Common problems and solutions in the construction of fire smoke exhaust fans

Fire exhaust fans are usually the main smoke control facilities in high-rise civil buildings, which ensure the safety of people's lives and property. However, due to the lack of understanding of the structure, function, and performance of smoke control facilities by some designers and construction personnel, as well as their incomplete understanding of national standards and regulations, there are often some problems in the design and construction process. Below, we will provide a detailed introduction to several aspects.

1、 Natural smoke exhaust facilities cannot achieve the purpose of smoke exhaust

Natural smoke exhaust is an economical, simple, easy to operate, and convenient maintenance and management method for smoke exhaust. However, due to some projects not following the standard requirements in the design and construction process, natural smoke exhaust facilities often do not have smoke exhaust function after the completion of the project. The main reasons for this are as follows:

1. The placement of natural smoke exhaust windows is improper. From the perspective of natural smoke exhaust effect, smoke exhaust windows should be set as close to the upper part of the wall as possible. Currently, a considerable number of natural smoke exhaust windows are not set at the upper part of the wall, but at the lower part, with a large distance from the ceiling and suspended ceiling, which is not conducive to natural smoke exhaust.

2. The opening area of natural smoke exhaust windows does not meet the standard requirements. The national standard "Code for Fire Protection Design of High rise Civil Buildings" has clear regulations on the area of windows that use natural smoke exhaust. However, due to some designers not carefully calculating according to the requirements of the specifications, or calculating the area of fixed windows within the area of smoke exhaust windows, the area of some engineering smoke exhaust windows does not meet the requirements of the specifications, directly affecting the smoke exhaust effect.

3. The structural form of natural smoke exhaust windows is unreasonable. Some make the smoke exhaust window as a fixed window that cannot be opened, while others make the upper part of the window as a fixed window and place the openable smoke exhaust window at the lower part of the window, seriously affecting the smoke exhaust function.

4. The smoke exhaust window installed at a higher height lacks a convenient operating mechanism for opening. According to the specifications, smoke exhaust windows should have devices that are easy to open. However, some smoke exhaust windows with high installation heights and difficult opening are not equipped with opening operation devices, which is not conducive to the opening of smoke exhaust windows in case of fire.

2、 The required residual pressure value for setting up mechanical pressurized air supply and smoke prevention facilities is difficult to form, and the smoke exhaust effect of mechanical smoke exhaust facilities is not obvious

The phenomenon of the air volume and residual pressure values of the air supply and exhaust outlets not meeting the standard requirements is quite common in engineering. Some air supply and exhaust outlets even have wind speeds close to zero. The reasons for this phenomenon are complex and mainly include the following aspects:

1. Improper selection of fan. According to the specifications, the mechanical pressurized air supply volume for smoke proof stairwells and lobbies, shared lobbies, and fire elevator lobbies should be determined by calculation. When the calculated value is inconsistent with the value specified in the specifications, the larger of the two should be used for determination. Some designs directly determine the value given in the specifications, which often results in the selected fan air volume and pressure being too small to meet the requirements; Some designs use a combined ventilation and mechanical smoke exhaust system, but during construction, dual speed fans were not selected according to the design requirements; Some construction units do not order according to the designed fan model and purchase fans with smaller power specifications, resulting in a serious shortage of fan air volume.

2. The mechanical pressurized air supply system is duplicated with natural smoke exhaust facilities. For public buildings with a height exceeding 50 meters and residential buildings with a height exceeding 100 meters, it is recommended to install mechanical pressurized air supply systems in accordance with the requirements of the "Height Regulations". Some projects also use natural smoke exhaust in the above areas, which may result in a fire situation where the mechanical pressurized air supply system and natural smoke exhaust windows are simultaneously opened, making it difficult to form positive pressure in the smoke proof staircase and failing to achieve smoke prevention effect.

3. The shared positive pressure air supply system is not designed with a differential pressure adjustment device. According to the specifications, smoke proof stairwells and shared front rooms with mechanical pressurized air supply should be equipped with independent air supply systems. When it is necessary to share a system, an automatic differential pressure adjustment device should be installed on the branch air duct leading to the shared front room. At present, many designed shared positive pressure air supply systems do not have pressure differential adjustment devices, which cannot create a residual pressure value in the stairwell higher than that in the front room.

4. Smoke prevention zones are not equipped with smoke blocking facilities in accordance with regulatory requirements. A considerable portion of the projects, especially in large shopping malls where mechanical smoke exhaust is installed, have not installed smoke barriers under the suspended ceiling as required by regulations. Although some basements use building beams as smoke barriers, the smoke exhaust outlets of the smoke exhaust system are not installed on the ceiling or near the ceiling wall as required by regulations, but are located below the beams.

5. The air supply outlet setting does not meet the requirements. Some vertical smoke control systems use fixed louver type normally open air vents for the air supply, with similar specifications and sizes, resulting in serious imbalances in air volume and speed among the air supply vents on each floor. The air volume at the air supply vents farther away from the fan is insufficient, and even the air speed and volume at the end of the air supply vents are close to zero.

6. The construction quality of the air duct shaft is poor, and there is severe air leakage. There are quite a few projects where the ventilation shaft is not plastered, the pipeline connection is not tight, the normally closed air outlet is not tightly closed, and the air leakage is very serious, resulting in the wind speed and volume of the air supply and exhaust outlets not meeting the standard requirements. Some construction units even cancel the air duct connecting the vertical shaft to the ceiling air outlet, and use the ceiling space instead of the air duct, which often causes the wind speed of the air duct air outlet to be close to zero.

At the same time, some smoke exhaust systems have their fan exhaust ports located indoors, which can cause smoke to be expelled from indoors in the event of a fire. This not only loses the smoke exhaust function, but may also lead to the spread and expansion of the fire.

3、 The power distribution of the smoke exhaust fan does not meet the regulatory requirements

One is that the power supply and distribution of the fan cannot meet the load level requirements of high-rise civil buildings. Some power supply lines are not connected to the fire power supply, but to the floor lighting distribution box. Some designs use single circuit distribution lines, while others do not have automatic end power switching devices, which cannot meet the requirements of dual circuit power supply loads at the first and second levels. Therefore, it is required to install automatic end switching devices.

The second issue is that the installation of exposed distribution lines does not meet the requirements. Some distribution lines of smoke exhaust fans are installed in PVC plastic pipes, while others are installed in metal pipes without fireproof coating, which does not meet the fire performance requirements for pipe installation.

4、 The parts that should be equipped with mechanical smoke control facilities are not set up according to the specifications

1. When smoke proof staircases and lobbies, fire elevator lobbies or shared lobbies in high-rise civil buildings with podiums are equipped with natural smoke exhaust through openable external windows, and the podiums do not have natural smoke exhaust conditions, the lobbies or shared lobbies are not equipped with positive pressure air supply systems according to regulatory requirements.

2. For interior corridors over 20 meters without natural smoke exhaust, some designers believe that they have the conditions for natural smoke exhaust due to the presence of natural smoke exhaust in the front room of the smoke proof staircase connected to them, and do not install mechanical smoke exhaust facilities as required by regulations.

countermeasure

In response to the above-mentioned problems, the fire supervision department should strengthen the supervision and management of the design and construction of building smoke control facilities from the following aspects:

1. We should strengthen the professional guidance and training of personnel involved in the design and construction of smoke control facilities. Fire departments in various regions generally attach great importance to the professional guidance and training of personnel in the design and construction of fire alarm, electrical, and fixed (automatic) fire extinguishing facilities. However, during the training process, the design and construction personnel of smoke control facilities are often overlooked.

2. The system should be implemented where the construction of smoke prevention and exhaust facilities is undertaken by the fire protection facility construction unit. Smoke control facilities are an important component of building automatic fire protection facilities, and according to regulations, their construction should be undertaken by the fire protection facility construction unit. However, currently many projects' smoke control facilities are installed by air conditioning professional construction units.

3. During the construction drawing review, the construction review personnel of the fire supervision department should attach great importance to the review of the design of smoke control facilities. They should focus on the setting and window area of natural smoke exhaust windows, the location of mechanical smoke control facilities, fan selection, the setting of air supply and exhaust outlets, smoke control zones, etc. At the same time, they should strengthen the supervision and management of smoke control facilities during construction, make smoke control facilities an important part of construction supervision and inspection, timely discover problems, guide and supervise the construction unit to rectify them.

Installation skills

1. Usually, the screw rod needs to pass through the floor slab, with pre embedded steel plates and double nuts welded together

2. Pay attention to the issue of double nuts. It is necessary to use stainless steel nuts for basement nuts

3. The flexible soft joints at both ends of the fire exhaust fan need to be made of non combustible materials, and the sealing gasket also needs to be made of non combustible materials

4. The foundation requirement level and consolidation of the fire smoke exhaust fan, and the foundation height is greater than or equal to 200mm

5. It is necessary to fix the steel bracket of the fan on the concrete foundation, and it is necessary to add rubber damping pads between the steel bracket of the fan and the foundation. All fan and motor components are installed on a single steel bracket, and the steel ground frame is installed on the vibration damping pad at the top of the foundation. The vibration damping pad is made of porous rubber plate

6. The depth of the foundation buried below the ground should be constructed according to the planning requirements, usually not less than 50mm. The concrete should be dense, the surface should be lubricated, and the concrete strength grade should meet the planning requirements

How to calculate the air volume of fire smoke exhaust fan and supply fan

The calculation method for the air volume of the fire smoke exhaust fan and the air volume of the supply fan is determined based on the building area. Each smoke control zone needs to calculate its smoke exhaust volume independently. When the smoke exhaust fan is responsible for one smoke control zone, it can be calculated based on an area of not less than 60m ³/h per square meter. If it is responsible for multiple smoke control zones, it can be calculated based on an area of not less than 120m ³/h. The air supply volume is calculated based on 50% of the smoke exhaust volume. Each smoke prevention zone should not exceed 500 square meters.

In common fire and smoke control systems, the smoke exhaust volume is generally calculated based on the maximum smoke control zone area of 500 * 120, which is 60000 cubic meters per hour. When designing the smoke exhaust volume, attention should be paid to the air leakage rate, which can generally be calculated as 10% of the minimum air leakage rate.

For example, when the building area is 1000 square meters, two smoke control zones need to be set up. The maximum area of each smoke control zone is 500 square meters, and the smoke exhaust volume is 500 * 120 ³/h=60000 ³/h. In addition, the minimum air leakage volume of 10% is 6000 ³/h. The total smoke exhaust volume should be 66000 ³/h, of which the air supply volume should be 50% of the smoke exhaust volume and 33000 ³/h.