MEC 351 REFRIGERATION &
AIR CONDITIONG
CHAPTER 5: Air Distribution Systems
Prepared by: Ahmad Najmie Rusli
CHAPTER 5 Faculty of Mechanical Engineering,
UiTM Kampus Pasir Gudang.
Email: ahmad7586@johor.uitm.edu.my
5.0 TABLE OF CONTENTS
5.1 Air Conditioning System
5.2 Air Distribution System
5.2.1 Air Distribution System Components
5.2.2 Air Distribution System Ducting System
5.3 Ducting System
5.3.1 Ducting System Fittings
5.3.2 Ducting System Diffusers & Grilles
5.4 Diffusers Selection & Consideration
5.5 Duct Sizes Consideration
5.6 Procedures for Design Centralised Air Conditioning System
5.7 Required Air Quantity in a Room (CFM)
5.8 Losses in Ducts System
5.1 Air Conditioning System
Air conditioning system depend on air ventilation and
refrigeration cycles.
The objective of air conditioning design system is:
1. Achieving desirable temperature
2. Achieving desirable humidity
3. Achieving right amount of conditioned air
4. Provides air movement
5. Distribute conditioned air within the space
5.2
Air Distribution System
The good of an air distribution in air condition system must:
1. No excessive draft (air motion) to users:
a. Avoid excessive throw near walls to prevent
bouncing.
b. Diffusers with adjusting features (for balancing of
flow)
2. No excessive room air temperature
5.2
Air Distribution System
3. No excessive fluctuation room temperature and air motion.
a. Row of diffusers may be located closer to the outside wall to
compensate extra heat load
4. No excessive room air noise
a. Adequate return grilles (suction noise)
b. Low level of CFM supply per diffuser (400 700 CFM)
5. Avoid short circuiting
a. Return grille not located where by-passing of supply air
5.2.1 Air Distribution System - Components
Figure: Schematic Diagram Air Ventilation
5.2.2 Air Distribution System Ducting System
AHU
SA
RA
Figure: Example of Air Ventilation Ducting Connection
5.2.1 Air Distribution System Ducting System
Figure: Examples of Air Ventilation Ducting Connection
5.3.1 Ducting System - Fittings
Figure: Fitting for air ventilation ducting (PanelDuct Product)
5.3
5.3.2
Ducting System Diffusers & Grilles
Diffuser: The supply outlet of SA. Symbol
1. Square (2x2, rooms / halls)
2. Round (nice looking, difficult to install)
3. Slot/linear (hotel ball rooms)
Grille: The return air inlet RA (rectangular, square).
Normally situated in the stagment zones (room corner or end)
Air distribution by square diffuser Air distribution by round diffuser
5.3
5.3.2 Ducting System Diffusers & Grilles
Square Diffuser
5.3.2
5.3 Ducting System Diffusers & Grilles
Round Diffuser Slot/Wall Mounted Diffuser
Swiral Diffuser Grille Return Air Diffuser
5.4
Diffusers Selection & Consideration
The selection of diffuser depend on:
1. Architectural requirement
2. Structural requirement
3. Diffusion temperature difference
4. Size of diffuser / grilles
5.5 Duct Sizes Consideration
1. Round or rectangular ducting size depend on the space
available as example ceiling height.
2. Return air ducts can be sized in the same manner as the supply
air units.
5.5 Duct Sizes Consideration
3. Round ducts has minimal friction compared to rectangular
ducts.
4. If youre choosing rectangular duct, try to be as square as
possible size.
5. The branches from main duct could be flexible round duct for
easy positioning.
Flexible round duct
5.6 Procedures for Design Centralised A/C System
1. Estimate the cooling load (Btu/hr)
2. Determine the supply condition.
3. Choose the type of system.
4. Plan the equipment and duct location.
5. Determine the duct size.
* See an examples provided in class room for the design and calculation stages
5.7 Required Air Quantity in a Room (CFM)
There are two methods to determine the required CFM for
each room.
1. Considering the diffusion temperature:
(/) (/)
= =
1.09 () 1.2 ()
2. Considering the rule of thumb: 22F
(/)
=
12 103
5.7 Required Air Quantity in a Room (CFM)
Diffusion
Temperature
22C
Figure: Diffusion temperature in a room
5.8 Losses in Ducts System
Friction loss from air flow in ducts divided to major and minor loss:
1. Major: Friction loss due to air flow along the ducting.
2. Minor: Friction loss due to air flow through the ducts fitting.
Friction loss of air through the duct could be determined by
using standard chart air flow in galvanized steel round ducts.
However, the chart only limited to:
1. Clean galvanized steel.
2. Air at standard conditions.
3. General HVAC temperatures.
4. Altitude up to 2000 ft.
5.8
Duct
diameter
Air velocity
L or W
Convert round to rectangular duct
5.8 Example
Question 1:
A 12 inch diameter galvanised duct 250 ft. has 1000 CFM of air flowing
through.
What is the pressure loss due to friction and velocity in the duct?
[hf = 0.55 in w.g, Velocity = 1300 FPM]
Question 2:
A 30 x 19 inch rectangular duct deliver 7000 CFM of air. What is the friction / 100
ft.?
[hf /100 ft = 0.17 in w.g]
5.8 Example
Question 3:
Figure below shows a symmetry ducting system from AHU to the diffusers
with equal friction hf = 0.4 in w.g per 100 ft.
What is the pressure loss due to friction and velocity in the duct section AB
and BC by neglecting the fitting loss?
14 9
Answers:
Section CFM Duct L FPM hf /100 hf (in wg)
AB 2000 10 ft 1950 0.4 0.04
BC 1000 3 ft 1480 0.4 0.012
END
