STARTING-UP-TIME AND -METHOD OF FANS FOR EMERGENCY TUNNEL VENTILATION SYSTEMS

Transcription

1 STARTING-UP-TIME AND -METHOD OF FANS FOR EMERGENCY TUNNEL VENTILATION SYSTEMS ABSTRACT F. van Vemden Zitron Nederland B.V. In case of an emergency situation in a tunnel, for instance fire, it is vital to have a ventilation system that responds to control commands as quickly and as reliable as possible. Furthermore it is essential to avoid that the fans operate in stall. Depending on the length and the size of the smoke extract ducting in a tunnel the mass of the air that has to be accelerated during start-up of the ventilating system can be substantial. The paper highlights the effect of system dimensions, fan types and starting methods on the starting time and the time the fans operate in stall during the start-up. 1. INTRODUCTION To be able to predict, especially during a fire or another emergency situation, if and how fast a desired volume flow can be reached is very important. As a supplier of tunnel ventilation systems we decided to develop a calculation method and tools to be able to determine the start-up behaviour of fans in tunnel ventilation systems. It is generally known that operating a fan in stall conditions should be avoided. During stall operation high dynamic loads on the blades and increased vibration levels can occur. A fan operates in stall if the actual volume is below the stall volume (See figure 1). During startup an axial fan will always operate in stall for a short period, however this stall period increases significant if a large mass of air has to be accelerated. Besides that there is a possibility that during parallel start-up/operation or sequential starting one (or more) of the fans remains operating in stall. This occurs if the system resistance line the fan feels or experiences is higher is that the saddle-pressure of the fan (See figure 5). With the developed method and tools it is possible to predict and compare the start-up behaviour of fixed blade angle fans to fans equipped with a variable pitch system. 2. CALCULATION METHOD The calculation method is based on the following assumptions: - The difference between fan pressure and system pressure accelerates the air mass. - The actual motor torque reduced by the aerodynamic torque of the fan is the torque that accelerates the rotating speed of the impeller. - The start-up is finished and the volume flow is stationary when the volume flow has a value at which the corresponding fan pressure equals the pressure loss of the system. - The air density is constant (incompressible flow). 4 th International Conference Tunnel Safety and Ventilation 28, Graz

2 The calculation is performed in small time steps and based on the following formulas: Pressure loss of the system P sys = P + Zeta sys x.5 x Rho x Q² P : System Pressure at zero Flow P sys : Pressure loss of the ventilation system Zeta sys : Pressure loss factor of the system Rho : Air density Q : Actual volume through the system. Aerodynamic power of the fan POW aero = Q x P fan / Eff POW aero : The aerodynamic shaft power of the fan Eff : The efficiency of the fan Aerodynamic torque T aero = POW aero / Ω T aero : Aerodynamic torque Ω : rotating speed of the fan Acceleration of the impeller T motor - T aero = J x dw/dt T motor : actual motor torque J : Inertia of the motor-rotor and the impeller dw/dt : acceleration of the impeller Acceleration of the airspeed (P fan P sys ) x A = dv/dt x M air A : Area of the ventilation duct dv/dt : acceleration of the airspeed : Mass of the air that has to be accelerated M air The fan pressure depends on the actual volume, the fan speed and the fan type (Fixed Pitch versus Variable Pitch). Fans with fixed pitch If the fan speed is (during start-up) below the nominal speed the fan curve of the fan for the actual speed can be calculated with the following fan laws: P fan = P fannominal x (RPM/RPM nominal )² P fan : Pressure at actual speed P fannominal : at nominal speed RPM : Fan actual speed RPM nominal : Nominal fan speed Q = Q nominal x (RPM/RPM nominal ) 4 th International Conference Tunnel Safety and Ventilation 28, Graz

3 Fan Pressure [Pa] Qstall Volume [m³/s] Figure 1: Fan curve fixed pitch fan If the actual volume is above Q stall (See figure 1) the fan operates in the stable area. If the actual volume is below Q stall the fan operates in the stall area. Fans with variable pitch If the fan speed is (during start-up) below the nominal speed the fan curve of the fan for the actual speed can be calculated as described above. During start-up the fan starts with a small blade angle. With this reduced blade angle the relation between fan pressure and volume is according to stall line in figure 2. If the actual volume is between Q u and Q stall (see figure 2) the blade angle should be regulated in a way that Q stall is/stays slightly smaller than the actual volume. Regulating the blade angle this way has the following advantages: - The fan does not operate in stall if the volume is between Q u and Q stall. - The fan pressure is higher than the pressure of a fixed pitch impeller. 6 5 Qstall Pfan at design blade angle Stall Line Fan Pressure [Pa] Qu, Pfan at reduced blade angles 1 Actual blade angle Actual Volume Volume [m³/s] Figure 2: Fan curve variable pitch fan 4 th International Conference Tunnel Safety and Ventilation 28, Graz

4 RESULTS AND DISCUSSION The above-mentioned method was used to determine the start-up time of the ventilation system of a tunnel in Switzerland, were smoke extraction is performed over a duct, which is equipped with extract dampers. Two parallel axial flow fans for extraction are located in a fan room. During a fire in the tunnel 3 dampers next to the fire are opened for smoke extraction. For this tunnel the required start-up time of the ventilation system is less than 9 second. For an overview of the main dimensions of the extract duct and the fans see table 1. Table 1: Dimensions of the extract duct and fan data Description Total length of the tunnel Maximum length between the fans and the fire Number of fans in parallel 2 Extract volume fans in parallel 32 m 2 m 1 m³/s per fan Remark Extract volume with one fan 14 m³/s In case one of the fans is not available. Total fan pressure with fans in parallel 36 Pa Static counter pressure due to atmospheric 26 conditions (wind pressure) Area of the extract duct 1 m² Air density Fan-motor drives Power of the Emotors Required start-up time of the system Impeller diameter of the fans 1.14 kg/m³ Variable speed drive 26 kw 6 seconds 2.1 m Fan curves See figure 1 and 2 Start-up calculations Based on the tunnel-data as a first step the start-up time is calculated for the fans equipped with fixed blades and with a variable pitch system. The calculations are based on the following assumptions: - Because of the variable speed drive the torque of the motor is equal to the nominal torque of the motor over the whole frequency range. - The fans start-up at the same time and during the whole start-up the both fans have the same speed. 4 th International Conference Tunnel Safety and Ventilation 28, Graz

5 Results See figures 3 and 4 for the variation in time results of the fan total pressure, system pressure, volume, and speed Pressure [Pa] / Speed [rpm] Volume flow System pressure Impeller speed Volume flow [m³/s] Time [s] Figure 3: Start-up fixed pitch fans Volume flow 1 35 Pressure [Pa] / Speed [rpm] System pressure Impeller speed Volume flow [m³/s] Time [s] Figure 4: Start-up variable pitch fans From the previous figures the following summary (Table 2) can be noted: Table 2: Compare Fixed Pitch versus Variable Pitch Variable Fixed Pitch Variable Pitch Total start-up time of the system 5 seconds 4 seconds Acceleration time of the impeller 1 seconds 1 seconds Time the fan operates in stall 3 seconds, from t= to 3 seconds 3 seconds, from t= to 3 seconds Critical margin between fan stall P fan P system = 335 Pa, No critical margin pressure and system pressure. at t=3 seconds 4 th International Conference Tunnel Safety and Ventilation 28, Graz

6 Influence of the start-up method. The start-up calculations described above are based on the assumption that both fans have the same speed during the start-up. But what happens if one fan is running at full speed and the second fan has to start-up due to a fire? The quickest and most simple way would be to startup the second fan while the first fan keeps running at full speed. To be able to perform this calculation the system-resistance line the second fan experiences has to be determined. If one fan is running the volume flow of this fan is 14 m³/s. At this volume the pressure loss of the ducting is 2 Pa, so at zero flow the system-resistance for the second fan is 226 Pa (2+26). At full speed and a volume of 1 m³/s the system-pressure is 36 Pa (pressure of the two fans in parallel). The fan curve and the system resistance line can be seen in the figure 5. Pressure [Pa] Pfan DPsystem Volume [m³/s] Figure 5: System resistance line and fixed pitch fan curve Start-up the second fan with a fixed pitch system. The above figure shows that between 7 and 75 m³/s the fan curve lies below the system resistance line, so in this area the critical margin between the fan pressure and the system resistance line becomes negative. This means that the fan will remain operating in stall at a volume of 7-75 m³/s (also see figure 6) Volume flow 7 4 Pressure [Pa] / Speed [rpm] System pressure Impeller speed Volume flow [m³/s] Time [s] Figure 6: Start-up second fan with fixed pitch system 4 th International Conference Tunnel Safety and Ventilation 28, Graz

7 Start-up the second fan with a variable pitch system. Also with a variable pitch fan the fan goes into stall if the volume is below Q stall. However during start-up the volume-pressure curve can be moved to the left if the volume is lower than Q stall. So by starting the fan with a small blade angle the blade adjusting system has to follow the volume by increasing the angle in a way that Q stall stays smaller than the actual volume. Doing so prevents stall and the maximum fan pressure is still available for accelerating the air. 6 Pfan at design blade angle Fan Pressure [Pa] Qu Qstall DPsyst Stall Line Pfan at reduced blade angles Volume [m3/s] Figure 7: System resistance line and variable pitch fan curve For the change of the fan pressure, system pressure, impeller speed and volume flow in time see figure 8. The figure shows that the start-up time of the second fan is about 55 seconds Volume flow 1 Pressure [Pa] / Speed [rpm] System pressure Impeller speed Volume flow [m³/s] Time [s] Figure 8: Start-up second fan with variable pitch fan 4 th International Conference Tunnel Safety and Ventilation 28, Graz

8 Influence of the length of the extract duct on the start-up time To determine the influence of the length of the extract duct the start-up time is calculated for different duct lengths. The results can be seen in the following graph: Start-up time [s] Extract duct length [m] Fixed pitch Variable pitch Figure 9: Influence duct length on start-up time Note that the start-up time is proportional to the length of the extract duct. 4. CONCLUSIONS - The calculation method gives a good understanding about the start-up of fixed and variable pitch fans in parallel. - The start-up time of variable pitch fans is shorter than with fixed pitch fans. The difference between the start-up time increases with the length of the extract duct. - The length of the extract duct has a considerable influence on the start-up time. To be able to start the system within 3 to 6 seconds the extract duct should not be much longer than 2 m. - With variable pitch fans parallel start-up or sequential start-up is possible. If due to a fire a second fan has to be started there is no need to reduce the speed of the already running fan first. - For any fan selection it should be thoroughly checked if and how the fixed pitch fans can be started. If the critical stall margin is relatively small the fans can remain operating in the stall area and do not reach the design volume. This can happen if the static counter pressure due to atmospheric conditions is (temporary) higher than estimated or if during start-up the speed of one fan is higher (in this case 5-1 rpm) than the other fan. 4 th International Conference Tunnel Safety and Ventilation 28, Graz