7.1 Terminal Devices, Effective Area, and Capture-Hood Checks
Key Takeaways
- Use an effective-area factor only for the exact terminal, configuration, instrument, and sampling method identified by the manufacturer.
- A capture hood can add resistance and change terminal flow; compensation and orientation checks follow the hood manufacturer’s procedure.
- Gross face area, neck area, free area, and effective area are different quantities and are not interchangeable.
- Induction devices and active chilled beams may require a primary-air measurement because room-side discharge includes induced air.
Terminal Devices, Effective Area, and Capture-Hood Checks
A terminal reading depends on the method
Diffusers, registers, grilles, slots, nozzles, induction units, and active chilled beams create different velocity profiles. A terminal measurement is valid only when the instrument, location, and conversion factor match the exact device and the manufacturer's stated method. Start with the device tag, model, size, neck, blade or pattern setting, and approved submittal. A visually similar outlet can have a different effective area or test procedure.
Identify the boundary being measured. A supply diffuser hood test attempts to capture the volume delivered through the terminal. A rotating-vane or thermal-anemometer method samples velocity at defined face locations. A duct or inlet traverse measures flow upstream of the terminal. These methods do not automatically produce interchangeable results because each interacts with the airstream differently.
Effective area
When the manufacturer supplies an effective area, often written as Ak, for a stated probe and sampling pattern, the corresponding relationship is:
Q = Vavg x Ak
Q is airflow in cubic feet per minute, Vavg is the average velocity obtained by the specified method, and Ak is the matching effective area in square feet. Ak accounts empirically for the device geometry and the way velocity is sampled. It is not simply gross face area, free area, or neck area.
Suppose the approved data lists Ak = 0.75 square foot for the exact diffuser and a four-position rotating-vane method. If the four readings average 640 feet per minute, the calculated airflow is 0.75 x 640 = 480 CFM. Using the four-square-foot gross face area would produce 2,560 CFM and would be a method error, not a different interpretation.
Before using Ak, verify:
- exact manufacturer, model, size, and discharge pattern;
- required instrument type and orientation;
- number and location of readings;
- units and whether a correction is already built into the factor; and
- damper or accessory position covered by the published data.
If the factor cannot be traced to the installed configuration, use an approved alternate method or report the limitation.
Capture hoods and backpressure
A capture hood encloses a terminal and directs the air through a calibrated measurement base. Select the correct hood base and skirt, fully cover the active terminal, avoid blocking adjacent outlets, support the hood without distorting the ceiling device, and allow the display to stabilize. Check zero, range, configuration, physical condition, and current calibration status before the sweep.
The hood adds resistance and can change the terminal flow. The size of that effect depends on branch pressure, terminal type, hood geometry, and airflow. A backpressure-compensated hood applies its manufacturer-defined procedure to estimate unhooded flow; compensation reduces a known bias but does not make every terminal measurement exact. Repeat an unusual value with the hood rotated when the method calls for it, or compare with an upstream branch measurement.
Slots, nozzles, swirl diffusers, very large grilles, low-pressure branches, and high-induction devices can be poor hood applications. A skirt that does not seal, a diffuser larger than the hood, or a hood that changes the sound and throw noticeably is evidence to question the method.
Face-velocity and alternate methods
A rotating vane averages velocity over its swept area and may be used for a defined face grid or continuous sweep when the manufacturer procedure supports it. Maintain the stated spacing and orientation and keep the probe from blocking the flow. A thermal anemometer can resolve low velocities but is sensitive to direction, temperature, contamination, and handling.
For a long linear slot, the approved method may use a specific probe position and factor, a calibrated inlet relation, or a branch-duct traverse. For an induction unit or active chilled beam, room-side discharge includes induced room air as well as primary duct air; a room-side hood reading therefore may not equal primary airflow. Use the manufacturer's primary-air method or a suitable inlet measurement.
Reconciliation and repeatability
A terminal sum is useful only when the devices, boundaries, and system state match. Compare the sum with a simultaneous or stable branch/main measurement while allowing for leakage and the uncertainty of both methods. Do not force terminal values to equal a traverse by multiplying every result by an unexplained correction.
When one terminal disagrees with the branch relationship:
- verify tag, design value, device configuration, and operating mode;
- repeat the instrument zero and physical setup;
- confirm the exact Ak, hood, or manufacturer method;
- inspect neck connection, damper position, and local restriction;
- compare with an independent method; and
- record the accepted method and remaining limitation.
The final field record should identify the terminal, design flow, initial and final flow, method, instrument, factor or hood configuration, operating mode, adjustment, and any correction supported by documented evidence.
A technician uses a calibrated 4-inch rotating vane anemometer to test a 24 in. × 24 in. supply diffuser. The manufacturer submittal lists an Ak factor of 0.75 sq ft for the model. If the average measured velocity across the four quadrants is 640 FPM, what is the calculated discharge airflow?