VENTILATION IN BUILDINGS
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PRESENTED BY:
LALIT SHARMA

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VENTILATION IN BUILDINGS
What is Ventilation ?

The process of removing used air by fresh air from buildings and spaces is called ventilation. This is done by Natural or Artificial methods
Effects of Ventilation :-
1. To remove the used air from spaces/buildings.
To supply oxygen for humans.
Remove excessive moisture.
Remove excessive heat.
Cool the surroundings and freshen up the atmosphere in the building.
Composition of Air
Nitrogen 78%
Oxygen 20.9%
CO2 0.04%
CO, NH3, Inert gases, water vapour 0.06%
What is the need for ventilation?
The quantity of O2 is reduced and CO2 is increased by combustion and respiration.
When coal is burnt it gives CO2, CO and other impurities.
During breathing an average adult gives off 0.017 cum/hr of CO2
Each cum of gas on burning gives 0.017 cum of CO.
Therefore combustion and respiration both impure the atmosphere which should be freshened by proper ventilation.
Other factors affecting the air in a space/room
Apart from increase in CO2 other factors that affect the atmosphere in a room/space are –
The odour and organic matter given by skin, clothing and mouth of occupants.
Rise in temp due to heat produced by the body’s metabolism.
Increase in humidity by evaporation of sweat.
In brief ventilation is necessary to :
Maintain the CO2 content and harmful substances within safe limits,
Provide sufficient oxygen content in air for respiration,
Remove combustible matter and control odour and maintain satisfactory thermal environment of the room.
Requirements of a good ventilation system
Sufficient quantity of fresh air be admitted and used air extracted.
The quantity and velocity of fresh air admitted should be properly controlled.
The change of air should be complete without leaving any stagnant pockets.
Maximum velocity should be 15 m/min.
The incoming air should be clean and humid.
The temp of air should be uniform and under control.
Types of Ventilation Systems
Natural System.
Artificial System.
Natural System :-
In this system the used air which is warmer and lighter is allowed to escape through the ventilators located near the ceiling while fresh air enters through doors/windows near floor level.
A good rule is to provide a window above floor level and keep the area at about 1/10th of floor area.
Second method is to provide fan lights which permit ventilation even when windows are closed. Current of fresh air gets mixed up with fresh air and is deflected upwards to move out through ventilators.
Drawbacks of Natural System
Reliance upon the wind direction.
No control of volume, temperature and velocity of incoming air.
Chances of smoke, dust , odour entering the room.
Artificial Ventilation System
The Vacuum or Extraction system.
The Plenum or propulsion system.
Balance system.
Air conditioning system.
The Vacuum system :-

is one in which the used air is exhausted to outside by means of suitable fans or blowers. This causes lower pressure inside the room and the used air being warmer and lighter rises up and is extracted through the electrical blower fitted near the ceiling.
This system is more efficient than natural system and involves less initial cost and running cost. It is commonly used in offices and public halls.
The Plenum system :-
This system involves forcing of fresh air to push out the used air through ventilators by use of an electric fan or blower.
This system has better control over the incoming air with regard to its quantity, humidity and temp. Extensively used in crowded places like auditoriums, cinema halls, theatres etc.
Balance System :-
Its combination of Plenum and vacuum systems. Electrical fans are fitted at the bottom to supply fresh air continuously while the used air is extracted continuously by exhaust fans fitted on top near ceiling.
Air Conditioning
Is the process of treating air so as to control simultaneously its temp, humidity, purity and distribution to meet requirements of the conditioned space. Air conditioner gives the feeling of comfort which comes by comfortable temp, humidity, air movement and elimination of body odours, gases, dust etc.
Application of air conditioning
Residential and office buildings.
Hotels, hospitals, cinema halls and departmental stores.
Libraries, museums, computer centres and research labs.
Transport vehicles –
Cars, buses, rail coaches.
Aircrafts, ships and submarines
Food and process units.
Printing, textiles and photographic products.
Production floors, labs, restaurants etc
Types of comfort air-conditioning
As the atmospheric conditions vary with the seasons, the requirements of occupants also change. In summers we need lower temp inside the room , in winters the temp inside is to be maintained higher than the atmosphere. Hence AC can be classified as follows as per the use
Summer AC :- Involves reducing the air temp and humidity by the process of cooling and dehumidification.
Winter AC :- used to provide comfort in winters. Involves an increase in sensible heat and water content of air by the process of heating and humidification.
How does AC work :-
Most people think that air conditioners lower the temperature in their homes simply by pumping cool air in. What's really happening is the warm air from your house is being removed and cycled back in as cooler air. This cycle continues until your thermostat reaches the desired temperature.
Vapour Compression Cycle :-
An air conditioner uses a material called a "working fluid" to transfer heat from inside of a room to the outside. The working fluid which is also called “Refrigerant” is a material which transforms easily from a gas to a liquid and vice versa over a wide range of temperatures and pressures. This working fluid moves through the air conditioner' s three main components, the compressor, the condenser, and the evaporator in a continuous cycle.
Vapour compression cycle
Working of Vapour Compression Cycle :-

The compressor compresses cool Freon gas coming out from the evaporator, causing the temp and pressure of gas to rise (red in the diagram above).
This hot gas enters condenser which is located outside the room and runs through a set of snake like coils to dissipate its heat, and this gas condenses into a liquid.
The Freon liquid runs through a narrowing pipe or an expansion valve , and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).
This cold gas enters the evaporator located inside the room and runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.
Mixed in with the Freon is a small amount of lightweight oil which lubricates the compressor.
Processes in the AC system/plant
Recirculation :- Since the temp of air inside room is higher than atmosphere in winter and lower in summers it is economical to recirculate portion of air from occupied space and take in fresh air from atm to prevent concentration of odours.
Filtration :- cleaning of air for removing dust & other solid particles from air entering the AC is called filtration. This is achieved by using dry or viscous filters made of felt or cloth through which air is forced to pass trapping the dust particles in the filter.
Dehumidification :- This can be achieved by first lowering temp of air below the dew point, condensing the moisture and reheating it to the desired temp.
Pre heating :- During winter season some of the heat lost from occupied rooms is made good by pre heating the outside air. This is done by blowing hot air or passing through coil heated by hot water/steam.
CYCLE OF OPERATION
Removal of dirt/dust by passing the air through air cleaner or filter, Cooling or heating air by refrigerator coils or heating coils respectively, Control the humidity by adding moisture in summers and removing moisture in winters or rainy season by passing air through humidifier. Forcing of conditioned air through ducts by fans and distributing it in rooms.
Cycle of operation
Humidification: -

This is an important part of AC system which is achieved by passing air through spray of water then through eliminator plates to remove water droplets. In winters the outside air may have high relative humidity . When heated the RH becomes low and dry leading to parched feeling and scaling of skin. On the other hand air too humid can result in feeling of discomfort. The proper control of humidity is therefore important to human comfort.
humidification
Comfort Criteria
Humidity :- The air should not be too dry or nearing saturation. The humidity should be between 45 to 65%.
Temperature :- It should be around surface temp of human body. A temp between 24 to 27 degree C is a comfortable temp.
Air velocity :- Should not exceed 15 m/min. (4.5-9% considered as ideal)
Organic matter :- The CO2 content should be below 0.06% or 600 ppm.
Type of AC Plants/Units
Package Units :-
Larger units used for commercial establishments having temp and humidity controls. Suitable for manufacturing and process units requiring controlled conditions. All the components, namely the compressor, condenser (which can be air cooled or water cooled), expansion valve and evaporator are housed in a single box. The cooled air is thrown by the high capacity blower, and it flows through the ducts laid through various rooms.
Central AC system :-
Plant of large capacity fitted in basements, terrace or any suitable place supplying conditioned air through supply ducts to entire building. Used air is returned to the plant through return duct for recirculation. Widely used in theaters, cinema halls, hotels, malls, offices etc
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