Natural Ventilationsite.iugaza.edu.ps/sammar/files/2014/03/Lecture-12... · 2019. 6. 2. · Natural Ventilation (Passive Cooling) Natural ventilation is necessary for: Fresh air supply

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Principles of Architectural and Environmental Design

EARC 2417

Islamic University-Gaza

Faculty of Engineering

Architectural Department

Instructor: Dr. Suheir Ammar

2019

Lecture 11Natural Ventilation

(Passive Cooling)


Natural Ventilation

(Passive Cooling)

1. Principles of Natural Ventilation NV

1.1 Understanding air flow

1.2 Stack effect

1.3 Wind effect

2. Ventilation in Interior Spaces

2.1 Ventilation Concepts in Interior Spaces

2.2 Window placement

2.3 Interior partitions

3. Vegetation and ventilation


Natural Ventilation (Passive Cooling)

Natural ventilation is necessary for:

Fresh air supply into the building,

Increasing, through air movement, the rate of

perspiration evaporation from the bodies of those in

the building,

Cooling the interior of the building by exchanging

relatively warm hot internal air with cooler outside air

1. Principles of Natural VentilationFor air to move into and out of a building, a pressure

difference between the inside and outside of the

building is required.

The pressure difference is caused by:

• wind (or wind effect)

• difference in air density due to temperature difference

between indoor and outdoor air (stack or chimney effect)

• combination of both wind and stack effects.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect

2. Ventilation in Interior

Spaces

2.1 Ventilation Concepts in

Interior Spaces



3. Vegetation & Ventilation


1.1. Understanding air flow

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Principle 1: Air will always flow from region of high

pressure to lower pressure

low pressurehigh pressure

barrier

Principle 2: Air has mass and it will tend to continue

in its direction until altered by an barrier or an

adjacent air flow.



Principle 3: The overall effect of wind at a site is so

large that locally deflected airflow (by trees or

buildings) will tend to return to the direction and speed

of the site wind.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




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1

2


Principle 4: Airflow is smooth with adjacent air moving

in similar direction and speed.

1. Slow, gentle alterations of flow direction will preserve

laminar* flow,

2. Sudden alterations results in “turbulent flow” whereby

adjacent air currents separate suddenly into swirling,

unpredictable directions.

3. When two currents of air traveling in opposite

directions, they will always be separated by eddies

because adjacent particles of air always move in the

same direction.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




3


Principle 5: when laminar airflow is constricted in

order to pass through an opening, the same volume

of air must pass through a smaller area, so it causes

an acceleration of air flow.


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




an acceleration air flow

windows


Principle 6: Cross-ventilation requires an outlet as well

as an inlet.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





Analogy: water cannot be

put into a bottle that

already full unless some

old water is removed first.



9

Principle 7: The stack effect

Stack effect is the result of air density decreasing

as temperature increase.

Warm air rises through upper openings and

replaced by cooler through low openings.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




The stack effect


1.2. Stack effect

Stack effect is the result of air density decreasing

as temperature increase.

Warm air rises through upper openings and

replaced by cooler through low openings.

If the wind shifts, the air circulation may reverse,

causing the solar-heated air to be blown down

into the living space, effectively heating the

building rather than cooling it. For this reason,

any solar chimney should have a wind-activated

ventilator cap to ensure that airflow is always

positive.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




heated air to be blown down Warm air rises


The Wind effect

1.1. Understanding air flow1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





1.3. Wind effect

When air flow is due to wind, air

enters through openings in the

windward walls, and leaves

through openings in the leeward

walls.

Wind pressures are generally

high/positive on the windward

side of a building and low/

negative on the leeward side.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




The occurrence and change of wind pressures on building surfaces depend on:

wind speed and wind direction relative to the

building.

the location and surrounding environment of the

building.

shape of the building.


Low pressure zones occur along the

sides parallel to the wind and on the

leeward side of the building.

1.3. Wind effect

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




The following diagrams describe principles of

ventilation airflow around buildings and the pressure

distribution patterns due to wind.

Plan


A low building

placed in the

windward path of a

tall building produce

a large amount

turbulence

“instability” between

the two buildings.

An opening on

windward side only

results in poor

ventilation.

1.3. Wind effect

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





Raising a tall building

reduce the high

pressure on the

windward side by

allowing airflow under

the building.

1.3. Wind effect

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





Airflow patterns through a) a normal, b) a scattered

c) a diagonal layout of buildings.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Wind & building arrangement³

Staggered building

arrangements result

in reduced wind

shadows.



Single Sided Ventilation:

This kind of ventilation is normal

and generally practiced, but it is

only useful up to a certain room

depth

Cross Ventilation:

In order to obtain the optimal

airflow with minimal draught,

the windows on the

windward side are opened

less than on the lee side.

Single Sided Double

Opening:

An advancement of the single

sided principle provides a

double opening, which is

considerably more efficient.

2. Ventilation in Interior Spaces

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





Passive Cooling:

As the external temperature drops at night, the building

can be cooled by partially opening the vents around the

building - often called night-cooling.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





night-cooling


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces






1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





A solar chimney uses the

sun's heat to provide

cooling, using the stack

effect. Solar heat gain

warms a column of air,

which then rises, pulling

new outside air through the

building.

Solar chimneys can also be

used for heating. If the top

exterior vents are closed,

the heated air is not

exhausted out the top; at the

same time, if high interior

vents are opened to let the

heated air into occupied

spaces, it will provide

convective air heating


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces





Solar chimney in buildings


1. On opposite walls in rooms, cross-ventilation creates the best airflow.

2. On adjacent walls in a room, cross-ventilation can be improved by positioning of wing walls adjacent to the window openings.

3. On one exterior wall in a room, with only wing walls adjacent to window have been found to increase ventilation.

22

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




2.2. Window placement


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




2.2. Window placement³

Air flow patterns around a

building and impact of

opening location on

ventilation


wing wall design

patterns for two

windows on the

same or adjacent

walls

showing probable

airflow patterns and

wind directions for

improved ventilation

performance

due to wing walls:

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




2.2. Window placement “on the same or adjacent walls“

excellent

good poor

poor poor

goodexcellent

good



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




lf the inlet is larger than the outlet, velocity in the room is

reduced (although velocity outside just to-leeward of the

outlet is increased). This has potential for cooling a

localized exterior area such as a patio

Maximum air exchange is created when the

inlet and outlet areas are equal, making the

optimum configuration when building cooling is

the goals.

cooling an

exterior area

“a patio”

1 2

Maximum air exchange



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces



3. Vegetation & VentilationMaximum interior air speed is created when the inlet

is smaller than the outlet, making the optimum

configuration when people cooling is the goal.

3

Maximum interior air speed



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




A baffle (wing wall, door, or casement window) placed

perpendicular to the opening changes the direction of

airflow through the space with only a small reduction in

velocity.

a small reduction

in velocity.


An inlet centered in the wall restricts airflow to a side

outlet due to an abrupt change in direction, flow is

increased by repositioning the inlet to a more diagonal

location and adding a baffle directs entering air

diagonally in the direction of the outlet.


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces






1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Openings located at the corner of the building allow the

inertia to continue the motion in the same direction in a

smooth curve until the outlet is reached.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




The vertical position of the inlet window is

important in maximizing the airflow through the

lower, occupied portion of the room.

The outlet location has little effect on flow within

the room

The low inlet is better for cooling.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces



3. Vegetation & Ventilation The addition of a slot separating the overhang

from the building redirects the flow down into the

room, increasing the useful cooling effect .

An overhang above the inlet window

directs the interior airflow along the ceiling

out of the occupied zone.


An overhang or an awning increases air

velocity through a window below them

Sashes configuration may have a significant

effect on the flow of air through an opening.


1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Effect of sashes



The location and orientation of interior partitions can

affect the velocity and direction.

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




The partition placed

perpendicular to the

direction of airflow redirects

the pattern and reduces the

velocity.

The partition placed

parallel to the direction

of airflow have least

effect.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Partitions placed blocking

the path create a “barrier”

effect, creating motionless

areas.

Partitions placed outside

of the main airflow path

have little effect.



Vegetation increasing, decreasing and directing airflow

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces



3. Vegetation & VentilationVegetation causes pressure

differences which shifts the

air path


3. Vegetation and ventilation2

36

1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces



3. Vegetation & Ventilation good ventilation poor ventilation


3. Vegetation and ventilation 2

37



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces



3. Vegetation & Ventilation Effect on ventilation of a low hedge (less than 3 ft high)

located parallel to the windward windows at different

distances.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Effect on ventilation of a medium hedge located

parallel to the windward windows at different distances.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Effect on ventilation of a high hedge located

parallel to the windward windows at different

distances.



1. Principles of NV


1.2 Stack effect

1.3 Wind effect


Spaces


Interior Spaces




Effect on ventilation

of a 30-ft tree on

the windward sides

at different

distances.


Thank you

Reference:

1. Moore, Fuller, 1993. Environmental control systems: heating, cooling,

lighting. New York : McGraw-Hill

2. Santamouris, M., Allard, F., Energy, E. C. D.-G. f., & Programme, A.

(1998). Natural Ventilation in Buildings: A Design Handbook: James and

James (Science Publishers) Limited

Documents

Natural Ventilationsite.iugaza.edu.ps/sammar/files/2014/03/Lecture-12... · 2019. 6. 2. · Natural Ventilation (Passive Cooling) Natural ventilation is necessary for: Fresh air supply