Steam Raising Blower

When I was setting about making a blower, economy was in mind. Most of the readily available, cheap, ex-equipment rotors seemed to be plastic which is the reason that I made my own. Aerodynamic design isn't really an issue. A fairly high speed gives adequate airflow with quite crude blading, but in turn (sorry!) gives rise to the need for a fairly well balanced assembly.

The old Avionics fan I bought on ebay has a metal rotor, aluminium I presume, but its put up with a lot of abuse so far. It's marked 1970 on the date tag, and 24v, so actually I guess thats more likely from an armoured fighting vehicle or similar rather than an aircraft, which are 28vdc.

I made a very novice mistake when I made a brass chimney adaptor which I then tried to solder onto the mild steel plate on the base of the fan.... which turned out to be mazac or a similar alloy.... I had to make a replacement....

One other thing to remember about centrifugal fans is that there is a danger of overloading the motor if the fan is run fully open (not connected to a 'system'

Radial and forward curved fans have a power curve that rises continuously with increasing flow, backward bladed have so called power limiting characteristcs, the power peaks and drops off. The normal way we select a fan in industry is based on the specified duty point not the maximum power, this is for commercial reasons, why fit a 200kw motor if a 150kw does the job required.

Of course you can fit a motor that covers any condition and I would expect fans that are sold off the shelf such as you buy from Machine Mart etc would be designed on this basis as they are sold for general purposes not a specific application.

This surprises many people not familiar with fans, it is common for people to think that if you shut a fan off it is overloading, the reverse is actually true, its power would be at a minimum. It is the case however that a fan that depends on its motor cooling by the fan airflow passing over the motor, axial flow fans and special purpose fans such as those in vaccum cleaners fall into that category, will be compromised if you shut off the airflow for too long.

So as far as steam raising blowers are concerned (probably more correctly called steam raising exhausters) the proprietary ones you buy with an external separately cooled motor are designed, as far as I can see, such that they will work without problem under all conditions from fully open to fully shut off.

Bill D.

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