VENTILATION

QSB 1714 Prepared by: Zhermeica Lim

INTRODUCTION Definition:

The process of changing air in an enclosed space.

A proportion of air within the enclosed space should be continuously withdrawn and replaced by fresh air.

Drawn in from a clean external source:

At as high an elevation as practical…where more polluted air occurs at the relatively low levels found in congested towns and cities.

Why do we need ventilation?

Ventilation is needed to maintain air purity…

which is......

1. Preservation of oxygen content.

This should be maintained at approximately 21% of air volume.

2. Removal of carbon dioxide

3. Control of humidity

30 – 70% relative humidity (RH) is acceptable for human comfort.

4. Prevention of heat concentrations from machinery, lighting & people.

5. Prevention of condensation

6. Dispersal of concentrations of bacteria

7. Dilution & disposal of contaminants

Smoke, dust, gases & body odours

8. Provision of freshness

An optimum air velocity lies between 0.15 & 0.5

m/s.

Natural ……… mechanical

It can be natural or mechanical ventilation.

Natural ventilation

affected by

pressure variations due to

wind & stack effects.

Wind

The movement of air takes place smoothly well away from the earth’s surface.

Velocity is reduced towards the ground, direction of movement is affected by relief of hills and mountains and vegetation resulting in turbulence of airflow.

Admiral Beaufort devised a scale in 1806 primarily for classification of winds at sea.

Typical Pattern of Pressure Distribution

In high buildings above ten floors, wind and

stack effect become apparent.

Stack Effect

Natural movement of air within a tall building

caused by temperature difference between

outside and inside of building.

o Becomes stronger as the building gets

taller and the temperature difference

become greater.

Stratification location: when light heated air

flows upward and reaches a point where it is

same temperature/weight as surrounding air.

The key requirement for ventilation is the

need to maintain adequate indoor air

quality.

An efficient ventilation system must meet

the following requirements:

1. Establish hazard-free air quality

2. Satisfy user comfort

3. Be energy efficient

4. Be cost effective

The air temperature and relative humidity

are major factors for maintaining comfort

and good health.

Extreme conditions can cause discomfort,

promote the spread of microorganisms

which cause disease and can seriously

irritate the respiratory systems.

Ventilation Requirement

Control of ventilation rates is influenced

by various authorities & codes of practice,

some statutory & other recommendations.

Recommended air change rate

Source : The society of Heating, Air Conditioning and Sanitary Engineers of Japan

The fresh air supply to a room can be calculated as :

Q = n V

where

Q = fresh air supply (ft3/h, m3/h)

n = air change rate (1/h)

V = volume of room (ft3, m3)

Q= nV

Example –

Fresh air supply to a public library with volume 1000m3

can be calculated as

Q = nV

Q=6x1000

=6000m3/h

Air change

rate

NATURAL VS MECHANICAL

Natural Ventilation Mechanical

Ventilation

Natural Ventilation

Depend on either:

Wind direction & pressure,

or

The stack effect of warm

air rising within a building,

while cooler air exists

outside.

Natural Ventilation

The system involves the provision of

background ventilation for:

periods of low occupancy, rapid ventilation for

temperature and air pollution during

occupancy, and the removal of local air pollution at source rather than allowing it to

escape to the general atmosphere.

e.g. cooking area or bathroom.

Cooking area Bathroom

Natural Ventilation

uncontrolled windward and

outward leakage through

cracks and interstices (i.e.

infiltration and ex-filtration).

The air entering and leaving the

enclosure through openings

provided intentionally e.g. open

windows and doors, and vents.

Infiltration and ex-filtration are

caused by the weather and

other pressure-difference forces

exerted on a building.

Infiltration Air is the unintentional and uncontrolled entry of outdoor air into an enclosed space. Infiltration occurs through cracks in the building envelope and due to pressure differences between inside and outside. The outdoor air entering through open doors and windows is considered infiltration although the purpose of opening the door or window might be ventilation.

Infiltration occurs mainly in winter when the air outside is colder and heavier than the air inside. It depends on wind velocity, wind direction and the air-tightness of the building envelope. In the case of high-rise buildings the stack effect also causes infiltration

Exfiltration Air refers to the flow of indoor air from an

enclosed building space to the outdoors.

Commercial air-conditioned buildings are designed

to be air-tight (the windows cannot be opened)

and pressurized.

In summer the air inside is colder (air-conditioned)

and therefore denser (heavier) than the hotter air

outside. The natural air flow direction is therefore

from inside to outside. Since commercial buildings

are pressurized, the air flow leakage is from the

inside to the outside.

Traditional building materials

& traditional design are

suitable for the hot humid

climate in Malaysia.

Due to competition for land and high demand for houses, manufactured houses are built from a new breed of building materials where orientation and location followed the bare minimum requirements of the by-laws.

For high rise buildings, increase in use

of steel structures and glass render a

different set of problems e.g. glare,

green house effect etc.

For natural ventilation, stack effect and wind effects are dominant.

Need to utilize windows, ventilators and air spaces especially for night cooling.

Windows should be narrow and keep (skinny) with controllable openings at different levels. Most houses are provided with side ventilation and the air-well is often renovated such that it is deemed useless for improving ventilation.

Mechanical Ventilation

Ventilation provided by electric fans.

These fans are often part of an air-

conditioning system and may drive the

supply airflow, or both the supply and

exhaust airflows.

Mechanical ventilation or air-conditioning system is suitable for

1. Internal rooms

2. Large closely populated rooms where distribution of natural ventilation is inadequate

3. Rooms where volume per occupant is too low for efficient natural ventilation.

4. Controlled environment e.g. clean room.

5. Windows cannot be opened either too noisy or too dusty

6. Tall buildings for stack effect and smoke control

7. Extract ventilation e.g. fumes from processes.

In designing for high standards of air quality,

attention needs to be paid to a wide range

of building features.

Ventilation Standards

ASHRAE (American Society of Heating,

Refrigeration and Air-Conditioning

Engineers) recommends for natural

ventilation, the air movement of 0.1 – 0.3

m/s (1.5 – 2.5 ach) for domestic, 1.5 – 2.5

ach for office, with air conditioner >3 ach.

Control of Air Quality and Ventilation

1. Measurement

It is very expensive to measure ventilation

rates to see whether specified standards

(refer to UBBL 1984) have been achieved.

One method is by using concentration

decay of a tracer gas e.g. SF6, NO2.

Rate of decay (in concentration versus

time) gives the air change per hour.

Age of air can be used to determine

ventilation effectiveness of a site.

Another method uses a manometer where

it involves pressurizing the interior of a

building , with all windows and doors shut.

A panel containing a fan and a measuring

equipment is sealed into the open front

door and a pressure of 55 Pa is maintained.

The procedure is repeated with negative

pressure. This method measures air

tightness.

2. Air Movement

Some degree of air movement is essential for feeling of freshness and comfort.

Desirable speeds vary with temperature and conditions. In domestic buildings, a velocity of 0.1 – 0.33 m/s is reasonable.

In air-conditioned buildings, a velocity of 1 m/s is the norm except for clean room or other localized extraction.

3. Fumes, smells, product of combustion

Where fumes are likely to be offensive or

injurious they should be removed by

extraction at source.

Hoods or extractors should be placed to

capture fumes as soon as they are

emitted. Fume cupboards provide source

of fume to be enclosed.

4. Bacteria

Special precautions may be required in hospitals or other places where bacterial concentrations could be critical. Precautions include no re-circulation of air, displacement ventilation and high rates of air change.

Patterns of air movement are such that bacteria are swept away from areas they may do harm, most critical at operating theatres.

5. Excess Heat

Excess heat can be removed by

ventilation at sources (1 m3 can convey

1.3 kJ for each o C of temperature)

difference between the heated control

and the outside air.

6. Relative Humidity

Relative humidity of 30 – 70% is

acceptable. Increase ventilation rate can

reduce relative humidity; caution may get

too draughty.

Sick building syndrome

Complaints of acute discomfort in office

building especially those equipped with air

conditioning.

Ailments such as headache, eye ache, sore -

throat, breathing difficulty, cold – like

symptoms were real and clinically proven.

When 20% of occupants or more complain of

these ailments then sick building syndrome

exists.

Potential causes include: thermal, aural, visual, cleanliness, hygienic and

medical considerations. Common places include swimming pool and recently refurbished building.

Discrepancy between designed performance of heating and air conditioning systems and actual operations.

Maintenance and regular housekeeping is important to ensure effectiveness of a system.

Sick building complaints have been reduced by a rigorous process of cleaning.

There is a need for minimum standards of installation, commissioning and operation of environmental systems to be established and incorporated into building contracts and legislation.

Flow caused by wind

Major factors affecting ventilation wind forces include:

average wind speed;

prevailing wind direction;

seasonal and daily variation in wind speed and direction;

local obstructing objects, such as nearby buildings and trees;

position and characteristics of openings through which air flows; and

distribution of surface pressure coefficients for the wind.

Natural ventilation systems are often designed for wind speeds of half the average seasonal velocity because from climatic analysis there are very few places where wind speed falls below half the average velocity for many hours in a year.

TUTORIAL TASK:

BY REFER TO UBBL 1984, STUDENTS ARE REQUIRED TO

HIGHLIGHT THE VENTILATION REQUIREMENTS IN A

BUILDING. (CLAUSES RELATED SHOULD BE STATED)

THANK YOU