CB503VENTILATION & AIR CONDITIONING 3

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

NAZRIZAM BINTI AB. WAHABpnnazz@gmail.com

017-612 5556

PSA/ CB503/ PNNAZZ

VENTILATION

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.

VENTILATION RATE

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 occupants2. 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 62.1-2007Refer 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 0.782A = 2.242 m2.

Note: The upper floor has less stack 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

3600Q =

(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 0.391A = 8.97 m2.

FAN

TYPES OF FANS

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

1. Propeller2. Axial flow3. Centrifugal4. 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.

2. 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 Fans1. 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.

VENTILATION DESIGN

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 fan6. 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 directioniv. 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 DESIGN1. Not noisy2. Concealed3. No draughts4. Efficient fan5. 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

EَكGِد Iْم EَحGِب Eَو Nَّم PُهNالَّل EَكEاَن EَحIْب Pُس EَتIَنE َأ NَّالGِإ EَهEلG ِإ Eَّال IْنEَأ Pِد Eُه IْشEَأ

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