APC ACRD101 User Manual

The primary function of the cooling unit is to capture the heat generated by IT equipment and neutralize 
it before it mixes with the room. The control strategies employed by the cooling unit depend upon the 
deployment strategy of the cooling group.

In an InRow environment, the cooling unit supplies constant-temperature supply air to the common cold 
aisle. The fan speed is modulated to ensure that the required volume of air reaches the IT equipment. The 
fan speed is determined by the difference between the cooling setpoint and the maximum rack air inlet 
temperature of the cooling group.

In a HACS or RACS environment, the cooling unit neutralizes the heat accumulated in the common hot 
aisle and expels it into the surrounding environmental space while maintaining the necessary 
temperature difference across the IT equipment.

In a Spot cooling environment, the equipment operates like traditional CRAC equipment, providing a 
constant return air temperature. In Discrete mode, the fans operate at a user-selectable speed (Fan Speed 
Preference) and the compressor operates at maximum capacity. In Prop (Proportional) mode, the fans 
are modulated to maintain the required return air temperature and the compressor output is modulated to 
maintain the necessary supply air temperature.

A setpoint is the target value that a cooling group will maintain. The default setpoints are appropriate for 
most cooling applications.

The setpoints for each mode must be within the following ranges:

•  Cool: 18.0 – 25.0°C (64.4 – 78.8°F)
•  Deadband: -17.7 – 5.6°C (0 – 10.0°F)
•  Supply Air: 13.8 – 23.0°C (57 – 73.4°F) Spot (Proportional) only

17.7 – 23.0°C (64 – 73.4°F) HACS, RACS or InRow

Note: The Supply Air setting is defined by APC authorized personnel only when the 
cooling group is commissioned. 
Note: The Supply Air setpoint must never be set higher than the Cool setpoint. 

•  Fan Speed: When the unit is programmed for HACS or RACS mode, this property specifies the 

necessary temperature difference (DT) across the IT equipment. When the cooling unit is 
programmed for Spot (Proportional), InRow, HACS or RACS with the Fan Control property set 
to Manual, this property defines the fan speed. When the cooling unit is programmed for Spot 
(Discrete), the fan speeds are also selected from the following table but the Fan Control property 
is irrelevant.

– Low: 16.7°C (30°F) DT (60% of maximum fan speed)
– Med-Low: 13.9°C (25°F) DT (70% of maximum fan speed)
– Med: 11.1°C (20°F) DT (80% of maximum fan speed)
– Med-High: 6.3°C (15°F) DT (90% of maximum fan speed)
– High: 5.6°C (10°F) DT (100% of maximum fan speed)