Forward curved blades

These impellers first became popular at the end of the 19th Century and almost superseded all other types. A diagram­matic representation of the impeller is shown in Figure 1.60. They are considerably smaller for a given duty than all other de­signs.

Flowrate can be as high as 2.5 times that of the same size of backward-bladed fan. This is now seen to be not necessarily an advantage since casing losses, which are a function of velocity, will therefore be about six times a great. Thus even with an im­peller total efficiency approaching the theoretical optimum of about 92%, the overall fan total efficiency would still be down to about 75%.

Such fans are now only used where space is at a premium, as they will be the most compact. Due to their smaller size they are usually cheaper, although the differences are much reduced with the greater possibility for automated manufacture of back­ward bladed fans. Nevertheless the scope for improvement has been appreciated and current designs achieve static efficien­cies of 63% and total efficiencies of 71 % at even lower speeds.

It will be noted that the performance curve has discontinuities due to stall and/or recirculation (see Figure 1.61). A large mar­gin over the absorbed power is necessary where the system re­sistance cannot be accurately determined, or where it is subject to variation, to take account of the rising power characteristic.

The impeller has a large number of shallow blades in widths from 0.25 to 0.5D and runs at lowertip speed for the duty. Struc­tural considerations have in the past limited the pressure devel­opment to about 1 kPa, but the narrower widths are now suit­able for pressures up to 14 kPa.

Apart from low-pressure ventilation requirements, these fans are widely used for mechanical draught on shell-type boilers, oil burners, furnace recirculation etc.

Posted in Fans Ventilation A Practical Guide