FAN PROTECTION AGAINST STALLING PHENOMENON Roberto Arias Álvarez 1 Javier Fernández López 2 2 1 ZITRON Technical Director roberto@zitron.com ZITRON Technical Pre Sales Management jfernandez@zitron.com STALLING ON AXIAL FANS One of the main problems that could be suffered by the fans installed in the definitive ventilation systems is stalling, this being one of the major causes of concern for the operation contractors in charge of the ventilation systems of tunnels. Stalling in axial fans is a phenomenon that can be compared with the situation experienced by aircraft wings where, as the angle of incidence of the airflow is increased, the lift force increases first, but falls abruptly when a certain critical angle is exceeded. Figures 1 and 2 show both flow conditions. Figure 1 - blade operating in ideal conditions Figure 2 - blade operating in stall 1
In the case of the fan blades, severe separation of the flow lines along the profile of the blade occurs, and this induces a strong decrease in the increasing pressure given by the fan, impacting negatively on fan efficiency and radiated noise. The fan stalling is a situation that should always be avoided. Below there is a list of some of the main problems created by fan stalling: - Potential risk of serious damage due to blade fatigue - Fan does not develop the design performance, air flow and pressure (Lower safety level of the system). - Efficiency is lower and hence power consumption is higher. - Dramatic increase in noise. - Higher vibration levels than at normal operation. In some severe cases, stall operation for any prolonged time may result in fan blade rupture due to metal fatigue, and consequently in the damage of the major parts of the fan. Figure 3 - progressive stall starting at blade tips Concerning the noise, when the fan is operating at or near the stall region, the noise generated by the fan will be increased. In some cases, this phenomenon, known as hammering, could generate a sound as if the blades were being hit by a solid object. 2
On the other hand, we should be aware that ideal situations are not always the reality, and therefore we should take into account in our designs that fans could face this problem during their operational life. The air flow delivered by the fan will be determined by the intersection of the fan performance curve with the system resistance curve. In order to know the maximum system resistance that the fan will be able to deal with, a real test at full power and nominal speed prior to the fan installation on site is totally necessary. The factory acceptance test in a proper test bench then becomes one of the first steps to avoid further stall problems on site. By testing the stalling point in the factory the real margins to the fan stall limit will be known. One fan performance curve will be obtained for each impeller pitch angle, and as shown in figure 4, as the system curve increases the fan air flow delivered by the fan will decrease. Figure 4 System resistance Vs fan performance curve It would be nice if system calculations were always accurate, but sometimes wrong calculations or unexpected changes in the system resistance could make the fan operate in the stall zone. 3
FIGHTING AGAINST FAN STALLING As explained above, stalling is a phenomenon that should be always avoided in the fans. In order to do that, below we present a table showing the main steps for an effective fight against fan stalling. FIVE EFFECTIVE STEPS AGAINST FAN STALLING 1 st Realistic system calculation of system pressure loss 2 nd Factory fan performance test at nominal speed 3 rd Fan stall detection system 4 th System evaluation for additional pressure loss sources removal 5 th Installation of anti-stall devices in the fan 1 st step: Realistic system calculation of system pressure loss The most frequent reason of fan stalling is the wrong system pressure drop calculation. As shown above (figure 4), when the real system resistance curve is higher than the calculated one, the fan will deliver lower air flow than originally expected, and if this error is big enough, the fan will operate in stall. The tunnel safety demands are being improved every day, and hence the proposed ventilation systems are more complex, making use of dampers or roof openings necessary to comply with the tunnel ventilation demands during normal and emergency conditions. Extreme care should be taken when determining the total pressure drop value for the design volume flow, and all the system pressure drop sources should be designed to create the lowest pressure drop possible. The most standardised method to calculate the system pressure drop is the coefficient method. This methodology calculates the required pressure drop by multiplying the dynamic pressure by the experimental factors published in different literature such as Idelchik or ASHRAE. Although the coefficient method takes into account important parameters such as flow regime, turbulent or laminar, it is sometimes difficult to get accurate results and additional CFD simulations may be required. ZITRON uses FLUENT software to confirm the fan selection in those cases where traditional numerical calculations are not providing reliable results. In the following images (figure 5) it can be observed how the flow lines across different elements are not occupying all the available section, so velocity and hence dynamic pressure may not be the ones expected from the numerical calculations. 4
90º bend after mine collar shaft Diffuser flow discharge before splitter silencers Flow discharge to tunnel after saccardo nozzle Figure 5 CFD simulations The main advantage of using CFD simulation is that real accuracy of the air flow performance in the system can be determined, and therefore the most suitable fan with enough margin to the stall region can be selected. 5
Although the CFD tool is very useful for the ventilation designers, care should be taken when using this software, the right choice of the input parameters and boundary conditions being of critical importance. The validation of the CFD simulation then becomes necessary to improve the accuracy of the results obtained. In this sense, ZITRON is the company with the largest number of validated CFD simulations ; all the CFD simulations carried out by ZITRON are later validated during the ventilation system commissioning, adjusting the software theoretical values to the real ones obtained in the tunnels. 2 st step: Factory fan performance test at nominal speed Once the fan is selected, designed and manufactured, a real test at full power and nominal speed should be carried out prior to site delivery. During the test, the fan performance curve and the exact stalling region are determined, and therefore the real fan limits are known. In order to find out the maximum pressure drop that the fan will be able to generate before stalling, the test should be carried out in a proper laboratory which is able to run the fan at full power. The ZITRON test bench is an AMCA accredited laboratory (see Figure 6 certificate), and is ready for the testing of fans of up to 4 m diameter and 1600 Kw motor rated power. The test is carried out by generating several system resistance curves and then by measuring the air flow and pressure delivered by the fan all over the selected impeller pitch angle curve. In addition to the air flow and pressure, the motor power consumption is also measured, so real fan efficiency for each operation point is obtained. Figure 6 ZITRON AMCA laboratory certificate In order to get real performance curves for fans running at full power and speed, the ZITRON test bench (figure 7) is an underground tunnel of almost 100 m long, with a cross section of 52 m2. By means of an auxiliary fan with a 2800 mm impeller diameter and a circular closing 6
damper with continuous adjustable blade angle, all kinds of possible circuit resistances are generated, and all the fan performance curve points are tested. When the damper is closed enough, the tested fan will start to operate in stall, making it possible to record the real performance delivered under this situation, such as air flow, pressure, efficiency, vibrations, noise With the ZITRON test bench it is possible to analyse in detail the fan stall behaviour before delivering the fans to site, knowing in advance the real risks and possible solutions. Figure 7 ZITRON Test Bench As shown in the below image (figure 8), the complete fan performance curve is obtained and the real margin from the design duty point up to the stall limit can be observed. The example on the next page is for a ZITRON 100% reversible fan, where both directions of impeller rotation are tested, forward (blue line) and reverse (red line). 7
Figure 8 100% reversible axial fan test 8
3 rd step: Fan stall detection system It is important to note that the stall may not always be apparent. A fan operating in stall will show increase in normal fan noise or excessive increase in power consumption, but without an effective warning device, it may be difficult to detect that the fan is really operating in stall. Even devices designed for anti-stall purposes, such as anti-stall rings, are not designed for stall operation warning. Anti-stall rings are used to minimise the fan stall effects but not to detect stalling itself. The most effective, simple and cost-effective system for stall detection is offered by ZITRON and consists in installing a Petermann probe on the fans (see figure 9). Figure 9 - ZITRON fan stall detection system, Petermann probe 9
The pressure variations occurring in the stalling zone express themselves by creating one or more zones in which the airflow through the vane channels is blocked or goes in the opposite direction. The Petermann probe uses the above phenomenon to detect the stall condition of an axial flow fan by measuring the pressure difference between the total air pressure acting in a direction opposite to the direction of rotation of the fan impeller (with of a hook-shaped tubular measuring probe) and a reference pressure corresponding substantially to the static pressure at the wall of the air duct in the same radial measuring plane immediately in front of the fan blades, upstream. This pressure difference will be approximately zero in the stable working range of the fan, but increases considerably at a point corresponding to the inflection of the pressure difference characteristic when the fan enters in stall condition. Thus, by means of a simple differential pressure transmitter it is possible to know when the fan is in the stall condition. Figure 10 represents the pressure difference between both pressure probes depending on the air flow delivered by the fan. In this curve it is possible to see the characteristic change at the junction between the stable working range and the unstable range in the fan characteristic. Figure 10 - point of inflection The Petermann probe system is used for stall-detecting purposes, warning the ventilation system operators when the fan is operating at stall and making a signal which can be treated as trip or alarm. 10
4 th step: System evaluation for additional pressure loss sources removal Once the stall is detected, the circuit where the fan is installed must be analysed looking for the reason of such stall operation. Although numerical calculations can be used to determine the reason of the stall problem, again CFD simulations can be very useful to detect the problem as well as to solve it. The major part of the pressure drop in the ventilation system occurs in the sudden changes of flow velocity. Figure 11 shows a real example of how it was possible to decrease by almost 40% the pressure drop generated by a 90º elbow, by installing guide vanes. Figure 11 different 90º elbow configurations Before using any system for safe fan operation in stall (i.e. anti-stall rings), it is strongly recommended to analyse in detail the complete ventilation circuit, looking for any possible modification in its original design which helps to bring the fan out of the stall region. 11
5 th step: Installation of anti-stall devices in the fan Anti-stall rings There are different methods to keep the fan operating in stall mitigating the risk of blade rupture, and one of those is the anti-stall ring. The Anti-stall ring consists in a chamber located around the impeller. This chamber is fitted with internal guide vanes that catch the turbulent flow generated at the blade tips during the stall. The turbulent flow is stabilised and returned to the main volume flow which is circulating through the impeller. This system could seem advantageous, but once it is analysed in detail, some disadvantages are found. The Anti-stall ring system reduces drastically the fan efficiency, and therefore the power consumption all along the fan design life will be higher than really required to deliver the design duty point. In addition, if the fans operates in stall it will not deliver the design air flow, and hence the safety level of the ventilation system will be much lower than originally expected. Figure 12 fan stall region Now that the patent has expired, this system is being offered by different fan manufacturers, but it should be noticed that fan stall will be avoided assuming a continuous efficiency loss. Depending on the type of axial fan, the anti-stall ring negative impact in efficiency could be between 4 % and 8 % during stable fan operation, but reaching values around 40% when the fan is working on its unstable part of the curve. 12
Figure 13 - anti-stall ring For some applications, other anti-stall systems could be used to bring the fan out of the unstable region. As it occurs with anti-stall rings, delivered air flow will be lower than originally specified, but without negative impact on efficiency. By means of pitch angle variation, which could be also carried out without stopping the fan, even for system resistance higher than expected, the fan could continue working without risk of damage. Figure 14 - blade adjustment for fan bring out of stall As shown in the above figure (14), in case the fan adjusted to pitch angle nº4 is operating in stall, pitch angle nº 3 could be set to bring the fan out of the stall region. We should note that this system avoids the risk of fan damage and without the typical stalling decrease of the fan efficiency. 13
CONCLUSIONS Stall is one the main problems that the fans could suffer. In order to avoid this kind of problem, there are five main steps that should be taken into account: - 1 st : Realistic system calculation of system pressure loss - 2 nd : Factory fan performance test at nominal speed - 3 rd : Fan stall detection system - 4 th : System evaluation for additional pressure loss sources removal - 5 th : Installation of anti-stall devices in the fan The most frequent reason for fan stall is the underestimation of the system pressure drop; CFD tools are very helpful to calculate the real system pressure drop. Stall problems due to wrong pressure drop calculations or any unexpected change of the circuit geometry should always be investigated and solved. It should be noticed that anti-stall rings does reduce the risk of blade rupture due to fatigue, but does not eliminate the risk. Even if the fan is not suffering from rupture problems, a fan operating in stall by means of anti-stall rings is never the best solution. When the fan is in stall, the air flow delivered is lower than originally specified and this means that the safety level of the system is lower than required. One of the risks of the use of anti-stall rings is that the stall may not be detected, and hence the fan will remain working on its unstable region for long periods of time without looking for the solution to the reason that has induced the stall. In addition to the reduced air flow delivered by the fans, it should be noted that the fan power requirements are directly related to the fan efficiency, and anti-stall rings will produce a negative impact on fan efficiency. An alternative method such as automatic blade pitch angle adjustment, which is reliable and energy-friendly, should always be considered before choosing poor efficiency anti-stall systems. It could be said that nobody would drive their car continuously with the handbrake on, because it would be necessary to stop at some point, and that s really what is occurring when energy is continuously wasted with anti-stall rings. REFERENCES EUROVENT 1/11, published 2007. 14