Applications Guide Tracer Graphical Programming - Trane

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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