Numerous building fires have shown that the high-temperature toxic smoke in a fire spreads quickly and through diverse channels, making smoke inside buildings one of the main causes of hindering people's escape and firefighting operations and of causing deaths. The smoke prevention design of buildings is very important for ensuring the safety of people inside the building and preventing the spread of smoke. Smoke prevention and smoke exhaust of the building should be given key consideration in the building fire safety design of hospital clean operating departments.

(1) The basic principle of smoke prevention and exhaust design is to prevent smoke from entering evacuation routes and to ensure evacuation safety. However, for high-rise buildings, windowless buildings, and similar places such as hospital clean operating departments, where evacuation and firefighting are difficult, it is necessary to carry out smoke prevention and exhaust design at their important parts, adopting facilities focused mainly on smoke control.

Smoke prevention and exhaust design should, in accordance with the characteristics of smoke and its movement laws, study in the plan layout the smoke flow direction of rooms that may catch fire; through different assumptions, find the most economical and effective smoke prevention design scheme and the routes for controlling smoke flow, select appropriate smoke prevention and exhaust equipment, and arrange reasonable areas and positions of air inlets, exhaust outlets, and ducts.

(2) Smoke prevention or smoke exhaust of a building can be achieved by means of natural smoke exhaust, mechanical pressurized air supply, or mechanical smoke exhaust. Which method to adopt should be determined through comprehensive consideration of the building's plan layout, ventilation conditions, space height, and the fire load density inside the building as well as its type, form, and distribution.

To meet the requirements of surgery and cleanliness, a clean operating department is generally relatively airtight, and the clean areas maintain an air pressure higher than that of the surroundings. The clean areas generally have no windows or only sealed fixed windows. In the design of its smoke prevention and exhaust system, when it is built as a standalone building, it should be treated as a windowless building; when it is built jointly with other buildings, it should be treated as a windowless room within the building. Figure 10.4　Schematic diagram of fire alarm detectors and alarm control devices Since the current Code for Fire Protection Design of Buildings does not yet contain provisions for the design of smoke control and exhaust systems in buildings, the relevant design may be carried out with reference to the relevant provisions of the Code for Fire Protection Design of High-rise Civil Buildings and the Code for Fire Protection Design of Civil Air Defense Works. However, the design of smoke control and exhaust systems for buildings has long been a focus of research on building fire protection and smoke control abroad, and many theories have been proposed; at present, however, there is no mature design method that is widely accepted. People are attempting to solve this problem by means of performance-based design methods, and a number of computer prediction and calculation models for smoke flow in buildings have already been developed.

[Reference]

① Smoke is a mixture of air with the gaseous, liquid, and solid substances, containing a large amount of heat, that are generated by the thermal decomposition of a substance during the combustion reaction process.

When a substance undergoes thermal decomposition and combustion, it gives off heat, light, and smoke. The composition and quantity of the smoke depend on the chemical composition of the combustible material and on combustion conditions such as the temperature and the oxygen supply during combustion. The combustible building materials, decoration materials, combustible furniture, and other stored combustible items in ordinary buildings are mainly organic compounds composed of elements such as carbon, hydrogen, oxygen, sulfur, phosphorus, and nitrogen. Under complete combustion conditions, the combustion products of a substance are mainly carbon dioxide, carbon monoxide, water vapor, sulfur dioxide, and nitrogen; see Table 10.7. Under incomplete combustion conditions, in addition to the above combustion products, organic compounds such as alcohols, ketones, ethers, and aldehydes are also generated. When substances with a high carbon content burn under oxygen-deficient conditions, a large amount of carbon particles is produced. The harm of smoke to the human body mainly consists of asphyxiation caused by the toxic gases produced by combustion, irritation of human organs, and the effect of high temperature; see Table 10.8.

Table 10.7　Toxic Gases Produced During the Combustion of Some Combustibles

Name of combustible Main toxic gases produced
Wood Carbon dioxide, carbon monoxide
Wool Carbon dioxide, carbon monoxide, hydrogen sulfide, ammonia, hydrogen cyanide
Cotton, man-made fibers Carbon dioxide, carbon monoxide
Polytetrafluoroethylene Carbon dioxide, carbon monoxide
Polystyrene Benzene, toluene
Polyvinyl chloride Carbon dioxide, carbon monoxide, hydrogen chloride
Nylon Carbon dioxide, carbon monoxide, ammonia, acetaldehyde
Phenolic resin Carbon monoxide, ammonia, cyanide
Epoxy resin Carbon dioxide, carbon monoxide, acetone

Table 10.8　Harm to the Human Body Caused by Thermal Decomposition Products of Common Combustibles

Gas name HCl NH₃ HF SO₂ COCl₂ NO₂
Toxic effect Irritates the eyes and upper respiratory tract; asphyxiation caused by rupture of the upper respiratory tract Irritates the eyes and upper respiratory tract, leading to pulmonary emphysema Irritates and corrodes the eyes and the mucous membranes of the upper respiratory tract Irritates the eyes, upper respiratory tract, and bronchial mucous membranes; asphyxiation due to pulmonary and glottic emphysema and tracheal blockage Irritates the bronchi and alveoli; asphyxiation due to breathing difficulty caused by pulmonary emphysema Irritates the bronchi and alveoli; asphyxiation due to breathing difficulty caused by pulmonary emphysema
Safe for 1h exposure 50～100 300～500 50～250 — — —
Dangerous for 0.5～1h exposure 1 000～2 000 2 500～4 500 — 50～100 25 117～154
Lethal for 0.5h exposure — — — — — —
Lethal for short-time exposure 1 300～2 000 5 000～10 000 — 400～500 50 240～775

(i) Asphyxiating effect. The concentration of carbon monoxide produced in a fire room differs significantly depending on the nature, quantity, and stacking arrangement of the combustibles and on the opening conditions of the room. It mainly depends on the ratio of the rates of the thermal decomposition reaction and the oxidation reaction of the combustibles. When When the indoor temperature keeps rising, the thermal decomposition of combustibles accelerates, and more free carbon is produced, an insufficient oxygen supply will cause the concentration of carbon monoxide to increase. The carbon monoxide concentration in the fire room can generally reach 4%～5%, and at most about 10%. The safe concentration of carbon monoxide for 1h of human exposure is 0.04%～0.05%; for personnel evacuation, this concentration should not exceed 0.2%.

When a fire burns vigorously inside a building, the carbon dioxide concentration can reach 15%～23%. Generally, exposure to a carbon dioxide concentration of about 10% causes dizziness; in severe cases, coma, difficulty breathing, and even respiratory arrest and loss of consciousness may occur. Exposure to a carbon dioxide concentration of about 20% causes paralysis of the human central nervous system, leading to death.

During a fire, combustion consumes a large amount of oxygen, and the oxygen concentration in the air drops significantly, reaching as low as 3%～4% when the fire is burning vigorously. Generally, when the oxygen concentration in the air falls below 11%, a person will lose the ability to reason, suffer convulsions, and turn blue in the face. When the oxygen concentration is below 6%, a person will die after a short exposure.

(ii) Irritant effect. Among the products of thermal decomposition of combustibles during a fire, there are some gases, such as hydrogen chloride, ammonia, hydrogen fluoride, sulfur dioxide, phosgene, and nitrogen dioxide, that have a strong irritant effect on the human body.

(iii) Effect of high temperature. When a fire occurs in a building and the temperature reaches the flashover point, the indoor temperature can exceed 500℃ and even reach as high as 800℃. High temperature not only increases the heart rate and causes profuse sweating, with fatigue and dehydration appearing quickly, but can also burn people, causing injury or death. In addition, high-temperature smoke is also a main cause of the rapid spread and expansion of the fire and the increase in fire losses. Smoke also reduces people's visual range, creates a sense of terror, and affects evacuation and firefighting operations.

② Movement patterns of smoke within a building. The high-temperature smoke produced by a fire has a lower density than cold air; driven by buoyancy it rises, and when it encounters the floor slab or ceiling, it changes direction, forming horizontal diffusion of the smoke. If the smoke temperature does not drop at this moment, the high-temperature smoke and the surrounding air will form a clearly separated laminar flow. In reality, however, as the smoke flows and diffuses, on the one hand cold air is constantly mixed into it, and on the other hand it is cooled and obstructed by building components such as floor slabs and ceilings, so its temperature gradually drops; when the smoke flowing and diffusing horizontally encounters surrounding objects, it is further cooled and flows downward. In this way, the gradually cooling smoke and the cooled air flow toward the fire zone, forming natural convection in the room, which makes the fire burn more and more vigorously.

The speed of smoke flow and diffusion is related to the smoke temperature and the direction of flow. The horizontal diffusion flow speed of smoke is relatively low: generally 0.1～0.3m/s in the early stage of the fire and 0.5～0.8m/s in the middle stage. The vertical diffusion flow speed of smoke is relatively high, usually 1～5m/s. In stairwells or pipe shafts, the draft force generated by the stack effect can raise the smoke flow speed to 6～8m/s.

③ Flow of smoke in the fire room. As the smoke rises in the fire room, it spreads horizontally in all directions after reaching the ceiling, and is obstructed and cooled by the surrounding building envelope, tending to flow downward along the walls. Smoke is continuously generated, and the upper smoke layer gradually thickens; when it descends below the door and window openings, it diffuses to the outdoors and the corridor through the open door and window openings. If the doors and windows are closed, the smoke layer will continue to thicken; when the indoor temperature rises to a certain level (generally 200～300℃), the glass in the doors and windows breaks, and the smoke diffuses to the outdoors and the corridor through the gaps in the doors and windows.

When smoke is ejected from a vertical window, the air carried away nearby can be replenished from both sides of the window, and its trajectory curves upward, so the danger of the fire spreading upward through the window is not great; when smoke is ejected from a horizontal window, the air carried away nearby cannot be replenished in time from both sides of the window, causing the flame to cling to the wall surface and extend over a relatively long distance; this situation poses a great threat to the floor above.

When the room door is closed, there are no other openings connecting with the corridor, and the windows on the exterior wall are open: when there is wind outside and the window is on the windward side, the neutral plane will rise; when the window is on the leeward side, the neutral plane will descend.

When the room door opens toward the corridor, the flow of smoke becomes more complicated; it will be related to factors such as the stack effect of the building, the smoke control and exhaust method, the outdoor wind speed, and the fire temperature.

④ Flow of smoke in the corridor. The smoke flowing from the room into the corridor at first hugs the underside of the ceiling as it moves; due to cooling and mixing with air, the smoke layer gradually thickens. The smoke near the ceiling and the wall surfaces is easily cooled and first flows downward along the wall surfaces; as the flow With the lengthening of the travel route and the intensification of mixing with the surrounding air, the smoke temperature gradually drops and loses its buoyancy, finally leaving a circular space at the center of the corridor.

Fire tests have shown that, during the vigorous stage of a fire, the smoke flowing out of a room travels along the corridor ceiling in a laminar state, and the thickness of the smoke layer remains unchanged over a distance of 20～30 m. However, during the flow, if the smoke layer is obstructed by beams and other protrusions, or disturbed by the inflow of outdoor air or by air currents from ventilation and air-conditioning systems, its laminar flow distance will be shortened and it will turn into a turbulent state.

⑤ Flow of smoke through vertical shafts such as stairwells, elevator shafts, and pipe shafts. When the indoor air temperature is higher than the outdoor temperature, the airflow, after entering through the openings in the exterior walls of the stories below the neutral pressure plane of the building, travels up through vertical shafts such as stairwells, elevator shafts, and pipe shafts, enters the rooms through the shaft openings above the neutral pressure plane of the building, and then flows out through the exterior wall openings of each story. Generally, the pressure distribution within a building follows such a relationship: when, in the lower part of the building, the outdoor air pressure ＞ the pressure of each story ＞ the pressure inside the shafts, outdoor air flows into the building; when, in the upper part of the building, the pressure inside the shafts ＞ the pressure of each story ＞ the outdoor pressure of the building, indoor air flows out of the building. In addition, owing to the pressure differences between the floor slabs of successive stories, air flows from the lower story to the upper story; under such conditions, once a fire breaks out on a lower story, the smoke flows upward, and the top story is even more dangerous than the story immediately above the fire story.

When the indoor air temperature is higher than the outdoor temperature, buoyancy is generated because of the difference in specific weight between the indoor and outdoor air. The pressure in the upper part of the building is greater than the outdoor pressure, while the pressure in the lower part is less than the outdoor pressure. When there are openings in the exterior walls, indoor air flows out through the openings in the upper part of the building, and outdoor air flows in through the openings in the lower part. This phenomenon is the stack effect (chimney effect) of a building. Under normal conditions, this phenomenon plays an important role in the air movement within buildings. During a fire, the large amount of heat released by combustion causes the indoor temperature to rise rapidly, the stack effect of the building becomes more pronounced, and the fire spreads more rapidly.

⑥ In general, smoke control and exhaust design includes one or more of the following design methods: the pressurization method; the exhaust method; the airflow method.

(i) Pressurization method. The pressurization method is the most common smoke control method; the most familiar example is pressurizing the stairwell relative to the rooms and the smoke lobby. This method is intended to create a pressure difference across the two sides of a smoke barrier; in most cases, a pressure difference of 25 Pa is created across the two sides of the smoke baffle. Usually, the smoke control and exhaust system creates a pressure difference of 25 Pa between the fire area and the adjacent area, or between the smoke lobby and the stairwell. For example, in a typical high-rise office building, the corridor of the fire area is exhausted to create a negative pressure of 25 Pa, while the rooms and the smoke lobby are kept at a positive pressure, and the stairwell is kept at a positive pressure.

In engineering practice, the pressurization method requires calculating the leakage areas of the walls, ceilings, floors, and doors on each story. For example, in a stairwell, the smoke exhaust zone usually creates a negative pressure of 25 Pa relative to the adjacent smoke zone or the stairwell. First, the areas of the stairwell walls, ceiling, and floor should be calculated; multiplying them by an air leakage coefficient gives the leakage area of the building envelope, and adding the leakage area of the stairwell doors yields the total leakage area of the stairwell. Using the total leakage area and the required pressure difference of 25 Pa, the supply air volume can be calculated, and thereby the pressurization rate can be determined.

(ii) Exhaust method. The goal of the exhaust method is to maintain a tenable environment in the space, keeping the lowest height of the smoke layer above the tolerance limit height required for the safe evacuation of occupants—generally 2.1 m above the floor level, or, according to the Japanese recommendation, $H_s geqslant 1.6+0.1H_f$ (where $H_f$ is the clear height of the room). The exhaust method is widely applied in atriums and large shopping center systems. There are several calculation models for the exhaust method, which will not be described in detail here.

(iii) Airflow method. The principle of the airflow method is to prevent smoke from moving through openings between smoke control zones by supplying a sufficient quantity of air. Factors affecting the calculation of the airflow method include the height of the opening, the temperature of the smoke, the temperature of the surrounding air, and the airflow velocity. One issue to note when applying the airflow method is that the airflow velocity must not exceed 1 m/s. If the required airflow exceeds this limit, a technical evaluation of its effectiveness should be performed to determine whether this method should be adopted.

⑦ Smoke exhaust outlets in clean areas shall be provided with measures to prevent backflow. The smoke exhaust outlets must be plate-type smoke exhaust outlets. ⑧ Smoke exhaust valves in the clean area shall be installed in an embedded (flush-mounted) manner, and the surface of the smoke exhaust valves shall be easy to clean and disinfect.

【Reference】 Smoke exhaust fire damper. The smoke exhaust fire damper is installed on the ductwork of the smoke exhaust system and remains open under normal conditions; during a fire, it closes automatically when the gas temperature in the duct reaches 280℃. It is a valve that, within a certain period of time, meets the requirements for fire-resistance stability and fire-resistance integrity and serves to block smoke and stop fire. Its composition and shape are similar to those of a fire damper. It is generally installed on the air ducts of the smoke exhaust system and is normally closed, with both manual and automatic functions. When a fire occurs, the fire detector sends out a fire alarm signal, and the controller energizes the electromagnet on the valve, causing the valve to open rapidly, or the valve can be opened manually for smoke exhaust. When the temperature reaches 280℃, the valve closes automatically and is reset manually. The valve can be interlocked with other equipment and can output an electrical signal after actuation.

After a fire breaks out in a room, the smoke exhaust outlets in the room open, the smoke exhaust fan is started at the same time to exhaust the smoke, and the occupants evacuate. When the smoke temperature in the smoke exhaust duct reaches or exceeds 280℃, the smoke already carries fire, but under normal circumstances the occupants in the room have already finished evacuating. If smoke exhaust is not stopped at this point, the fire and smoke are in danger of spreading to other areas and may cause new hazards. Therefore, it is very important to install, on the smoke exhaust ducts, smoke exhaust fire dampers that can close automatically when the smoke temperature exceeds 280℃.

The installation of smoke exhaust fire dampers shall comply with the following provisions:

① A smoke exhaust fire damper shall be installed on the smoke exhaust branch ducts of the smoke exhaust system.

② A smoke exhaust fire damper shall be installed where the smoke exhaust duct enters the smoke exhaust fan room, and it shall be interlocked with the smoke exhaust fan.

③ A smoke exhaust fire damper shall be installed on smoke exhaust ducts that must pass through fire walls, and it shall be interlocked with the smoke exhaust fan.