Daylight Factor

Daylight Analysis

Daylight and Shading Devices

Ventilation and Building Design

Types of Ventilation

DAYLIGHTING AND NATURAL VENTILATION

TYPE OF VENTILATION

Natural Ventilation

Air Flow, Bernoulli Effect, Stack Effect,

Ventilation and Building

Mechanical Ventilation

VENTILATION AND BUILDING DESIGN

VENTILATION

VENTILATION

A process of removing or supplying air by natural or mechanical

means to and from a air source or any space

Adequate ventilation is essential to maintain the temperature limits

inside the building and to remove the air, vitiated by the products of

respiration, bacteria and all objectionable or unpleasant odours.

Poor ventilation gives rise to a feeling of discomfort to the

inhabitants because it causes increase in temperature and humidity

which leads to perspiration or sweating. The want of fresh air

produces nausea, headache, sleepiness, laziness and unattentiveness

An unventilated room is subjected to unsuitable living and working

condition because of increased dust amount, excessive content of

carbon dioxide, unsuitable humidity or relatively uncontrollable air

movements present.

NECESSITY OF VENTILATION

To prevent an undue concentration of body odours, fumes dust and other industrial products

To prevent an undue concentration of bacteria carrying particles

To remove products of combustion, and in some cases to remove body heat and the heat liberated by the operation of electrical and mechanical equipment

To create air movement so as to remove the vitiated air or its replacement by the fresh air

To create healthy living conditions by preventing the undue accumulation of carbon dioxide and moisture and depletion of the oxygen content of the air. For comfortable working conditions the content of carbon dioxide should be limited to about .6% volume

To maintain conditions suitable to the contents of the space

To prevent flammable concentration of gas vapour or dust in case industrial buildings

FUNCTIONAL REQUIREMENTS OF A

VENTILATION SYSTEM1. Rate of supply of fresh air

2. Air movements or air changes

3. Temperature of air

4. Humidity

5. Purity of air

GENERAL CONSIDERATIONS AND RULES FOR NATURAL VENTILATION

1. Inlet openings in the buildings should be well distributed and should be located on the windward side at a low level. The outlet openings should be located on the leeward side near the ceiling in the side walls and in the roofs

2. Inlet and outlet openings should be equal size for greatest air flow, but when outlet is in the form of a roof opening the inlet should be larger in size.

GENERAL CONSIDERATIONS AND RULES FOR NATURAL VENTILATION3. Where the wind direction is variable, openings should be provided in

all walls with suitable means of closing them

4. Inlet openings should not be obstructed by adjoining buildings, trees,

signboards, partitions or other obstruction in the path of air flow

5. Increased height of the room gives better ventilation due to stack

effect

6. The long narrow rooms should be ventilated by providing suitable

openings in short sides

7. The rate of air-change in a room mainly depends on the design of

opening location of inlet and outlet and the difference in temperature

between the inside and outside air. The cooler air enters from the

bottom and after becoming hot during its stay in the room, it leaves

from the top.

GENERAL CONSIDERATIONS AND RULES FOR NATURAL VENTILATION8. The efficiency of roof ventilation depends on their location, wind

direction and the height of the building.

9. The ventilation through windows can be improved by using them in

combination with a radiator, deflector and exhaust duct.

10. For cross-ventilation, the position of outlets should be just opposite to

inlets. The openings over the doors of back walls create good

conditions for cross ventilation

11. Windows of living rooms should either open directly to an open space

or open space created in buildings by providing adequate courtyards

12. If the room is to be used for burning gas or fuel, enough quantity of air

should be supplied by natural ventilation for meeting the demands of

burning as well as ventilation of the room

Natural Ventilation

Air Flow, Bernoulli Effect, Stack

Effect, Ventilation and Building

Mechanical Ventilation

TYPES OF VENTILATION

Air Flow

Bernoulli Effect

Stack Effect

Ventilation and Building

NATURAL VENTILATION

AIR FLOW

Types of Air Flow – Laminar, Separated, Turbulent and eddy currents

Air flow changes from laminar to turbulent when it encounters sharp

obstructions such as buildings

Eddy currents are circular air flows induced by laminar air flows

Inertia – Since air has some mass moving air tends to go in straight

line. When forced to change direction, air streams will follow curves

but never right angles

Conservation of Air – Since air is neither created nor destroyed at

the building site, the air approaching a building must equal the air

leaving the building. Air streams should be continuous

To design successfully for ventilation in the summer or for wind protection in the winter, the following

principles of air flow should be understood

Air flows either because of natural convection currents, caused by differences in temperature, or

because of differences in pressure.

High and low pressure area –As air

hits the windward side of a building it

compresses and creates positive pressure

(+). Air sucked away from the leeward side

at the same time is negative pressure (-).

Air deflected around the sides will also

create negative pressure.

Pressure – not uniformly distributed

The type of pressure created over the roof

depends on the slope of the roof.

These pressure areas around the building

determine how air flows through the

building.

High and low pressure areas are not

necessarily places of calm but also of air

flow in the form of turbulence and eddy

currents.

These currents reverse the air flow in

certain locations.

BERNOULLI EFFECT

An increase in the velocity of a fluid decreases its static

pressure. Because of this phenomenon, there is negative

pressure at the construction of a venturi tube.

A gabled roof is like half a venturi tube. Air will be sucked out

of any opening near the ridge.

The effect can be made even stronger by designing the roof to

be like a full venturi tube.

The velocity of air increases rapidly with height above ground.

The pressure at the ridge of a roof will be lower than that of

windows at ground level. Consequently even without the help

of the geometry of a venturi tube, the Bernoulli effect will

exhaust air through roof openings.

STACK EFFECTThe stack effect can exhaust air from a building by the action of

natural convection.

The stack effect will exhaust air only if the indoor temperature

difference between two vertical openings is greater than the

outdoor-temperature difference between the same two opening.

To maximize the weak effect, then openings should be as large as

far apart vertically as possible.

The air should be able to flow freely from the lower to the higher

opening.

The shape of the roof and the increased wind velocity at the

roof can all combine to ventilate a building naturally.

Roof monitors and ventilators high on the roof are especially

helpful because of stratification, the hot test indoor air is

exhausted first.

SOLAR CHIMNEY

Stack effect is a function of temperature differences heating

the indoor air increases the air flow.

The solar chimney heats the air after it leaves the buildings

The stack effect is increased but without additional heating of

the building.

STACK EFFECT

The stack effect causes the lower part of a building

with an atrium to have a negative pressure and the

upper part to have a positive pressure.

In between will be the neutral axis.

Hot air from the lower stories enters the upper

floors.

To avoid this problem, the neutral axis must be raised

above the top floor.

ADVANTAGE

The stack effect over the Bernoulli effect is that it does

not depend on wind.

DISADVANTAGE

It is a very weak force and cannot move air quickly.

VENTILATION AND BUILDING LAYOUT FACTORS DETERMINING THE PATTERN OF AIR FLOW

THROUGH A BUILDING Pressure distribution around the buildings

Direction of air entering windows

Size, location and details of windows

Interior Partition details

SITE CONDITION Adjacent buildings, walls and vegetation on the site will greatly affect the air flow

through a building.

WINDOW ORIENTATION AND WIND DIRECTION Winds exert maximum pressure when they are perpendicular to a surface and

the pressure is reduced about 50 percent when the wind is at an oblique angle of

about 45o.

The indoor ventilation is better with the oblique winds because they generate

greater turbulence indoors.

VENTILATION AND BUILDING LAYOUT

WINDOW ORIENTATION AND WIND DIRECTION The need for summer shade and winter

sun calls for a building, orientation with the long axis in the east-west direction

The range of wind directions that works well with that orientation.

When winds are east-west the solar orientation usually has priority because winds can be rerouted more easily than the sun

WINDOW LOCATION Cross-ventilation is so effective because air is both pushed and pulled through

the building by a positive pressure on the windward side and by a negative

pressure on the leeward side.

Ventilation from windows on adjacent walls can be either good or bad

depending on the pressure distribution which varies with wind direction.

VENTILATION AND BUILDING LAYOUT WINDOW LOCATION

Ventilation from windows on adjacent walls can be either good or bad depending on

the pressure distribution which varies with wind direction.

Some ventilation is possible in the asymmetric placement of windows because the

relative pressure is greater at the center than at the sides of the windward walls.

FIN WALLS Fin walls work best for winds at 45o to the window wall. Casement windows can

act as fin walls at no extra cost.

Fin walls can increase the

ventilation through windows on the

same side of a building by changing

the pressure.

VENTILATION AND BUILDING LAYOUT FIN WALLS

Ventilation from windows on adjacent

walls can be either good or bad

depending on the pressure

distribution which varies with wind

direction.

A fin wall can be used to direct the airstream through the center of the room