HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumin terms of percent proportional band (PB), whereKc = 100PBFigure 3-2. Proportional-only control.A wide band, high percentage of PB corresponds to less sensitive controllersettings, and a narrow band, low percentage, corresponds to moresensitive controller settings.The correction generated by the proportional control mode is proportionalto the error. The proportional controller responds only to thepresent. It cannot consider past history of the error or possible futureconsequences of an error trend.It responds to all errors in the same manner, in proportion to them.When a small error results in a large response, the gain (Kc) is said tobe large or the proportional band is said to be narrow. When it takes alarge error to cause a small response, the controller is said to have asmall gain or a wide proportional setting.The gain in DCS control units is usually adjustable from 0 to 8,while in analog controllers the range can usually be adjusted from 0.02to about 25.Proportional OffsetThe main problem with proportional control is that it cannot keepthe controlled variable on setpoint. The proportional controller can onlyrespond to a change by moving away from its setpoint. The process willbe on setpoint for only part of the time. At all other times the operatinglevel travels up and down on its operating line as a function of the load.The difference between the actual value and the setpoint is called©2001 by The Fairmont Press, Inc. All rights reserved.