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ASHRAE Design Criteria
CIBSE Design Criteria
CARRIER Handbook
Mahmoud Farahat
1-9-2023
ASHRAE Design Criteria
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 / Table8, 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
35
17.8
12.7
25.4
17.8
25
8.6
12.7
45
35
12.7
8.9
22.9
15.2
25
6.1
10.2
45
35
10.2
7.4
19.8
13.2
25
4.8
8.6
Above suspended acoustic ceiling
Duct located within occupied space
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
senior mechanical engineer
+201146089454
ASHRAE Design Criteria
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
35
14.2
10.2
20.3
14.2
25
6.9
10.2
45
35
10.2
7.1
18.3
12.2
25
4.9
8.2
45
35
8.2
5.9
15.8
10.6
25
3.8
6.9
Above suspended acoustic ceiling
Duct located within occupied space
(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
Duct located within occupied space
(b)
45
<6.4
<11.5
35
<4.1
<7.6
25
<3.1
<5.1
45
<5.1
<9.9
35
<3.7
<6.6
25
<2.4
<4.3
Velocities in final runouts to outlets should be 50% of values listed for main duct or less
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
senior mechanical engineer
+201146089454
ASHRAE Design Criteria
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
45
3.8
40
3.4
35
3.0
30
2.5
25
2.2
Return air opening
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
senior mechanical engineer
+201146089454
ASHRAE Design Criteria
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
40-45
9.0
7.0
4.5
45-55
10.0
8.0
5.0
Department stores,
shops, cafeterias
Industrial buildings
supermarkets,
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
senior mechanical engineer
+201146089454
ASHRAE Design Criteria
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
360 – 1800
1800 - 5400
>5400
8
9
11
15
9
11
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
Riser or above plasterboard
ceiling
Above suspended ceiling
Duct type
Rectangular
Critical
5
Normal
7.5
Non-critical
10
Circular
7
10
15
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)
Critical
Normal
Supply
1.5
2.5
3
Return
2
3
4
Supply or return air
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
Uncritical
senior mechanical engineer
+201146089454
ASHRAE Design Criteria
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
5.1
7.6
6.6
6.1
5.1
6.1
10.2
7.6
8.1
6.1
4.1
6.6
5.6
5.1
4.1
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
Apartments
Hotel bedrooms
Hospital bedrooms
Private offices
Directors rooms
Libraries
Theaters
Auditoriums
General offices
High class restaurants
High class stores
Banks
(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.
Mahmoud Farahat
mahmoudmohamedfarahat@gmail.com
senior mechanical engineer
+201146089454