True CFM (Cubic Feet per Minute) VVT-C

| 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 of VVT-C in the Carrier system.

The feature is introduced in the following terminal profile.

VVT-C

| Configuring CFM

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

Along with the already existing parameter in the VVT-C terminal profile, the user intent parameters such as K Factor and min CFM for IAQ is introduced.

| 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 Carrier system hardware is used.

Carrier 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 VVT-C.

CFM for VVT-C damper control

When the system is operating using a VVT-C terminal profile,

The Algorithm tries to maintain the min CFM (minCFMIAQ) value.

If the CFM is below the minimum threshold, then the damper opens up to maintain the min CFM (minCFMIAQ) value
If the CFM is above the target, then the damper closes to maintain the min CFM (minCFMIAQ) value

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

| CFM Related Widgets & Visualization

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

Predefined Widgets:

Portal User Intent

CCU Zone User Intent

Note: The default value of the following tuners has been changed for efficient air flow balancing to give more weightage to the Integral side of the loop and also to reduce the amount of time it takes for the integral to ramp up. The new tuner values are as follows:

airflowCFMproportionalKFactor = 0.2
airflowICFMntegralKFactor = 0.8
airflowCFMintegralTime = 5

browse