True CFM (Cubic Feet Per Minute) VAV

| 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 with Series Fan

VAV Reheat with Parallel Fan

| Configuring CFM

VAV

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

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 of measuring and communicating the Airflow in CFM. At the ductwork, end of the system the following setup from the 75F system hardware is used.

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, the following are the changes that are made to the Algorithm and operation.

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 case the system is in heating, the min CFM is maintained, and the damper position is used as per standard DAB operation using a normalization strategy.

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

Note: The system does not maintain the Minimum CFM levels during auto-away and forced occupied modes.

| 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

Portal User Intent

VAV

CCU Zone User Intent

VAV

| CFM Influence on Damper Position Calculation in VAV Terminal Profiles

The above configuration is a room temperature & CFM-based control. The influence of CFM on the VAV terminal sequences for the Damper position calculation is like the example illustration below.

The PI loop (Cooling/ Heating), which uses the current and desired temperatures (Cooling/Heating) is used to determine the Airflow setpoint.

The Loop output signal is scaled for configured Minimum and Maximum CFM to determine the Airflow setpoint.

Example:

Let us Assume the PI loop output signal is 80%, the same is translated to Airflow Setpoint as

Min CFM(Cooling/Heating)=50 cfm
Max CFM(Cooling/Heating)=250 cfm
Airflow CFM Range = 200 cfm
80% loop output signal scaled for Airflow CFM Range (200)= 200*80/100= 16000/100= 160 cfm
Actual Airflow Setpoint= 50 (Min CFM(Cooling/Heating))+ 160= 210 cfm

The Damper position is scaled from the Minimum to the Maximum position ( Configured parameters), based on the resultant CFM Loop Output Signal ( Cooling/Heating), which uses the Airflow Setpoint and calculated actual CFM, as below.

Zone/Room Demanding Cooling

Zone/Room Demanding Heating

On the cooling side of the operation, it is straightforward scaling, the damper positions are more or less directly proportional to the Cooling CFM Loop Output Signal.

On the heating side, the Damper remains at a minimum, till a maximum Discharge Air Temperature (DAT) is reached. This is defined by the tuner parameter VAV-reheatZoneMaxDischargeTemp, which is by default 90F.

This also means for half of the Heating CFM Loop Output signal the Damper position remains at the minimum specified, and then the position is proportionally scaled for the rest of the Loop output if the required conditioning is not achieved.

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