NATE Air Distribution Service flashcards
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31 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Diffuser
- A supply air outlet that spreads and mixes conditioned air into the space, typically at the ceiling, using fixed or adjustable vanes to control throw and drop.
- Return grille
- A louvered opening that admits room air into the return duct system without a damper; it protects the duct and distributes return airflow across the face.
- Volume damper
- An adjustable blade inside a duct that throttles airflow to balance branches; opposed-blade types maintain straighter flow while single-blade types are cheaper.
- Fire damper
- A listed closure installed at rated wall or floor penetrations that closes on heat to preserve the fire-resistance rating of the assembly and limit smoke spread.
- Turning vane
- Curved blades inside a square elbow that guide airflow around the corner, reducing turbulence and pressure loss compared with an unvaned elbow.
- Plenum
- A box or duct section that distributes air to several outlets or collects return air; an extended plenum serves multiple branch takeoffs from one trunk.
- Total external static pressure
- The sum of supply and return static pressures measured at the air handler, representing the resistance the blower must overcome across the entire duct system.
- VAV terminal box
- A variable-air-volume terminal with a damper and airflow sensor that modulates primary airflow to a zone; reheat or fan-powered versions add heating capability.
- Equal-friction method
- A duct sizing approach that assigns the same friction rate to every section, commonly 0.08 to 0.10 in. w.c. per 100 ft for residential supply trunks.
- Available static pressure
- Total external static pressure minus losses from filters, coils, and accessories; the remaining pressure drives duct sizing and airflow through the distribution system.
- Friction rate formula
- Friction rate in inches of water per 100 ft equals available static pressure multiplied by 100 and divided by the total effective length of the longest run.
- Total effective length
- The straight-run duct length plus the equivalent lengths of all elbows, tees, and takeoffs on the longest path from fan to outlet.
- Equivalent length
- A published value in feet that converts a fitting's pressure loss into an equivalent straight-duct length, allowing fittings to be added to measured duct length.
- Velocity method sizing
- A sizing method that assigns target velocities: roughly 700 to 900 fpm for supply trunks, 500 to 700 fpm for branches, and 600 to 900 fpm for returns.
- Duct area formula
- Cross-sectional duct area in square feet equals airflow in cfm divided by velocity in fpm; equivalent round diameter derives from area equal to pi times radius squared.
- Velocity-friction relationship
- Friction loss per unit length rises approximately with the square of velocity, so doubling duct velocity roughly quadruples friction loss and noise.
- Duct traverse
- A measurement averaging velocity readings across a duct cross-section with a pitot tube or anemometer to calculate total airflow in cfm.
- Flow hood
- A calibrated balometer placed over a diffuser or grille that directly reads delivered airflow in cfm during balancing and diagnostic checks.
- Temperature-rise method
- Estimating airflow by dividing sensible heat in Btu/h by 1.08 times the temperature difference in degrees Fahrenheit across the equipment.
- Balancing sequence
- Set the air handler to design airflow, balance the branch farthest from the fan first, then adjust progressively toward the trunk.
- Static pressure profile
- A series of static readings taken along the air path; a large drop across a filter or coil reveals a restriction, while low readings suggest leakage.
- Duct leakage test
- A calibrated fan and pressure gauge test with all registers sealed that measures leakage cfm at a specified test pressure against a leakage class limit.
- Evaporator delta T
- The temperature difference between return and supply air across the coil; values outside roughly 16 to 22 degrees Fahrenheit suggest charge, airflow, or duct problems.
- UL 181A sealant
- A listed sealant for rigid fibrous glass duct board and closures; flexible duct and its connections require UL 181B listed tape or mastic.
- Flex duct bend radius
- Flexible duct should bend with a centerline radius of at least one duct diameter and be supported every 4 ft to prevent crushing and airflow loss.
- Vapor retarder orientation
- Duct insulation vapor barriers face the warm side of the assembly to keep the cold duct surface above dew point and prevent condensation and mold growth.
- Duct leakage class
- A SMACNA rating expressing allowable leakage in cfm per 100 square feet of duct area at 1 in. w.c., ranging from class 3 to class 30.
- MERV rating
- Minimum Efficiency Reporting Value from 1 to 16; MERV 8 captures about 20 percent of 3 to 10 micron particles, while MERV 13 captures roughly 85 percent.
- ASHRAE 62.2
- The residential ventilation standard requiring whole-house ventilation of about 0.03 cfm per square foot plus 7.5 cfm per occupant, with local exhaust in kitchens and baths.
- Energy recovery ventilator
- A ventilation device that transfers both sensible heat and latent moisture between exhaust and incoming outdoor air, reducing heating and cooling loads.
- Filter pressure drop
- The resistance a filter adds to the air path; it rises as the filter loads, so higher-MERV filters must fit within the equipment's rated external static pressure.