VENTILATION AND AIR-MOVEMENT

CONTENTS

Function of ventilation Provision for ventilation – Stack effect Provision for air-movement – Wind effect Air-flow through buildings

Building orientation External features Cross-ventilation Position of openings Size of openings Control of openings

Air movement and rain Air-flow around buildings Air flow and humidity control

FUNCTION OF VENTILATION

Supply of fresh air Removal of internal heat – convective cooling Heat dissipation from skin – physiological

cooling

PROVISION OF VENTILATION – STACK EFFECT

Natural air flow is caused by pressure difference: it will flow from a zone of high pressure towards a zone of low pressure.

Pressure differences may be due to two effects:

Stack effect: Occurs when the air inside a vertical stack

is warmer than the outside air (provided that there are both inlet and outlet openings). The warmer air will rise and will be replaced at the bottom of the stack by cooler outside air.

Ventilating shafts are often used for internal bathrooms or toilets, which are quite successful in a cool climate.

VENTILATION DUCT ARRANGEMENTS

PROVISION OF VENTILATION – STACK EFFECT

PROVISION OF VENTILATION – STACK EFFECT

Stack effect can also occur within a room of significant height, if it has both a high level outlet and a low level inlet.

The air flow will be proportional to the height difference between inlet and outlet openings and to the temperature difference between the air within the stack (or room air) and the outdoor air.

In low-rise buildings such stack effects are quite small, but – for example – in the staircase of a multi-storey building it can develop into a strong air-current.

In warm climates the outdoor air may be just as warm as the stack air, so there will be no air flow, or if the stack air is cooler, it can produce a down-draft.

A special case that could be considered as an ‘enhanced stack effect’ is the solar chimney, where at least one side of the stack is exposed to solar radiation and has a high absorptance.

This will be heated. It heats the air inside, thus the inside–outside

temperature difference is increased, which in turn would increase the air flow.

SOLAR CHIMNEY

A solar chimney utilizes the stack effect, as already described, but here the air is deliberately heated by solar radiation in order to create an exhaust effect.

The chimney can therefore be designed to maximize solar gains and the ventilation effects. The parameters effecting the ventilation rates are: height between inlet and outlet; cross-sectional area of the inlet and the outlet; geometrical construction of the solar absorbing

plate; inclination angle.

The use of solar chimneys is advisable for regions where very low wind speeds exist.

SOLAR CHIMNEY EFFECT

WIND TOWER - PRINCIPLE

The hot ambient air enters the tower through the openings in the tower and is cooled when it comes in contact with the cool tower and thus becomes heavier and sinks down.

When an inlet is provided to the rooms with an outlet on the other side there is a draft of cool air.

After a whole day of heat exchange, the wind tower becomes warm in the evening.

During night the reverse happens, i.e., the cooler ambient air comes in contact with the bottom of the tower through the rooms; it gets heated up by the warm surface of wind tower and begins to rise due to buoyancy, and thus an air flow is maintained in the reverse direction.

WIND TOWER - APPLICATION

This system can work very effectively in hot and dry types of climate, where daily variations in temperatures are high with high temperature during day time and low temperature during night time.

PROVISION FOR AIR-MOVEMENT – WIND EFFECT

Wind-shadow Effect of direction on width of wind-shadow

AIR-FLOW THROUGH BUILDINGS – EFFECT OF BUILDING ORIENTATION

Effect of wind direction and inlet opening size on air velocity distribution inside room

AIR-FLOW THROUGH BUILDINGS – EFFECT OF EXTERNAL FEATURES

AIR-FLOW THROUGH BUILDINGS – EFFECT OF CROSS-VENTILATION

AIR-FLOW THROUGH BUILDINGS – EFFECT OF POSITION AND SIZE OF OPENINGS

AIR-FLOW THROUGH BUILDINGS – EFFECT OF CONTROL OF OPENINGS

AIR-FLOW AROUND BUILDINGS

Wind-shadowEffect of direction on width of wind-shadow

AIR-FLOW AROUND BUILDINGS

EFFECT OF HEIGHT OF BUILDING ON THE BEHAVIOR OF EDDY

EFFECT OF DEPTH OF BUILDING ON THE BEHAVIOR OF EDDY

EFFECT OF LENGTH OF BUILDING ON THE BEHAVIOR OF EDDY

Behavior of Eddy on a straight block with different orientation along and against the wind direction

BEHAVIOR OF EDDY WHEN STRUCTURES ARE TWO STOREY HIGH

BEHAVIOR OF EDDY WHEN STRUCTURES ARE TWO STOREY HIGH

AIR FLOW AROUND BUILDINGS

AIR FLOW AROUND BUILDINGS IN LAYOUTS

DESIRABLE ORIENTATION OF THE LONGER SIDE OF THE BUILDING WITH LARGER AREA OF WINDOWS

Thank you………