Flow Velocity Function Block

This function computes the flow and velocity based on the measured pressure and the K factor. K-Factor is often used in terminal unit controls to calculate actual airflow. From iteration to iteration, the Function Block keeps track of the last state of the auto set offset. On power up/reset this is set to "true" so that auto offset cannot be executed on a reset or power up.

Figure 1: FlowVelocity Formula

Figure 2: FlowVelocity Function Block

Figure 3: FlowVelocity Property Sheet

Inputs

Input Name

Description

Engg Unit (Metric/Imperial)

Offset

(In & Par)

Input pressure offset correction to adjust the Flow to Zero.

Numeric: 32-Bit Floating Point value >=-inf-<inf.

Pa OR inW

Pressure

(In & Par)

Air pressure which is measured via a pitot tube.

Numeric: 32-Bit Floating Point value >=-inf-<inf, Default: Null.

Pa OR inW (inches water gauge)

AutoSetOffSet

(In & Par)

Input correction pressure offset

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

Pa OR inW

ClearOffSet

(In & Par)

Reset correction pressure offset to value 0.

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

Pa OR inW

Area

(In & Par)

Area (ft^2)

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

(m2/hr) )/ft^2

KFactor

(In & Par)

Actual flow corresponding to a velocity pressure sensor output of 1" w.g. or 249Pa. Please describe what it is, where it comes from.

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

(m3/hr) )/ft^3/min (CFM)

Table 1: Inputs of FlowVelocity Function

Output

Output Name

Description

Engg Unit (Metric/Imperial)

Flow

Airflow in CFM/(ft3/min)

Numeric: 32-Bit Floating point value, >=-inf-

(m3/hr) )/ft^3/min (CFM)

Velocity

Air flow velocity (ft/min).

Numeric: 32-Bit Floating point value, >=-inf-

m/hr OR ft/min

Table 2: Output of FlowVelocity Function

Parameters

Parameter Name

Description

Engg Unit (Metric/Imperial)

Flow Par

Airflow

Numeric: 32-Bit Floating point value, >=-inf-

(m3/hr) )/ft^3/min (CFM)

Offset Par

Input press Offset correction (Pa/inW). Stored Flow offset value for display.

Numeric: 32-Bit Floating Point value >=-inf-<inf.

Pa OR inW

Velocity Par

Air flow Velocity (m/hr/ft/min). Numeric: 32-Bit Floating point value, >=-inf-<inf.

m/hr OR ft/min

Pressure Par

Air Pressure Pa OR inW

Numeric: 32-Bit Floating Point value >=-inf-<inf, Default: Null.

Pa OR inW

AutoSetOffSet Par

Input correction pressure offset (Pa OR inW)

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

Pa OR inW

ClearOffSet Par

Reset correction pressure (Pa OR inW)offset to value 0.

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

Pa OR inW

Area Par

Area

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

(m2/hr) )/ft^2

KFactor Par

Actual flow in corresponding to a velocity pressure sensor output of 240Pa/ 1" w.g.

Numeric: 32-Bit Floating value: >=-inf-<inf, Default: Null.

(m3/hr) )/ft^3/min or CFM

Table 3: Parameters of FlowVelocity Function

Examples

If Pressure is 2.02 in/wc, offset is 0.02 in/wc, KFactor is 1015 cfm, and the area is 0.54 square feet (10-inch diameter), the flow will be 1015 ft/min, and velocity will be 1879 ft/min.

Figure 3: Example FlowVelocity Function Block

Refer to General Description for general information about IRM N4 Function Blocks.