Duct Design Air Flow Velocities

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Duct Design Air Flow Velocities

HVAC System and Components Design and Engineering Data
Engineering Fluid Flow Design and Engineering

Duct Design Air Flow Velocities - Typical

HVAC - air conditioning and heating systems usually require some form of duct system to channel air to places where the conditioned air is needed. A poor HVAC dust design, faulty installation installation job will result in poor performance, bad air flow, leaky duct systems, and higher than usual energy costs. It is important that the installation process is to make sure the duct work is sized properly. An over sized duct system will cost more and may not not maintain the desired air flow and an undersized duct system can cause the system to inadequately flow the HVAC air to the desired destinations.

The following should be considered in an effective duct system design.

Tradeoff's between the costs of the duct system and the energy cost of the air distribution system.
space available,
noise level requirements,
capacity for expansion,
maintainability
Ductwork system be designed for the air conditioning load.
Each room or space should be evaluated and a determination of how much air flow will be required

Table - HVAC Ducts Air Velocities

Intake Louvers	Velocity (FPM)
7000 cfm and greater	400
Exhaust Louvers
5000 cfm and greater	500
Panel Filters
Viscous Impingement Dry-Type, Pleated Media: Low Efficiency Medium Efficiency High Efficiency HEPA	200 to 800 350 500 500 250
Renewable Media Filters Moving-Curtain Viscous Impingement Moving-Curtain Dry-Media	500 200
Electronic Air Cleaners Ionizing-Plate-Type Charged-Media Non-ionizing Charged-Media Ionizing	300 to 500 250 150 to 350 500 to 600
Steam and Hot Water Coils	200 min 1500 max
Electric Coils Open Wire Finned Tubular	Refer to Mfg. Data Refer to Mfg. Data
Dehumidifying Coils	500 to 600
Spray-Type Air Washers	300 to 600
Cell-Type Air Washers	Refer to Mfg. Data
High-Velocity, Spray-Type Air Washers	1200 to 1800

Adapted from ASHRAE “Pocket Guide” 1993

Typical Air Design Velocities for HVAC Components (Ducts)

Duct Element	Face Velocity, m/s
Louvers
Intake
7000 cfm3300 L/s and greater	2
Less than 7000 cfm3300 L/s	See Figure 1.0
Exhaust
5000 cfm 2400 L/s and greater	2.5
Less than 5000 cfm 2400 L/s	See Figure 1.0
Filters
Panel filters	1 to 4
Viscous impingement
Dry-type, extended-surface	Duct velocity
Flat (low efficiency)
Pleated media (intermediate efficiency)	Up to 3.8
HEPA	1.3
Renewable media filters
Moving-curtain viscous impingement	2.5
Moving-curtain dry media	1
Electronic air cleaners	Ionizing type
Heating Coils
Steam and hot water	2.5 to 5 1 min., 8 max.
Electric
Open wire	Refer to mfg. data
Finned tubular	Refer to mfg. data
Dehumidifying Coils	2 to 3
Air Washers
Spray type	Refer to mfg. data
Cell type	Refer to mfg. data
High-velocity spray type	6 to 9

Parameters for Figure	Intake Louver	Exhaust Louver
Min. free area (1220 mm² test section	45	45
Water penetration, mL/(m² · 0.25 h)	Negligible (< 0.3)	N/A
Max. static Pressure Drop, Pa	35	60

Figure 1.0 (Table + Chart Data) Criteria for Louver Sizing ( 2017F, Ch 21, Fig. 19)

Adapted from: ASHRAE Pocket Guide for Air Conditioning, Heating, Ventilation, Refrigeration SI 9th Edition