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.89�F(13 � 3) psig4psig10Because the sensor spans are equal, the % authority cannot becalculated using the definition that the % authority is the ratio of theproportional band values.It is necessary to calculate the % authorityusing throttling ranges as follows:% Authority = TR1/TR2 = 6�F/1.89�F = 317%.Next, assign a value of SP1 = 110�F and calculate SP2 using equa-tion 6-18:0 + 1.89 �F 180 � 110 �FSP2 = + =68�F(8 � 3 psig)8 psig10 psigNote that the difference between the scheduled and the calculatedvalues is equal to about one-half the throttling range.This is a reasonable value.The dual-input controller can be programmed as follows:Action = DA/DAPB = 3%% Authority = 317%SP1 = 110�FSP2 = 68�FThe dual-input controller is calibrated as follows:1.Set the controller to direct-acting.2.Set proportional band (PB) to 3%.3.Set % authority to 317%.4.Using test instruments, set pressure input to port 1 for 110�F or8.5 psig.5.Using test instruments, set pressure input to port 2 for 68�F or 6.4psig.6.Adjust the controller until its branch pressure is 8 psig.7.Change input pressure to port 1 for 112�F or 8.6 psig.BranchHVAC CONTROL SYSTEM SET-UP 151(output) pressure should rise to 13 psig.8.Change input pressure to port 1 for 108�F or 8.4 psig.Branch(output) pressure should drop to 3 psig.9.If branch pressures in steps (7) and (8) do not occur, the PB scaleis inaccurate.Adjust PB setting and redo steps (7) and (8).10.To verify % authority setting, change inputs to both ports 1 and2 to Condition A from the reset schedule.Set input 1 to 180�F or12 psig and set input to port 2 to 0�F or 3 psig.Adjust the con-troller branch pressure to some middle pressure, about 8 to 10psig.111.Change inputs to port 2 and adjust input to port 1 to values forCondition B from the reset schedule.Set input 2 to 70�F or 6.5psig and then set input 1 to 110�F or 8.5 psig.The branch outputpressure should increase then return to the branch pressure set instep 10 above.12.Redo steps 10 and 11 as required until branch pressures reachedin both steps are equal.(This rarely happens on the first try.)13.Calibrate each sensor so that the pressure inputs to the ports arecorrect for the measured temperatures.14.Disconnect test instruments and reconnect sensors to input ports.15.Measure temperature of hot water supply and of outdoor airclose to sensor location.16.Solve the pneumatic dual-input controller equation for Pout usingtemperatures obtained in step 15.Note that if this step results inbranch output pressures greater than 15 psig or less than 3 psig,the temperatures are outside the sensor span and test instrumentinputs must be used for controller set-up.17.Adjust controller calibration knob until the output predicted instep 16 is obtained.18.Adjust the setpoint scale plate or indicator to 110�F.Discharge air reset from space temperatureA typical reset schedule for this type system is:Primary Variable Secondary Variable ControllerDischarge Air Conditioned Space OutputCondition Temperature Temperature PressureA50�F78�F 13 psigB55�F70�F 3 psig152 HVAC CONTROLS OPERATION & MAINTENANCEAccording to the rules discussed above for this system, the control-ler action must be DA/DA to give reverse reset.Try a throttling range of 4�F for the primary variable, which is thedischarge air temperature.Next, try a discharge air sensor with a range of 0 to 100�F and aspan of 100�F, then calculate PB using equation 6-10:PB1 = (TR1/span1) � 100 PB1 = 4%.Calculate TR2 using equation 6-14:70 � 78 �F 55 � 50 �FTR2 = � =18�F(3 � 13) psig4�F10An 18�F throttling range is generally too large to provide satisfac-tory space temperature control.It is necessary to try other combinationsof sensors and primary-secondary variable designations to get an ac-ceptable throttling range.An  acceptable throttling range may bethought of as being equal to the range of acceptable temperature swingfrom full-load to no-load conditions on the HVAC system.To reduce the calculated throttling range, redesignate the tempera-ture in the conditioned space as the primary variable on sensor 1 andmake the discharge air temperature the secondary variable on sensor 2as shown in this revised reset schedule:Primary Variable Secondary Variable ControllerCond- Conditioned Space Discharge Air Outputtion Temperature Temperature PressureA78�F50�F 13 psigB70�F55�F 3 psigAssign a throttling range for sensor 1 as TR1 = 4�F, then calculatea new TR2 using equation 6-14:55 � 50 �F 70 � 78 �FTR2 = � =5�F(3 � 13) psig4�F10HVAC CONTROL SYSTEM SET-UP 153The new value of 5�F for TR2 appears to be reasonable [ Pobierz całość w formacie PDF ]