Air volume and number of changes

ASHRAE Standard 62 recommends a certain volume of fresh air be supplied to various areas in a building, which is dependent on the number of people present and the nature of the activity taking place. This is typically expressed as cubic feet per minute (cfm), cubic meters per hour (m3/hr) or liters of air per minute (l/min) per person.

Proper volume flows and air exchanges per hour should be verified any time changes or renovations occur that may affect the HVAC system. Volume flow verification should also be done when there are increases in occupant complaints, higher than normal operating costs, odors, abnormal ventilation noise or when changes in building differential pressures create noticeable conditions such as unexpected drafts and difficulty opening doors. Fresh air volume and air changes can be compared to recommendations in ASHRAE Standard 62. If measurements conform to these guidelines, it is a good general indicator that the system is performing properly.


Measuring volume flow can be accomplished in several ways:
· Performing duct traverses with a thermoanemometer or micromanometer with pitot probe and then doing the necessary conversions,
· Using a capture hood directly on the supply diffuser or exhaust grille,
or
· With a micromanometer connected directly to a pressure based flow station or diffuser taps to calculate flow using Kfactors provided by the manufacturer of the flow station and diffuser.


First, the percentage of outside air being supplied to an area must be determined. Air velocity is rarely uniform in an air duct since the shape of the duct, frictional forces, bends, branches, dampers and transitions all affect the movement of air. For this reason, when average air velocity is used to determine volume flow, a special technique called the log Tchebycheff method should be employed. As shown in the diagrams below, several velocity measurements should be taken in the cross-sectional area of a duct to ensure the most accurate estimate of average velocity is determined. This average velocity can then be multiplied by the cross-sectional area of the duct (in square feet) to give an estimate of volume based on velocity. ASHRAE Standard 111 has additional details on measuring air flow in ducts.


Air flow can also be used to determine the number of air changes that occur in a space over a period of one hour. This is accomplished by determining the supply air flow rate in CFH (cubic feet per hour) and dividing it by the total volume of a given space (length x width x height) to come up with the number of air exchanges per hour.

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