fans, as a common ventilation equipment is widely used in various industries, in the cement industry is more widely used. with the development of science and technology, the human habitat is challenged by industrial pollution, and the downward pressure on the development of the world economy is increasing, and people are increasingly concerned about the environmental impact of industrial equipment applications. low-carbon, emission reduction, reduce the cost of the requirements, so that people pay more attention to the efficient industrial centrifugal fan, which will be efficient industrial fan research and development, selection, transformation into a new chapter.
in the cement production process, with the upgrading of process requirements, often have a higher demand for the fan, the need to increase the size of the fan to improve the fan speed, to meet the greater processing capacity and transportation process requirements. therefore, in the fan procurement, the use of wind turbine energy costs than the previous procurement costs more attention. "consumption is tantamount to make money" people began to put the transformation of low-efficiency fan included in the cost of reducing the weight of some high-power process air blower, high temperature cycle fan transformation year to two years to save the cost of electricity is equal to a new the cost of putting the fan.
first, how does the centrifugal fan work?
centrifugal fans do not rely on axial fans to rely on centrifugal force to transform energy, but by the blade movement to produce energy. in order to explain the working principle of the centrifugal fan, when the impeller rotates, a vacuum zone is formed at the bottom of the blade, and the air flow fills the vacuum area and then flows to the blade surface. this shows an important fact that the lower surface of the blade has no decisive influence on the efficiency of the fan. so the fan manufacturers usually in the blade backplane plus welding inside and outside the strengthening board, strip to strengthen the weld, increase the strength of the impeller. and increase the parallel fixing bolts, the pad as wear protection. this also means that high efficiency wing blades are not much more efficient than efficient curved blades. the main advantage of the airfoil blade is that for larger, wider impellers, its hollow structural blades are stronger in strength than conventional curved blades.
when the fan is spinning, the speed of the impeller varies with the radius of the fan. so, the best efficiency of the blade should be the back of the spiral blade. in practical applications, usually use the curved blade, with the inlet and outlet angle, to achieve the required performance parameters of the fan. these angles are achieved by defining the curvature and the inclination angle. for the wing-type leaves, leaves wide, curvature gentle, so this section of the blade efficiency will be relatively improved. the tilting (forward / backward) blades are a compromise option when the fan is transported with gas. in the larger dust transport system, self-cleaning tilting (forward, backward) leaves become preferred, but to make the blade efficiency more than 80%, you need to combine more careful fan design achieve. for example, halifax fan bfbi (bf backwards) series. this series of fan efficiency win over curved blade fan, blade curvature and tilt angle set the perfect combination of fan inlet and outlet tilt, the fan efficiency far more than 80%.
the air flow through the inlet cone into the fan into the process of the impeller, there is no rotary valve to promote the gas, only about 50% of the air is pushed into the fan backplane / central hub side, so only a certain amount of energy conduction to the impeller, while the other part of the air flow in the impeller around the cycle into energy loss. this part of the number of circulating air directly determines the efficiency of the impeller.
we can change the impeller recirculating airflow by changing the design distance of the inlet duct, changing the depth of the impeller into the impeller or the impeller inlet reinforcement ring size will have an impact on the performance of the fan. another approach is to design a parabolic or slope of the impeller cover to reduce the impeller around the gas circulation area. the wider the impeller, the more obvious the gas shunt, if the impeller is very wide, the performance of the fan will be difficult to predict. so when the impeller is very wide, we have to rely on a large slope of the front cover to maintain a stable fan performance.
second, how to improve the efficiency of the fan
accurate knowledge of the use of the fan in order to better choose the appropriate fan design, however, a simple fan design will not fully take into account the fan application process all the problems, including the impact of manufacturing accuracy.
verification of performance testing as a means of performance testing is still widely used by many fan manufacturers and users. there are a lot of fan performance tests from iso and amca test standards, the basic theory of these tests are the same. customize the pipe according to the test standard for standard testing, followed by correcting the test conditions. and then through the fan standard law to calculate the same impeller shape of different sizes of fan performance. e.g:
pressure and density are proportional, and the speed, the size of the size of the square ratio
flow and speed are proportional and size of the size of the establishment ratio
the actual manufacturing process will have limited tolerances (eg ± 1mm). this means that when the fan is getting smaller and smaller, the manufacturing tolerances do not decrease with the size of the fan (making a 250mm fan ± 0.25mm tolerance is very difficult to make a 1000mm fan limit tolerance of ± 1 mm tolerance not difficult). the welding process and the surface roughness have a greater impact on the small fan. in addition, the airflow boundary layer and the turbulence effect do not increase with size. these characteristics are called size effects - the size of the fan directly affects the performance of the fan, making an efficient small fan is much more difficult than making an efficient wind turbine. this fan size effect is also recognized by amca feg and iso12759 fan shaft absorption power ratings.
when testing a new r & d model, in order to make the test results more perfect, the fan manufacturing and assembly process more detailed, far more than the average fan production level, which is a factor in reducing fan performance. halifax fans use the test method is to test the small fan, and the use of ordinary production process to create a fan, fan test to 380mm diameter impeller model. large tolerances of small fans for the test standards to ensure that the production of small tolerances of large fan performance is more superior and more efficient.
there is also a test method, is through the cfd to fan modeling analysis (computational fluid dynamics). 10 years ago, relying on cfd analysis of transmission machinery and equipment for the fan manufacturer is very expensive. over the past 10 years, the use of cfd has been slowly reduced. using only one ansys function in cfd software, but the analysis can only achieve 10% accuracy, a cfd model requires at least 32-bit processor 12-hour analysis. there are additional theories and modules for similarity analysis. this only 10% of the accuracy can predict the fluid changes, but not enough for the fan performance prediction, so even with the support of cfd results, or to rely on physical testing method for performance testing.
halifax fans incorporate theoretical development into model manufacturing to quickly prototype prototypes. this test method is faster than cfd analysis, once the theoretical model through physical testing and then into the cfd analysis to improve the fan design. in general, cfd is more important to the analysis of geometries, but it is not intelligent to combine the prototype theory, cfd is also used in the analysis of easy to test flow theory and physical structure effects, which are difficult to detect the physical test.
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third, the fan selection
the pressure of the fan is defined as two aspects, full pressure and static pressure;
full pressure boost = outlet full pressure - inlet full pressure
static pressure boost = outlet static pressure - inlet full pressure
the total pressure increases the total energy of the fan, so it is often used in the specification and standard to measure efficiency - amca feg and iso12759. however, the static pressure is increased by most factories for selection.
many engineers have to establish the required static pressure and volumetric flow of 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 fan inlet. it can also be used to determine the static pressure change throughout the fan. however, as mentioned above, the static pressure rise is not the static pressure of the tuyere minus the static pressure of the air inlet, and the total pressure of the air inlet of the fan is the most accurate and should be used. if the inlet and outlet have a similar (equal) area, the required value should be the total pressure rise. so the use of static pressure difference to choose us a hidden safety factor.
when we change the replacement of a fan, the fan intake / exhaust speed will be due to fan inlet / outlet area changes. for this situation, it is best to use the total pressure rise to select the fan. make sure that the static pressure in the downstream of the new fan is the same as the original fan pressure.
in addition to pressure and flow, the operation of the fan is also one of the factors that need to be considered to influence the selection of the fan. it may affect the curve of the fan, the characteristics of the fan and how to control the fan.
for many fan operating systems are stable, so the fan can safely operate at about 5% of the maximum pressure point. however, not all systems are stable. for example, the kiln cement clinker into the grate cooler after cooling in the grate plate, grate bed thickness and density will change. for these less stable systems, the operating point of the fan needs to run away from the peak of the curve, usually between at least 10% and 15% of the peak pressure. in other cases, the system designer would want the fan pressure operating point below the highest pressure peak to ensure that there is still some safety margin in the case of unpredictable pressure rise.
if the fan impeller sticks some dust, it will accumulate on the impeller into a dust layer. the dust layer thickening the final part of the loss, when the fan will lose dynamic balance. this happens when the fan is running, or when the fan is parked and restarted. when the fan starts the motor starting the impact force can knock off the dust. in addition, the dust may absorb moisture and lose weight during parking, so it will be easier to fall off. we can reduce this imbalance by using large angle blades or radial blades. for the large air volume of small dip angle fan, the use of backward blade is the best choice.
dust in addition to stick to the fan, can also lead to blade wear. this wear can be corrected using impeller welding repair. if the erosion is severe or the fan has been repaired many times, the impeller will need to be replaced. in order to avoid this situation the impeller can be made with a hard surface at the time of manufacture. this hard surfacing can be applied to the impeller surface can also be added to wear the ribs. but hard surfacing to the inner liner can cause impeller cracks, which means that the anti-friction layer or the baking coating is more suitable for fan blades and the liner is preferably welded to the impeller surface. anti-friction bolts are also common, but can not be used for airfoil blades.
the choice of flow control depends on the system resistance line. the most efficient form of flow control can be frequency control. however, as the speed control of the pressure and volume flow with the speed changes;
pressure = constant x speed 2
volume flow = constant x velocity
according to the square law relationship;
pressure = constant x volume flow 2
in these laws, if the fan design selection flow is close to the maximum efficiency of the fan curve, then increase the ratio of this fan in any air volume will maintain the highest efficiency.
not all process systems follow the law of squared rule. in some systems with constant pressure, the system keeps working on a constant flow. one typical case is the fluidized bed coal-fired boiler clinker extraction process. in this system, if you simply use the inverter to reduce the fan speed will make the fan deviate from the efficient operating point. worse still, the fan may not be able to produce enough pressure due to changes in the speed of the fan. in other words, for such a process, the best choice for controlling the flow is the damper.
outlet air valve: the use of fan outlet valve control fan flow, due to increased system resistance and increase the system pressure loss. if the early fan selection fan efficiency is not high, the use of outlet air valve adjustment can reduce the fan shaft absorption power to improve fan efficiency. however, the efficiency of the fan and the damper at the same time is always less than the efficiency of the efficient selection of the engine through the correct selection. outlet valve on the system control is not particularly ideal, the wind valve at least 50% to close the real effect on the system. another drawback of the outlet air valve is that the fan may run in a low flow surge zone with severe surge.
inlet air valve: the inlet air valve is very close to the fan impeller, which can change the flow direction of the guide vane to change the flow direction of the fan, which is a more efficient flow control method, but also lead to system loss. the closer the inlet valve is to the impeller, the more effective the fan flow control. the control efficiency of the inlet air valve is relatively high, but the disadvantage is easy to wear and damage. under the control of the air valve, can be done to reduce the total wind turbine 10% of the conditions of stable operation. the damper from full open to 100% off the pressure loss of about 10%. whether it is imported air valve or outlet valve control fan efficiency should be far less than the correct choice of a highly efficient fan efficiency. therefore, the main purpose of the application of the inlet valve is to maintain the fan pressure in the case of adjustable fan flow.
(editor: yu yao yi)
article source: http://www.ccement.com/
