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.c.Measure return air temperature, outdoor air temperature andmixed air temperature.Make certain that measurements representa representative or average measurement for each duct.Read thethermometer to an accuracy of 0.1°F.d.Use the air mixture formula to calculate the actual percentage ofoutdoor air as follows:TReturn Air ±TMixed Air%OA= × 100TReturn Air ±TOutdoor AirEvaluation:Evaluation of Test 1.a.If dampers do not close properly or are open more than 60% at fullstroke, adjust dampers and damper motors so this condition isachieved.Eliminate any binding through lubrication or adjust-ments of the damper and linkage.b.Verify that return air dampers open to about 60% opening at fulltravel.Observe damper action through full stroke.At end of thestroke, verify that the return air damper is closed and outdoor airdamper is open about 60%.Evaluation of Test 2.a.If mixed air section control pressures show short term variations ofmore than 1 psi, the control system is considered unstable.Evaluation of Test 3.a.Compare the measured and calculated outdoor air percentage withthe desired outdoor air percentage.The outdoor air percentagevalue should be justified by review of building occupancy type,number of occupants, and special requirements for makeup air toexhaust ventilation systems.FINE-TUNING PROGRAM FOR PNEUMATIC CONTROL SYSTEMS 225Adjustments:Adjustment 1.a.If the blades are not within 1/8 of closing when the actuator is atfull stroke, they must be adjusted or repaired to provide closurewithin 1/8 , except for performance leakage dampers, which mustbe adjusted in accordance with the manufacturer s specific instruc-tions.b.If the dampers do not open to the desired position at full travel,adjust the damper linkage.Be certain that fastenings are securebefore leaving damper.c.Verify the setting of the minimum positioning relay which is in-tended to open the outdoor air damper to minimum position with-out positioning return air and relief air dampers.Adjustment 2.a.If mixed air section controls are evaluated to be unstable, adjustthrottling range of mixed air temperature controller in about 25%increments of initial throttling range until control stability isachieved.Adjustment 3.a.If outdoor air percentage is not equal to the desired value, adjustthe mixed air section dampers to bring the outdoor air percentageas close to the desired value as is possible with the system as in-stalled.It may be necessary to perform additional air balance workor sheet metal work to achieve the desired outdoor air percentage.Procedure No.5 Discharge Air Control SystemObjective: Evaluate stability and reset schedule of discharge aircontrol systems.Discussion: The emphasis here is on fine-tuning of the dischargetemperature control of heating coils, because heating coils require goodcontrol to maintain stability in discharge applications and constitutegood candidates for fine-tuning.When discharge air temperature con-troller is reset from outdoor air temperature, the reset schedule can befine-tuned.Test conditions: Conduct tests on heating coils when the controlsystem for the heating coils is active and during moderate load condi-tions, such as with entering air to heating coil below 65°F.226 HVAC CONTROLS OPERATION & MAINTENANCETest procedures:Check of Reset Schedule.a.Observe performance of a representative sample of terminal unitsduring extremely heavy loads and again during extremely lightloads.Note the branch pressures of the terminal unit temperaturecontrollers during both periods.b.Observe the discharge temperature of a heating coil being resetduring periods of both extremely heavy loads and light loads.Record the temperature extreme values.Evaluation procedures:a.If the terminal unit is not calling for full heating during extremelyheavy heating load, the supply air temperature is too high.Fine-tuning requires that the coil discharge temperature be set to a tem-perature just high enough to supply heat to maintain conditionswhen the terminal unit having highest load is fully open.b.If the discharge temperature of the heating coil does not showextreme values which are expected with the reset schedule, thereset sequence setpoints are not correct.Adjustment procedures:a.Measure the branch pressure to the actuators and the sensor pres-sures, then recalculate the set-up parameters using formulas givenin Chapter 5,  Performance Prediction in ATC Systems. Set up thecontrols using the new parameters and retest.Procedure No [ Pobierz caÅ‚ość w formacie PDF ]