Use of High Pressure Stages in the Design of New Axial Fans for High Performance Blocks in Coal Electricity Power Plants

D3 - Fan Performance (i)

Our paper includes several key research results relating to three new axial, high pressure gain and high flow rate, axial flow fan stages designed for the manufacture of modern axial flow fans used in electricity power plants. Two stages were designed with the hub/tip ratio of 0.60 and two design flow coefficient options: Option No. 1 – 0.60 and Option No. 2 – 0.35. Third option had the hub/tip ratio of 0.50 and flow coefficient of 0.45. Design pressure coefficient values of new stages were within the range of 0.43 to 0.83. Design software was used to design the axial compressor stages and the flow simulation to determine the aerodynamic performance map was carried out using dedicated software package.
New stages had high aerodynamic loading of the rotor and stator blade row cascades. The rotor row cascade diffusion factor near the hub was within the range of 0.56 to 0.58. Thus the loading was subcritical from the viewpoint of the flow separation onset and consequently the maximum efficiency observed in tests at a design point was slightly above 90%. Further increase in aerodynamic loading would decrease the stage efficiency as it follows from acquired test and flow simulation data.
Aerodynamic performance map of the new high pressure stages, with the rotor blades stagger angle adjustment of ± 20 deg, generated large work area with good efficiency of 85% to 90% over a wide range of flow coefficient values of 0.15 to 1.2. For comparison, we present measurement results of a standard, less aerodynamically loaded, stage (design flow coefficient of 0.4, pressure coefficient of 0.3 and the hub/tip ratio of 0.5), which currently is used in design of fans designated for coal fired power plants. Differences between the maximum pressure coefficient of new stage and conventional standard stage become larger with the flow coefficient increase.
Axial flow fans with new stages, inlet chambers and exhaust diffusers were tested on ISO test rig with external diameter of 600 mm. As an example the paper presents comparison of measured and calculated aerodynamic characteristics of single, Option 1, stage fan. We note acceptable agreement between the measured and calculated values within the rotor blades adjustment of ± 20 deg.
The new stage aerodynamic characteristics allow optimising the design of modern axial fans in terms of production costs and attaining maximum pressure gain and efficiency. In this connection, analysis based on one dimensional flow model of axial flow fan, is able to clarify the effect of energy losses in the inlet chamber and outlet diffuser and stage parameters relating to the axial flow fan efficiency.
Our paper gives example of optimisation of the flue gas axial fan with a diameter of 4.3 m and peripheral speed of 167 m/s destined for a large coal fired 660 MW power plant block. Required volumetric flow rate of the flue gas and fan specific work at a nominal point were Q = 830 m³/s and Y = 8,500 J/kg or at a point of maximum flow Q = 1,250 m³/s and Y = 10,800 J/kg. Use of standard stage with the hub ratio of 0.5 in two stage fan is able to meet the assignment requirements.
Using analysis it was found that one, new Option 1, stage with the design pressure coefficient of 0.83, may be used as a replacement for two standard stages. In this design option the same peripheral speed and outside diameter were used as in the original fan design. Denoted working points in measured performance map of modelled fan show projected efficiency values. In estimating the prototype fan efficiency we used results of similarity analysis of modelled fan (external diameter of 600 mm) and prototype fan (external diameter of 4,300 mm) and with the use Reynolds number and relative surface roughness.