With a fan impeller with forward-curved blades, a high air performance is achieved with the smallest possible installation space due to the high power density (see figure 3.2.1. gray area). The impeller generates almost exclusively kinetic energy, which is converted into static pressure in the fan housing. However, the lower efficiency and the high power consumption must be mentioned as disadvantages.
In the case of a fan impeller with backward-curved blades, the generated flow energy is already largely converted into static pressure in the impeller, and the proportion of kinetic flow energy is comparatively low (see Figure 3.2.1. red and green area). In addition to the resulting higher efficiency, these impeller types can also be used without a scroll housing without any major loss of performance. Typical applications are e.g. in AHUs, roof fans or air circulation in industrial plants. For detachment-free flow through the impeller, the use of a suitable inlet cone is essential (optimum gap flow). The inlet cone and fan impeller are matched to each other in terms of flow, so it is essential to use the dedicated inlet cone. If this is not the case, this will have a significant negative influence on the fan characteristics.
A forward-curved fan impeller generates a given pressure at about half the circumferential speed of a backward-curved fan impeller and is therefore much quieter. In addition, due to the higher blade counts, the noise spectrum is more broadband for forward-curved fan impellers and more tonal (few blades) for backward-curved fan impellers.
In the usual range of application, the total pressure characteristic is flat for the forward-curved fan impeller. For a backward curved bladed impeller, the pressure curve can be rather steep but also flat, depending on the diameter-to-width ratio of the impeller. This results in different changes of the volume flow rate in case of pressure fluctuations at the fan in installed condition.
For fan impellers with a steep characteristic curve, the error in the pressure demand calculation can be greater, since a change in pressure here causes a smaller change in volume flow compared to a flat characteristic curve. Thus, fan impellers with a steep characteristic curve are more suitable if fluctuating pressure changes are to be expected during operation. At constant speed, the power requirement for the forward-curved type increases progressively with the volume flow, whereas for the backward-curved type it increases only up to a defined maximum.
In summary, the forward curved fan impeller is 10 to 25% smaller than a backward curved one for the same application and runs quieter because of the lower speed. However, it is also less efficient and requires more drive power.
