Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSthan 7 minutes. The operating tower valves shall modulate tomaintain the above noted valve control setpoint at anytimeregardless of fan operation. If, after a fan is running greater than4 minutes in the on-off mode of operation, the water temperaturerises to a value equal to the “fan on” setpoint plus 1 kelvin, thefan shall revert back to EPID control with a start value of 25%and a ramp duration of 120 seconds. If the single fan operates toa speed exceeding 60%, the “off” fan shall start at an initial speedof 25% and shall ramp into unison control over a 240 secondduration (during which time the operating fan will slow down asthe starting fan slowly assumes part of the load).Dual pump operation with below freezing OA temperature:For both towers; valves shall position closed to the sump andthe tower fans shall modulate in unison, to maintain commoncondenser water temperature, under EPID control down to aminimum fan speed of 25%. If the condenser water load dropsbelow the capacity of both fans operating at minimum speed,the fan that has operated longest shall stop, and the remainingtower fan shall assume the full load. If the condenser waterload drops below the capacity of the remaining fan operatingat minimum speed, the fan shall stop and revert to an on-offalgorithm alternating the two tower fans operating at minimumspeed in the “on” mode. In the on-off mode the fan shall stop atthe fan control setpoint noted above, and shall restart after arise in temperature of 2 kelvins, but to no higher than 29.5°C,and shall run no less than 7 minutes. If either fan is on and thewater temperature drops to the valve control setpoint plus 0.5kelvins, the valves of the tower in which the fan is on shallposition open to the sump until the fan stops, at that time thevalves shall position back to the tower.If both fans are off and the water temperature drops to the valvecontrol setpoint plus 0.5 kelvins, the valves of Tower 1 shallposition open to the sump, and the Tower 2 valves shall alternatelycycle open to the sump with a 1.5 kelvins temperature differential.If, while one valve is open to the sump, the water temperaturedrops to the valve control setpoint, the other valve shall positionopen to the sump, and the valves shall alternate open to the towerwith a 1.5 kelvins differential. If, after one valve is open to thetower for 2 minutes, and the water temperature rises above thevalve control setpoint plus 2 kelvins, the remaining valve shallposition open to the tower and the valves shall revert to alternatelyopening to the sump at the valve control setpoint plus 0.5 kelvins,and opening to the tower after a 1.5 kelvins differential. If, afterboth valves position open to the tower for 2 minutes, thetemperature rises to the valve control setpoint plus 2 kelvins,control shall revert back to alternately cycling the fans on atminimum speed as before. If, after a fan is running greater than 2minutes at minimum speed in the on-off mode of operation, thewater temperature rises to a value equal to the “fan on” setpointplus 1 kelvins, the fan shall revert back to EPID control with astart value of 25% and a ramp duration of 120 seconds. If thesingle fan operates to a speed exceeding 60% for greater than 3minutes, the “off” fan shall start at an initial speed of 25% andshall ramp into unison control over a 120 second.NOTE: This “Dual pump below freezing OA temperature”sequence gets quite complex as the condenser water loadincreases and decreases. Table 1 may help understandthe shifting sequence.Table 1. Temperature Control Sequence for Dual Pumps Running in Below Freezing OA.Load SequenceLoad: Decreasing IncreasingBetween both fansrunning at minimumspeed and full load.Seq. 1: Modulates both fans in unison to controlat Setpoint + 1 kelvin.—Less than both fans atminimum speed, greaterthan one fan at minimumspeed.Less than one fan atminimum speed, greaterthan both fans Off.One fan running atminimum speed greaterthan 2 minutes and watertemperature less thanSetpoint + 0.5 kelvin.Both fans Off greater than2 minutes & watertemperature less thanSetpoint + 0.5 kelvin.One set of valves open totower (Seq. 5) greaterthan 2 minutes, & watertemperature less thanSetpoint.Seq. 2:Seq. 3:Seq. 4:Seq. 5:Seq. 6:Fan running longest stops.Remaining fan modulates to maintainSetpoint + 1 kelvin.Fans alternately cycled:At Setpoint + 2.5 kelvin first fan On atminimum speed.At Setpoint + 1 kelvin first fan Off.Fans run 6 minutes minimum.Repeat cycle for second fan.Positions valve set of the operating towerto sump until fan stops.Repositions valve set back to tower.Alternately positions valve sets:At Setpoint + 0.5 kelvin to sump.At Setpoint + 1.5 kelvin back to tower (oneset of valves always open to tower).Fans locked Off.At setpoint positions valve sets of bothtowers to sump.At Setpoint + 1.5 kelvin alternatelypositions valve sets to tower (one set ofvalves always closed to sump).Seq. 10: If one fan runs greater than 60%speed greater than 2 minutes:Remaining fan ramps on in unisonfrom 25% speed over 240 seconds.Revert to Seq. 1.Seq. 9: If either fan is on at minimum speedgreater than 2 minutes & watertemperature is greater thanSetpoint + 2 kelvin:Revert to Seq. 2 via EPID with startvalue of 25% and ramp duration of120 seconds.—Seq. 8:Seq. 7:If both valve sets position open to thetower greater than 2 minutes &water temperature is greater thanSetpoint + 2 kelvin:Revert to Seq. 3.If one valve set positions open to thetower greater than 2 minutes &water temperature greater thanSetpoint + 2 kelvin:Revert to Seq. 5.322ENGINEERING MANUAL OF AUTOMATIC CONTROL