HVAC Air Distribution flashcards
HVAC Air Distribution · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Total pressure
- The sum of static pressure and velocity pressure at a point in an air system; it represents the energy available to overcome downstream resistance.
- Velocity pressure formula
- VP = (V/4005)^2 for standard air at 0.075 lb/ft^3, where V is velocity in fpm and VP is in inches of water column.
- Continuity equation
- Q = V x A relates volumetric airflow to velocity and cross-sectional area; use consistent units such as cfm, fpm, and square feet.
- Hydraulic diameter
- For a rectangular duct, D_h = 1.30 x (W x H)^0.625 / (W + H)^0.25, allowing round-duct friction charts to be used for rectangular ducts.
- Aspect ratio
- The width-to-height ratio of a rectangular duct; ratios above about 4:1 increase friction, surface area, and installation cost.
- Equal friction method
- A duct sizing approach that selects sizes so pressure loss per unit length is constant throughout the system, simplifying design but not optimizing fan energy.
- Static regain method
- A sizing method where each downstream section is sized so the static pressure regained from velocity reduction equals the friction loss of the preceding section.
- Equivalent length
- A method of expressing fitting pressure loss as the length of straight duct that would produce the same pressure drop at the same velocity.
- System effect
- Additional pressure loss caused by non-ideal fan inlet or outlet connections, such as elbows close to the fan, that is not captured by fan curve ratings.
- Fan laws
- For a given fan, flow varies with speed, pressure with the square of speed, and power with the cube of speed; used to predict performance at other speeds.
- Duct leakage class
- A classification specifying allowable duct leakage as a percentage of design airflow at a specified test pressure, with seal classes defining which joints must be sealed.
- Fire damper
- A device installed where a duct penetrates a fire-rated assembly that closes on heat to prevent fire spread through the duct; requires an access door.
- Smoke damper
- A damper that closes on a smoke detector signal to limit smoke migration through ducts; it is often combined with a fire damper in rated assemblies.
- Flexible duct compression
- Compressing flexible duct increases its pressure loss sharply; even moderate sag or compression can multiply friction compared with the fully extended rating.
- Vapor retarder placement
- In cooling applications, the vapor retarder must be on the warm side of duct insulation to prevent condensation within the insulation.
- Diffuser throw
- The horizontal distance from the diffuser to the point where jet velocity falls to a specified terminal value, typically 50 or 100 fpm.
- Diffuser drop
- The vertical distance a supply jet travels from the diffuser before reaching a specified terminal velocity, important for heating operation and ceiling effects.
- Coanda effect
- The tendency of a supply jet to attach to and follow an adjacent surface such as a ceiling, extending throw and improving mixing in the occupied zone.
- ADPI
- Air Diffusion Performance Index: the percentage of occupied-zone measurement points meeting acceptable effective draft temperature and velocity criteria; 80% or higher is a common target.
- Effective draft temperature
- EDT = (T_local - T_room) - 0.07 x (V - 30), with V in fpm; values between about -3 and +2 F are considered comfortable.
- Air change rate
- ACH = Q x 60 / room volume, expressing how many times the room air volume is replaced per hour; used for ventilation and dilution design.
- Mixing ventilation
- A strategy that supplies high-velocity air to induce room air entrainment and create uniform temperature and contaminant concentration throughout the occupied space.
- Displacement ventilation
- A strategy that supplies low-velocity, slightly cool air near the floor and relies on buoyant plumes to carry contaminants upward to a high return.
- Face velocity
- The average air speed through the free area of a grille or diffuser; it influences noise generation, pressure drop, and entrainment of room air.
- Noise criteria
- NC or RC curves rate the sound level of air distribution devices; diffusers and grilles are selected to meet a space's NC limit at design airflow.
- Static pressure reset
- Reducing duct static pressure setpoint when the critical VAV damper is mostly open; fan power drops approximately with the cube of speed.
- Critical zone
- The VAV zone requiring the highest duct static pressure to deliver its design airflow; its damper position drives the supply fan static pressure setpoint.
- Relief air path
- A designed route for excess return air to exit the building, preventing overpressurization and ensuring outdoor air intake is maintained.
- Return air plenum
- A ceiling cavity used as a return air path; plenum velocity and pressure drop must be limited to avoid ceiling tile lift and dirt staining.
- Pitot tube traverse
- A method of measuring duct airflow using velocity pressure readings at multiple points across the duct; accuracy requires adequate straight duct upstream and downstream.
- Flow hood
- A calibrated instrument placed over a diffuser or grille to measure airflow directly; commonly used for balancing and commissioning air systems.
- Balancing damper
- A damper used to adjust airflow among branches during commissioning; it should be set once and not used as a routine flow-control device because of noise.