1 Common problems of mechanical pressurized air supply system

1.1 Building components protrude into the civil shaft, increasing the air duct resistance

During the design review, it was found that some beams and columns in the air duct shaft of some smoke prevention systems partially protrude into the civil shaft, resulting in large local resistance. The designer did not consider this factor, and the air pressure of the selected pressurized air blower was insufficient, resulting in that the air volume of the pressurized air supply system could not meet the requirements of fire acceptance.

for example, a 28-story office building is 3.1 m high, and the pressurized air supply volume of the front room or shared front room is 22 111 m3/h, and the shaft size is 811 mm×811 mm Fig. 1 shows that there is no protrusion in the air shaft, fig. 2 shows that there is a floor beam protruding by 125 mm in the air shaft, and fig. 3 is a cross-sectional view of fig. 2. The wind well resistance under three conditions is calculated as follows. In order to simplify the calculation steps, only the resistance of the 26-story main air shaft is calculated, excluding the resistance of the air inlet section of the hot-pressing and pressurized blower, the positive-pressure air outlet and some air shafts behind the air outlet. The calculation formula and method are based on the Practical Heating and Air Conditioning Design Manual (hereinafter referred to as the Manual).

the formula for calculating the resistance loss δ P1 along the air duct:

where k is the absolute roughness correction coefficient; δ PM is the frictional resistance along the unit pipe length, Pa/m; L is the length of air duct, m.

calculation formula of local resistance loss Δpj of air duct:

where ζ is the local resistance coefficient; V is the air velocity where local resistance occurs in the shaft, m/s; ρ is the air density, kg/m3.

1.1.1 smooth shaft resistance calculation

Calculate the resistance loss δ P1 along the 26th floor shaft of pressurized air supply shaft in Figure 1. Substituting the known data into equation (1) gives Δ P1 = 173.2 Pa. In Figure 1, the local resistance of the pressurized air supply shaft is 1, so the total resistance loss δ Δpz1 of the 26-story shaft is 173.2 Pa.

1.1.2 calculation of the resistance of building components protruding into the shaft

In Figure 2, the protruding beam width of the pressurized air supply shaft is 1.25mm, the beam height is 611 mm, the shaft size is 811 mm×811 mm, and the height of each floor is 3 m.

1) Calculate the resistance loss along each floor of the shaft. According to the above calculation method, it is concluded that the resistance Δp11 along the pipeline section 1 is 5.55 Pa；; The section size of section 2 is 675 mm×811 mm, the height of each layer is 1.6 m, and the resistance Δp12 along the way is 1.72 Pa.

2) Calculate the local resistance loss of each floor of the pressurized air supply shaft. There is no completely corresponding pipe fitting in the Manual for the local resistance coefficient of beam protruding into the shaft. Look up the table according to the variable diameter of rectangular air duct, and the abrupt change is 181 according to the gradual change angle.

duct section 1: the area ratio is 1.84, and ζ 1 = 1.18 according to the insertion method; The local resistance δ Δpj1 is 4.38 Pa.

duct section 2: the area ratio is 1.19, and ζ 2 = 1.26 according to the insertion method; The local resistance δ Δpj2 is 19.99 Pa.

in fig. 2, each floor of the pressurized air supply shaft protrudes into the beam, and the total resistance loss δ Δpz1 of the 26-floor shaft is 822.64 Pa.

1.1.3 Calculation of resistance of shaft with smooth inner wall of reduced section

In Figure 4, the cross-sectional area of shaft is reduced, but the inner wall of shaft is smooth and the local resistance is 1; When the size of the pressurized air supply shaft is 811 mm×675 mm, Δ P14 = 268.4 Pa. The total resistance loss Δpz4 of the 26-story air shaft with reduced cross-sectional area and smooth inner wall is 268.4 Pa.

fig. 4 schematic diagram of shaft cross-sectional area reduction

After calculation, it is concluded that the total pressure loss of pressurized air supply shafts in figs. 1, 2 and 4 is 173.2, 822.6 and 268.4 Pa respectively.

It can be seen that the resistance loss varies greatly with different shaft designs; Obstacles protrude into the air shaft, and the local resistance increases obviously. If the resistance of the air supply system is not calculated in detail, the air pressure of the selected blower will be insufficient, and the pressurized air supply volume will be insufficient during operation, which will affect the smoke prevention effect.

when designing, it is necessary to closely cooperate with architecture, structure and other specialties, and give priority to the selection of air shaft with flat and smooth inner wall, which can appropriately reduce the area and increase the wind speed, and handle it as shown in Figure 4. We should also pay attention to the influence of the change of the thickness of the upper and lower sections of the transfer beam and shear wall on the resistance of the wind well.

1.2 Multi-leaf air supply outlets protrude into the civil shaft to increase the air duct resistance

Some air shafts have a wall thickness of only 111 mm, while the length of multi-leaf air supply outlets is 275 mm. Installing multi-leaf air supply outlets on thin walls will protrude into the air shaft by 175 mm, thus forming a large local resistance.

as shown in fig. 5, the multi-leaf air supply outlet protrudes into the shaft by 175 mm, forming two local resistance parts, namely sudden contraction and sudden expansion. According to the above conditions and calculation method, the pressure loss of the shaft is calculated to be 581.3 Pa, which is 417.1 Pa higher than that of the shaft with smooth inner surface.

the following methods can be adopted: 1) when selecting a fan, increase the wind pressure of the fan by 411 Pa to overcome the local resistance; 2) The wall of the multi-leaf air supply outlet is thickened to 251 mm；; 3) As shown in Figures 6 and 7, the remote control multi-leaf air supply outlet is installed in the ceiling, and the air duct is connected, but the section of air duct should be made of 2.1 mm thick steel plate fire-proof air duct.

1.3 Pay attention to the construction and acceptance of civil shaft

During the acceptance, the author found that the internal surface of many civil air shafts was not plastered, and the resistance of air shafts would increase, even the scaffolding holes on the partition wall of air shafts were not blocked, and the air leakage and channeling were serious, and the air volume of the system could not be guaranteed.

if the air volume of the pressurized air supply outlet can't meet the requirements of fire inspection and acceptance, and there are no other failure reasons, it is advisable to check the sealing performance and make rectification from the sealing aspect of the civil shaft.

The construction space of civil air duct is narrow, which brings great difficulties to construction, acceptance and rectification, and all management departments must pay attention to it. The retaining structure of air shaft should be dense, especially under beams and slabs, and the plastering should be continuous; The plastering of the inner wall should be plastered as you build by laying bricks or stones, and the thinnest part should not be less than 11 mm；; The inner wall should be flat and smooth and airtight.

1.4 Suggestions

1) There are many water wells and air shafts in public buildings, and their mutual influence should be considered in the layout. For example, the air supply shaft should not be separated from the smoke exhaust shaft to avoid air channeling and air leakage; When the water pipe shaft is adjacent to the air supply shaft or the smoke exhaust shaft, a solid wall with a thickness of more than 211 mm should be used for separation.

2) For public buildings with a height of more than 111 m, a refuge floor (room) should be set up, and the pressurized air supply and smoke prevention system that closes the refuge floor (room) is generally set up on this refuge floor, and the pressurized air supply outlet is directly set on the external wall of the refuge floor. If the upper floor of the refuge floor is on fire, it can be considered that it will not be affected; If the floor below the refuge floor is on fire, with the spread of smoke, the air intake is threatened by fire and smoke. If the air intake is set in different directions, it can adapt to the influence of fires in different directions and outdoor wind direction, but this method is complicated and difficult to control. In the design, the air intake of the pressurized air supply system should be located at the lower part of the service area of the system as far as possible to avoid the threat of fire and smoke to the outdoor air intake.

2 Common problems of mechanical smoke exhaust system

2.1 Insufficient air volume of smoke exhaust system

Because the smoke exhaust system is not used at ordinary times, in order to save space when designing, the wind speed of smoke exhaust duct is usually high. Without detailed calculation, the air pressure of smoke exhaust fan will be lower, resulting in insufficient smoke exhaust. As shown in Figure 8, the floor 1 of the smoke exhaust system is the most unfavorable loop. The length of the horizontal pipe from the farthest smoke exhaust port to the shaft is considered as 61 m long, with 2 elbows and 1 smoke exhaust fire dampers. The wind speed of the smoke exhaust port is 9 m/s, and the smoke exhaust system resistance is calculated in four working conditions: A, B, C and D (the local resistance coefficient is selected according to the Manual, and the calculation process is omitted).

fig. 8 schematic diagram of smoke exhaust system

1) Working condition A: the horizontal main wind speed of smoke exhaust is 17 m/s, the shaft adopts civil shaft, and the shaft wind speed is 13 m/s. The calculation shows that the resistance of the horizontal main pipe section of the 1 # floor is 812 Pa, and the local resistance accounts for 81%, which is mainly the local resistance of the smoke exhaust fire damper, the smoke exhaust tee, the elbow and the main air duct entering the vertical tee (calculated by 91 confluence tee); The resistance of the civil shaft section is 145 Pa, and the local resistance accounts for 75%, which is mainly the local resistance where the shaft leads into the fan duct section. The resistance of exhaust fan section is 217 Pa, which is mainly the local resistance of exhaust fire damper, fan inlet and outlet. The total resistance of the smoke exhaust system under working condition A is 1.164 Pa.

2) working condition b: the shaft is changed to be lined with galvanized steel duct (the main duct is calculated as a 45 junction tee when it enters the shaft), the wind speed of the main duct is 17 m/s, and the resistances of the horizontal main duct section, the shaft section and the fan section are 711 pa, 35 pa and 217 Pa respectively. The total resistance of the smoke exhaust system under working condition B is 953 Pa.

3) working condition c: the wind speed of the horizontal main pipe of smoke exhaust is 12 m/s, the civil shaft and fan section are the same as working condition a, and the resistances of the horizontal main pipe section, shaft section and fan section are 449,145,217 pa respectively. The total resistance of the smoke exhaust system under working condition C is 811 Pa.

4) working condition d: the wind speed of the horizontal main pipe of smoke exhaust is 12 m/s, the shaft is lined with galvanized steel duct, and the resistances of the horizontal main pipe section, the shaft section and the fan section are 399, 35 and 217 Pa respectively. The total resistance of smoke exhaust system under D condition is 651 Pa.

From the above calculation, it can be seen that:

1) Reducing the wind speed of the exhaust duct is an effective measure to reduce the resistance of the exhaust system. In order to save building space, the cross-sectional area of air duct is generally reduced, but the wind speed is increased. When the wind speed of the smoke exhaust system is close to the limited wind speed (metal duct 21 m/s, nonmetal duct 15 m/s), the large resistance of the smoke exhaust system is close to the maximum pressure head reached by common smoke exhaust fans. When the wind speed of the main air duct is about 15 m/s, the resistance is high, and the smoke exhaust systems are quite different. It is necessary to calculate the resistance of the air duct in detail before choosing a suitable smoke exhaust fan.

2) The local resistance is about 81% of the air duct resistance. Therefore, diversion devices should be used at the inlet and outlet of the smoke exhaust shaft as far as possible, and long current-sharing buffer sections should be set at the inlet and outlet of fire dampers and fans as far as possible, and pipe fittings with small local resistance coefficient should be selected. If the height and space of some engineering projects are limited, and the cross section of the air duct must be reduced, the optimal design can be carried out to reduce the local resistance. For example, under the working condition of A, the horizontal air duct enters the shaft, occupying the local space of the ceiling, and the height of the air duct locally changes from 411 mm to 631 mm, and the wind speed locally decreases, and the wind speed decreases from 17 m/s to 11.8 m/s by gradually expanding the pipe; The top of the air shaft is connected with a fan duct and an elbow. After two optimization designs, the calculated local resistance can be reduced by 299 Pa. See Figure 9 for measures to reduce local resistance of smoke exhaust system, and the calculation process is omitted.

fig. 9 measures to reduce local resistance of smoke exhaust system

3) For a large smoke exhaust system with a long horizontal exhaust duct, the negative pressure of the smoke exhaust shaft is high, and the air leakage is also large. In order to reduce the air leakage and resistance of the shaft, the method of lining the steel plate air duct in the shaft can be adopted.

2.2 There are too many smoke exhaust valves (ports) in the smoke exhaust system, and the normally closed smoke exhaust valves (ports) have a large air leakage

According to article 6.12.3 of GB 15931—2117 Fire Prevention Valves for Building Ventilation and Smoke Exhaust Systems, the static pressure difference between the two sides of the fire prevention valves or smoke exhaust valve blades shall be kept at (311±15) Pa within the specified fire-resistant time.

calculate the air leakage and air leakage rate of a single smoke exhaust valve as shown in fig. 8 according to the above air leakage standard.

the maximum smoke-proof partition is calculated as 411 m2, the calculated smoke exhaust capacity of the smoke-proof partition is 24 111 m3/h, the smoke exhaust speed of the smoke outlet is 9 m/s, the calculated area of the smoke outlet is 1.74 m2, and the calculated air leakage of one smoke outlet is 518 m3/h. The calculated smoke exhaust capacity of the smoke exhaust system is 48 111 m3/h; The air leakage rate of the smoke outlet is 1.18%, which is about 1%.

for the convenience of calculation and simplification of the system, smoke outlets with the same size are used in each smoke prevention zone. The maximum wind speed of smoke outlets in smoke prevention zones is 9 m/s, and the pressure difference between the inside and outside of normally closed smoke outlets is 311 Pa. The air leakage of a normally closed smoke outlet meeting the allowable air leakage standard is about 1% of the total smoke discharge of the smoke exhaust system.

From the above calculation method and checking calculation, it is known that the air volume of the smoke exhaust system, the smoke exhaust volume of the smoke exhaust port and its size are directly proportional to the maximum smoke prevention area, but the air leakage rate of the smoke exhaust port is basically unchanged; When the wind speed of the smoke outlet is small, the size of the smoke outlet will become larger and the air leakage will increase; The larger the service radius of the smoke exhaust system, the greater the negative pressure in the smoke exhaust system, and the air leakage will naturally increase.

The complex smoke exhaust system shown in Figure 8 is generally used in large commercial buildings. When smoke control is accepted, the terminal smoke exhaust port is opened, and sometimes the wind speed at the smoke exhaust port is only 3 ~ 4 m/s, which obviously leads to insufficient smoke exhaust. In the design, too many smoke outlets of a single smoke exhaust system should be avoided. For the smoke exhaust system with more smoke exhaust ports, the air leakage of smoke exhaust ports should be determined by calculation.

At the time of acceptance, we also saw a simple smoke exhaust system one floor at a time, and the smoke exhaust capacity of the smoke exhaust port can generally meet the design requirements.

2.3 nonstandard installation and construction of smoke exhaust system in fire fighting acceptance

During the acceptance of fire fighting projects, thin steel plate flanges were adopted for smoke prevention and exhaust ducts in some projects, and it was clearly pointed out in the drawing 17K133 "Manufacture and Installation of Thin Steel Plate Flanged Ducts" that thin steel plate flanges were not suitable for circular ducts and fire fighting exhaust ducts, and it should be made clear in the construction instructions that the smoke exhaust ducts were connected with angle steel flanges.

the acceptance inspection found that the construction unit paid the least attention to the material of the hose of the smoke exhaust system, which should be implemented according to the requirements of the specification. The hose of the smoke exhaust system should be made of non-combustible materials with a fire resistance temperature above 281℃ and continuous operation for more than 31 min.

3 The name, function and control of the valves of the smoke control system should be specified in the design description.

The valves (vents) of the smoke control system include fire dampers, smoke prevention fire dampers, smoke exhaust fire dampers, multi-leaf air supply outlets and multi-leaf smoke exhaust outlets. The valve functions are normally open and normally closed, manual and electric, air volume adjustment, 71℃ and 281℃ fuse closure, signal feedback, interlock control and so on. During the review of drawings, it is found that some drawings are marked with irregular valves, and there is a lack of specific function and control instructions for various valves (tuyeres).

Paragraph 4 of Article 9.3.13 of GB 51116-2114 Code for Fire Protection Design of Buildings stipulates that fire valves and smoke exhaust valves should meet the requirements of GB 15931—2117 Code for Fire Prevention of Ventilation and Smoke Exhaust System of Buildings, and their names and functions should be in Article 9.3.14 of GB 51116-2114 Code for Fire Protection Design of Buildings.

the linkage design of electrical specialty of smoke control system lacks the control principle of smoke control, especially when air exhaust and smoke exhaust are used together at ordinary times, the control principle should be clarified, as shown in Figure 8, where the exhaust outlet and smoke-proof fire valve for ordinary air exhaust are set at the end of smoke exhaust at floor 1, and it should be clear that the smoke-proof fire valve should be closed by electrical signal when smoke is exhausted before it can be converted into smoke control system. Perfect linkage control of smoke control system is the basis of normal operation of smoke control system, and fire linkage is one of the most important parts of fire acceptance.

4 Conclusion

The inner wall of mechanical pressurized air supply shaft should be smooth, and there should be no protrusions such as beams and multi-leaf air supply outlet valve bodies in the shaft. When there are protrusions in the shaft, the air duct resistance increases obviously, so the air duct resistance should be calculated. All kinds of pipeline wells