4-12
Sequence of Operation
µ
P-D Controller
CSR-20 & CSR-40, September 1999
Chill Loads
Frozen Loads
Setpoints at -9.9 C
Setpoints at -10.0 C
(14.4 F) and Above
(14.0 F) and Below
Cool Mod
Null
Heat Defr
Cool
Null
Defr
Unit Function
•
•
•
•
Evaporator Fans HIGH SPEED1
•
•
Evaporator Fans LOW SPEED1
•
•
Evaporator Fans OFF1
•
•
•
•
Proportional-integral Derivative (Supply Air) Control
•
•
Return Air Sensor Control
•
•
Evaporator Coil Sensor Control
•
•
•
Compressor ON
•
•
•
Compressor Liquid Injection ON (valve energized)2
•
Warm Gas Bypass Solenoid Valve OPEN (energized)3
•
•
•
•
•
Condenser Fan ON4
•
•
Modulation Valve OPEN (de-energized)5
•
•
•
Modulation Valve MODULATING (energized)5
•
•
•
Electric Heaters ON (energized)
1Return air temperature determines the evaporator fan speed (except when Economy Mode is ON):
• At return air temperatures of -9.9 C (14.4 F) and above, the evaporator fans operate on high speed.
• At return air temperatures of -10.0 C (14.0 F) and below, the evaporator fans operate on low speed.
2Controller OPENS (energizes) the liquid injection valve:
• For 5 minutes whenever the compressor starts.
• Continuously when the unit is in Modulation Cool.
• Continuously when the unit is in Power Limit mode.
• When the compressor discharge temperature exceeds 138 C (280 F).
3Controller OPENS (energizes) the warm gas bypass valve:
• When the calculated temperature differential is less than 0.6 C (1.0 F), the controller pulses the bypass valve OPEN and
closed. The amount of OPEN (energized) time increases as the modulation valve closes.
• When the modulation valve is closed (PCVAL = 100%), the bypass valve remains open (energized) continuously.
4Condenser fan operation:
• Cooling Mode: Fan operation is impossible to determine exactly because the controller uses a control algorithm to cycle the
condenser fan between ON and OFF. However, the condenser fan will typically be ON when:
- The compressor discharge temperature is above 50 C (122 F) and increasing, or
- The condenser temperature is above 35 C (95F).
• Null Mode: If the condenser fan was ON when the unit shifted to NULL, it will operate for 30 seconds and stop.
• Water-cooled Condenser Option: When water supply pressure opens the water pressure switch, the controller stops the con-
denser fan.
NOTE: The condenser fan operates continuously if either the compressor or condenser temperature sen-
sor is defective.
5Modulation valve MODULATES:
• When the Power Limit mode is ON.
• When the calculated temperature differential is less than 2.5 C (4.5 F) above setpoint.
Operating Mode Function Chart — Standard Operation
