VENTILATION

24.3.1 Necessity of ventilation

1. To prevent an undue concentration of body odours, fumes, dust and other industrial by products.
2. To prevent an undue concentration of bacteria-carrying particles.
3. 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.
4. To create air movement, so as to remove the vitiated air or replace it by fresh air.
5. 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 0.6 per cent volume (in air).
6. To maintain conditions suitable to the contents of the space.
7. To prevent flammable concentration of gas vapour or dust in case of industrial buildings.

24.3.2 Functional requirements of a ventilation system

A ventilation system should meet the following functional requirements.

1. Rate of supply of fresh air
2. Air movements or air changes
3. Temperature of air
4. Humidity
5. Purity of air

24.3.2.1 Rate of supply of fresh air

The quantity of fresh air to be supplied to a room depends upon the use of the building to which it is subjected. The rate of supply of fresh air is decided by considering several factors such as the number of occupants, type of work and age of occupants.

Type of building	Minimum rate of fresh air supply to buildings (m3 per person per hour)
Assembly halls, canteens, shops, restaurants	28
Factories and workshops
i) work rooms	23
Residential buildings
ii) living rooms	3 air changes per hour
iii) kitchens	6 air changes per hour
iv) bathrooms	6 air changes per hour
v) halls and passages	1 air changes per hour
Hospitals
i) wards	3 air changes per hour
ii) theatres	10 air changes per hour

24.3.2.2 Air movements (or air changes)

At workplaces, air has to be moved or changed to cause proper ventilation of the space. The minimum and maximum rates of air change per hour are 1 and 60 respectively. If the rate of air change is less than one per hour, then it will not create any effect on the ventilation system. While, on the other hand, if the rate of air change is more than 60 per hour, it may lead to discomfort due to high velocities of air. For effective working of the ventilation system, 5-6 air changes per hour are considered alright. Moreover, the air movements should be uniform and should not allow the formation of pockets of stagnant air at any spot in the room.

In naturally ventilated buildings, cross ventilation is provided to secure air movement, whereas in the case of mechanically ventilated buildings air movement is obtained by either increasing the rate of fresh air supply or by recirculation of a part of air in water. The air movement or rate of air change will depend upon the velocity of incoming fresh air, disposition of inlets, type of activity in the premises, number of occupants, etc. The air movement should be varied both in velocity and direction and this can be achieved by means of fans.

24.3.2.3 Temperature of air

It is desirable that the incoming air for ventilation should be cool in summer and be warm in winter before it enters the room. Whenever the velocity of the incoming air is high, its temperature should not be lower than the room temperature. The usual temperature difference between inside and outside should not be more than 8°C.

The effective temperature should, therefore, be maintained with regard to the comfortable conditions for different seasons of the year. This effective temperature indicates a most suitable temperature to the majority of people, considering the comfort of human body under the probable conditions of humidity and air motion. The value of effective temperature depends upon the type of activity, geographical conditions, amount of heat loss from the body, age of occupants, etc.

24.3.2.4 Humidity

A relative humidity within the range of 30–70 per cent at a working temperature of 21°C is considered desirable and, therefore, should be maintained. When work is required to be done at a higher temperature, low humidity and greater air movements are necessary for removing a greater portion of heat from the body. The value of relative humidity can be obtained by comparing dry bulb and wet bulb temperatures.

Any water vapour within a given space or volume is known as humidity and its ratio with the water vapour it had contained had it been saturated is known as relative humidity. The relative humidity depends only upon the vapour pressure of water vapour and dry bulb temperature. The relative humidity of saturated air is 100 per cent.

24.3.2.5 Purity of air

The purity of air plays a significant role in the comfort of people affected by a ventilation system. Hence, it is essential that the ventilating air should be free from any impurities. To get pure ventilated air, the entry point of the ventilation system should not be situated in the neighbourhood of chimneys, latrines, urinals, etc.

24.3.3 Systems of ventilation

The systems of ventilation are basically divided into the following two categories:

1. Natural ventilation or aeration
2. Mechanical ventilation or artificial ventilation

24.3.3.1 Natural ventilation

In this system of ventilation, the outside air is supplied into a building through windows, doors, ventilators and other openings due to the wind outside and convention effects arising from temperature or vapour pressure differences or both between the inside and the outside of the building.

This system of ventilation may be developed where precise control over the air conditions and the rate of air changes are not required. Natural ventilation is usually considered suitable for houses and flats (i.e., small buildings) and it cannot be adopted for big offices, assembly halls, theatres, auditoriums, large factory workshops, etc. This system is very economical and the desired ventilation can be achieved by providing sufficient windows and other openings which open to the external air. An opening area equal to not less than one-twentieth of the floor area of the room should be provided in view of proper ventilation. The top of this opening area should be not more than 45 cm below the ceiling.

The rate of ventilation by natural means through doors, windows and other openings depends upon the following effects.

Wind effect (or wind action)

In this, ventilation is affected by the direction and velocity of wind outside and the sizes and position of the openings. Wind creates pressure differences and when it blows against a building a positive pressure is created on the windward side and leeward side. Suction will occur on the other side and the wind will blow from the windward side to the other side if there is an opening.

When wind blows at right angles to one of the rectangular faces of the building or an exposed site, a positive pressure is produced on the windward face and a negative pressure on the leeward face. If the wind direction is at 45° to one of the faces, positive pressure will be produced on the two windward faces and negative pressure on the two leeward faces.

The rate, at which the air change or airflow will occur, depends upon the pressure difference between the inside and the outside. The greater the wind speed the greater will be the pressure difference and sometimes the air changes can occur quickly.

Stack effect

In this, the ventilation rate is affected by the convection effects arising from temperature or vapour pressure difference or both, between the inside and outside of the room, and the difference of height between the outlet and inlet openings. If the air temperature inside is higher than that of outside, the warmer air tries to rise and pass through the opening in the upper part of the building.

At the same time, the incoming cooler air from outside through the opening at the lower elevation replaces it. The rate of air flow, in addition to the temperature or pressure difference and height difference, also depends upon the ratio between the areas of the two openings.

General considerations and rules for natural ventilation

The following considerations and rules should be followed for promoting natural ventilation in buildings:

1. Inlet openings in the buildings should be well distributed and should be located on the windward side at a low level. 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 preferably be of equal size for greatest air flow, but when the outlet is in the form of a roof opening the inlet should be larger in size.
3. 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 obstructions 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 the room mainly depends on the design of the opening location of the inlet and outlet and the difference in temperature between the inside and outside air. Generally, the outside air is cooler than the inside air. Hence, the cooler air enters from the bottom and after becoming hot during its stay in the room leaves from the top. It would, therefore, be advantageous to provide ventilators as close to the ceilings as possible.
8. The efficiency of roof ventilators depends on their location, wind direction and the height of the building.
9. It is found that 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 the 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 for ventilation of the room.

24.3.3.2 Mechanical or artificial ventilation

In this system of ventilation, outside air is supplied into a building either by positive ventilation or by infiltration by reduction of pressure inside due to exhaust of air, or by a combination of positive ventilation and exhaust of air. The supply of outside air by means of a mechanical device such as a fan is termed as ‘positive ventilation’, whereas the removal of air and its disposal outside by such a device is termed as ‘exhaust of air’. For positive ventilation, centrally located supply fans of centrifugal type, and for exhaust of air, wall- or roof-located exhaust fans of propeller type are normally used. So, this ventilation involves the use of some mechanical arrangement for providing enough ventilation to the room.

Mechanical ventilation is recommended in all the cases where a satisfactory standard of ventilation in respect of air quantity, quality or controllability cannot be obtained by natural means. A mechanical system is capable of meeting the requirements of air quantity and quality (of air) regarding humidity, temperature, etc and produces the comfortable conditions at all times during the year. Though this system is comparatively costly, it results in the considerable increase in the efficiency of the persons under the command of this system. This system is adopted for big offices, banks, assembly halls, auditoriums, theatres, large factories, workshops, places of entertainment, etc. This system may be regarded as generally desirable in all rooms occupied by more than 50 persons, where the space per occupant is less than 3 cu.m.

The following methods of mechanical or artificial ventilation are in common use.

1. Extract or exhaust systems
2. Supply or plenum systems
3. Combination of exhaust and supply systems or balanced systems
4. Air conditioning

Extract or exhaust systems

In this system, a partial vacuum is created in the inside of the room by exhausting or removing the vitiated inside air by means of propeller type fans. The extraction of air from inside permits the fresh air to flow from outside to inside and thus it becomes possible to provide fresh air to flow from outside into the room through doors and windows.

These fans for exhaust are installed at suitable places in the outside walls or roofs and they are further connected to different rooms through a system of duct-work.

These exhaust systems are best confined to situations where it is essential to create an air flow towards the ventilated rooms, such as in kitchens, lavatories and industrial plants. This system is useful for removing smoke, odours, fumes, dust, etc from the above-mentioned rooms. In this system, the ducts are placed near the place of formation of smoke, fumes, odours, dust, etc.

Supply or plenum systems

As the name implies, in this system the space is filled with air by means of fans, but no special provision is made to remove it. In plenum ventilation, the air inlet is selected in the side of the building where the air is purest. In this opening, screens or filters may be fixed and fine stream of water may be impugned in the path of the incoming air. The disinfection of incoming air is achieved by adding ozone at the point of inlet. Thus, by this system of mechanical ventilation, it is possible to control the quality, humidity and temperature of incoming air.

Ventilation by plenum process may be downward or upward. In downward ventilation, the incoming air is allowed to enter at the ceiling height and is taken out through outlets situated at the floor level. In upward ventilation, the fresh air is allowed to enter at the floor level and the outlet is provided at the ceiling height.

These ventilation systems are costly and are used for factories, big offices, theatres, etc. and also for supplying air to the air-conditioned buildings.

Combination of exhaust and supply systems or balanced systems

This balanced system is a combination of the above-said two systems and makes use of fans to supply and extract air (i.e., input fans and exhaust fans). This system enables full control over the air movement and conditions to be obtained and should be used where accurate performance is desired. In most buildings, it is desirable to extract only 75 per cent of the quantity of air supplied so that positive pressure is maintained within the rooms. This is essential to prevent the entry of hot air when the doors are opened and also to prevent the infiltration of dust and air-borne contaminants. Moreover, the recirculation of air is possible in this system.

Air conditioning

This is the most effective system of artificial ventilation, in which provision is kept for humidifying or dehumidifying, heating or cooling, filtrations, etc. of the air to meet the possible requirements.