I. The significance of ventilation
To make the indoor concentration of pollutants to meet the relevant standards.
II. Main sources of pollutants
In the human-dominated indoor environment, the main pollutants are.
- Carbon dioxide from human metabolism and skin surface metabolites;
- 2. Harmful substances emitted from construction materials, such as benzene, aldehydes and other organic substances;
- Radioactive substances such as radon in the surrounding soil;
- Dust and sulfur dioxide in outdoor air.
III. Types of the ventilation system
Basis: The power source of air.
(1) Natural ventilation system
Natural ventilation relies on the heat pressure caused by the temperature difference between indoor and outdoor air ((actually the density difference), or the wind pressure caused by outdoor wind, so that the air inside and outside the room can be exchanged, thereby improving the indoor air surroundings.
Natural ventilation does not require additional power equipment.
It is an economical and effective ventilation method for workshops with a lot of waste heat.
The disadvantage is that it cannot handle the air entering the indoor and outdoor spaces, and it is also difficult to purify the dirty air discharged from the indoor to the outdoor; secondly, natural ventilation is affected by outdoor weather conditions and the ventilation effect is unstable.
(2) Mechanical ventilation system
The method of a mechanical ventilator to make airflow, resulting in room ventilation method, is called mechanical ventilation.
Since the air volume and air pressure of the fan can be determined according to the needs, this ventilation method can ensure the required amount of ventilation, control the direction and speed of the airflow in the room, and carry out the necessary treatment to the air inlet and exhaust air, so that the air in the room can reach the required parameters.
Therefore, the mechanical ventilation method is widely used.
Natural ventilation system
The working principle of natural ventilation
For a building or a room, if it has two openings (doors or windows, etc.), and the air pressure on both sides of each opening is not the same, then the air flows at each opening under the effect of the pressure difference.
Natural ventilation under outdoor wind pressure:
Mechanical ventilation system
- Overall ventilation
1) Principle: Overall ventilation is to ventilate the entire room.
The basic principle is to dilute (dilute) the concentration of harmful substances in the indoor air with clean air, and continuously discharge the polluted air to the outside at the same time, to ensure that the indoor air environment meets hygienic standards.
Overall ventilation is also called dilution ventilation.
Location of air supply and exhaust vents for general ventilation:
When designing a comprehensive ventilation system, a basic principle should be observed: clean air should be sent directly to the staff’s location or to a place with a low level of pollutants.
Common types to send and exhaust air include top delivery with top exhaust, bottom delivery with top exhaust, middle delivery and two-exhaust etc.
For specific applications, the following principles should be as followed.
The air inlet should be located upwind of the exhaust vent;
The air intake should be located close to the location of staff or in an area with a low concentration of pollutants;
Exhaust vents should be located in areas of high pollutant concentration;
In the entire control space, trying to make the indoor airflow uniform and reducing the existence of eddy currents are needed to avoid the accumulation of pollutants in local areas.
- Local ventilation
(1) Principle: Local ventilation is divided into the local air intake and local exhaust, its basic principle is to control the local airflow, so that the local working area is not polluted by harmful substances and to create an air environment that meets the requirements.
Theoretical mechanism of building and natural ventilation
Natural ventilation in the usual sense is to produce airflow by purposeful openings.
This flow is directly influenced by the pressure distribution on the building’s exterior surface and the characteristics of the various openings.
The pressure distribution is the driving force, while the characteristics of the individual openings determine the resistance to flow.
In terms of natural ventilation, there are two main reasons for air movement in buildings:
Wind pressure and air density difference between indoor and outdoor.
These two factors can work alone or together.
- Natural ventilation under wind pressure
The formation of wind is due to the pressure difference in the atmosphere.
If the wind encounters obstacles on the passage, such as trees and buildings, it will produce energy conversion.
The dynamic pressure is transformed into static pressure, so positive pressure (about 0.5-0.8 times of wind speed dynamic pressure) is generated on the windward side, and negative pressure (about 0.3-0.4 times of wind speed dynamic pressure) is generated on the leeward side.
The pressure difference that occurs when passing through the building encourages air to flow into the room from the windows and other gaps on the windward side, while indoor air is discharged from the leeward opening, forming natural ventilation with full ventilation.
The wind pressure around a building is related to the geometric shape of the building, the position of the building relative to the wind direction, the wind speed and the natural topography around the building.
- Natural ventilation under thermal pressure
Hot-pressing is caused by the temperature difference between indoor and outdoor air, which is known as the “chimney effect”.
As a result of the temperature difference, a difference in density between indoor and outdoor is created and a pressure gradient is created along the vertical direction of the building wall.
If the indoor temperature is higher than the outdoor temperature, there will be a higher pressure in the upper part of the building and lower pressure in the lower part of the building.
When orifices exist at these locations, air enters through the lower opening and exits through the upper part.
If the indoor temperature is lower than the outdoor temperature, the airflow is in the opposite direction.
The amount of hot-pressing depends on the difference in height between the two openings and the difference in air density between inside and outside.
In practice, architects often use chimneys, ventilation towers, patio atriums and other forms to provide favorable conditions for the use of natural ventilation, so that the building can have good ventilation.
- Natural ventilation under a combination of wind and hot-pressures
The natural ventilation in the actual building is the result of the joint action of wind pressure and hot-pressures, but the effect of each has its own strength or weakness.
Since wind pressure is influenced by the weather, outdoor wind direction, building shape, surrounding environment and other factors, the joint action of wind pressure and hot-pressures is not a simple linear superposition.
Therefore, architects should take all factors into consideration to make the wind pressure and hot-pressures complement each other and work closely together to achieve effective natural ventilation of the building.
- Mechanically assisted natural ventilation
In some large buildings, due to the long ventilation path, large flow resistance simply relying on natural wind pressure and heat pressure is often not enough to achieve natural ventilation.
For cities with serious air pollution and noise pollution, direct natural ventilation will also bring polluted outdoor air and noise into the room, which is not good for human health.
In this case, a mechanical-assisted natural ventilation system is often used.
The system has a complete set of air circulation channels, supplemented by air treatment means in line with ecological thinking (such as soil pre-cooling, pre-heating and deep water heat exchange, etc.), and indoor ventilation is accelerated with the help of certain mechanical methods.
Equipment and components of the ventilation system
Natural ventilation systems generally do not require equipment.
The main equipment for mechanical ventilation includes fans, air ducts, air-valve, tuyere and dust removal equipment, etc.
- The role of the fan in the pipeline: transport air.
- The basic structure of the fan: impeller, motor and shell.
- Types of fans:
- 1) Centrifugal fan;
- 2) Axial fan;
- 3) Mixed flow fan.
Centrifugal fan: used for low-pressure or high-pressure air supply systems, especially low-noise and high-pressure systems.
There are four types of impeller blades: streamline blades, back curved blades, forward curved blades and radial blades.
Fans in comfort air conditioners typically use centrifugal fans.
Four impeller designs make up the four basic forms of a wind turbine:
(1) Backward-blade fan: Straight backward-curved blade, curved blade or winged blade.
- Straight backward curved blade: a straight single metal .
- Backward curved blade: curved metal blade
- Winged blade: Double-layered metal blades increase the efficiency of air flow through the impeller.
It is mainly used for operating investment savings can be higher than the initial investment.
(2) The fourth type is the forward-curved blade, which has a curved, single-layer metal blade.
Four types of impeller
Forward curved impellers vs backward curved impellers
(1) Forward curved impeller
A large number of small, lightweight blades make up the rest of the turbine, and the rest of the turbine is also light material.
These are lighter than the winged impellers.
Features: Depending on the design, a forward-facing fan can move more air at a lower speed than a backward-facing fan of the same diameter.
Any type of backward-facing fan can operate at half the speed of a backward-facing fan when delivering the same volume of air.
Thus the forward curved fan has a lower noise level and low price, which makes it the best choice for low to medium pressure operation.
(2) Backward curved impeller
The fan is more efficient than the forward curved fan under large capacity and large differential pressure, so the backward curved fan is used in many cases of medium pressure operation.
Two typical fan impellers
The structure of the axial fan is shown in the figure.
The impeller consists of a wheel and a blade riveted to it, and the blade is mounted at an angle to the plane of the wheel.
The type of blade has the airfoil twisted blade or straight blade; equal-thickness twisted blade or straight blade.
It occupies a small area, is easy to maintain, has a low air pressure, and has a large air volume.
It is mostly used for large air volume systems with low resistance.
Schematic diagram of the axial fan structure
Small site area, easy maintenance, lower air pressure, larger air volume, which is mostly used in large air volume systems with low resistance.
(3) Mixed flow fan
Concentrates the characteristics of centrifugal fan with high pressure and axial flow.
(4) Common fans for building
High-temperature smoke control and extraction fan: can be used for daily ventilation under normal conditions.
When encountering fire, it extracts high-temperature indoor flue gas to enhance indoor air circulation.
It has the characteristics of high-temperature resistance.
It is suitable for ventilation and smokes exhaust in high-rise buildings, ovens, garages, tunnels, subways, underground shopping malls and other places.
This series of fans can be divided into single-speed fan and double-speed fan.
It has the advantages of compact structure, small volume and convenient maintenance etc.
According to different occasions, it is possible to change the installation angle, the number of blades, the rotational speed, the machine number and other methods to meet the requirements of various uses.
The roof and the side-wall fan: It has the common centrifugal roof fan and the low noise centrifugal roof fan, which applies to air exchange in the workshop, the warehouse, the high building, the laboratory, the theater, the hotel and the hospital etc.
Air-conditioning ventilation fan: Centrifugal air-conditioning fan has the advantages of large performance and application range, low noise, light weight, convenient installation and reliable operation.
It can be matched with the combined air conditioning units of various air conditioning plants.
Smoke exhaust cabinet fan
II. Air duct
- Form: round and rectangular;
- Material: brick and concrete; thin steel plate; glass fiber board; aluminum polyvinyl chloride board; hose material.
III. Partial components
- Air duct supports
(1) Function: anti-vibration, load-bearing;
(2) Form: connection of air duct and bracket: fixed and not fixed.
Bracket support methods: brackets, hangers and brackets.
Right-angle elbow and arc elbow: to change the direction of airflow.
- (a) Double blade
- (b) Single blade
- Tee: merging tee; shunttee
- Pipe reducer
1) Sudden expansion and contraction: a change in wind volume. (See left figure below)
2) Gradient tube: airflow change. (See right figure below)
- Air duct valves
- Regulate air flow, open or shut off the wind system: butterfly valve, opposed multiblade damper, 3-way regulating valve;
- Fire damper: when a fire occurs, cut off the airflow path, which is to prevent the fire from spreading along the air ducts;
(3) Check valve: to prevent airflow reversal after the fan stops.
Precautions in the design of the air duct system:
- Air duct layout
The arrangement of air ducts should be straight to avoid complicated partial components like elbows and tees etc. is arranged properly.
The connection with the air duct should be reasonable to reduce resistance and noise.
The air duct should be provided with necessary adjustment and measuring devices or reserved for the interface of the measuring device.
The adjustment and measurement device should be located in a place that is convenient for operation and observation.
- The cross-sectional shape of the air duct
In the same area, the resistance of the circular duct is smaller than that of the rectangular duct.
When the rectangular duct is designed, the ratio of long and short sides is below 3.0.
- The layout of the inlet and outlet of the fan
The connecting pipe of the inlet and outlet of the fan has a great influence on the performance of the fan.
Because the dynamic pressure of the air at the inlet and outlet is very high, improper connection pipe practice will cause considerable head loss, and severely punished loss of air volume.
For this reason, attention must be paid to this problem in the pipeline design.
1) The distance from the inside of the turn or elbow of the duct to the inlet of the fan shall be greater than the diameter of the inlet of the fan, which is to ensure uniform airflow into the fan impeller.
When the turning radius of curvature is not enough, deflector vanes should be added at pipe bends, as shown in the following figure.
2) When the air duct enters the fan with altered diameter, the requirement (cierta) ≤45° shown in the following figure, generally ≤30 is better.
3) For double-inlet fans, it needs to ensure B≥1.25D, as shown in the figure below.
4) The turning near the outlet of the fan must be consistent with the direction of rotation of the fan impeller to make the air flow unobstructed and even and avoid unnecessary energy loss.
5) There should be a straight pipe section with a diameter of less than 3D (D is the diameter of the fan inlet) from the outlet of the fan to the turn to avoid unnecessary static pressure loss.
6) The flexible joint should be added at the inlet and outlet of the fan to reduce the influence of vibration; the material of flexible joint should be artificial leather or canvas.
- Location of tuyere
The air inlet is the inlet of the ventilation and air conditioning system to collect fresh outdoor air, and its location should meet the following requirements:
(1) Locate in a location with clean outdoor air.
(2) To prevent the exhaust air from being sucked back into the system, the air inlet should be located on the upwind side of the exhaust air outlet and lower than the exhaust air outlet.
(3) The distance between the bottom of the air inlet and the outdoor ground is generally not less than 2cm to avoid inhaling ground dust.
(4) The air inlet of the cooling system should be located on the external wall with the sun at the back.
Dust removal equipment
In order to prevent air pollution, the exhaust system should be purified according to the actual situation before the air is discharged into the atmosphere, so that the dust is separated from the air.
The equipment for this treatment process is called dust removal equipment.
There are many types of dust removal equipment, including baffle dust collector, cyclone dust collector, bag type dust collector and spray tower dust collector and electric dust collector etc.
Fireproofing and smoke exhaust
In order to prevent the spread of fire and hazards, in high-rise buildings, fire and smoke exhaust design must be carried out.
The purpose of fire prevention is to prevent the spread of fire and extinguish the fire.
The purpose of the smoke exhaust is the fire smoke will be eliminated in a timely manner, which is to prevent the spread of smoke to the outside and ensure the success of evacuation for indoor people.
In the design for fireproofing and smoke exhaust in a high-rise building, the building is usually divided into several fire and smoke partitions and the partitions are separated by firewalls and fire doors to prevent fire and smoke from spreading from one partition to another.
Mechanisms of smoke diffusion
The smoke refers to the floating state of solid and liquid particles in the air produced by incomplete combustion of substances.
The flow and diffusion of smoke is mainly influenced by factors such as wind pressure and thermal pressure.
Wind pressure refers to the pressure generated when the wind blows to the external surface of a building, due to which the air flow is hindered, the speed is reduced and part of the kinetic energy is transformed into static pressure.
On the windward side, the outdoor pressure is greater than the indoor pressure, and air permeates from outside to inside.
During a fire, if a window is on the windward side of the building, the wind pressure effect can cause smoke to quickly spread throughout the lost floor and even blow it into other floors.
The hot-pressure or chimney effect is caused by the force created by the difference in density between indoor and outdoor air and the height of the air column.
The effect of heat-pressure increases with the increase of the temperature difference between indoor and outdoor and the height of the shaft.
When a fire occurs, the temperature inside a high-rise building is much higher than the outdoor temperature.
Coupled with the greater influence of the height of the high-rise building shaft, the hot-pressure increases significantly, and the smoke will spread upward along the building shaft.
The lower the fire floor, the more obvious the chimney effect.
It can be seen that when a fire occurs in the lower part of a building or a room on the windward side, due to the effects of wind pressure and heat pressure, the fire is more harmful than the upper part of the building or a room on the leeward side.
In addition, when a fire occurs, the power provided by the air conditioning system fans and the chimney effect created by the vertical ducts can cause smoke and fire to spread along the ducts, quickly spreading as far as the ducts can reach.
Therefore, the smoke prevention and exhaustion of high-rise buildings need to adopt various forms such as natural smoke exhaust, mechanical smoke prevention, and mechanical smoke exhaust to prevent the spread of smoke in the evacuation passages inside the building and ensure safety.
In addition, the building’s ventilation and the air-conditioning system should take fire and smoke prevention measures.
Forms of building fire and smoke exhaust:
- Natural smoke exhaust
The natural smoke exhaust is a smoke exhaust method that uses wind pressure and hot-pressure as power.
It has the advantages of simple structure, energy-saving and high operational reliability etc.
In high-rise buildings, buildings with anti-smoke stairwells and front rooms against external walls, front rooms of fire elevator rooms and shared front rooms should adopt natural smoke exhaust methods.
The location of the smoke outlet should be located on the leeward side of the building’s dominant wind direction throughout the year.
- Mechanical smoke prevention
Mechanical smoke prevention is a smoke prevention technology that adopts a mechanical pressurized air supply method to control the flow direction of the flue gas by the gas flow and pressure difference generated by the fan.
When a fire occurs, the pressure difference caused by the airflow of the fan prevents the smoke from entering the safe evacuation passage of the building, thereby ensuring the needs of evacuation and fire fighting.
For no spread out balcony and concave corridor, smoke-proof stairwell and front room with different orientations capable of opening external windows, fire elevator front room and both-shared front room, mechanical smoke prevention facilities should be provided.
When the refuge floor is a fully enclosed refuge floor, pressurized air supply facilities shall be provided.
- Mechanical exhaust:
The mechanical exhaust is a mechanical exhaust method, which uses the gas flow and pressure difference produced by the fan to exhaust the flue gas or dilute the concentration of the flue gas by using the exhaust pipe.
The mechanical exhaust method is suitable for internal walkways, rooms, atriums and basements that do not have natural exhaust conditions or are difficult to perform natural exhaust.
It should be designed and built-in strict accordance with the requirements of the mechanical exhaust. (Such as the setting of the exhaust port, the selection of the exhaust fan and the selection of air duct materials etc.)
The control procedures of the mechanical smoke exhaust system can be divided into two types: no fire control room and a fire control room.
- Fire protection of ventilation and air conditioning systems
After a fire occurs, It needs to control the fire spread to other fire compartments.
Therefore, fire dampers must be installed in the ventilation ducts of the ventilation and air conditioning system, and certain fire prevention measures must be taken.
The fire damper should be set at:
- The partition wall of the crossing fire compartment;
- Crossing machine room and important rooms or partition walls and floor slabs of rooms with fire hazards;
- Junction of horizontal ducts connected to vertical ducts;
- The sides of the crossing deformation joints
The operating temperature of the fire damper is 70°C.
The pipes, thermal insulation materials, noise-absorbing materials and adhesives used in the ventilation and air-conditioning pipeline engineering shall be made of non-combustible or non-combustible materials.
Fire and smoke prevention equipment and components:
It mainly include fire dampers, smoke exhaust valves and smoke exhaust fans.
- Fire damper
Fire dampers can be controlled by thermal components, smoke sensing thermostats and compound control etc.
When the fusible ring is used, the fusible ring will fuse and fall off in case of fire, and the valve will be closed by spring force or self-gravity.
When using thermistor, thermocouple and bimetal etc., the valve is closed by driving the micro motor controlled with sensors and electronic components.
The electromagnet and motor action of the control actuator or the control pneumatic actuator can close the valve under the action of the spring force or close the valve by the rotation of the motor.
The valve closing drive mode of fire damper has four types:
- Spring force driven (or electromagnetic)
- Motor driven
- Pneumatic driven
The commonly used fire dampers are:
- Gravity fire damper
- Spring fire damper
- Spring fire control valve
- Fire vent
- Pneumatic fire damper
- Electric fire damper
- Electronic self-control smoke
Structure of temperature fuse
- Fire damper
Installed in the smoke exhaust system, usually closed.
When a fire occurs, the signal from the control center will control the work of the actuator to realize the opening of the valve under the action of spring force or motor torque.
The exhaust smoke valve with a temperature sensor device will act when the fire temperature reaches the action temperature, and the valve will be closed under the action of spring force to prevent the fire from spreading along the exhaust duct.
Smoke evacuation valves can be divided as follows:
- According to the control mode, there are two types: electromagnetic type and electric type;
- According to structure type can be divided into decorative smoke evacuation valve, flap smoke evacuation valve, smoke evacuation and fire damper;
- According to structure type, it can be divided into decorative smoke vent, flap smoke vent, smoke and fire vent;
- According to shape, it can be divided into rectangles and round valves.
- Anti-smoke ventilator
The anti-smoke exhaust fan can use a general-used fan, or a special fan for fire and smoke exhaust.
When the smoke temperature is low, it can be operated for a long time, and when the smoke temperature is high, it can be continuously operated for a fixed time, and usually there are more than two grades of rotation speed;
The commonly used fire and smoke exhaust special fans are HTF series, ZW series, W-X series and other types.