrigerant
temperatures can cause the coil to frost.
The Tracer AH540/541 controller
provides two methods of detecting low
evaporator refrigerant temperatures:
1. One uses the evaporator refrigerant
temperature (analog input IN13) to
measure suction temperature
2. The other uses a binary thermostat
device (binary input IN7 or IN12)
applied to the evaporator suction line
Two-circuit split-system DX cooling
applications should provide an evapora-
tor refrigerant sensor or binary thermo-
stat device on the first circuit or the circuit
with the first cooling stage. As an option,
sequence of
operation
both an evaporator refrigerant sensor
and binary thermostat can be used on a
unit with one device installed on each
circuit. The controller will respond
according to either frost input.
Two binary thermostat devices can also
be used with one device installed on each
circuit. The devices are separately hard-
wired to coil defrost inputs IN7 and IN12,
or the thermostat devices are wired in
series to coil defrost input IN7 or IN12.
Evaporator refrigerant temperature
A 10kΩ thermistor can be hard-wired to
the universal analog input IN13. The
controller has the ability to directly
measure the evaporator refrigerant
temperature. If evaporator refrigerant
temperature drops below the defrost
setpoint (30°F , default) the controller will
unload one compressor stage every 120
seconds. If the refrigerant temperature
rises above the defrost setpoint, DX
cooling will stop unloading and each
stage that frosted will be enabled after a
minimum of 10 minutes. If the refrigerant
temperature increases above the defrost
setpoint + 10°F , an integrating function is
initiated. When the evaporator
refrigerant time-temperature satisfied,
defrost operation is terminated. When
defrost is terminated, DX cooling capacity
is allowed to once again stage On (see
Table O-SO-12).
Coil defrost binary input
The controller provides two binary inputs
that can optionally be configured for coil
defrost, IN7 and IN12. These inputs
should be used if the universal analog
input is not available or cannot be
configured for evaporator refrigerant
temperature.
When the coil defrost binary input is
active, the controller de-energizes the last
active DX cooling stage. All subsequent
binary outputs will deenergize; one stage
every 120 seconds or until the coil defrost
binary input resets to a normal state.
LPC-SVX01C-EN