Chapter 4 Sequence of operation
64 CNT-SVX05B-EN
DX cooling operation
This sequence applies to both space temperature control and discharge
air temperature control.
The Tracer AH540/541 controller provides four DX cooling binary outputs
to control up to four stages of cooling. A cascade control algorithm is used
for space temperature control. Valid discharge 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 temperature plus
5°F. DX cooling can also be suspended by communicated mechanical cool-
ing 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 set-
points for DX cooling and economizer operation based on outdoor air tem-
perature, DX and economizer operation will be initiated when outdoor air
temperature falls below 60°F. DX cooling will be disabled below 50°F out-
door air temperature and economizing will be disabled above 65°F out-
door 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.
DX cooling is normally disabled if:
• System mechanical cooling lockout is active
• Outside air temperature is less than compressor cooling lockout set-
point
• Defrost condition exists
• Supply fan is Off
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