CB503 VENTILATION AIR FLOW DESIGN

CB503
VENTILATION & AIR CONDITIONING 3
TOPIC 2 : AIR FLOW DESIGN
(VENTILATION RATE & FAN)

NAZRIZAM BINTI AB. WAHAB
pnnazz@gmail.com
017-612 5556

PURPOSES OF VENTILATION

The purposes of ventilation are:

1. To provide a continuous supply of oxygen necessary for
human existence.

2. To remove the products of respiration and occupation,
that is; heat, moisture and carbon dioxide from people.

• At rest a normal adult inhales between 0.10 and 0.12
litre/s of air.
• The exhaled breath contains between 3% and 4% of
carbon dioxide, which is equal to 0.003 to 0.005 litre/s.
• The amount of heat from occupants is about 100 Watts
sensible and 40 watts latent heat from a sedentary
worker.
• The amount of moisture produced by a sedentary person
is about 59g of water vapour per hour.

3. To remove contaminants such as:

• Water vapour
• Heat and smells from cooking
• Gases and vapours from industrial processes.
• Formaldehyde from; insulation foam, furnishings,
wallpaper, carpets, resin in wood products and
plasterboard.
• Outdoor aerosol pollutants such as; smoke, soot, mist,
fumes, pollen, plant fibres, mould spores, viruses and
bacteria
• Indoor aerosol pollutants such as; carpet fibres,
furniture fibres, clothing fibres, skin flakes, mites,
viruses and bacteria.

NATURAL VENTILATION

A habitable room requires one or more ventilation openings,
the total area of which must not be less than 1/20 th. of
the floor area of the room, and some part of the opening
must be more than 1.7 metres above floor level.

When ventilation is by mechanical means, one air change per
hour (1 ACH) must be provided to habitable rooms and three
air changes per hour (3 ACH) to bathrooms and kitchens.

Design Criteria

To design a ventilation system, the engineer has to meet two
basic requirements:

1. To supply fresh air for the occupants
2. To change the air in the room sufficiently so that smells,
fumes and contaminants are removed.

VENTILATION RATES IN CIBSE GUIDE

The following table gives Ventilation Rates for buildings.
Table 3.1 CIBSE Guide B2 (2001) Summary of
recommendations (Refer http://www.bsenotes.com)

The following table from ASHRAE Standard 62.1-2007
Refer http://www.hei-ohio.com/minimum%20ventilation%20rates.pdf

The Table below gives Ventilation rates required to limit
CO2 concentration where level of activity is known.

Table 3.2 CIBSE Guide B2 (2001) Ventilation rates
required to limit CO2 concentration for differing activity
levels.

The following table gives fresh air rates.

Table 3.3 CIBSE Guide B2 (2001) Recommended outdoor air
supply rates for sedentary occupants.

The table below is an extract from Table 3.6 and gives rates
for Assembly Halls and Auditoria

VENTILATION CALCULATIONS

The following formulae may be used:

1. For General Mechanical Ventilation

Ventilation rate (m3/h) =

Air Change Rate (/h) x Room Volume (m3)

Air Change Rate (/h) comes from CIBSE Guide B2 Table 3.1

Ventilation rate (m3/s) = Ventilation rate (m3/h)
3600

2. For Calculating Fresh Air Ventilation Rates

Fresh Air Rate (m3/s) =

Fresh Air rate per person (l/s/p) x
number of
occupants

Fresh Air rate per person (l/s/p) comes from CIBSE Guide
B2 Table 3.3.

NATURAL VENTILATION SYSTEMS

• Natural ventilation is ventilation without the
assistance of fans or other mechanical air moving
equipment.
• Natural ventilation uses no energy or little energy
therefore reduces building running costs.
• Air moves naturally due to the buoyancy effect when a
temperature difference exists and less dense air
rises.
• This is called the stack effect.
• Air also moves unassisted by wind.
• These effects can be utilised in a building to create a
‘free’ ventilation system that requires no fans.
• Some systems incorporate fans and are partially
natural ventilation but with a greater degree of
control.

NATURAL VENTILATION SYSTEMS IN USE

• In large buildings where large amounts of air need to be changed in
rooms, natural ventilation can be used if special features are used.
• Also is commercial building the ventilation system should be
controlled to meet comfort criteria in rooms.
• This can be achieved by careful building design and using
technology to provide adequate ventilation.
• The diagram below shows a building layout that uses the stack
effect to increase natural ventilation.

Exhaust outlets at roof
level can be disguised or
used as a feature of
building design as shown
below.

Low level vents are shown below

Another natural ventilation system uses
the sun to assist air movement.

The vertical shafts in the building are
glass fronted so that the sun heats up the
air inside and causes it to rise out the
openings at roof level.

The high level openings in this case are
stainless steel chimneys.

As air flows out of the chimneys at roof
level replacement air is drawn from the
rooms into the shaft and thus naturally
ventilated.

EFFECTIVENESS

• The effectiveness of natural ventilation for commercial
buildings depends on several criteria.

• These are wind strength and direction, size of openings, air
temperatures and height of building.

• For effective controlled ventilation the designer should not
rely solely on the wind but more on the stack effect and air
controls.

• Dampers can be used to control air entering and/or exiting a
natural ventilation system.

• These dampers could be linked to occupancy sensors,
temperature sensors, time switches and other weather
sensors to give automatic control of ventilation which is the
key to a useful system.

• The diagram below shows some of the features of a Natural
Ventilation system for a four-storey building.

DESIGNING NATURAL VENTILATION SYSTEMS

CIBSE guide Applications Manual AM10 (1997) Natural
Ventilation in non-domestic buildings gives design details.

Section 4.2 of AM10 gives details of how to control
Natural ventilation systems.

Section 5 gives methods of calculating flow rates for
wind driven and stack effect ventilation.

STACK DRIVEN VENTILATION CALCULATIONS

Stack ventilation calculations in the simplest form ignore wind
effects, although these can be allowed for in a more complex
analysis. The pressures developed in stack systems can be
determined from the following formula:

The equation below can be used to determine air flow rates in
stack driven ventilation or the opening areas required.

NEUTRAL PRESSURE LEVEL

This is where the outside pressure equals the internal
pressure.

At this level there would be no flow of air in or out of the
building.

This is usually high up in a building otherwise the stack effect
would not work.

The neutral pressure level for most buildings is about 0.25
metres above the level of the top floor ceiling.

TEMPERATURES

• The internal room temperatures need to be calculated since in
summer heat gains elevate the room temperature.
• This can be done using software where summertime temperature
can be predicted along with required air flow rates to keep room
temperatures to acceptable levels.
• The HEVACOMP software package and other programmes may be
used.
• Outside summer temperatures may be obtained from the CIBSE
guide A section 2.
• It would seem that the outdoor temperature in summer rarely
exceeds 27oC, and if the temperature does rise above 27oC it is
only for a maximum of 4 days in the south of England and less
than one day in the north of the U.K.
• If a solar chimney is used to assist stack suction pressure then
the temperature inside the stack would have to be altered.
• It is important to obtain accurate inside and outside
temperatures since this difference creates the driving force
inside the stack or the pressure difference to move air up the
stack to outside.

EXAMPLE 1

Calculate the ventilation opening area required in a Stack
ventilation system for the building shown below. The flow rate
required each room is 4 air changes per hour. Each lecture room
measures internally 24 m x 10 m x 4m high.

ANSWER:

Air flow rate for each room, Q = Room volume x Air change rate
3600
= (24 x 10 x 4) x 4
3600
= 1.07 m3/s

Rearranging above formula for Area (A) gives;

For Ground floor room;
A = 1.07/0.61 [(2/1.1656) 1.1656 x 9.81 (9 – 1) (301 – 298/ 301)
A = 1.07/0.61 [ 1.716 x 1.1656 x 9.81 x 8 x 0.00997]
A = 1.07/0.61 [ 0.19557 x 8]
A = 1.121 m2.

For First floor room;
A = 1.07/ 0.61 [(2 /1.1656) 1.1656 x 9.81 (9 – 5) (301 – 298/301)
A = 1.07/ 0.61 [ 0.19557 x 4 ]
A = 1.07/ 0.61 x 0.782 Note: The upper floor has less stack
A = 2.242 m2. suction pressure so openings are larger.

EXAMPLE 2

Calculate the ventilation opening area required and the size of fresh
air louvre required in a Stack ventilation system for the building
shown below. The flow rate required for the Class room is 10 air
changes per hour. The Class room measures internally 18m x 10m x 4m
high. The Fresh air louvre has a 50% free area.

ANSWER:

Air flow rate for each room, Q = Room volume x Air change rate
3600
Q = (18 x 10 x 4) x 10
3600
= 2.0 m3/s

Rearranging above formula for Area (A) gives;

A = 2.0/0.61 [ (2 / 1.1605) 1.1605 x 9.81 (8 – 1) (299 – 296/299)
A = 2.0/0.61 [ 1.723 x 1.1605 x 9.81 x 7 x 0.01003]
A = 2.0/0.61 [ 1.377]
A = 2.38 m2 fresh air area required

The fresh air louvre has a 50% free area so the size of louvre is;
Louvre area = fresh air area / (percent free area /100 )
Louvre area = 2.38 / (50/100)
Louvre area = 4.76 m2.

STACK OUTLET

• The opening at the top of a stack can be sized in a similar manner
to the fresh air inlets.
• The height difference in the formula is between the NPL and the
stack outlet.
• The flow through the stack outlet is the sum of all the flows
through the rooms in a building feeding the stack.

EXAMPLE 3

Calculate the stack outlet opening area required in the system given
in Example 1. The flow rate required each room is 4 air changes per
hour.cEach lecture room measures internally 24 m x 10 m x 4m high.

Air flow rate for each room, Q = 1.07 m3/s
(already calculated in Ex. 1)

Total air flow rate, Q total = 1.07 x 2
= 2.14 m3/s

A = 2.14 /0.61 [ (2/1.1656)1.1656 x 9.81 (11 – 9) (301 – 298/301)]
A = 2.14 / 0.61 [ 1.716 x 1.1656 x 9.81 x 2 x 0.00997 ]
A = 2.14 / 0.61 [ 0.19557 x 2 ]
A = 2.14 / 0.61 x 0.391
A = 8.97 m2.

TYPES OF FANS

There are several types
of fan to choose from in
ventilation. These are:

1. Propeller
2. Axial flow
3. Centrifugal
4. Mixed flow

1. Propeller Fan
• Used in situations where there is minimal resistance to air flow.
• Typical outputs are; up to 4 m3/s and up to 250 Pa pressure.
• Fan efficiency is low at about 40%.
• Suitable for wall, window and roof fans where the intake and
discharge are free from obstacles.
• Can move large volumes of air.
• Low installation cost.

1. Axial Flow Fan
• High volume flow rate is possible with this type of fan with
high efficiency, about 60% to 65%.
• Typical outputs are; up to 20 m3/s and up to 700 Pa pressure.
• The fan is cased in a simple enclosure with the motor housed
internally or externally.
• Aerofoil blades can be used to increase efficiency.
• Adjustable pitch blades can be used for greater flexibility.
• Ductwork can be simply connected to the flange at either end
of the fan.

3. Centrifugal Fan
• High pressure air flow is
possible with this type of
fan.
• Used in air handling units
and other situations to
overcome high resistance
to air flow.
• The impeller is made of
thin blades which are
either forward or
backward curved.
• The air changes direction
by 90 degrees in a
centrifugal fan so more
space is required.
• Usually the motor is
placed external to the
casing and a vee belt and
pulley drive is commonly
used.

Centrifugal Blades

• Centrifugal curved fan blades generally have higher efficiencies
than if a plain flat blade is used.
• The efficiency of a fan with forward curved blades is about
50% - 60%.
• The forward curve has a scoop effect on the air thus a higher
volume may be handled.
• Backward curved blades offer even better efficiency, 70% to
75%.
• This improves airflow through the blade and reduces shock and
eddy losses.

• High pressures can be developed with backward curved blades.
• Even further improvements may be made by using an aerofoil
section blade in which case the efficiency may be 80% to 85%.
• Another feature of backward curved blades is their non-
overloading characteristic.
• A disadvantage is the high blade tip speed, necessary to obtain a
comparable rate of discharge to forward curved blades, makes
the fan noisy.

4. Mixed Flow Fan
• Mixed Flow fans can be used for return air, supply, or
general ventilation applications where low sound is critical.
• As compared to similarly sized axial fans, a mixed flow fan
can be 5-20 dB quieter.

CHARACTERISTICS OF AXIAL FLOW AND CENTRIFUGAL FANS

Axial Flow Fans
1. Axial flow and backward-curved centrifugal fans have similar
characteristics as shown below.

2. The axial flow fan is very convenient from an installation point of
view, it can be directly duct mounted even in restrictive areas but
they tend to be noisy. This is because they run at a higher speed
compared to a centrifugal fan.
3. Like the Backward-bladed centrifugal fan, the Axial flow fan has a
self-limiting power curve as shown above.

Centrifugal Fans

4. The backward curved centrifugal fan runs at a higher speed than
the forward curved fan for the same output.

5. A forward-curved centrifugal fan may be liable to overloading
because the power rises as the volume increases. An example of
this in practice is if the main dampers are left wide open when
the fan is first started up, too much air will be handled and the
excessive power absorbed will overload the driving motor.

6. The backward–curved fan is less
liable to over-loading than the
forward-curved centrifugal
fan and it is also able to deliver
a relatively constant amount of
air as the system resistance
varies. The power of a backward
curved fan reaches a peak and then
begins to fall, this is called the
self-limiting characteristic. This is
shown beside.

7. A backward-curved centrifugal fan must run at higher speed to
deliver the same amount of air as a forward-curved fan
because of the shape of the impeller blades and the direction
of rotation.
8. The backward-bladed fan is used in high velocity systems
where high pressures are required and is often made with
aerofoil blades to increase efficiency.
9. Up to about 750 N/m2 fan pressure, the forward-curved
centrifugal fan tends to be quieter and cheaper. Above this
value of pressure backward-curved fans take over.

CHOOSING A FAN

• To choose a suitable fan one must
look at the performance curves.

• Performance curves are found in
fan catalogues.

• These curves show the pressure
developed by a fan at a given flow
rate.

• The pressure to be developed by
the fan is found from duct sizing
data and the flow rate is found
from design data.

• The operating point of the system
is marked as a point on the curve.

EXAMPLE 1

The example below shows a system operating point of 250 Pascals
(Pa) pressure and 0.48 (m3/s) flow rate.

Go to the curve above the operating point, this is the fan curve for
the appropriate fan.The fan size is chosen as a 250mm-dia. fan
(1350 r.p.m. speed)

EXAMPLE 2

The example below shows a system operating point of 320 Pascals (Pa)
pressure and 1.25 (m3/s) flow rate.

The fan performance curve for a 400mm-dia. fan will be suitable for
the requirements for this example since the curve is above the
operating point. The fan size is chosen as a 400mm-dia. fan (1350
r.p.m. speed).

EXAMPLE 3

An axial flow fan is required for a ventilation system for a Workshop. Four fans are
represented below in the four curves – 2 green and 2 red curves. The left-hand
diagram shows fans with 4-pole electric motors, and the right hand diagram shows
fans with 2-pole electric motors. Four pole electric motors are slower than two pole
motors, in this example 4-pole is at 1420 r.p.m. and 2-pole is at 2840 r.p.m. The
system operating point requirements are 100 Pascals (Pa) pressure and 0.60 (m3/s)
flow rate.

The fan size is chosen as a 350mm-diameter fan (1420
r.p.m. speed).
The electric motor for this fan has a 4-pole winding
and will run at 1420 r.p.m. which will be slower than a 2-
pole motor and therefore quieter.

DESIGN METHODOLOGY

When considering ventilation design the following approach could be adopted
before sizing begins and the following questions should be considered:

1. What areas need ventilation? The contaminants should be listed for
these areas.
2. What type of system should be used, supply, extract or balanced?
3. Are there any alternative systems to consider?
4. Is air conditioning necessary in the building? If air conditioning is
necessary then should it be incorporated into the ventilation system?
5. Where should the fan(s) and plant be installed?
6. What type of fan(s) and plant should be used?
7. Is a separate heating system necessary?
8. What type of control system should be used?
9. What type of air distribution system should be used, upward or
downward?
10. Have I considered what will happen in the event of a fire in the building?
11. Have I considered the noise from fans?

After all the above questions have been answered the sizing process may
commence.

SIZING

The sizing procedure is as follows:
1. Calculate Ventilation rates.
2. Decide on number of fans and grilles/diffusers.
3. Draw scale layout drawing:
• Position fan(s).
• Lay out ductwork.
• Lay out grilles and diffusers.
• Indicate flow rates on drawing.
4. Size ductwork
5. Size fan
6. Size grilles and diffusers.

DESIGN CRITERIA

To design a ventilation system, the engineer has to meet two basic
requirements:
1. To change the air in the room sufficiently so that smells, fumes
and contaminants are removed. (See Table 3.1 CIBSE GUIDE B2)
2. To supply fresh air for the occupants. (See Table 3.3 CIBSE
GUIDE B2)

1. Ventilation Rates
• Table 3.1 CIBSE Guide B2 (2001) Summary of
recommendations Ventilation Rates for buildings.
• Table 3.3 CIBSE Guide B2 (2001) Recommended outdoor
air supply rates for sedentary occupants (fresh air rates)
• Table 3.6 CIBSE Guide B2 (2001) and gives rates for
Assembly Halls and Auditoria

2. Number of Fans and Grilles
• Several fans are often better than one since its makes the
ventilation system more flexible.
• Also the air to be supplied or removed may be in different
areas of a room or building where individual fans can be
more effective.
• The number of grilles or diffusers may depend on the
ceiling layout, lighting layout and amount to air to be
transferred.
• Sometimes it is necessary to complete a preliminary grille
size to decide on the final number in a room.

3. Drawings
• Accurate, scaled plan drawings are necessary for installation,
fabrication, estimating and commissioning a ventilation scheme.
• Drawings should show:
i. Flow rates of air.
ii. Ductwork to scale with sizes indicated.
iii. Air flow direction
iv. Items of plant
• Other details such as; builder’s work, support details, fan
specification, grille and diffuser details, louvre details, plant
details, insulation, ductwork specification may be given on a
drawing or in a specification document.

4. Size duct work
• Will be discussed in next lecture notes.

5. Size fan
• Shown in Example 1, 3 & 3.

6. Size Grilles and Diffusers.
• Will be discussed in next lecture notes.

GOOD VENTILATION DESIGN
1. Not noisy
2. Concealed
3. No draughts
4. Efficient fan
5. Good control of air flow with dampers and appropriate diffusers.
6. Good control of room temperature.
7. Appropriate duct sizes.
8. Well supported ducts and equipment.
9. Prevent spread of smoke in the event of a fire with smoke/fire
dampers.
10. Ensure that supply air is clean by using a filter.
11. Ensure that vermin cannot enter the duct system by using a
bird/insect screen in the fresh air intake.
12. Minimise risk of infection in some buildings (e.g. hospital) by having
no recirculation duct.
13. Use recirculation duct in some buildings to save energy.
14. Use appropriate air change rates to meet room requirements.
15. Use appropriate fresh air rate to meet room occupants’
requirements.
16. Use suitable system to fit in with building aesthetics.
17. Avoid duct leaks by using proper jointing method.

TASBIH KIFARAH

(Maha Suci Engkau Ya Allah dan Segala Puji
BagiMu, aku bersaksi bahawa tiada Tuhan
melainkan Engkau, aku memohon keampunan dan
taubat daripada Engkau)

CB503 VENTILATION AIR FLOW DESIGN

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