Velocity
A quick spot check at the supply diffuser will show if sufficient air is entering a space. This will assure there are no unexpected blockages in the air system, such as a closed damper. Velocity is also a good indicator that air is being appropriately distributed or balanced throughout the building and reaching all the intended spaces. Measurements should also be taken in the actual occupied “zones” to assess how air velocity affects individuals. The instruments used are air velocity meters, rotating vane anemometers and the multi-parameter ventilation meters.
Volume
ASHRAE Standard 62 lists recommended outdoor air requirements expressed in terms of Cubic Feet per Minute (CFM) per person depending on the type of space and activity. The percentage of Outdoor Air must be calculated (see ventilation section). This percentage can then be multiplied by the measured airflow to calculate the amount of outdoor air being supplied. Air volume or flow into an area affects the air change rates or exchange of air between outdoors and indoors. This results from leakage in natural or mechanical ventilation systems. The exchange of air can have a large impact on indoor air quality as it may increase the amount of outdoor pollutants being introduced or, conversely, dilute and help remove contaminants generated indoor.
The average air velocity can be determined using a straight average for both round and rectangular ducts using the log-Tchebycheff method, a method that accounts for velocity losses due to friction. As the figures indicates, velocity measurements should be taken at a minimum of 25 points for rectangular ducts and, for round ducts, symmetrically disposed diameters with at least 6 points on each should be used. For the greatest accuracy, take these measurements at least 7.5 diameters downstream or 3 diameters upstream from any disturbance such as an elbow, venturi or take-off. ASHRAE Standard 111 has additional details on measuring flow in ducts.
To determine volumetric flow rate, the average measured air velocity is multiplied by the cross-sectional area of a duct. For example, if a duct is 2ft by 2ft (cross-sectional area = 4sqft) and the average measured air velocity is 150ft per minute, the resulting flow rate is 150ft/min x 4sqft or 600 CFM. Multi-parameter ventilation meters are capable of calculating automatically the volumetric flow rate when the cross-secti
onal area of a duct is entered.
An air capture hood can also be used to determine air flow. Capture hoods provide quick, direct measurements of air flow from diffusers, vents or grills. They are capable of collecting and storing real-time flow measurements and they are also valuable when balancing the system for proper flow in all areas. Air velocity meters, multi-parameter ventilation meters and air capture hoods all provide a fast, accurate means for measuring volumetric airflow.
Ventilation
The introduction of outdoor air helps dilute unwanted pollutants and gets them out of the building faster. ASHRAE Standard 62 presents recommendations pertaining to ventilation, or the amount of outdoor air introduced into a given area. It recommends a minimum volume per person over time, depending on the type of space and activity being performed, expressed in cubic feet per minute per person.
A good indicator of proper ventilation is the level of CO2 present in a space. Carbon dioxide is a normal by-product of respiration, combustion and other processes. Elevated levels of CO2 may indicate that additional ventilation is required. ASHRAE Standard 62 recommends an indoor level not to exceed about 700 ppm above outdoor ambient air which is typically about 300 to 400 ppm. Under normal conditions, even elevated CO2 levels are rarely a health hazard since levels up to 10,000 parts per million can be tolerated without ill effects by healthy people. Measurements should be taken between different areas, in air distribution zones, at varying heights and between indoor and outdoor areas to ensure that the building is properly ventilated.
