Steam Raising Blower

[Paul LousickParticipant @paullousick59116]

Another source of an impeller is one from an old car alternator which is a similar design to that shown in Model Engineer magazine.

Paul

[Bill DawesParticipant @billdawes]

You see very simple impellers on all sorts of things such as car alternator that Paul mentioned, oven circulating fans, cooling fans on motors etc. A circular plate cut through at intervals and a blade folded out is a classic. just needs a a piece of round bar fixed in the centre and bobs your uncle. Not very sophisicated and hardly in the 'fan engineering' category but it will produce an air flow.

Bill D.

[Speedy Builder5Participant @speedybuilder5]

What sort of flue temperatures would you expect a fan to work at (choice of fan case and bearing materials) ?

[Paul LousickParticipant @paullousick59116]

The temperature that the fan can reach depends on how long you leave it on the chimney. I start my fire with kindling and then add coal/charcoal and remove the blower after the coal is burning on its own. The blower is hot and and has to be handled with gloved hands.

Because of the heat, the case and fan should be made from metal and the motor and bearings should be separated from the hot flue gasses

Paul

[duncan webster 1Participant @duncanwebster1]

I know this is nit picking, but I wonder how many of those little motors have decent thrust bearings to carry the weight of the fan? They seem to last long enough in any case.

[Paul LousickParticipant @paullousick59116]

The blower motors do not operate for any length of time to wear out. probably fail because of the heat and dirt first. The motors in cheap cooling fans last for years.

[Peter BellParticipant @peterbell11509]

Thanks everyone, wealth of information! As I have no experience of these fans I have been drawing up the design put together by Brian Baker with view to cnc'n the parts and building one.

Thanks for the info on gas boiler exhaust fans, just realised I know someone who services has boilers for a living, he's bound to have something….

The fans on the boiler exhaust fans look rather small and curved, don't they soot up easier than the straight blades?

Peter

[Chris GunnParticipant @chrisgunn36534]

Duncan, in my case the fan does not run for many hours in a normal rallying year, and the exhaust fans are light. I am using an ex lorry wiper motor which is pretty chunky with decent bearings.

Peter, my exhaust fan is quieter and seems to run cleaner than my home made one, and there is much more clearance around the impeller of the exhaust fan, which may help in this regard.

Chris Gunn

[Bill DawesParticipant @billdawes]

Hi Duncan I would have thought the axial load load on such small impellers will be negligible as both mass and suction thrust will be very small. A ball bearing will take a good thrust load, we accommodate very high thrust loads on deep grove ball bearings, around 600 lbsf or more ( in old money). Bearings slightly bigger than the steam raising blower ones of course.

On the subject of blade shape again, curved blades both forward or backward are potential problems where dirty gas is involved. Radial blade the best, straight backward sloping not as good but a more efficient compromise.

As a fan engineer I feel I should be making my own blower, not at he stage where I need one just yet but my Emma Victoria is inching (painfully) towards completion so I guess I will have to start thinking about it. Any thoughts on best place for a 12v motor guys.

[Paul LousickParticipant @paullousick59116]

Old car windscreen wiper or door window motor. Old battery operated equipment / toys  with motors, too many to specify.  Also lots of inexpensive motors on ebay.

Paul

Edited By Paul Lousick on 26/09/2020 07:55:43

[Peter BellParticipant @peterbell11509]

Thanks everyone for the info.

Still looking for my motors but no progress, perhaps they may have gone in a tidyup.

Got some wiper/door motors but the majority don't have a plain output shaft so difficult to attach anything to also some door motors have a limited duty cycle and got very hot on previous use. A heater fan looks the most attractive and the older Smiths or Lucas have metal casings which are easier to mount, some of the modern variety seem to be built into the plastic mouldings. Lots on ebay but was intending visiting a local breakers and have a look. New motors on ebay abound but many look smallish and appear to be higher speed.

Peter

[Paul LousickParticipant @paullousick59116]

The latest edition of Model Engineer No 4648 has an article with a steam raising blower.

Paul

[Peter BellParticipant @peterbell11509]

Thanks Paul, saw that and bought one Friday,

Peter

[SillyOldDufferModerator @sillyoldduffer]

Just some theoretical observations about motor impellers that might avoid a really unlucky selection!

Most impellers help cool the motor without putting a significant load on it, ie most of the motor's output is still available on the shaft, not absorbed moving air. While it's reasonable to use impellers as a fan they won't shift air efficiently – too small, and generally crudely made. A table fan has whacking big aerofoil blades on the front while the motor inside cooled by a tiny impeller made of bent metal at the back.

Some electric motors, like the Universal type, can run away off-load and damage themselves. On these the impeller may be made deliberately inefficient so that it absorbs enough power at high speed to stop the motor running away. This type of impeller might disappoint in a blower.

Fans work more efficiently at high-speed rather than low, but the blade design becomes more critical. They're better at moving large volumes of low pressure air rather than creating a vacuum or compressing air. Efficiency goes to pot when either the input or output is constricted. So a top-notch design optimises the blade, motor power, rpm, volume, and pressure differential.

But for steam raising, I don't think fan efficiency matters much, and it might even be bad – too powerful a blower might cool the fire so much it goes out or lift loose fuel out of the firebox before it's delivered heat.

As electric blowers have generally been doing a good job on steam locos for donkey's years I don't think their design is critical, but the above might explain occasional failures and why some blowers outperform others.

Wild guess on my part, but I suggest:

• An aerodynamic ran running with its tips close to the casing.
• Fan speed close to it's design rpm, with the facility for the operator to adjust speed for best results.
• Flared top on the outlet.
• An inlet tube long enough to allow an adjustable air-intake near the smoke box and away from the fan. Purpose to allow the operator to bleed air into the system so the fan has enough to bite on. Rather than relying on the fan struggling against back-pressure to pull a vacuum, it would allow the operator to create an updraft, or not, for best results.

Probably all been tried already, a hundred years ago!

Dave

[duncan webster 1Participant @duncanwebster1]

If you partially or fully block the inlet or outlet of a fan it will speed up. That's because it is doing less work, I know it's counter-intuitive. Having said that, to regulate the suck it is much better to use an electronic controller, cheaply available off ebay.

It's just dawned on me that a ready supply of steam raising blowers could be the motor/fan combinations out of these battery powered vacuum cleaners that you see advertise all the time. I reckon the most common failure will be the batteries giving up, and they might start appearing at the tip. I've got one put away that I was going to press into use for an organ. It looks as tho the air is ducted back round the outside of the motor to cool it, but if running under less voltage it might well survive without.

I use one of those ex military radar cooling fans which are advertised as working off 12v. Not for a medium sized 5"g loco they don't. but 12-24v converters are again readily available.

[noel shelleyParticipant @noelshelley55608]

The old smiths motors had a volume control to vary the amount of noise it made, this may have been the delux model, old ish cars before the 80s had this set up. Noel.

[Clive Brown 1Participant @clivebrown1]

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.

[Jon LawesParticipant @jonlawes51698]

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….

[Bill DawesParticipant @billdawes]

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&#39

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.

[Bill DawesParticipant @billdawes]

Don't know where that emoji came from?

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