Capture hoods are popular instruments to use for measuring flow rates from supply grilles. Since they obtain measurements quickly, provide a direct air volume reading, are easy to use, and come with a variety of hood sizes to match various grilles, capture hoods are often the tool of choice (when measuring supply).

ASHRAE recommends performing a duct traverse to determine if a correction factor is needed for air capture hood measurements. Different diffuser styles, elbows attached directly to the diffuser, and dampers located just upstream of the diffuser can impair the uniformity of the flow patterns coming out of the diffuser and affect the hood reading. Be sure to choose the hood size that most closely matches the outlet size being measured. Many capture hoods on the market today include an A_k-factor function.

Utilizing these instruments effectively can help you ensure the proper balance of mechanical HVAC system supply airflows.

Nonuniform Flow Corrections

Using a capture hood on a supply opening with no diffuser or deflecting vanes may not provide accurate results due to a nonuniform air pattern and jet effects. Capture hoods are generally calibrated on a wind tunnel with a 2 x 2 ft (61 x 61 cm) hood attached to a diffuser or grille. The diffuser or grille spreads out the airflow and the capture hood will see more uniform flow. Other instruments are often used instead of capture hoods for nonuniform air flow applications.

Other suitable instrument types for this application include rotating vane, thermal anemometer, velocity matrix, and swinging-vane anemometers.

Measuring air velocities at a supply opening is easily accomplished when no grille or louvers cover the opening. Divide the opening into sections of equal area and take a measurement in each section. Instruments used for this application can be a rotating vane anemometer, velocity matrix, deflecting vane anemometer, or thermo-anemometer. Add the readings and divide the sum by the total number of readings to determine the average.

If the supply opening is covered with a simple grille which provides at least 70 percent free opening and there are no turning vanes, a rotating vane anemometer is a common choice. The best procedure is to divide the grille into equal areas and to take a reading at each grille section for at least 30 seconds for a mechanical instrument and 10 to 15 seconds for a digital instrument. The rotating vane anemometer will average the velocity during this timing interval, giving a more stable result. However, the lower the velocity, the longer it may take for the vane to stabilize.

A velocity matrix provides a larger measurement area, reducing the total number of samples resulting in quicker measurements. The velocity matrix can be used on a supply opening with no grille or louvers, but may not be the best choice if there are turbulent airflows. They can cause some turbulent air patterns which could affect the averaging function of the grid pressures.

Many diffuser manufacturers have established A_k-factors and procedures applicable to their outlets for correcting measurements taken with a deflecting vane anemometer equipped with diffuser probe. Probe positioning performed in the same manner as was done in the laboratory is critical in achieving reliable measurements in the field using this technique. The flow rate is determined by multiplying the diffuser manufacturer's A_k-factor by the average velocity in fpm (m/s) to obtain cfm (m³/h).

Q = A_k x V

Where:
Q = flow rate in cfm (m³/h)
A_k = outlet manufacturer's correction factor
V = average velocity in fpm (m/s)

Since the introduction of the capture hood, the deflecting vane measurement has become less prevalent. It is a good option for low flow applications.

Static Pressure Measurement

Static pressure measurements are an important method of determining system operating efficiency and identifying problems such as leaky ducts, clogged filters and coils, and blocked inlets. Any of these can negatively affect occupant comfort and system operating efficiency, which can waste significant utility dollars. Unnecessary increased loads on fans and other system components can cause premature wear and untimely failure, resulting in increased maintenance costs.

Static pressures in air distribution ducts can be measured using a digital micromanometer, incline manometer, or deflecting vane anemometer with static pressure probe. A digital micromanometer or incline manometer can use the standard "L-shaped" pitot probe with a hose connected from the positive port of the manometer to the static pressure leg of the pitot probe. The total pressure leg of the pitot probe and the negative port of the manometer would be open to the atmosphere.

Exhaust Grille Measurements

To adequately balance a mechanical HVAC system, it is necessary to also verify exhaust flows. Instruments such as a capture hood, rotating vane, or velocity matrix are common choices to measure exhaust flows.

Capture hoods calibrated for exhaust mode can be used on return grilles without an A_k-factor. For the best accuracy, choose the hood that most closely matches the outlet size being measured and set the hood to exhaust or return mode. Also, as with supply diffusers, an alternate means of measuring flow (such as a duct traverse) is advised when determining if a correction factor needs to be applied to the hood readout.

Rotating vanes can be used on return grilles provided the vanes of the grille are straight with no deflection. Place the rotating vane approximately 1 to 2 inches (2.5 to 50 cm) away from the grille and divide the face of the grille into equal areas with the vane head taking a measurement in each location. The readings should be added and the sum divided by the total number of readings to obtain an average velocity. The average velocity can be multiplied by the free area of the grille in square feet (square meters) to obtain a cfm (m³/h) volumetric value. Rotating vanes are a good alternative if the grille is too small or too large to accommodate a capture hood.

A velocity matrix is a good choice for large return grilles with high flow rates that a capture hood cannot accommodate. The face of the return grille should be divided into sections and the velocity matrix placed in each section to obtain a reading. These readings should be added and then divided by the total number of readings to produce an average velocity. Stand-offs located on the corners of the velocity matrix aid in positioning the matrix for repeatable results by keeping the distance between the matrix and the grille consistent for each measurement.

If the exhaust outlet does not have a grille or louvers, divide the face into a grid pattern with equal areas. A larger number of measurement locations may be required to increase the accuracy of the average flow. Measurements can be taken in the center of each area using a velocity matrix or rotating vane anemometer. However, the blockage effect caused by the size of the instrument when placed in the air stream at the exhaust opening will vary from model to model, and the accuracy may be impaired.

Duct Traversing: Low to High Velocities and at High Air Temperatures

Air velocity measurements are often taken within ducts to determine flow rate. Since velocity distribution is not uniform even in the best of circumstances, a duct traverse is necessary. This involves taking a number of readings within the duct and determining average velocity.

For selecting the most appropriate technique, here are some general guidelines. If air velocity in ducts normally exceeds 1,000 fpm (5 m/s), a manometer with pitot tube, thermo-anemometer, or deflecting vane anemometer with pitot probe can be used for making these measurements. However, each instrument has its limitations and may not be applicable for all jobs.

If velocities are below 700 fpm (3.5 m/s), the choice of instruments would be between either the deflecting vane anemometer with pitot probe or a thermal anemometer.

If velocities are below 300 fpm (1.5 m/s), the thermal anemometer may be the only choice. An example is a vertical stack or flue that functions as a result of temperature differences and may contain combustion or industrial process byproducts.

Measuring airflow in extreme air temperatures found in chimneys or drying oven ducts will require a manometer with stainless steel pitot tube that is rated up to 1,500°F (815°C).

Note: The manometer is not considered to be a flow through device. This means the elevated air temperature seen by the pitot tube will not pass down through the hoses to the manometer and damage components or electronics. However, if the probe is left in the high temperature airstream for extended periods, high temperatures may eventually affect the tubing connecting the manometer to the pitot tube.

Measuring Air Velocity in Open Areas

Air velocities (and movement) in a room or space are often felt as a draft and, depending on the intensity, can be uncomfortable to most people. Cross-drafts in areas having spray booths or lab hoods can interfere with air measurement by the permanently installed air velocity monitors on the booth or hood. These and similar situations call for an air velocity measurement in the range of 50 to 200 fpm (0.25 to 1.00 m/s).

Thermal anemometers are generally the first choice for accuracy, but are inappropriate for determining airflow direction. A smoke source can be used in conjunction with the thermo-anemometer to provide an indication of flow direction.

A deflecting vane anemometer can also be used in most open areas for measuring velocity. To obtain an accurate reading, hold the instrument in the air stream with the proper port facing upstream. If the flow direction is not known, orient the instrument in various positions until you obtain maximum deflection.

An air velocity transducer capable of outputting an analog signal (voltage or current) along with an omni-directional thermo-anemometer sensor can be used to measure room currents or draft over a period of time. The analog output signal can be connected to a remote datalogger to track trends in air patterns throughout the day, week, or month in order to get a better understanding of where and when room air currents are unacceptable.

Excerpted and reprinted from the Alnor HVAC Handbook from Alnor/TSI Incorporated, Shoreview, Minn. Alnor manufactures instruments for the measurement of indoor environments including air velocity, volume, pressure, and temperature. For more information, visit www.alnor.com/REDIRECTS/DIRECTORIES/ElecAds/00001/.

Publication date: 09/04/2006

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