Emerson Liebert ICOM-DO User Manual download pdf (Page 35)

Operation

27 Liebert

iCOM

for Liebert DSE

™

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 (27°C), 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.

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.

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

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Emerson Liebert ICOM-DO User Manual Page 35

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