Lec 7-8 Ventilation & Air Movement

8/8/2019 Lec 7-8 Ventilation & Air Movement

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Environmental Control Systems 1

Ventilation

PROF. DR. SYED TAUSEEF AHMAD

DEPARTMENT OF ARCHITECTURAL ENGINEERING & DESIGN

UNIVERSITY OF ENGINEERING & TECHNOLOGY, LAHORE


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Ventilation

Ventilation is the process of "changing" or

replacing air in any space to provide

high indoor air quality (i.e. to control

temperature, replenish oxygen, or remove

moisture, odors, smoke, heat, dust, airborne

bacteria, and carbon dioxide).


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Ventilation includes both the exchange of air

to the outside as well as circulation of air

within the building. It is one of the most

important factors for maintaining acceptable

indoor air quality in buildings. Methods forventilating a building may be divided into;

mechanical or forced ventilation

natural ventilation.


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Ventilation

In any occupied space ventilation is necessary to

provide oxygen and to remove contaminated air.

Fresh air contains about 21% oxygen & 0.04%

carbon.

Expired air contains 16% oxygen & 4% carbon

dioxide.

Ventilation has a great effect on the heat loss frombuildings.


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The normal process of breathing gives

significant quantities of latent heat and water

vapor to the air.

Body odours, bacteria, and the products of

smoking, cooking, and washing also

contaminate household air.

In places of work, contamination may be

increased by a variety of gases and dusts.


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Types of spaces Recommended air-supply

Residences, offices, Shops 8 liters/s per person

Restaurants, bars 18 liters/s per person

Kitchens, domestic toilets 10 liters/s per person

Typical Fresh air-supply rates

The ventilation rate, for aII buildings, is normallyexpressed by the volumetric flow rate of outside air

being introduced to the building. The typical units

used are cubic feet per minute (CFM) or liters per

second (L/s). The ventilation rate can also be

expressed on a per person or per unit floor areabasis, such as CFM/p or CFM/ft, or as air changes

per hour

Ventilation Rate


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NaturalVentilation

Natural Ventilation involves harnessing

naturally available forces to supply andremoving air through an enclosed space.


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Need for Natural Ventilation

Wide spread use of air-conditioners

Very high prices of electricity.

Brownouts and load shedding. Air-conditioned buildings become

intolerable.


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Philosophy Of Natural Ventilation

What one is trying to achieve with natural

ventilation?

Design the building and the air paths through

it to meets those ends.


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It is necessary to understand in relation to a

particular building:

Reason for using ventilation.

Thermal comfort requirements of building occupants.

Thermal performance of the building structure.

Impact of building form and its openings.

Constraints of the local and regional climate.

Unless all of these aspects are considered at thedesign stage, it is unlikely that a good natural

ventilation system in a building can be achieved.


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Functions of Natural Ventilation

The supply of fresh air

Physiological cooling

Removing heat from, or adding it to, thethermal mass in the building structure


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Types of Natural Ventilation

Natural ventilation is driven by the natural forces

of wind and temperature. It is created by

pressure differences between inside and outside

a building and is induced by wind and

temperature differences.

Wind driven ventilation,

Wind pressure on a building depends on wind

direction, speed and shape of the building.

temperature-driven ventilation,

Temperature differences between indoors and

outdoors causes density differences in the air which in

turn causes pressure differences.


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Reason for the air to flow:1. Natural convection currents caused by differences in

temperature

2. Differences in pressure


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Ventilation and Buildings

There are four main ways in which buildings are

naturally ventilate:

Single sided Ventilation

Cross Ventilation

Stack Effect

Reverse stack Effect


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Cross Ventilation


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Fin walls can

significantly increase

ventilation throughwindows on the same

wall.

Poor ventilation

results from fin walls

placed on the same

side of each window or

when two fins are used

on each window.


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Stack Effect


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ASSISTED STACK EFFECT


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Reverse Stack Effect


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The End

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Lec 7-8 Ventilation & Air Movement