Duct Airflow Velocities – HVAC Design

Fig. 01 - Ductwork design is an essential and demanding task of any HVAC system, that requires the compliance with many design criteria and construction specifications. For extended air supply and return duct systems, the velocity criteria play an important role on the system efficiency (pressure drop) and occupants comfort (acoustic performance).
Photo by T.Moumiadis

Ductwork system is an essential part of most small to large scale commercial and industrial HVAC systems. Scope of the ductwork is to provide a contained path for the conditioned air coming out from the cooling / heating units to the air supply terminals (diffusers, FCUs, VAVs) within the occupant’s space. The same happens for the return air paths where ductwork systems connect the air return grilles and openings with extraction fans or the air handling units.

One of the design parameters that define the sizing of ductwork is the maximum allowable air velocity. According to CIBSE Guide B – HVAC / 2005, clause 3.3.2, the duct air velocity is not a major factor in the constructional specifications of the ductwork. On the other hand the velocity of air flowing through a duct can be critical, particularly where it is necessary to limit noise levels and has a major impact on the pressure drop.

The selected maximum air velocity within each ductwork section of a network depends on the following criteria:

•         System pressure class
•         Acoustic design criteria
•         Building usage
•         Exact location of the ductwork section in the system topology

This post aim is to summarize the above criteria in compliance with CIBSE and ASHRAE requirements. Thus become an accurate and quick reference tool of duct air velocity for any future HVAC project.

  

Reference Standards and regulations

All information about duct airflow velocities is in accordance with the following references:

[1] CIBSE Guide B – Heating, ventilating, air conditioning and refrigeration / 2005ed,

[2] ASHRAE Handbook – Fundamentals 2017ed / Section 22 – Duct Design,

[3] ASHRAE Handbook – HVAC Applications 2015ed / Section 48 – Noise and Vibration Control,

[4] Carrier Handbook of Air Conditioning system design / Chapter 2 Air duct design.

ASHRAE Design Criteria

Any ductwork system can be designed with certain maximum allowable values for the air velocity by using the design criteria below ⁽¹⁾.

According to ASHRAE Handbook – HVAC Applications 2015 / Section 48 Noise and vibration control / Table-8, the maximum recommended duct airflow velocities in order to comply with certain acoustic design criteria is:

Table 1. Maximum recommended main duct airflow velocities to achieve

specified acoustic design criteria

		Maximum Airflow velocity (m/s)
Main Duct Location	Design RC(N)	Rectangular Duct	Circular Duct
In shaft or above drywall ceiling	45	17.8	25.4
	35	12.7	17.8
	25	8.6	12.7
Above suspended acoustic ceiling	45	12.7	22.9
	35	8.9	15.2
	25	6.1	10.2
Duct located within occupied space	45	10.2	19.8
	35	7.4	13.2
	25	4.8	8.6

Table 2. Maximum recommended branch duct airflow velocities

to achieve specified acoustic design criteria

		Maximum Airflow velocity (m/s)
Branch Duct Location(a)	Design RC(N)	Rectangular Duct	Circular Duct
In shaft or above drywall ceiling	45	14.2	20.3
	35	10.2	14.2
	25	6.9	10.2
Above suspended acoustic ceiling	45	10.2	18.3
	35	7.1	12.2
	25	4.9	8.2
Duct located within occupied space	45	8.2	15.8
	35	5.9	10.6
	25	3.8	6.9

(a)     Branch ducts should have airflow velocities of about 80% of values listed for main duct

Table 3. Maximum recommended runout duct airflow velocities

to achieve specified acoustic design criteria

		Maximum Airflow velocity (m/s)
Runouts to outlets(b)	Design RC(N)	Rectangular Duct	Circular Duct
In shaft or above drywall ceiling	45	<8.9	<12.7
	35	<6.4	<8.9
	25	<4.3	<6.4
Above suspended acoustic ceiling	45	<6.4	<11.5
	35	<4.1	<7.6
	25	<3.1	<5.1
Duct located within occupied space	45	<5.1	<9.9
	35	<3.7	<6.6
	25	<2.4	<4.3

(b)     Velocities in final runouts to outlets should be 50% of values listed for main duct or less

According to ASHRAE Handbook – HVAC Applications 2015 / Section 48 Noise and vibration control / Table-9, the maximum recommended air velocities at neck of supply diffusers or return grilles, in order to comply with certain acoustic levels are:

               

                Table 4.0 Maximum air velocities at supply diffusers and return registers

Type of Opening	Design RC(N)	“Free” opening airflow velocity (m/s)
Supply air outlet	45	3.2
	40	2.8
	35	2.5
	30	2.2
	25	1.8
Return air opening	45	3.8
	40	3.4
	35	3.0
	30	2.5
	25	2.2

CIBSE Design Criteria

According to CIBSE Guide B - HVAC & Refrigeration 2005 / Section 3.10 Ductwork / Table 3.2, the maximum recommended duct velocities for low pressure ductwork systems, depending on acoustic criteria and project type.

                Table 5.0 Maximum airflows for low pressure systems in relation to noise levels

		Velocity (m/s)
Typical applications	Typical noise rating (NR)	Main ducts	Branch	Runouts
Domestic buildings (bedrooms)	25	3.0	2.5	<2.0
Theaters, concert halls	20-25	4.0	2.5	<2.0
Auditoria, lecture halls, cinemas	25-30	4.0	3.5	<2.0
Bedrooms (non-domestic buildings)	20-30	5.0	4.5	2.5
Private offices, libraries	30-35	6.0	5.5	3.0
General offices, restaurants, banks	35-40	7.5	6.0	3.5
Department stores, supermarkets, shops, cafeterias	40-45	9.0	7.0	4.5
Industrial buildings	45-55	10.0	8.0	5.0

According to CIBSE Guide B - HVAC & Refrigeration 2005 / Section 3.10 Ductwork / Table 3.3, the maximum recommended duct velocities for medium and high pressure ductwork systems, depending on duct volume flow shall be:

                               

                Table 6.0 Maximum airflows for medium and high pressure systems

	Velocity (m/s)
Volume flow in duct (m3/h)	Medium pressure systems	High pressure systems
<360	8	9
360 – 1800	9	11
1800 - 5400	11	15
>5400	15	20

According to CIBSE Guide B - HVAC & Refrigeration 2005 / Section 3.10 Ductwork / Table 3.4, the maximum recommended duct velocities in risers and ceilings in accordance to the application type (critical, normal, non-critical) shall be:

                Table 7.0 Maximum velocities for risers and ceilings

		Duct air velocity (m/s) for stated room type
Duct location	Duct type	Critical	Normal	Non-critical
Riser or above plasterboard ceiling	Rectangular	5	7.5	10
	Circular	7	10	15
Above suspended ceiling	Rectangular	3	5	6
	Circular	5	7	10

According to CIBSE Guide B - HVAC & Refrigeration 2005 / Section 3.10 Ductwork / Table 3.5, the maximum velocity for supply and return air openings shall be:

                                Table 8.0 Maximum velocity for supply and return air openings

	Permitted air velocity (m/s)
Supply or return air	Critical	Normal	Uncritical
Supply	1.5	2.5	3
Return	2	3	4

CARRIER Handbook

According to Carrier Handbook of Air Conditioning design / Chapter 2 Air duct design, the recommended duct velocities for low velocity systems shall be:

                               

		Controlling factor the duct friction
		Main ducts		Branch ducts
Application	Main Duct (Controlling factor the noise generation)	Supply	Return	Supply	Return
Residencies	3.1	5.1	4.1	3.1	3.1
Apartments Hotel bedrooms Hospital bedrooms	5.1	7.6	6.6	6.1	5.1
Private offices Directors rooms Libraries	6.1	10.2	7.6	8.1	6.1
Theaters Auditoriums	4.1	6.6	5.6	5.1	4.1
General offices High class restaurants High class stores Banks	7.6	10.2	7.6	8.1	6.1
Average store Cafeterias	9.1	10.2	7.6	8.1	6.1
Industrial	12.7	15.2	9.1	11.2	7.6

(1)     These criteria can be used in isolation or combined based on the type of project, client requirements and system specifications. All tables are collections of tables and figures existing within sections of references above and in any case do not express personal results or experience values. The reader is really encouraged to read in detail the referred sections in order to have a more well- rounded view of the matter.

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