Hi Rolphill,
You asked, so my advice is as follows.
But first I shall explain (for uninitiated readers) all I know about "chimneys"..
A "Chimney" seems to be any simply "pipe" - often brick or stone construction - when in a factory, house, or other building, and these latterly were tapered internally to accelerate the flue gases from smoky coal or wood fires and increase draught though the fire - and power from the boiler.
A flue is any passage carrying exhaust gases to where they can be discharged.
A chimney, or funnel, can have a decorative or plain exterior.
Titanic had 3 funnels for boiler exhaust, and one "dummy" funnel containing other vents - e.g. from kitchens, etc, - purely as a decorative device to make it look more powerful than sister ships with 3 funnels.
What "decorative style" of funnel do you propose to use? - A simple straight pipe to get flue gases above your head when in the boat?
Why do chimneys draw?
The purpose of a chimney is to take the products of combustion (smoke and gasses) from the appliance to the atmosphere outside the building. At the same time, to draw air for combustion into the appliance. This movement of combustion air and exhaust is called
draft.
The difference in pressure and temperature between the air/gasses inside the chimney flue and the outside air creates movement. (warmer, lighter gasses in the flue tend to move upward).
To keep the pressure conditions favourable, we need a tall column of warm air inside the chimney, and cooler air outside. The warm air will tend to rise, drawing the exhaust from the appliance out. As air exits the chimney, fresh air for combustion is drawn into the appliance.
FOR THE STEAM BOILER:
We need to "draw the fire" when first lighting the fuel by various methods, as there is insufficient heat/power in the natural draught to get the fire going quickly, and raising steam. Generally, an efficient boiler will cause some restriction to flue gases as it extracts heat from the gases. But we want maximum performance from the boiler - as we are not crossing oceans with limited fuel supplies! So we have boilers with less restriction, and, like with railway locomotives, usually force the fire by use of engine exhaust steam as a blast up the chimney to increase air drawn through the fire and hence increase the rate of combustion, and the quantity of available steam.
But before the fire gets going properly, and to speed-up steam production when the engine is not running, or lightly loaded, we have an additional blast-pipe feeding steam to a Nozzle/jet at the base of the funnel to increase the draw....
The blast nozzle can be: as you describe " a ring with just some holes in it"..
The whole effectiveness of a blower is achieved by the effect of a jet in an annulus, of specific dimensions, to extract as much flue gas as possible with the minimum of "blower steam".
So the "ring with holes in it" is precisely positioned and with specific hole sizes as designed and developed by the original designer.
And there are a number of designs you can use:
- A single jet centred in the chimney/funnel = a long narrow tube - This needs to be at least 7 x the diameter of the tube to have optimum effect. - This may be best for your Chimney/funnel?
- A venturi that starts as wide as the chimney/funnel/flue is planned to be... Tapering for 1 1/2 diameters: reduces to about 2/3rds the diameter, then expands to full diameter over the next 5 1/2 diameters.... This can give a large increase on the efficiency of the blower, but the minimum diameter must not restrict the fire when there is no blower, unless the fire is only meant to be forced by either the blower or the engine exhaust steam. (As in a loco!),
- Instead of a single jet, a ring with a very small gap can be used to blow up the walls of the chimney/flue/funnel....
- Or a ring of very small jets can achieve the same thing, either around the central exhaust from the engine, or around the periphery of the funnel, chimney/flue..
To make jets/nozzles these can simply be a small hole drilled in the end of a piece of threaded bar - may be hexagon? - so you can remove it and fit a different size?
It may be a finely tapered hole (from the inside) to a flat end face, or finely tapered outer part - such as to maximise the exiting steam velocity (as in steam nozzles feeding steam into a turbine). - These double tapered nozzles should follow the same rules as a venturi in the funnel.
- Incidentally, it is called a "funnel" on ships as it was tapered (internally if not externally) to accelerate the exhaust smoke so it passed up and clear of the deck of a ship. Captains hated "smoke in their eyes" when there was a following wind... so the Funnel was developed to shorten the original tall narrow smoke-stacks seen on early ships that had steam and sail... or Mississippi paddle boats, etc.
Locomotive funnels have complex interior shapes, often with a decorative or aerodynamic outer shape.
https://images.app.goo.gl/sd4yKgn9ebFY7NzC6
https://images.app.goo.gl/gCNViyGYf5stpozT6
https://images.app.goo.gl/9ro3pCL59QQqZ8fa9
https://images.app.goo.gl/FK9dE1ybXCeqAEpFA
https://images.app.goo.gl/njTk8Eubhd335KXn8
I hope this is informative, and of some use so you can decide how to arrange your blast pipe/nozzle(s)?
I suggest you consider the funnel exterior that you want. Then that may be affected by the internal shape of the funnel...
WITHOUT any blast it should be as much as 4 x the cross-sectional area of the grate opening area. Bet you had not considered that one! So then you have to consider if you will have a forced blast when the engine is running using the exhaust steam, or if on salt water (sea) then you may have a condenser on the exhaust from the engine and thus no engine exhaust to make the blast. A condenser increases total efficiency and engine delivered power... by effectively increasing the pressure different between steam-in and exhaust-out of the engine by the vacuum created in the condenser.... Both my model boats have condensers, but I use gas fire in the boilers..
WITH blast up the funnel, it can be whatever you decide from the above explanations.
As to "SIZE" of funnel (cross-section area) and blast pipe nozzle CSA, then that depends on the engine, permissible back-pressure (creates a small loss of shaft power), etc.
For a recent "tuning" of a loco - 5 inch gauge Simplex:
Problem: While it had an excellent blower, that could work at very low steam pressure to draw the fire to quickly raise steam, the steam test failed because the Safety Relief valves (as per original design) could not cope with the fully fuelled fire, with full blower, and the pressure rose well above the safety limit very quickly. The remedy was to rduce the maximum blower by reducing the hole at the nozzle. (previously drilled larger than original design by someone to increase the "blow"!). At first we reduced the blower jet hole too much. It was fine for the steam test, and when more than 50% of steam pressure was available. But in service, the engine lost steam pressure on one part of the track, and needed extra "Blow" when the engine slowed considerably on an incline. So it was common practice to turn on the blower for this incline. But at less than 50% of normal steam pressure, there was insufficient "blow" with the small jet. So we increased the jet by the next numbered drill until we had a satisfactory blower at 30% of "normal" pressure, and running was acceptable. Also the re-test of the steam test showed the Safety Relief valves to be adequately sized when this blower was turned full-on and the fully fuelled and watered boiler was not using ay steam for the engine. It then passed the steam test.
So I cannot tell you what size of blower jet/nozzle you need, until we discuss further with actual dimensions of the boiler, fire-box, flues, etc.
OK? (Sorry if this was a bit long... but "you asked"!)
K2
