Fans, as general ventilation equipment, are widely used in various industries, especially in the cement industry. With the development of technology, the human living environment is challenged by industrial pollution, and the downward pressure on the world economy is increasing. People are paying more and more attention to the impact of industrial equipment applications on the environment. The demand for low-carbon, emission reduction, and cost reduction has led to an increasing emphasis on high-efficiency industrial centrifugal fans, thus pushing the research and development, selection, and transformation of high-efficiency industrial fans into a new chapter.
In the cement production process, with the improvement of process requirements, there are often higher requirements for fans. It is necessary to increase the size and speed of fans to meet the requirements of larger process processing capacity and conveying technology. Therefore, in the procurement of wind turbines, the energy consumption cost of using wind turbines is more valued than the initial procurement cost. Reducing consumption equals making money ". People have started to prioritize the transformation of low energy efficiency fans in cost reduction. The cost savings from the transformation of some high-power process gas fans and high-temperature circulating fans within one to two years are equivalent to the cost of a newly invested fan.
Centrifugal fans do not rely on centrifugal force to convert energy like axial fans, but rather generate energy through blade movement. To explain the working principle of a centrifugal fan in a different way, when the impeller rotates, a vacuum zone is formed at the bottom of the blade, and the air flow immediately fills this vacuum zone and flows towards the surface of the blade. This illustrates an important fact that the lower surface of the blade does not have a decisive impact on the efficiency of the fan. Therefore, fan manufacturers usually weld internal and external stiffening plates on the blade back plate to strengthen the weld bead in strip shape to increase the strength of the impeller. And add parallel fixing bolts and pads as wear-resistant protection. This also means that high-efficiency wing shaped blades are not much more efficient than high-efficiency curved blades. The main advantage of airfoil blades is that for larger and wider impellers, their hollow structure blades have stronger strength than traditional curved blades.
When the fan rotates, the speed of the impeller changes with the radius of the fan. So, the most efficient blade should be a spiral blade with a backward bend. In practical applications, curved blades are commonly used to achieve the required performance parameters of the fan by adjusting the inlet and outlet angles. These angles are achieved by defining the curvature and tilt angle radius. For wing shaped blades, the blades are wide and the curvature is gentle, so the efficiency of this type of blade will be relatively improved. The inclined (forward/backward) blades are a compromise solution that must be chosen when the fan transports gas and dust. In conveying systems with high dust content, self-cleaning inclined (forward leaning, backward leaning) blades are preferred, but to achieve an efficiency of over 80% for inclined blades, more meticulous fan design is required. For example, the BFBI (BF Backward Tilt) series of Halifax wind turbines. This series of fans outperforms curved blade fans in terms of efficiency, with blade curvature and tilt angle perfectly aligned with the inlet and outlet tilt angles, resulting in fan efficiency far exceeding 80%.
During the process of airflow entering the fan through the inlet cone tube and flowing into the impeller, there is no rotating valve pushing the gas. Only about 50% of the airflow is pushed into the fan back plate/center hub side, so only a certain amount of energy is transmitted to the impeller, while the other part of the airflow circulates around the impeller and is converted into energy loss. The amount of circulating airflow in this section directly determines the efficiency of the impeller.
We can change the impeller circulation airflow area by changing the design spacing of the inlet air duct, and changing the depth of the inlet air duct embedded in the impeller or the size of the impeller inlet reinforcement ring will have an impact on the performance of the fan. Another approach is to design impeller cover plates with parabolic or sloping shapes to reduce the area of gas circulation around the impeller. The wider the impeller, the more obvious the gas diversion will be. If the impeller is very wide, the performance of the fan will be difficult to predict. So when the impeller is wide, we rely entirely on the steep front cover plate to maintain stable fan performance.
2、 How to improve the efficiency of the fan
Accurately knowing the operating conditions of the fan is essential for selecting a suitable fan design. However, a simple fan design may not fully consider all the issues in the fan application process, including the impact of manufacturing accuracy.
The verification performance testing method, as a means of performance testing, is still widely used by many wind turbine manufacturers and users. At present, many fan performance tests are extended from ISO and AMCA testing standards, and the basic theories of these tests are the same. Customize the pipeline according to the testing standards for standard testing, and then correct the testing conditions. Calculate the performance of fans with the same impeller form but different sizes using the fan standard law. For example:
Pressure is proportional to density, and proportional to velocity and size squared
The flow rate is directly proportional to the speed, and the size is proportional to the square ratio
In the actual manufacturing process, there will be limited tolerances (such as ± 1mm). This means that as the fan becomes smaller, manufacturing tolerances do not decrease with the size of the fan (making a 250mm fan with a tolerance of ± 0.25mm is difficult, while making a 1000mm fan with a tolerance limit of ± 1mm is not difficult). The welding process and surface roughness have a greater impact on small fans. In addition, the boundary layer and turbulence effects of the airflow do not widen with increasing size. These characteristics are known as size effects - the size of a fan directly affects its performance, and manufacturing an efficient small fan is much more difficult than manufacturing an efficient large fan. This fan size effect has also been recognized by AMCA FEG and ISO12759 fan shaft absorption power rating.
When testing a newly developed fan model, in order to make the test results more perfect, the manufacturing and assembly processes of the fan are more detailed, far exceeding the production level of ordinary fans, which is also a factor that reduces the performance of the fan. The testing method for Halifax fans is to test small fans and manufacture them using ordinary production processes. The fan testing is based on a 380mm diameter impeller model. The large tolerance small fan is used as the testing standard to ensure that the large fan produced with small tolerance has superior performance and higher efficiency.
Another testing method is to use CFD to model and analyze the wind turbine (computational fluid dynamics). Ten years ago, relying on CFD analysis to drive mechanical equipment was very expensive for wind turbine manufacturers. In the past 10 years, the scope of using CFD has gradually decreased. Using only one ANSYS feature in CFD software, the analysis can only achieve an accuracy of 10%, and a CFD model requires at least 12 hours of analysis with a 32-bit processor. There are other theories and modules to conduct similar accuracy analysis. The fluid changes that can be predicted with only 10% accuracy are not sufficient for predicting wind turbine performance, so even with the support of CFD results, physical testing methods are still needed for performance testing.
Halifax wind turbines integrate theoretical development into model manufacturing and rapidly produce prototype testing. This testing method is faster and easier to implement than CFD analysis. Once the theoretical model passes physical testing, CFD analysis is integrated to improve the fan design. Overall, CFD focuses more on the analysis of geometric shapes, but cannot intelligently combine with prototype theory. CFD is also used to analyze flow theory and physical structure effects that are easy to test, which are difficult to detect in physical testing.
3、 Fan selection
The pressure of a fan is defined in two aspects, total pressure and static pressure;
Full pressure boost=outlet full pressure - inlet full pressure
Static pressure boost=outlet static pressure - inlet total pressure
Full pressure rise promotes an increase in the total energy of the fan, so it is commonly used in specifications and standards to measure efficiency - AMCA FEG and ISO12759. However, static pressure rise is mostly used by most factories for selection.
Many engineers first establish the required static pressure and volumetric flow rate for the system and then evaluate the pressure loss of the system. The pressure loss will be combined with the static pressure required by the system given by the engineer. Static pressure is used to define the properties of the process gas at the inlet of the fan. It can also be used to determine the static pressure changes throughout the entire fan. However, as mentioned above, static pressure rise is determined by subtracting the static pressure at the inlet from the static pressure at the outlet, and the total pressure at the inlet of the fan is the most accurate value that should be used. If the inlet and outlet have similar (equal) areas, the required value should be the total pressure rise. So using static pressure difference for selection gives us a hidden safety factor.
When we modify or replace a fan, the intake/exhaust speed of the fan will change due to changes in the area of the fan's inlet/outlet. For this situation, it is best to use the total pressure rise of the fan for selection. Ensure that the static pressure in the downstream pipeline of the new fan is the same as the original fan pressure.
In addition to pressure and flow rate, the operation process of the fan is also one of the factors that need to be considered in the selection of the fan. It may affect the fan's curve, characteristics, and how to control the fan.
For many fan operating systems, they are stable, so the fan can operate safely at around 5% of the maximum pressure point. However, not all systems are stable. For example, when cement clinker from the kiln enters the grate cooler and cools on the grate plate, the thickness and density of the grate bed will change. For these unstable systems, the operating point of the fan needs to operate away from the peak of the curve, usually choosing at least 10% to 15% below the pressure peak. In other cases, system designers may want the fan pressure operating point to be below the peak pressure, ensuring that there is still some safety margin for the fan in the event of unpredictable pressure increases.
If some dust adheres to the impeller of the fan, it will accumulate into a dust layer on the impeller. The dust layer becomes thicker and heavier, and eventually part of it falls off, causing the wind turbine to lose dynamic balance. This situation often occurs when the fan is running or when the fan is stopped and restarted. When the fan starts, the impact force of the motor can knock off dust. In addition, dust may absorb moisture and become heavier when parking, making it easier to fall off. We can reduce the occurrence of this imbalance by using large angle blades or radial blades. For high air volume and low inclination fans, using rear inclined blades is the best choice.
Dust can not only stick to the fan, but also cause wear and tear on the blades. This type of wear can be corrected using impeller welding repair. If the erosion is severe or the fan has been repaired multiple times, the impeller will need to be replaced. To avoid this situation, the impeller can be equipped with a hard surface during manufacturing. This type of hard welding can be applied to the surface of the impeller or wear-resistant ribs can be added. However, hard welding onto the inner lining plate can cause cracks in the impeller, which means that the anti friction layer or baking coating is more suitable for the fan blades and the gasket is preferably welded on the impeller surface. Anti friction bolt washers are also common, but cannot be used for wing blades.
The choice of flow control depends on the system resistance line. The most effective form of flow control can be variable frequency control. However, as the pressure and volumetric flow rate of the variable speed control change with speed;
Pressure=constant x velocity 2
Volume flow rate=constant x velocity
According to the square law relationship;
Pressure=constant x volume flow rate 2
Among these rules, if the selected flow rate of the fan design is close to the highest efficiency point of the fan curve, then increasing this fan proportionally will maintain the highest efficiency at any air volume.
Not all process systems follow the law of squares. In some systems with constant pressure, the system operates by constantly changing flow rates. A typical case is the clinker extraction process of fluidized bed coal-fired boilers. In this system, simply using a frequency converter to reduce the fan speed will cause the fan to deviate from the high-efficiency operating point. Even worse, the fan may not be able to generate sufficient pressure due to changes in speed. That is to say, for this type of process, the best choice for controlling flow is the air valve.
Outlet air valve: Using the outlet air valve of the fan to control the flow of the fan will increase the pressure loss of the system due to the increase in system resistance. If the efficiency of the selected fan is not high in the early stage, adjusting the outlet air valve can reduce the power absorbed by the fan shaft and improve the working efficiency of the fan. However, the efficiency of the fan and damper working simultaneously is always lower than that of the high-efficiency fan selected correctly. The control of the system by the outlet air valve is not particularly ideal, and the air valve needs to be closed by at least 50% to truly function on the system. Another disadvantage of the outlet damper is that the fan may experience severe surge when operating in a low flow surge zone.
Imported air valve: The imported air valve is very close to the fan impeller, and it can control the flow direction of the fan by changing the angle of the guide vanes. This is a relatively efficient flow control method, but it can also cause system losses. The closer the imported valve is to the impeller, the more effective the flow control of the fan. The control efficiency of imported air valves is relatively high, but the disadvantage is that they are prone to wear and damage. Under the control of the air valve, stable operation can be achieved while reducing the total air volume of the fan by 10%. The pressure loss of the air valve from fully open to 100% closed is around 10%. Whether it is the efficiency of the fan controlled by the imported or exported air valve, it is much lower than the efficiency of selecting an efficient fan correctly. So, the main purpose of using imported air valves is to obtain adjustable fan flow while maintaining fan pressure.
4、 The benefits of selecting the correct fan type
The selection of fans often requires a balance between early installation costs and later operating costs. The installation cost includes fan cost, motor cost, and fan installation size. A good system design should balance installation costs, operating costs, and other factors involved in the appeal. In the early design stage, many factors are not yet fully understood, so we collect as much available information as possible, even if it may not be accurate. During the design phase, engineers often add many safety factors to the selection process. There are many reasons to increase the safety margin, but the main reason is to avoid unknown factors in the system that may cause the fan selection to be too small and require the replacement of a new fan. These unknown factors include uncertainty about the future system capacity (such as system upgrades), or deterioration of operating conditions (including system blockages).
However, some systems have excessively high safety margins. In some bad cases, the actual operating pressure of the fan is only 50% of the expected pressure. Causing the fan to operate at a very high efficiency point, and even causing the operating point to deviate too far from the selection point, resulting in motor overload. Although it is possible to change the speed of the fan to solve the problem of motor overload. However, it also resulted in a significant loss of air volume, thereby reducing the production capacity of the system. A simple solution in practical applications is to choose high-power motors, but this will also increase installation costs and later operational costs.
Instead of only considering increasing the pressure safety margin in the design, it is better to combine the safety margin with the selection of the fan, increase the operating pressure to the highest point of the fan curve pressure range, so that there is a greater margin for operating pressure. For example, if it is considered ideal to reserve a safety factor of 20% in the initial system design, the fan selection should choose a fan operating point pressure that is at least 15% lower than the maximum pressure, while also reserving a safety factor of 10%. This will ensure that the selected wind turbine is closer to the desired operating point and thus closer to the efficiency point during operation. If it is an on-site modification of the fan, we will have the opportunity to measure the current usage of the fan on site and select a new one based on the measurement results. We will increase the selection margin according to the customer's requirements for future system improvements.
Ensuring that the fan operates close to its maximum efficiency point can effectively save operating costs. Assuming a wind turbine operates continuously for 95% of its time, for every 1 kW increase in power, the annual electricity cost increases by approximately 5000 yuan. For a wind turbine with a power consumption of 100KW, every 1% decrease in efficiency will result in an additional electricity cost of 5000 yuan. For a wind turbine with a power consumption of 400KW, every 1% decrease in efficiency will result in an additional annual electricity bill of about 20000 RMB. It is obvious that the larger the fan, the more significant the energy-saving effect will be. It can be imagined that the cost savings here are huge when all the fans on site are running simultaneously.
The research on cement fans indicates ("Mastering the Efficiency of Fan Systems", World Cement, 2012) that the operating cost of fans accounts for 32% of the total expenditure of cement plants. The following table is an analysis of the results;
If the efficiency of the above fan is increased to 75%, it will reduce energy loss by at least 15% and energy expenditure by 5%. For a cement production line with a daily output of 1000 tons, the annual energy-saving cost will be reduced by between 1100-1200 million.
However, electricity is not the only cost loss. Reducing the failure rate of fans, minimizing parking time, and saving labor and material costs for repairs have a crucial impact on cost reduction. Choosing high-quality fans and reducing the repair rate can save costs equivalent to or even higher than those saved by energy-saving fans.
