In recent years, due to the rapid increase in the number of cars in cities in my country, the design projects of underground parking garages have also increased rapidly. The design of ventilation and smoke exhaust ducts in underground garages is also changing, especially the problem of using both ducts has attracted the attention of HVAC designers.
1.Changes in smoke control and exhaust
1. Changes in smoke control partitions The increase in the maximum allowable building area of underground garage fire compartments is a remarkable change. Generally, for buildings with first and second level fire resistance, the maximum allowable area is 2000m², and if there is an automatic fire extinguishing system in the garage, the area can be doubled. An important result of this is the substantial increase in the area of smoke control partitions.
Since smoke control partitions cannot cross fire compartments, the maximum building area of smoke control partitions can also reach 2000m². The increase in this area naturally reduces the number of smoke exhaust systems in the garage. For example, a garage with an area of no more than 2000m² now only needs one smoke exhaust system, which is much simpler in design, construction, and operation management.
2. Changes in smoke exhaust volume According to the latest regulations, the exhaust volume of the smoke exhaust fan should be calculated based on a ventilation frequency of not less than 1 time/6 hours, and the calculated value of the smoke exhaust volume has changed significantly compared to before. Still taking a 2000m² underground garage as an example, in the past, at least 4 independent smoke exhaust systems were required. Assuming that each system bears a 500m² smoke partition, the smoke exhaust volume of one system is 30,000m³/h. If the garage is equipped with an automatic fire extinguishing system, the fire partition can be expanded to 1,000m². At this time, at least two independent smoke exhaust systems are required.
Assuming that each system bears two 500m² smoke partitions, the smoke exhaust volume of this system is 60,000m³/h. According to the "New Garage Regulations", in this example, only one smoke exhaust system needs to be designed to meet the requirements. Assuming that the calculated net height of the garage is 2.8m, at this time, regardless of whether there is a fire extinguishing system in the garage, the smoke exhaust volume is 33,600m³/h. Therefore, the more far-reaching significance is that it can enable designers to combine the exhaust system and smoke exhaust system of the underground garage into one.
2.Design of traditional underground garage ventilation, smoke exhaust system
(ventilation and smoke exhaust system)
Traditional design method:
1. The garage air supply system also serves as air supply during smoke exhaust
2. Exhaust and smoke exhaust systems share equipment and air ducts
3. The smoke exhaust port is used together with the upper exhaust port
4. During exhaust, the upper exhaust port bears 1/3 of the air volume, and the lower exhaust port bears 2/3 of the air volume
5. Since the 4 lines will cause the wind speed of the upper smoke exhaust port to be too high during smoke exhaust, in order to solve this problem, a certain number of smoke exhaust ports (normally closed) will be added at the top, and smoke and fire damper regulating valves will be installed on the lower exhaust port branch pipes. In case of fire, the smoke and fire damper regulating valves on all the lower exhaust port branch pipes will be electrically closed to open the upper smoke exhaust port (normally closed) and the exhaust port to exhaust smoke together.
3.Disadvantages of traditional underground garage ventilation and smoke exhaust system design:
A large number of electrically controlled valves are selected, which greatly increases the project cost and makes the control system complex. Once the smoke and fire damper regulating valve fails during a fire, smoke will be sucked from the bottom. Not only can the smoke not be effectively controlled and exhausted, but the smoke will spread to the lower space due to the suction of the lower exhaust port, which directly threatens the evacuation of personnel. Therefore, the reliability of this ventilation and smoke exhaust system as a smoke exhaust system is low.
4.Improvements in the design of traditional underground garage ventilation and smoke exhaust system
1. The garage air supply system also serves as the air supply during smoke exhaust
2. The smoke exhaust system only shares the main engine and main section with the exhaust system, and then has a branch road. A smoke exhaust fire damper (280°C, normally closed) is installed on the smoke exhaust branch road, and a smoke and fire regulating valve (70°C, normally open) is installed on the exhaust branch road. In case of fire, the fire control center only needs to open and close these two valves to realize the conversion of the exhaust system to the smoke exhaust system. Thus, the number of electric control valves is reduced, the engineering cost is reduced, and the stability of the system is improved. Compared with the traditional system, the smoke exhaust and the exhaust air are each provided with a branch pipe. Since the smoke exhaust port is no more than 30m away from the farthest smoke exhaust point to meet the requirements, the smoke exhaust branch pipe will not be too long, and a higher flow rate can be used when exhausting smoke, and the cross-sectional size of the smoke exhaust pipe can be made smaller.
5.Precautions for underground garage design:
Since the underground garage itself does not have very high requirements for noise, the size of the air duct should be reduced as much as possible to reduce the cost and save space.
Summary: Since the car that emits harmful substances is a moving object, and the exhaust gas containing most of the harmful substances is a high-temperature jet, the harmful gas cannot stay stably in the lower part of the garage. Therefore, the daily exhaust of the garage is all discharged from the top, and it is feasible to place all the air outlets in the upper part of the garage. At the same time, a smoke exhaust fire damper that can close automatically when the temperature exceeds 280℃ is installed on the branch pipe, so that the fire smoke exhaust duct system and the daily exhaust duct system can be completely integrated.
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