AIR DISTRIBUTION SYSTEM/DUCTING OR DUCT DESIGN
What is a duct?
A duct in HVAC (Heating, Ventilation and Air conditioning)
is used as a medium or path to transfer air from air conditioning units to conditioned space. Its used for exhaust and ventilation purpose as shown in figure below. A
duct is constructed from a material with very low surface friction properties
to reduce static pressure on air conditioning unit blower. A ducting system is
used for supply, return, exhaust (kitchen and toilets), ventilation and fresh
air supply to conditioned space. Duct may be of different shapes spiral
(rounded), rectangular or square shape. A rounded duct may be flexible to make easy
connection with air terminals like diffusers or registers. The duct which
carries cool air from air conditioning unit is properly sealed and insulated
with high thermal resistance property material to reduce cooling loss, both
supply and return air ducts are insulated. But exhaust air ducts don’t need
insulation.
A duct is created from joining different pieces like
rectangular straight pieces, transducers (concentric or in-centric), bends and
offsets.
Air
distribution is combination of Air delivering source, Duct distribution
including fittings, dampers, registers, grills etc. In this article duct design
calculations, duct routing best selection will be thoroughly studied. SMACNA standard is followed
for duct construction. 
Types
of Ducting in HVAC:-
1. Supply air duct: - Supply air duct is the
path through which we deliver conditioned air to the space which is needed to
air conditioned. The air is dropped through grills, diffusers or registers at
different points. Supply air ducts are always shown in the system with green
color.
2.
Return air duct: -
Return air duct as name suggests it’s a path through which the cool air after
gaining heat from the conditioned space is taken back to the air source like
FCU, AHU etc. Return air duct has always
5-10% more duct size as compared to supply air duct. Return air ducts are drawn
in drawing with red colors.
3.
Exhaust air duct: -
For ventilation purpose or to maintain air quality in the conditioned space
exhaust of air is needed and this is done through a duct installed
separately.
4.
Fresh air duct: -
Fresh air duct is installed to maintain air quality and this duct does not
bring fresh air from outside directly inside the room. Fresh air is always
dropped first at the suction side of air source.
5.
Kitchen exhaust duct:
- The kitchen exhaust duct through the oily air and
smoke to the environment but there is a bit of trick to do that. This air is
first treated and then left in the environment following the norms of green
building.
6.
Toilet exhaust duct:
- This duct simply carries away all exhaust from the
toilets to the environment.
How
to design or size a duct?
Before duct sizing we need to calculate heat
load and from heat load we get TR, CFM and a check figure is considered as
follows: -
CFM/Area and sq.ft/TR
For example – we have calculated heat load
for a building which is 10 TR, CFM are 300 CFM and area is 600sq.ft. What will
be TR/Sq.ft and CFM/sq.ft.
CFM/Sq.ft = 600/300 = 2 cfm/sq.ft
And
sq.ft/TR means how much are will be covered
by 1TR.
600/10 = 60 sq.ft/ TR
This total are 600 sq. ft is for all parts like there may three bed rooms and a dining room and a guest room. So what we will do we will calculate the areas separately for each and then calculate the CFM for each room.
For example - area of a bed room is 10 X 20 = 200 sq.ft
Then, CFM = 200 X 2 cfm (From chech figure) = 400 CFM
In the same way each separate area and CFM will be calculated and using friction chart we will get sizes of ducts.
Important Note: - The
above example was only for reference but actual calculation will give accurate
cfm/sq.ft and sq.ft/TR. Which is recommended as 2-4cfm/sq.ft and 200-350 sq.ft/TR.
HOW TO SIZE?
Before sizing a
duct you should be aware about some standards.
1.
Aspect ratio 4:1 means the ratio of width and height should
not go above this ratio because higher the aspect ratio higher will be the
surface area and higher the Static pressure. Higher aspect ratio increases the
cost of installation and operation cost as well.
2.
The duct class is
considered for duct sizing. Duct class varies from class 1 to 6. Higher the
duct class higher will be installation cost, more weight of material, thickness
will increase which will be difficult to bend and at site no fabrication can be
done. So lowest duct class is considered at smallest aspect ratio.
3.
Rating of galvanizing for duct material is recommended to
G75 or G90.
4.
Type of class of fittings. There are two types of ducting
fittings one is class A and other is Class B.
In class A we use collars, bends, shoe-pieces etc for
braches. This type of fitting has lowest cost of installation but higher
operational cost.
In class B we use only y-pieces for branch ducts and this
type of fitting has higher cost of installation and lower operating cost.
For sizing a linear equation and a friction chart is considered as below
from which we get duct sizes for main and branch air ducts. From chart we get
duct size as equivalent dia and afterwards we convert it to rectangular size
with help of a software which is duct sizer.
Linear equation
Q = AV
Where Q is air flow (CFM)
A is area of duct cross
section sq.ft.
V is velocity (FPM)
Feet/minute
1. ACCORDING TO VELOCITY
1.1. Low velocity or High
velocity system.
1.2. Low pressure or High
pressure system.
These systems are further classified as following:-
COMFORT AIR CONDITIONING
SYSTEM - LOW VELOCITY (1200-2200FPM) & HIGH VELOCITY (2500-3000FPM)
INDUSTRIAL AIR CONSITIONING SYSTEM- 2200-2500FPM & 2500-5000 FPM
Air velocity is taken
according to the sound levels which is called Noise criteria and recommended as
follow: -
APPLICATION
|
NC levels (DB)
|
Bed Room
|
20-25
|
Hall or Office
|
30
|
Toilet/kitchen
|
35-40
|
Workshop
|
50
|
Where 200 FPM = 1 M/s
For return air ducts only low
velocity is considered and 20 % less than supply air ducts.
2. ACCORDING TO PRESSURE (TOTAL P = SP+VP)
2.1. Low Pressure – Upto 3 ¾”
WG.
2.2. Medium Pressure 3 ¾” – 6
¾” WG.
2.3. High Pressure 6 ¾” – 12
¾” WG.
Where 1’’ of water
gauge = 250 pascals
There
are three methods to size a duct
1.
Equal Friction method: -
1.1.
This
method of sizing is used for supply, exhaust and return air duct.
1.2.
In
this method same friction loss per ft is taken for entire length.
1.3.
The
velocity is selected from the table with sound level being limiting factor.
1.4. The
velocity is selected from the first piece of duct from starting from the fan
and considered same for entire length.
1.5.
Only
this method is most reliable and used for duct sizing in HVAC.
2. Velocity reduction method
3.
Static regain method
Best route for ducting in a building: - Duct rout is always
selected where we avoid diversions, offsets and bends. A straight path is
always considered like corridor is the best place to run the duct and supply branches
are dropped same as shown in figure below. A symmetrical diffuser placement is
always preferred for best cooling and design. For exhaust air ducts shafts and
cut outs are considered by the co-ordination of civil, architect and HVAC
engineers and all other agencies.
Recommended thickness according to size of
duct
Gauge
|
Thickness(mm)
|
Size (Width)
|
26 g
|
0.6
|
0-400
|
24g
|
0.7
|
401-750
|
22g
|
0.8
|
751-1050
|
20g
|
1.0
|
1100-1500
|
18g
|
1.2
|
Above 1500
|
Recommended air guiding vanes criteria
Upto 400 width
|
No vanes
|
Short radius L-bow 90’
|
401-800
|
1 vane
|
|
801-1200
|
2 vanes
|
|
Above 1200
|
3 vanes
|