smoke Smoke is defined as well mixed conglomeration

smoke

�Smoke is defined as well mixed conglomeration of following substance which surround the flame portion of the fire. �Unburnt decomposed and/or condensed matters including smoky soots, splintering matter and flying brands. �Hot gases and vapours given off by the fuel �A quantity of air heated by fire and entrained by the rising plume of smoke.

Volume of smoke

Volume of smoke

Volume of smoke

Time taken for smoke to fill the entire room can be calculated by following equation

Quality of smoke �The quality of smoke in terms of its density and darkness can be best demonstrated in respect its obscurity value and mostly measured in terms of its strength of reducing the visibility of human eyes.

Visibility and obscuration �Visibility in smoke is affected by following factors ◦ Colour and size of smoke particles ◦ Density of smoke ◦ Physiological effect of the smoke ie. Its irritant nature ◦ Size of object being observed, illumination level of the surrounding. ◦ Physical and mental state of the observer, age and sex.

LIGHT OBUSCURATION Sx = percentage obscuration Io = intensity of an incidental parallel beam of light

OPTICAL DENSITY

SPECIFIC OPTICAL DENSITY

SMOKE MOVEMENT • Due to its buoyancy • The force exerted by the surrounding air

The buoyancy force is generated due to pressure differentials developed by the �Expansion of gases as they are heated by fire �By the difference in density between the hot gases above the flames and the cooler air which surrounds the fire. Similarly the air movement in the building is caused by three separate factors • Stack effect • Wind effect • Mechanical ventilation

WIND ACTION

STACK EFFECT

Smoke Vents �Smoke vents are devices designed to remove combustion products from a buildings during a fire to prevent smoke from entering adjacent premises, to stop the formation of explosive mixtures of products of complete combustion with air, and to facilitate the work of the fire fighters. �Windows and skylights perform the function of smoke vents in many buildings.

�However, in buildings that do not have such openings, fire fighting is very difficult. Among such closed buildings and occupancies are: theatres, clubs and cultural centres, basements, buildings without skylights, warehouses, cold storages etc. �Fires in buildings without smoke vents are therefore long-lasting and produce large losses.

Smoke control during building design �Any building can be looked on by a designer as having three distinct portions as regard to smoke control during fire: ◦ All the occupied rooms which are occupied 24 hrs or some part of day/night by people. ◦ All common areas which are structurally separated from the occupied rooms. These are called protected escape routes. These includes

Smoke control in buildings �natural and mechanical ventilation �pressurization

NATURAL VENTILATION Building direction is so selected that maximum possible inlet openings are located on opposite sides. § Inlet and outlet opening should not be obstructed by adjoining buildings. § Outlet , ventilators should be always at top so as to cause stack effect § In case of industrial buildings wider than 30 m, ventilation may be augmented by roof ventilation by roof heat extract units. §

Mechanical ventilation �Internal circulation ◦ Predominantly vertical(by ceiling fans) and horizontal(air circulators) �By roof-heat extract units and hoods. �Positive ventilation ◦ Plenum systems ◦ Balanced systems �Evaporative cooling �Ventilation by impulse turbo fans

SMOKE CONTROL DURING BUILDING DESIGN �All the occupied rooms which are occupied 24 hrs or some part of the day/night by people. �All common areas which are structurally separated from the occupied rooms. These called protected escape routes. These include corridors, lobbies, stairs shaft for services. �Occupied areas which are open to atmosphere or/and does not have complete structural separation from the fire area, like

Pressurization of protected escape routes �A fire escape routes starts from the occupied room to corridor to floor, lobby, and continued through staircase unto ground floor lobby. The entire escape route is made of fire resisting construction. All the doors opening into escape routes are made of sufficient fire resistance. �Manually open windows �Ventilation in one wall of staircase

Single stage and Two stage pressurization �The pressurization system can be designed to operate only in an emergency. At all normal times there will be no excess pressure developed in any of the spaces chose for pressurization. This is called a single-stage system. �Alternatively, a continuously operating low level of pressurization of appropriate space can be incorporated as part of the normal ventilation arrangement for the building and an increased level of pressurization is then brought in to operation in an emergency. This is called a two stage system.

TOXICITY OF FIRE GASES Fire gas toxicants are usually considered as belonging to one of three basic classes: 1. Asphyxiants, or narcosis-producing toxicants 2. Sensory/upper respiratory irritants or pulmonary irritants 3. Toxicants exhibiting other or unusual effects; although always a possibility, this class has few documented examples

Asphyxiants �In combustion toxicology, the term narcosis refers to the effects of asphyxiant toxicants that are capable of resulting in central nervous system depression, with loss of consciousness and ultimately death. �Effects of these toxicants depend on the accumulated dose, that is, both concentration and duration of the exposure. The severity of the effects increases with increasing dose.

�Carbon Monoxide. -The toxic effects of carbon monoxide are those of anemic hypoxia. Hypoxia refers to the condition in which there is an inadequate supply of oxygen (O 2) to body tissue, with anemic hypoxia being characterized by a lowered oxygen-carrying capacity of the blood. �Hydrogen Cyanide. Hydrogen cyanide (HCN), whose lethal dose is approximately 25 times smaller than that of carbon monoxide, is a very rapidly acting toxicant.

Table - Physiological Response to Various Concentrations of Hydrogen Cyanide Parts of HCN per million parts of air Threshold limit value. 10 Slight symptoms after several hours 20 to 40 of exposure. Maximum amount that can be inhaled for 1 hour without serious disturbance. 50 to 60 Dangerous in 30 minutes to 1 hour. 120 to 150 Rapidly fatal. 3000

Carbon Dioxide Carbon dioxide (CO 2) is produced in quantity at most building fires. Inhalation of carbon dioxide stimulates respiration and this in turn increases inhalation of both oxygen and possible toxic gases and vapours produced by the fire. Stimulation is pronounced at 5 per cent (50, 000 ppm) concentration, and 30 -minute exposure produces signs of intoxication; above 70, 000 ppm unconsciousness results in a few minutes. The threshold limit for CO 2, that is the concentration that can be tolerated by workers day after day without adverse effect, is 5, 000 ppm.

Irritants �Both inorganic irritants (e. g. , halogen acids and those formed from nitrogen oxides) and organic irritants (e. g. , aldehydes) can be formed in fires. Irritant effects, produced from exposure to essentially all fire atmospheres, are normally considered by combustion toxicologists as being of two types: Sensory 1. irritation, including irritation of the eyes and the upper respiratory tract 2. Pulmonary irritation affecting the lungs

Hydrogen Chloride Hydrogen chloride (HCl) is produced when polyvinyl chloride (PVC) is decomposed at fires. If inhaled, HCl will damage the upper respiratory tract and lead to asphyxiation or death. The physiological response of man to various concentrations of HCl is given in Table - Physiological Response to various Concentration of HC 1 Parts of HC 1 per million parts of air Threshold limit value Maximum concentration allowable for short exposure (½ to 1 hour). 5 50

Nitrogen Dioxide There are three common oxides of nitrogen: nitrous oxide (N 2 O), nitric oxide (NO), and the two forms of the dioxide (NO 2 and N 2 O 4). Nitrogen dioxide, which is very toxic, can be produced from the combustion of cellulose nitrate. Nitric oxide does not exist in atmospheric air because it is converted into dioxide in the presence of oxygen. These compounds are strong irritants, particularly to mucous membranes and thus when inhaled will damage tissues in the respiratory tract by reacting with moisture to produce nitrous and nitric acids.

The physiological response of man to various concentrations of nitrogen dioxide is given in Table Parts of NO 2 per million parts of air Threshold limit value. Least amount causing immediate irritation to the throat. Dangerous for even short exposure Rapidly fatal for short exposure. 5 62 117 to 154 240 to 775

Supertoxicants �Concern regarding potential “supertoxicants” (materials generating smoke of extremely high potency) in fire atmospheres was prevalent in the 1970 s and 1980 s. With more experience in the testing of materials, this concern has diminished, since there have been few documented examples. � One involved the formation of a neurotoxin from thermal decomposition of a noncommercial rigid polyurethane foam, whereas another concerned the extreme toxic potency exhibited by polytetrafluoroethylene in some laboratory tests.