Using the PFC object in an MPM-VA

Issue

Unsure how to use the settings in the VAV Flow object

Product Line

SmartStruxure Lite

Environment

SmartStruxure Lite Multi-Purpose Manager

MPM-VA

Cause

Cannot change the units of the output

Resolution

For Variable Air Volume (VAV) Flow configuration, it is recommended to use the Kf method when setting up an MPM-VA

In the PFC calculation, the Kf factor represents the flow (Q) for a measured dP on the flow ring of 1 Pascal (Pa). The flow sensor on the MPM controller (AI21) measures dP in inches of water column (in w.c.). With a conversion of 1 in w.c. = 250 Pa, we obtain this formula:

Kf = Q [cfm] / (sqrt(250 x dP[in w.c.])) or,

Q [cfm] = Kf * (sqrt(250 x dP[in w.c.]))

 

Calibration/Balancing:

A typical VAV box should have a theoretical Kf factor provided by the manufacturer. This Kf value must be used in the in the PFC object when balancing VAV systems. Next, using instruments such as an air flow capture hood, technicians must compare the output flow from the PFC object and the actual measures air volume. If the values do not match, the measured value along with the value of AI21 must be entered in the above equation to produce a new Kf factor. The new Kf value will then be used in the PFC object to finalize the calibration

For example if the measured air volume is 200 cfm and AI21 is 1.2 w.c., the new Kf will be:

Kf = 200 / (sqrt(250 x 1.2)) =11.55

Important notes:

-Even though the Kf units show "Liters per second", the "Flow output value" will be in "Cubic feet per minute" (CFM) if the value of Kf is based on CFM values. If the Kf is based on LPS, the output will be in LPS

-Every time the Kf factor is changed in the PFC object in the MPM, it will take up to 2 minutes before the output flow changes to the new value