Emerson Liebert iCOM Manual De Usuario

Static pressure control normally refers to the control of a variable air flow device that will maintain a 

differential static pressure between two points. The two points of measurement are typically a 

point(s) below the raised floor in comparison to a point(s) above the raised floor, also known as ESP 

(External Static Pressure). There are other static pressure points within the data center that have 

also been used for control like the pressure differential between the inlet of a server rack as it 

compares to the outlet of a server rack in a containment application. No matter where the differential 

pressure is measured for control, the objective behind the control is to achieve the same result. Since 

static pressure measures the potential to flow air from the area of a higher pressure to a lower 

pressure, the result static pressure is trying to achieve is air flow. 
The control method of static pressure is to define a setpoint in either inches of water column or in 

Pascal’s that a controller will use to compare the actual differential pressure reading from either a 

single pressure sensor or multiple pressure sensors. The actual reading from the sensor is then 

compared to the static pressure setpoint and the fan speed is adjusted to keep the actual pressure 

reading at setpoint. The static pressure setpoint is selected by determining the amount of pressure it 

takes to produce air flow across a specific component. The components in a data center are normally a 

perforated floor tile or a contained hot / cold aisle.

Static pressure control in a data center has been derived from the building air management systems 

with the promises of a consist CFM or air flow from perforated floor tiles as they are added or removed 

from the white space. Using static pressure to control fan speed within an air handler with VAV 

(Variable Air Volume) systems has been applied successfully for many years. The key component that 

allows the system to work so well within an air handler system is the VAV box. In this type of system 

the VAV box controls the flow of air by adjusting a damper based on the local temperature that the 

VAV box serves. As the VAV box opens and closes to maintain its zone temperature a secondary 

control speeds up and slows down the supply fan to maintain a specific supply duct static pressure. As 

the zone heats up, the damper opens which lowers the static in the duct and the fan speeds up. Keep 

in mind that the zones are still controlled by temperature while the static pressure is only providing 

the potential to flow air by keeping a higher pressure in the duct than the area being conditioned. 
For a data center to be as effective using static pressure control to maintain fan speed each perforated 

floor tile would need to act as a VAV damper box. Unfortunately, many of the data centers operating 

today do not have automatically adjusting dampers that adjust the air flow from each perforated tile. 
Without the VAV part of the system, a raised floor data center application becomes a manual process 

to fine tune perforated tile locations based on server CFM demand. Static pressure control is also very 

dependent on the sensor position under the floor as floor obstructions like piping and cables can affect 

static pressure readings. In addition CRAH unit location and perforated tile placement can cause 

vortexes under the floor that would also affect the static pressure readings. 
Understanding the limitations for static pressure to automatically adjust the fan speed based on the 

IT equipment demand should prompt an initial CFD (Computational Fluid Dynamics) evaluation of 

the site to determine high and low pressure areas under the floor. In addition CFD’s should be 

performed when the floor tile arrangement changes or the IT equipment demand changes. The under 

floor environment will also change based on the floor tile arrangement changes and may not relate to 

above floor air flow expectations. The SixSigma

®

 and Tile Flow

®

 simulations in the next sections will 

illustrate under floor airflow in relation to tile flow and how the two variables change based on a unit 

being turned Off or in standby.