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| Overview

True CFM (Cubic Feet per Minute) is a unit of airflow that is been used by the ASHRAE 62.1 indoor air quality standards. It mandates that a minimum amount of airflow is maintained as long as the space is occupied. One of the primary reasons for which True CFM is adopted since legacy systems and senior facility management personnel use airflow measurements in CFM for ease of understanding. This further helps in maintaining common standards of measurements across HVAC systems. It is also incorporated in accordance to the Title 24 building codes which are building energy efficiency standards are designed to ensure new and existing buildings achieve energy efficiency and preserve outdoor and indoor environmental quality.

The CFM measurement is a product of velocity volume of air in the duct and the cross-sectional area of the duct.

To calculate Air Flow in Cubic Feet per Minute (CFM), we have to determine the Flow Velocity in feet per minute, then multiply this value by the Duct Cross Sectional Area. i.e.,
 
Air Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x Duct Cross Sectional Area (A)

 

| True CFM Introduction

As a part of the adoption towards enabling the CFM in profiles for comfort and indoor air quality standards, a toggle button is introduced in the terminal profiles VAV in the 75F system. 

The feature is introduced in the following terminal profile.

VAV

  • VAV Reheat with No fan

VAV_Reheat-No_fan.png

  • VAV Reheat with Series Fan

VAV_Reheat-Series.png

  • VAV Reheat with Parallel Fan

VAV_Reheat-Parallel.png

| Configuring CFM

VAV

Enabling the CFM for the VAV system profile supplied terminal profile, would introduce additional configurable parameters in the configuration as below:

CFM_Enabled_Series.png

Along with the already existing parameter in the VAV terminal profile, the user intent parameters such as K Factor, max and min CFM Cooling, max and min CFM Reheating are introduced.

The additional parameters introduced are the same for VAV Reheat with no fan and VAV Reheat with parallel fan terminal profiles.

| CFM Measurement Setup

As a part of the adoption towards measuring and communicating the Airflow in CFM. At the Duct work, end of the system, the following setup from the 75F system hardware is used.

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75F has developed a new differential pressure sensor that connects to a pitot tube and directly measures the airflow pressure difference between velocity pressure and static pressure. This allows a translation into velocity of the airstream. Coupled with knowing the cross-sectional area of the duct, the actual cubic feet per minute flow rate can be inferred. 

| Operation

With the Introduction of CFM, following are the changes that are brought into the Algorithm and operation of the terminal profiles for VAV.

CFM for VAV damper control

  • When the system is in cooling mode the CFM parameters used are  

maxCFMCooling and  minCFMCooling (when the terminal needs cooling) 

maxCFMReHeating and minCFMReHeating (when the terminal needs reheating) 

  • When the system is in heating mode the CFM parameters used are :

maxDamperPosHeating and minCFMReHeating

Note: In the system heating case, the min CFM provides for a floor IAQ, and the max damper position is used per standard DAB including normalization. 

Once the current CFM per damper that is controlled by the SmartNode is calculated, the system shall adhere to the the min and max value. If the CFM is below the minimum threshold, then the damper opens and vice versa if it is above the maximum CFM the damper closes.

Note: The system does not maintain the Minimum IAQ levels while in auto away or forced occupied mode.

| CFM Related Widgets & Visualization

As a part of the adoption towards visualizing information related the Airflow in CFM. At the portals and the CCU end of the system, the following are made available.

Predefined Widgets:

  • VAV

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Portal User Intent

  • VAV

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CCU Zone User Intent

  • VAV

mceclip2.png

 

 
 
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