Emerson Liebert iCOM User Manual download pdf (Page 64)

Operation

Liebert

iCOM

The integration (I) time works with the proportional band in PI Control to stage call for cooling before

the actual temperature increases far above the temperature setpoint. The “I” portion does this by

doubling the current “P” deviation for every time the “I” has elapsed.

Example: If the “P” deviation is 50% within the “I” time (e.g., 5 min), the call for cooling will be

doubled, giving 100% in 5 minutes.

While this happens, the real temperature “P” deviation will decrease. If the “P” deviation drops to

40% and the previously calculated “I” was already on 55%, it will shift back to 45% following “P.”

From this point the “I” will continue to calculate the call for cooling, but slower at a slower rate

since the actual room temperature has dropped.

Opening the valves or staging compressor(s) to 62% may decrease the temperature from 40% to

20%, which would drive the “I” down from 62% to 42%.

At this point, “P” is 20% and “I” is 42% (call for cooling, valve opening, compressor[s] stage).

This continues until “P” is 0—on the setpoint or within the deadband.

Setting Up PI control

Generally 3-5 minutes integration time is good for most applications. The proportional band may

depend on the amount of heat load and other factors. Use Table 8 as a guide for making the PI

control settings.

Intelligent—If Intelligent Control is selected, the air temperature/humidity is controlled at or near

the setpoint. The percent temperature/humidity adjustment required is calculated based on logic that

is programmed into the control. This logic simulates the actions that a human would take if manually

controlling the system. This control type is used only on chilled water units.

3.7.2 Humidity Sensor Reading Control Types

The Liebert iCOM has four humidity sensor control types: Dew Point, Relative, Compensated and

Predictive. The humidity sensor adjusts how the Temperature and Humidity Control determines the

percent requirement for humidification/dehumidification. The humidity sensor control type

parameter, Humidity Control Type, is in both the User and Service menus under Setpoints.

Dew Point—The actual return temperature and humidity measured by Liebert iCOM is used

internally to determine the dew point. When dew point control is selected, the user will set the

humidity setpoint in degrees dew point and will also adjust the proportional band in degrees dew

point.

Relative—The actual return air humidity sensor reading is sent to the Temperature and Humidity

Control to determine if and how much humidification/dehumidification is required. The actual return

air humidity reading is displayed on the Status menu. Unnecessary dehumidification can result when

overcooling occurs during a dehumidification cycle. This is because a higher than normal relative

humidity (RH) reading is caused by overcooling the room. This extends the dehumidification cycle.

Later, when the dehumidification ends and the return air temperature rises to the setpoint, the RH

reading falls. The final RH reading will then be lower than actually desired. If significant overcooling

occurred, the RH could be low enough to activate the humidifier.

Table 8 PI control troubleshooting

Problem Solution

Unit is too slow to activate

cooling.

Decrease the proportional band slightly and monitor operation.

Compressor short cycle alarm.

Increase the proportional band slightly and adjust the integration time to 3-5 min.

Monitor compressor run time. Run must be higher than 3 minutes to avoid short

cycle from occurring.

Excessive valve oscillation or

hunting.

Widen the temperature proportional band and/or consider increasing the

integration time.

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Emerson Liebert iCOM User Manual Page 64

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