User manual

Operation
53 Liebert
®
iCOM
®
3.7 Control Types
3.7.1 Temperature and Humidity Control Types
The Liebert iCOM has three Temperature and Humidity Control Types:
• Proportional
•PI
• Intelligent
Each control type affects the timing and intensity of the cooling/heating and
humidifying/dehumidifying operations. The Control Type parameter is in the Service menu under
Setpoints.
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. 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
(See 3.2.1 - 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. The
Proportional control type is commonly selected on compressorized units.
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 in a manner 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.
Example: Temperature Setpoint= 70°F (21°C)
Temperature Proportional Band= 20°F (11.11 K; see Note below)
Room Temperature= 75°F (24°C)
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%. Proportional Control requires a much tighter proportional band to hold the unit
closer to the setpoint, which often results in compressor(s) short cycling or valve hunting.
NOTE
k is the unit of measure used when U404 ‚ Temperature Indication is set for degrees Celsius.
K is the conversion from degrees F to degrees C without the -32°F offset. If the proportional
band is 20°F, then (20/1.8) = 11.1K