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10
General
Information
With Lead/Lag enabled, HGBP operation (if applicable) will be bypassed and
the system will go directly into pump down when the last subtract command
is initiated.
When the UCM is powered up (after a power loss), or any time the
compressor's start time and run time are equal, the lead compressor for
single circuit units (20 through 30 Tons), will be the first "On" and the lag
compressor will be the first "Off". For dual circuit units (40 through 60 Tons),
the "On" sequence will be A-C-B-D and the "Off" sequence will be D-B-C-A.
Loss of Flow (Refrigerant) Control
Is accomplished using a binary input device (LPC) located on the suction
lines near the scroll compressors.
The LP cutouts are designed to open if the suction pressure approaches 7 ±
4 psig. If the LP cutout opens after a compressor has started, all
compressors operating on that circuit will be turned off immediately and will
remain off for a minimum of three minutes.
The LPC contacts are designed to close when the suction pressure exceeds
22 ± 4 psig. If the LP control is open when a compressor is requested to
start, none of the compressors on that circuit will be allowed to operate.
They are locked out and a manual reset diagnostic is initiated.
If the LP cutout trips four consecutive times during the first three minutes of
operation, the compressors on that circuit will be locked out and a manual
reset diagnostic is initiated.
Saturated Condenser Temperature Sensors (Standard)
Are analog input devices mounted inside a temperature well located on a
condenser tube bend. They monitor the saturated refrigerant temperature
inside the condenser coil and are connected to the compressor module
(SCM/MCM). As the saturated refrigerant temperature varies due to
operating conditions, the condenser fans are cycled "On" or "Off" as required
to maintain acceptable operating pressures.
Head Pressure Control (Standard)
Is accomplished using two saturated refrigerant temperature sensors.
During a request for compressor operation, when the condensing
temperature rises above the "lower limit" of the controlband, the Compressor
Module (SCM/MCM) begins sequencing condenser fans "On". If the
operating fans can not bring the condensing temperature to within the
controlband, more fans are turned "On". As the saturated condensing
temperature approaches the lower limit of the controlband, fans are
sequenced "Off". The minimum "On/Off" time for condenser fan staging is
5.2 seconds. If the system is operating at a given fan stage below 100% for
30 minutes and the saturated condensing temperature is above the
"efficiency check point" setting, a fan stage will be added. If the saturated
condensing temperature falls below the "efficiency check point" setting, the
fan control will remain at the present operating stage. If a fan stage cycles
CSRA-SVX02A-EN
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