Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSOUTDOOR AIRTEMPERATURESENSORFig. 35. Discharge Air Control Loop with Reset.Compensation can either increase or decrease the setpoint asthe compensation input increases. Increasing the setpoint byadding compensation on an increase in the compensationvariable is often referred to as positive or summer compensation.Increasing the setpoint by adding compensation on a decreasein the compensation variable is often referred to as negative orwinter compensation. Compensation is most commonly usedfor temperature control, but can also be used with a humidityor other control system.Some controllers provide compensation start point capability.Compensation start point is the value of the compensationsensor at which it starts resetting the controller primary sensorsetpoint.COMPENSATION AUTHORITYRETURNTEMPERATURECONTROLLERSENSORAuthority =DISCHARGEAIRFAN=SUPPLYC2720Authority =Change in setpointChange in compensation input x 100Change in setpoint – TRChange in compensation input x 100Change in compensation input x 100Compensation authority is the ratio of the effect of thecompensation sensor relative to the effect of the primary sensor.Authority is stated in percent.The basic equation for compensation authority is:Authority =For proportional controllers, the throttling range (TR) isincluded in the equation. Two equations are required when thethrottling range is included. For direct-acting or positive reset,in which the setpoint increases as the compensation inputincreases, the equation is:Authority =Direct-acting compensation is commonly used to preventcondensation on windows by resetting the relative humiditysetpoint downward as the outdoor temperature decreases.For reverse-acting or negative compensation, in which thesetpoint decreases as the compensation input increases, theequation is:Change in setpoint + TRAuthority =In an application requiring negative compensation, a changein outdoor air temperature at the compensation sensor from –18 to 16°C resets the hot water supply temperature (primarysensor) setpoint from 94 to 38°C. Assuming a throttling rangeof 7 Kelvins, the required authority is calculated as follows:Authority = 185%The previous example assumes that the spans of the twosensors are equal. If sensors with unequal spans are used, acorrection factor is added to the formula:Compensation sensor spanPrimary sensor spanCorrection FactorAssuming the same conditions as in the previous example, asupply water temperature sensor range of 5 to 115°C (span of110 Kelvins), an outdoor air temperature (compensation) sensorrange of -30 to 30°C (span of 60 Kelvins), and a throttlingrange of 5 Kelvins, the calculation for negative reset would beas follows:Authority = 60 94 – 38 + 5x110 16 – (–18)Authority = 98%Change in setpoint + TRChange in compensation input x 10094 – 38 + 716 – (–18)x 100x 100The effects of throttling range may be disregarded with PI resetcontrols.PROPORTIONAL-INTEGRAL (PI) CONTROLxChange in setpoint ± TRChange in compensation input x 100In the proportional-integral (PI) control mode, reset of thecontrol point is automatic. PI control, also called “proportionalplus-reset”control, virtually eliminates offset and makes theproportional band nearly invisible. As soon as the controlledvariable deviates above or below the setpoint and offset develops,the proportional band gradually and automatically shifts, and thevariable is brought back to the setpoint. The major differencebetween proportional and PI control is that proportional controlis limited to a single final control element position for each valueof the controlled variable. PI control changes the final controlelement position to accommodate load changes while keepingthe control point at or very near the setpoint.ENGINEERING MANUAL OF AUTOMATIC CONTROL23