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Steam Raising Blower

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Peter Bell24/09/2020 13:43:03
320 forum posts
146 photos

Just pm'd

J Hancock24/09/2020 14:39:59
452 forum posts

Another very good source of ready made impellers are scrap, car water pumps.

Right size,shape,weight up to 2.5L cars.

Chris Gunn24/09/2020 16:28:18
337 forum posts
24 photos

Bill, thanks for the info on blades, I went to a lot of trouble to make a 6 bladed impeller with forward facing blades, and after 2 or 3 outings it collected enough soot to make it vibrate, bits would break off and jam it. At rallies it whined so much that folks would shout "all clear" when i switched it off.

I adapted a central heating exhaust fan in a tenth of the time, and it is much better. British gas service engineers are replacing these daily, so if you know someone on a maintenance contract, get a request in early.

Chris Gunn

Bill Dawes24/09/2020 23:07:18
363 forum posts

Hi Peter, fan engineering is both a science and a black art. I have always had a saying with fans, if you change something and think it will work it probably won't, or if you think it won't make any difference it will.

Blade type is selected for various reasons, efficiency or application mainly. The ideal is of course the most efficient, especially in these days of ErP regulations (Energy related Products) however sometimes an efficient blade would be not be fit for purpose for some heavy industrial applications. Cost is also a consideration, straight radial blades are less costly than fancy curved blades so as with many things it's all a balance (excuse the pun)

I wouldn't worry about blade type too much, a simple radial blade will give good performance for a steam raising blower, number of blades will be restricted by the space available on a tiny impeller like this, I would probably aim for about 6 blades.

I am talking here about centrifugal fans of course not axials.

The fan casing (volute) converts the energy off the blades into usable energy at the outlet and of course is directional, impellers can also run in a circular housing, probably with guide fans as found on large furnace fans, discharging 360 degrees around its periphery, some losses in performance compared to a volute. The part of the volute where it is closest to the impeller, callled the cutoff, influences the pressure development, too much clearance looses pressure, very close however creates more noise. Again all a bit academic really for this application.

Good luck.

Bill D.

Paul Lousick25/09/2020 00:15:15
1546 forum posts
580 photos

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


Bill Dawes25/09/2020 10:20:40
363 forum posts

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 Builder525/09/2020 11:08:47
2109 forum posts
146 photos

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

Paul Lousick25/09/2020 12:06:22
1546 forum posts
580 photos

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


steam raising blower 2b.jpg

duncan webster25/09/2020 12:41:48
2799 forum posts
41 photos

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 Lousick25/09/2020 13:45:11
1546 forum posts
580 photos

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 Bell25/09/2020 13:47:24
320 forum posts
146 photos

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?


Chris Gunn25/09/2020 22:28:54
337 forum posts
24 photos

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 Dawes25/09/2020 23:05:34
363 forum posts

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 Lousick26/09/2020 07:46:38
1546 forum posts
580 photos

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.


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

Peter Bell26/09/2020 08:57:15
320 forum posts
146 photos

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.


Paul Lousick27/09/2020 04:12:17
1546 forum posts
580 photos

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


Peter Bell27/09/2020 07:46:17
320 forum posts
146 photos

Thanks Paul, saw that and bought one Friday,


SillyOldDuffer27/09/2020 10:38:24
6331 forum posts
1389 photos

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!


duncan webster27/09/2020 17:58:47
2799 forum posts
41 photos

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 shelley27/09/2020 19:13:08
116 forum posts

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.

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