Applications Guide Tracer Graphical Programming - Trane
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Applications Guide Tracer Graphical Programming - Trane

Applications Guide Tracer Graphical Programming - Trane Applications Guide Tracer Graphical Programming - Trane

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28.09.2014 Views

® Chapter 7 Constant-volume AHU example Figure 143: Discharge air reset the output minimum and maximum values, as well as the disable and fail safe positions accordingly. See the PID properties in Figure 144. • Use wireless connections to pass the supply fan status and the calculated discharge air setpoint to other parts of the program. • Assign the discharge air setpoint to a variable for display purposes. Figure 144: Discharge air reset PID properties Implementing humidification and dehumidification You will use the discharge air setpoint, later, in the program to control the heating and cooling valves. Now, determine whether to humidify or to dehumidify. Use this decision to control the cooling and heating valves. The sequence of operation contains the following information about humidification and dehumidification. 166 CNT-APG001-EN

® Writing the discharge air control program Enable humidification and modulate the humidifier to maintain the space relative humidity setpoint when all of the following conditions are true: • The air handler is in occupied mode. • The supply fan is on. • The outdoor air temperature is less than 55°F. If the duct relative humidity exceeds 85%, turn off the humidifier to prevent condensation in the duct work and indicate an alarm. Enable dehumidification, modulate the cooling valve to maintain space relative humidity, and modulate the heating valve to maintain the discharge air temperature when all of the following conditions are true: • The air handler is in occupied mode. • The supply fan is on. • The outdoor air temperature is greater than 55°F. • The space relative humidity is greater than the space dehumidification setpoint. Use the module in Figure 146 on page 169 to implement the decision but not the control actions. • Use the Deadband blocks to provide the on/off mechanism to activate and deactivate humidification or dehumidification. • Use a network variable input to access the space relative humidity, which is communicated from another Comm5 device. The Comm5 device in this scenario is a Tracer MP503 I/O module that is bound to the Tracer MP580/581 controller. For more information about network variable bindings with the Tracer MP580/581, see the Tracer MP580/581 Programmable Controller Programming guide (CNT-SVP01A-EN). • Because the Tracer MP503 provides the space relative humidity in units of current (mA), use a Reset block to convert it to units of percent relative humidity. ➤ Using a Reset block The Reset block calculates a reset schedule or a linear equation. In this program, use the Reset block to determine the linear equation necessary to calculate the relative humidity based on the network variable nviCurrent_mA01 that supplies current. The inputs to the block include the following: x1 = 4.0 mA, y1 = 0.0% x2 = 20.0 mA, y2 = 100.0% The resulting linear equation is as follows: Relative humidity (%) = 6.25 × Current (mA) – 25.00 The Reset block uses this equation internally to calculate the relative humidity output with the current input. For example, when the network variable input nviCurrent_mA01 communicates the value of 14.0 mA, the Reset block calculates a relative humidity of 62.5%. See Figure 145 on page 168 for a plot of this equation. CNT-APG001-EN 167

®<br />

Chapter 7 Constant-volume AHU example<br />

Figure 143: Discharge air reset<br />

the output minimum and maximum values, as well as the disable and<br />

fail safe positions accordingly. See the PID properties in Figure 144.<br />

• Use wireless connections to pass the supply fan status and the calculated<br />

discharge air setpoint to other parts of the program.<br />

• Assign the discharge air setpoint to a variable for display purposes.<br />

Figure 144: Discharge air reset PID properties<br />

Implementing humidification and dehumidification<br />

You will use the discharge air setpoint, later, in the program to control the<br />

heating and cooling valves. Now, determine whether to humidify or to<br />

dehumidify. Use this decision to control the cooling and heating valves.<br />

The sequence of operation contains the following information about<br />

humidification and dehumidification.<br />

166 CNT-APG001-EN

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