3. Choice of ventilation equipment
3.1 Calculation of relevant parameters of the ducting
3.1.1 Wind resistance of tunnel ventilation ducting
The air resistance of the tunnel ventilation duct theoretically includes the friction air resistance, the joint air resistance, the elbow air resistance of the ventilation duct, the tunnel ventialtion duct outlet air resistance (press-in ventilation) or the tunnel ventilation duct inlet air resistance (extraction ventilation), and according to the different ventilation methods, there are corresponding cumbersome calculation formulas. However, in practical applications, the wind resistance of the tunnel ventilation duct is not only related to the above factors, but also closely related to the management quality such as the hanging, maintenance, and wind pressure of the tunnel ventilation duct. Therefore, it is difficult to use the corresponding calculation formula for accurate calculation. According to the measured average wind resistance of 100 meters (including local wind resistance) as the data to measure the management quality and design of the tunnel ventilation duct. The average wind resistance of 100 meters is given by the manufacturer in the description of the factory product parameters. Therefore, the tunnel ventilation duct wind resistance calculation formula:
R=R100•L/100 Ns2/m8 (5)
Where:
R — Wind resistance of tunnel ventilation duct, Ns2/m8
R100 — The average wind resistance of the tunnel ventilation duct 100 meters, wind resistance in 100m for short, Ns2/m8
L — Ducting length, m, L/100 constitutes the coefficient of R100.
3.1.2 Air leakage from the ducting
Under normal circumstances, the air leakage of metal and plastic ventilation ducts with minimal air permeability mainly occurs at the joint. As long as the joint treatment is strengthened, the air leakage is less and can be ignored. The PE ventilation ducts have air leakage not only at the joints but also on the duct walls and pinholes of the full length, so the air leakage of the tunnel ventilation ducts is continuous and uneven. Air leakage causes the air volume Qf at the connection end of the ventilation duct and the fan to be different from the air volume Q near the outlet end of the ventilation duct(that is, the air volume required in the tunnel). Therefore, the geometric mean of the air volume at the beginning and end should be used as the air volume Qa passing through the ventilation duct, then:
(6)Obviously, the difference between Qf and Q is the tunnel ventilation duct and the air leakage QL. which is:
QL=Qf-Q (7)
QL is related to the type of tunnel ventilation duct, the number of joints, the method and management quality, as well as the diameter of the tunnel ventilation duct, wind pressure, etc., but it is mainly closely related to the maintenance and management of the tunnel ventilation duct. There are three index parameters to reflect the degree of air leakage of the ventilation duct:
a. Air leakage of tunnel ventilation duct Le: The percentage of air leakage from the tunnel ventilation duct to the working air volume of the fan, namely:
Le=QL/Qf x 100%=(Qf-Q)/Qf x 100% (8)
Although Le can reflect the air leakage of a certain tunnel ventilation duct, it cannot be used as a comparison index. Therefore, the 100 meter air leakage rate Le100 is commonly used to express:
Le100=[(Qf-Q)/Qf •L/100] x 100% (9)
The 100 meter air leakage rate of the tunnel ventilation duct is given by the duct manufacturer in the parameter description of the factory product. It is generally required that the 100 meter air leakage rate of the flexible ventilation duct should meet the requirements of the following table (see Table 2).
Table 2 The 100 meter air leakage rate of the flexible ventilation duct
| Ventilation distance(m) | <200 | 200-500 | 500-1000 | 1000-2000 | >2000 |
| Le100(%) | <15 | <10 | <3 | <2 | <1.5 |
b. The effective air volume rate Ef of the tunnel ventilation duct: that is, the percentage of tunnel ventialtion volume of the tunneling face to the working air volume of the fan.
Ef=(Q/Qf) x 100%
=[(Qf-QL)/Qf] x 100%
=(1-Le) x 100% (10)
From equatation (9): Qf=100Q/(100-L•Le100) (11)
Substitute equation (11) into equation (10) to get: Ef=[(100-L•Le100)] x100%
=(1-L•Le100/100) x100% (12)
c. Air leakage reserve coefficient of tunnel ventilation duct Φ: That is, reciprocal of the effective air volume rate of tunnel ventilation duct.
Φ=Qf/Q=1/Ef=1/(1-Le)=100/(100-L•Le100)
3.1.3 Tunnel ventilation duct diameter
The selection of the diameter of tunnel ventilation duct depends on factors such as the air supply volume, the air supply distance and the size of the tunnel section. In practical applications, the standard diameter is mostly selected according to the matching situation with the diameter of the fan outlet. With the continuous development of tunnel construction technology, more and more long tunnels are excavated with full sections. The use of large diameter ducts for construction ventilation can greatly simplify the tunnel construction process, which is conducive to the promotion and use of full-section excavation, facilitates one-time formation of holes, saves a lot of manpower and materials, and greatly simplifies ventilation management, which is the solution to long tunnels. Large-diameter tunnel ventilation ducts are the main way to solve long tunnel construction ventilation.
3.2 Determine the operating parameters of the required fan
3.2.1 Determine the working air volume of the fan Qf
Qf=Φ•Q=[100/(100-L•Le100)]•Q (14)
3.2.2 Determine the working air pressure of the fan hf
hf=R•Qa2=R•Qf•Q (15)
3.3 Equipment selection
The choice of ventilation equipment should first consider the ventilation mode and meet the requirements of the ventilation mode used. At the same time, when selecting equipment, it is also necessary to consider that the required air volume in the tunnel matches the performance parameters of the above calculated tunnel ventilation ducts and fans, so as to ensure that the ventilation machinery and equipment achieve the maximum working efficiency and reduce energy waste.
3.3.1 Fan choice
a. In the selection of fans, axial flow fans are widely used because of their small size, light weight, low noise, easy installation and high efficiency.
b. The working air volume of the fan should meet the requirements of Qf.
c. The working air pressure of the fan should meet the requirements of hf, but it should not be greater than the allowable working pressure of the fan (the factory parameters of the fan).
3.3.2 Choice of tunnel ventilation duct
a. The ducts used for tunnel excavation ventilation are divided into frameless flexible ventilation ducts, flexible ventilation ducts with rigid skeletons and rigid ventilation ducts. The frameless flexible ventilation duct is light in weight, easy to store, handle, connect and suspend, and has a low cost, but it is only suitable for press-in ventilation; In the extraction ventilation, only flexible and rigid ventilation ducts with rigid skeleton can be used. Because of its high cost, large weight, not easy to store, transport and installation, the use of pressure into the pass is less.
b. The selection of the ventilation duct considers that the diameter of the ventilation duct matches the outlet diameter of the fan.
c. When other conditions are not much different, it is easy to choose a fan with low wind resistance and low air leakage rate of 100 meters.
To be continued......
Post time: Apr-19-2022
