Emerson Liebert iCOM Manual De Usuario

Proportional—If Proportional control is selected, the percent cooling/heating requirement is 

determined by the difference between the air temperature sensor reading and the temperature 

setpoint. The Temperature Proportional Band is a user-defined range that is divided into two equal 

parts for cooling and heating. Figure 25 illustrates how the temperature proportional band is evenly 

divided on either side of the temperature setpoint.

The Temperature Setpoint is between these two equal parts. As the air temperature rises above the 

temperature setpoint, the percent cooling required increases proportionally (from 0 to 100%) over half 

the programmable temperature proportional band.
The percent heating requirement (0 to -100%) is determined the same way when the air temperature 

falls below the setpoint. The humidifying/dehumidifying operations are controlled in the same 

manner as the cooling/heating operations; however, the humidity sensors, setpoints and proportional 

bands are utilized.
When the air temperature reaches the end of the proportional band, either 100% or -100%, full cooling 

or full heating capacity is provided. No operation is performed when a 0% call is calculated or the 

temperature is within the deadband. The control varies the call for cooling and heating in 1% 

increments as the air temperature moves through the proportional band halves.
PI—PI Control functions with all type of compressors and valves. If PI Control is selected, the percent 

cooling/heating requirement is calculated by combining two methods, proportional and integral. The 

proportional term is calculated similar to the previously described Proportional control. The integral 

term (sometimes called “reset action”) is calculated by measuring how much and how long the air 

temperature/humidity has been above or below the setpoint. If the actual air temperature/humidity is 

above the setpoint, the percent requirement is slowly but continuously increased until the total is 

sufficient to bring the return room air back to the setpoint. The control helps to ensure temperature 

setpoint is achieved while efficiently operating the equipment. The Proportional and Integral 

concepts work together to prevent excessive valve oscillation, compressor short cycle and temperature 

fluctuations. From an idle state (compressor[s] Off, chilled water valve closed, etc.), the call for cooling 

is based on proportional temperature increase from setpoint. As temperature deviates further from or 

closer to setpoint, the call for cooling is increased or decreased, respectively. The amount of cooling 

staged depends on how far the actual temperature has deviated from the setpoint on the proportional 

(P) band.

Because cooling accounts for half of the proportional band, the call for cooling in this example is 50%. 

If the room temperature increases to 80°F, the call for cooling would be 100%. Proportional only 

control will not function satisfactorily in some applications as shown in this example, because the 

room temperature would increase 10°F (5.5 k) above the setpoint before cooling is staged to 100%.