Measuring Air Flow (continued)
Air volume is measured to determine the total amount of air going
through an area in a given amount of time. We are typically looking
for cubic feet per minute (CFM) as it relates to heating or cooling
equipment. To determine this, you must either direct all of the air
through a known area, or use an instrument to measure velocity that
can calculate the CFM from the known area.
To understand we can compare to something more common. Consider
a small stream compared to a large river. A river has a huge area, but
the water may be moving at a low speed, while a stream may have
a higher velocity, but not be very big. If measuring the velocity of
the water only the stream would have a higher number. Some large
rivers move quite slowly, but due to their size move a tremendous
volume of water. If we think about all of the small steams flowing into
a river, we could arrive at the volume of the river by adding all of the
small streams velocity times their area (volume). Also, if we knew the
area that the river was occupying, we could determine the volume by
measuring the average speed of the water flow and multiplying by the
rivers area. This would then give us the total volume of water flowing
past a specific point.
Free Area (Ak value for a grill)
Grills or registers all have an area associated with all of the openings.
Grill manufactures refer to this as free area or effective area. They
perform extensive testing to determine the total area that is open to
air flow, and is effectively free for the air to flow through. Without this
number it is difficult to determine airflow by measuring velocity, but it
can be estimated for comparison purposes.
What exactly is free area, and how does it relate to airflow and registers
or grills? In open ductwork the free area is simply the open area of the
duct. For a rectangular grill this is determined by length times width.
A 12” by12” rectangular duct that is open has an area of 144 square
inches (length x width). If we want this in square feet we can divide the
result by 144 (144 square inches in each square foot). In this example
the free area is one square foot.
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