Interaction of Cool Setpoint and Supply Air Setpoint in Inrow environment (i. e. ACRC103)

[LALM]

Hello world,

I am trying to understand the control mechanism of InRows operating in Inrow environment (open & common cold aisle). Maybe one of you can help me?

First, let's begin with InRows programmed for HACS (Hot Aisle Containment System) environment. Relevant settings are Supply Air Set Point and Fan Speed Preference. While the Supply Air Set Point is determining the temperature of the air leaving the cooling unit, the Fan Speed Preference is defining the maximum delta of temperature between supply air and return air. In other words: the fan speed will only increase once the return air temperature would otherwise exceed the Supply Air Set Point plus the Fan Speed Preference.

Now, coming to InRows programmed for Inrow environment. Relevant settings are Supply Air Set Point and Cool Set Point. While the Supply Air Set Point is still determining the temperature of the air leaving the cooling unit, the Cool Set Point is the temperature the cooling group should maintain (so Supply Air Set Point must always be lower then the Cool Set Point). The fan speed is controlled by comparing the rack inlet temperature to the cooling setpoint. If the rack inlet temperature increases compared to the Cool Set Point the fans increase output to maintain the setpoint. If the rack inlet temperature decreases compared to the Cool Set Point the fans decrease output to maintain the setpoint. Up until this point all seems still fine to me. However, since the rack inlet temperature is representing the air entering the rack, it should be nearly identical to the supply air leaving the cooling unit. If the Supply Air Set Point is set to for example 20 °C and the Cool Set Point to 24 °C, does it mean that the fan speed would stay at minimum speed up until the cold water valve in the InRow opened up to 100% and the actual supply air starts to deviate from the setpoint increasing to 24 °C until it matches the Cool Set Point? Also, what would be typical / appropriate delta between those to setpoints?

Thanks in advance for clearing this up for me!

[CiaranR]

Hello and thank you for your question.

Regarding your query what if the Cool Setpoint was set higher than Supply setpoint we would just like to state that the Supply Air setting is defined by Schneider Electric authorized personnel only when the cooling unit or group of units are commissioned. We do not recommend that the Supply Air setpoint is ever set higher than the Cool setpoint.

Regarding your query “what would be typical / appropriate delta between those to setpoints?”

We recommend that the Cool setpoint is a minimum of 0.5⁰C greater than the Supply Air setpoint.

Also, please see below some information regarding the control operation of the InRow RC units:

The InRow RC continually adjusts its cooling output to accommodate varying loads. The cooling output is determined by the difference between the supply air temperature setpoint and the actual supply air temperature as well as the airflow (for each of the cooling units in the cooling group if more than one cooling unit is present).

In-row (open aisle) mode

The In-row (open aisle) mode is characterized by the free circulation of air, quantified by mf (mass flow) and T (temperature). The IT equipment draws in cool air from the cold aisle and rejects it to the hot aisle. The CRAC takes hot air from the hot aisle and supplies cool air to the cold aisle. There may be mixing of the cold and hot aisles over the top of the racks, around the sides, etc.

InRow RC Airflow Management

Changes in fan speed are always limited to a maximum of 2% per second. This limitation prevents the unit airflow from changing too quickly when the operating mode or active environmental control function changes, thereby improving control stability.

InRow RC-CW Cooling Control

The supply temperature controller regulates the chilled water (CW) valve and is responsible for maintaining the leaving air temperature at the desired set point. This works identically for InRow, HACS, and RACS. Minimum valve position is where there is essentially no flow through the coil. By preventing the valve from closing entirely while in operation, the unit can be more responsive to increases in heat load. The valves’ maximum position is where there is no additional flow through the coil. By limiting the maximum position, the controls can respond faster to a drop in the heat load.

Fan control

In the graph, illustrated above, the mass airflow is determined by T (remote), which is the maximum reading of the unit’s three Remote Temperature Sensors. The minimum fan speed is 30%. This speed was chosen to prevent fan bearing wear, and to ensure that there is always some airflow over the temperature sensors so that readings are accurate.

Supply air temperature control

The chilled water valve is controlled by the supply temperature controller which is responsible for maintaining the leaving air temperature at the desired set point. The controller’s input is the leaving air temperature T supply, measured by the integral supply temperature sensor, and its output is a commanded CW valve position. The design of the supply temperature controller is the same for all rack configurations.

InRow RC Group Cooling Control

Supply air temperature control

Each unit controls their chilled water valve (RC-CW) independently based on their own supply temperature sensor reading. In a group configuration the supply temperature setpoint is shared among all the units, with each unit managing its own chilled water valve or compressor based on its own supply temperature. Thus, there is no need for group control of the supply temperature.

[CiaranR]

[LALM]

Thank you @CiaranR for your very elaborate answer! Now there is only one question left on my side. In section 'Fan Control' you speak of three remote temperature sensors. As far as I know our units have three temperature sensors: 1 each for supply air, rack inlet and return air. Are those the ones you speak of? If that's the case, will the latter not always be the maximum reading (assuming no major screw ups happen to mix air between hot and cold aisle significantly)?

Have a great weekend!

[CiaranR]

Hi LALM,

Yes, the three sensors I was referring to are the Supply air, Return air and Rack inlet sensors. To be clear I was referring to the air temperature sensors specifically as there are also fluid sensing temperature probes. However, technically only one is "remote"...the rack inlet temperature sensor. The remote temperature sensor comes coiled inside the equipment and must be mounted on a rack server inlet, or the equipment will not operate properly. The remote temperature sensor monitors the room temperature, monitoring the environment surrounding the cooling equipment to ensure that the conditioned air is cooling the area.

With regards to your question "will the latter (return air sensor I assume?) not always be the maximum reading (assuming no major screw ups happen to mix air between hot and cold aisle significantly)?

The answer is yes, in theory the Return air temp should always be the maximum reading as it is monitoring the air temp on the warm side of the evaporator coil prior to any cooling has been performed.

• Supply Air: The air output from the cooling unit. The average temperature of the air leaving the cooling unit, as measured by the upper and lower supply air temperature sensors.
• Return Air: The air entering the cooling unit. The average temperature of the air entering the cooling unit, as measured by the upper and lower return air temperature sensors.
• Rack Inlet (Remote Temperature Sensor): The air entering the server rack at the temperature sensor.

[CiaranR]

[LALM]

Hey @CiaranR,

what would happen if one would choose in-row (open aisle) mode in an actual hot aisle containment system environment and thus,  there should be no (significant) potential of warm and cool air actually mixing? Reasons for wanting to operate the InRows in in-row mode instead of HACS would be a low IT load inside a room, leading to the units never running above minimum fan speed, which for the model in use is different for HACS (50%) and in-row (30%) mode (energy saving potentials). How to set the set points properly?

[CiaranR]

Hi @LALM 

The short answer is I have no idea what the outcome would be in such a scenario as I have not encountered this before however, if it is a low thermal load which is concerning you then there is always the option of placing a number of units on Standby mode (dependent on the number of units installed vs the load). Note: Newer units have a “Load Assist” and “Runtime Balancing” functionality to address such a situation.

To hazard a guess however I can see no reason to suggest that this would not be a successful strategy as 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 desired 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. In a HACS environment, the cooling unit neutralizes the heat accumulated in the common hot aisle and expels it back into the surrounding environmental space while maintaining the desired temperature in the cold aisle.

I wish to note also that switching to InRow mode from HACS to make energy savings based upon the lower fan speed may inadvertently cause excessive condensation. The condensate pumps in these units are relatively small and are not designed to handle significant amounts of water.