Flow Control- VAV flow control Function Block
The Flow Control function is a Damper Flow Controller with Variable Air Volume (VAV). This is typically the second half of a pressure independent VAV box cascade control strategy, with the input typically coming from the output of a PID block that controls space temperature.
The flow control function determines an effective flow control set point (effFlowSetPt) and generates a 0 - 100 percent command to control a VAV box damper. The effective flow setpoint is mapped to the commanded flow setpoint (in percent) from a temperature control signal, with 0 % mapping to the min flow setpoint and 100 % mapping to the max flow setpoint.
If the sensedFlowVol input is invalid (the sensor fails), the damper will be driven in a "pressure dependant" mode, with the output equal to 100 percent *(minSP/maxSP)+ (1- minSP/maxSP)*cmdPercent. The output = 20 % +.8*cmdPercent if either flow MinSP or MaxSP is invalid.
Figure 1: FlowControl Function Block |
Figure 2: FlowControl Property Sheet |
Figure 3: FlowControl Behaviour
Input
Input Name |
Description |
CmdFlowPercent |
Command flow percent. Numeric: 32 Bit Floating value, 0-100, Default: Null |
SensedFlow |
Sensed flow. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: Null |
MinFlowSp |
Minimum flow setpoint. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: Null |
MaxFlowSp |
Maximum flow setpoint. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: Null |
ManualFlowOverride |
The manual Flow Override input allows the flow setpoint to be selectively overridden based on the following codes:
Numeric: 32 Bit Floating value, >=-inf-<inf, Default: Null |
ManualFlowValue |
Manual flow value. Numeric: 32 Bit Floating value, 0-<inf, Default: Null |
DuctArea |
Duct area. If Units = 0 then duct area is in ft2, If Units = 1, 2 then duct area is in m2. Numeric: 32 Bit Floating value, 0-<inf, Default: Null |
Table 1: Inputs of Flow Control Function
Output
Output Name |
Description |
Eff Flow Sp |
Effective air flow setpoint. Numeric: 32 Bit Floating value, 0 - inf. |
Damper Pos |
Damper position in percent. Numeric: 32 Bit Floating value, 0 - 100. |
Table 2: Output of Flow Control Function
Parameter
Parameter Name |
Description |
CmdFlowPercent (In & Par) |
Command flow percent. Numeric: 32 Bit Floating value, 0-100, Default: 0 |
SensedFlow (In & Par) |
Sensed flow. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: 0 |
MinFlowSp (In & Par) |
Minimum flow setpoint. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: 0 |
MaxFlowSp (In & Par) |
Maximum flow setpoint. Numeric: 32 Bit Floating value, >=-inf-<inf, Default: 0 |
ManualFlowOverride (In & Par) |
Manual flow override.
Numeric: 32 Bit Floating value, >=-inf-<inf, Default: 0 |
ManualFlowValue (In & Par) |
Manual flow value. Numeric: 32 Bit Floating value, 0-<inf, Default: 0 |
DuctArea (In & Par) |
DuctArea. Numeric: 32 Bit Floating value, 0-<inf, Default: 0.1 |
Units |
Units 0 = cfm(flow) and ft2 (area), 1 = L/s(flow) and m2(area), 2 = m3/hr(flow) and m2(area) Numeric: 32 Bit Integer value, 0-2, Default: 0 |
MotorSpeed |
MotorSpeed in seconds. Numeric: 32 Bit Floating value, 1-255, Default: 90 Sec |
Out Save |
Master Sync Enabled : If you set it to "true" the last output will be set as output for one cycle after the controller restarts. The application can use it to return to the same state before the controller restart. Eff Flow Sp: To enable or disable the Effective Flow Setpoint feature. Damper Pos:To enable or disable the Damper Position feature. |
Table 3: Parameter of Flow Control Function
Example:
When there is no manual override.
When the temperature control output is 21 and fed as Command flow percent, minimum flow setpoint is 100CFM, maximum flow setpoint is 200CFM and Duct Area is 0.5sq.ft., the effective flow setpoint will be 121CFM and damper will be commanded to a position that will depends on the sensed flow feedback..
Figure 4: Example -No manual override
When there is manual override value set to 2 = ManFlowValue input.
When the temperature control output is 21 and fed as Command flow percent, minimum flow setpoint is 100CFM, maximum flow setpoint is 200CFM with ManualFlowValue of 160CFM and ManualFlowOverride set to 2 = ManFlowValue input and Duct Area is 0.5sq.ft., the effective flow setpoint will be 160CFM and damper position will modulate accordingly.
Figure 5: Example -With manual override
When there is manual override value set to 6 = MinFlowSP..
When the manual override is set to 6, the effective flow setpoint will be 100CFM and damper position will modulate accordingly.
Figure 6: Example -With manual override
When there is manual override value set to 7 = MaxFlowSP.
When the manual override is set to 7, the effective flow setpoint will be 160CFM and damper position will modulate accordingly.
Figure 7: Example -With manual override
Refer to General Description for general information about IRM N4 Function Blocks.