Loco boiler funnel

[steamjohn248]

Ok its not a locomotive but its still a locomotive boiler!

We are commissioning a new steam plant in a 25' boat and the boiler is loco type, short barrel (17" long 16" diameter), 51 fire tubes 3/4" ID giving a total cross section of around 22 sq. ins

The existing funnel is only 4 1/4" ID, area around 14" sq. ins. (56" high).

The boiler is shy for steam on natural draught and since the boat is intended to run on salt water most of the time the plant must run in condensing mode and I don't want to have the blower on all the time if it can be avoided.

Does anyone have a view on the benefit of increasing the ID of the funnel to nearer or in excess of the tube area. (an increase in height seems an obvious bonus).

I have posted the question on the Steam Boat Ass. Website but this forum has a wider readership and seems more reactive.

[alanstepney]

Extra height will give a major improvement in "draw".

However, the key is not the diameter of the funnel but the internal shape. You really want a double-taper to give an entry and an exit cone (venturi) , better to increase the speed of the gas flow. Although there are plenty of designs for them, and to get perfection dimensions are critical, almost anything will be better than a straight plain tube.

[cotswold]

alanstepney said:

Extra height will give a major improvement in "draw".

In view of what you say about minimal use of the blower, it may well be worth while doing something specifically aimed at increasing the efficiency of the blower when it is in use (more blow for less steam). Thus, if you take Alan's advice about a venturi, the positioning of the blower nozzle in relation to the venturi constriction could be of some importance and experiments may be worth while.

[Shawki Shlemon]

A rough guide the size of the venturi throat should be a little more than 3 times the dia of the blast nozzle and likewise the hight of the venturi from the blast nozzle . Make the blast nozzle hight adjustable and then trial and error . This formula works for me .

[steam4ian]

G'day John.

Your topic is getting hijacked by loco reasoning. You don't want to use the blower due to loss of water. That most likely means exhaust steam is condensed so there is no draft.
The only way you have of getting draft is the funnel. This needs to be higher to get more buoyancy, it is your only natural draft option. There are formulae for draft vs chimney height in old texts.
The tube length diameter ratio is a bit over the 20:1 often quoted as optimum for loco boilers with induced draft (blast pipes). IMHO the tubes are too small for natural draft.

You are left with inducing the draft. A centrifugal blower driven off the engine and connected to a "blast pipe" might be one solution. There are designs for much better venturi inducers than the conventional blast pipe. Alternately an ID fan on top of the smoke box may be another.

Photos would be great.

Regards,
Ian

[steamjohn248]

Thanks for all your interest

As a Loco builder I am familiar with blast nozzles blower jets, petticoat pipes and venturis. problem is they only relate to forced draught.

As Ian says we are looking to steam the plant on natural draught since all the exhaust steam needs to be condensed back into boiler feed water.

Using steam in a blower is not a preferred option either.

Masefields' "Dirty British Coaster with a Salt Caked Smoke Stack" typically steamed return tube boilers on natural draft. (Though some of the ones I remember had quite tall funnels). However tugs and launches in which the boiler top was only just below the casing had relatively short funnels. (I fired a coal burner with a quite squat funnel and she made steam ok, I never looked down the chimney to see how big the hole was though).

My library of old books on marine steam make mention of funnel ID re grate area but not tube area.

Simpson Stricklands' catalog of 1906 shows a loco type boiler in a launch fitted with quite a short funnel but the cutaway of the boiler is not dimensioned though the funnel appears to be to be quite a bit bigger than the one we have inherited .

I once surveyed a launch with a gunboat boiler (similar to a loco but shorter with a round furnace) and that had an ashpan blower (driven off the engine) with an interlock to shut the fan duct off before the firedoor could be opened.

That may be our only option but I'd like to get the chimney as near right as possible before we go down that road.

If the information is not forthcoming I guess we'll fit a much fatter funnel and see if there is any improvement.

Regards to everyone

John

[steam4ian]

G'day John.

If you go down the engine driven blower route you could look at an inducer on the funnel. I have seen pictures of units using relatively low pressure air for ventilation extraction. Instead of a central nozzle you have an annular slot around the periphery if the stack. This could be put on without mods to the smoke box.

Tubes on scotch boilers are relatively short. Even a short funnel is probably two decks high

Still waiting on pictures.

Regards,
Ian

[steamjohn248]

Hi Ian

Further reading indicates a funnel cross section/grate area ration should be around 1/7 which means we should at least increase the funnel to 5 1/2" to 6" ID.

The other tie in is fire tube area to grate area and it seems the builder of the boiler got that right.

Your idea of a low pressure air supply fed directly in at the chimney base sounds interesting, would save a lot of fannying around with interlocks on fire doors. (I'm not sure how to post a pic though I know it has to be via another server or summat, I'll ask SWMBO).

Cheers John

[steam4ian]

G'day John

Thanks for replying, I'm surprised others haven't joined in. the 1/7 ratio sounds like something I recall. But the tubes worry me. As I said earlier the length/diameter ratio for loco boilers is about 20/1, but these have induced draft. The resistance of a tube is a function of both its diameter and length, thinner the tube the higher the resistance. I have just looked up Harris "Model boilers and Boiler Making". He suggests Tube area / grate area of 1/7 for loco boilers (induced draft) and 1/4 for natural draft' that is more tube are is need for a natural draft boiler. Another reason is the slower gas speeds with natural draft allow better heat transfer.

Watch out if the funnel gets too big, it may start to get turbulent flow and back drafting, if it does put a cone to reduce the diameter at the top to increase the velocity at that point.

Regards,
Ian