Applications Guide Tracer Graphical Programming - Trane

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® Chapter 7 Constant-volume AHU example Controlling the outdoor air damper position Writing the discharge air control program The discharge air control program differs considerably from its VAV air handler counterpart. This program in your constant-volume air-handling unit must perform the following tasks: • Discharge air setpoint calculation • Cooling valve control • Heating valve control • Dehumidification • Humidification Create a new program and set its properties as shown in Figure 142 on page 165. 164 CNT-APG001-EN
® Writing the discharge air control program Figure 142: Discharge air control program properties Calculating the discharge air setpoint Unlike the VAV air handler, the constant-volume air handler does not have defined discharge air cooling and heating temperature setpoints. Thesequenceofoperationstatesthefollowing: Compare the space temperature and space temperature setpoint to determine the discharge air temperature setpoint. Reset the discharge air temperature setpoint according to heating or cooling demand. Calculate the discharge air temperature setpoint based on the difference between the space temperature and the effective cooling or heating setpoint. Use a method commonly known as discharge air reset, or cascade control. Use a PID loop to calculate the discharge air setpoint. Then apply the resulting discharge air setpoint in combination with additional PID loops to control the cooling and heating valves (Figure 143 on page 166). • Use a Switch block to select the appropriate setpoint based on the heat/cool mode. • Set the properties for the discharge air reset PID block differently than the PID loops seen in previous chapters. Although the program executes every 10 seconds, this PID loop executes every 60 seconds because the space temperature and setpoint do not change rapidly. The output is a temperature instead of a percentage. Be sure to set CNT-APG001-EN 165
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Applications Guide Tracer Graphical
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CNT-APG001-EN ®
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® Special notifications and format
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® Contents Arrangingblocks........
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® Contents Chapter 5 Cooling tower
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® Contents Chapter7 Constant-volum
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® Contents Summary-question answer
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® Figures Chapter3 Modifyingtheexh
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® Figures Figure 96: Input blocks
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® Figures Chapter 8 Constant-volum
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® Figures xx CNT-APG001-EN
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® Tables Chapter7 Constant-volumeA
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® Chapter 1 Using the Tracer Graph
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® Chapter 6 VAV AHU example Review
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® Chapter 6 VAV AHU example Table
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® Chapter 6 VAV AHU example Figure
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® Summary-question answers closed
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® Summary-question answers PROGRAM
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® Index dehumidification, 166-169,
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