exhaust’s

[BoldminerParticipant @boldminer]

[BoldminerParticipant @boldminer]

can anyone please advise me of the ratio of blast nozzle dia. to chimney "choke" dia. and the distance between them

[61962Participant @61962]

Recommended starting point as Martin Evans is as follows:

Blast nozzle 1/7th of cylinder diameter. Evans is silent on the size of the choke, but in full size practice it was around three times the size of the nozzle. In models the nozzle is usually much smaller than scale and the nozzle/choke ratio ends up around 5.

A 'V' of ratio 1 to 6 placed in the blast nozzle should intersect the chimney just below the top. If it is too high then the nozzle needs to be lowered.

A 'V' of ratio 1 to 3 should intersect the petticoat near the bottom. If it misses then the chimney liner is too short.

It is essential for the nozzle and chimey to be concentric

It's easier to draw than to explain in words.

If the locomotive steams freely it is possible to increase the size of the nozzle until it makes just enough steam when running to maintain pressure when linked up. If the steaming is poor then reducing the nozzle size might improve matters but the cause is more likely to be associated with air or steam leakage into the smokebox or inadequate air supply to the grate.

There are many theories on blast pipe/chimney design, and many famous locomotive engineers, not least Chapelon, Kylala, Le Maitre, and Ell all had lots to say on the subject, but if you follow the above you will get a result.

Hope it works out well

Eddie

[BoldminerParticipant @boldminer]

Thanks a lot Eddie, that was just the information that I was looking for.

Regards, Boldminer

[julian atkinsParticipant @julianatkins58923]

whilst i dont disagree with Boldminer, who follows the 'Greenly' formula, it would be useful to know what loco you have and cylinder and boiler sizes. also what type of loco, as smokebox volume and dia and height of chimney have a significant effect, as often the 'Greenly' formula wont work with 'squat' chimneys in big express locos with large dia boilers and smokeboxes.

cheers,

julian

[BoldminerParticipant @boldminer]

Hello Julian, thank you for your intrest/reply to my query. With regards to the type, size, and spec. of the engine, I hope that the following can be of some help.

The engine is a 31/2" K4 to Matin Evans design. The cylinders have a diameter of 1.125" x 1.625" stroke and the boiler barrel is 4.375" dia, barrel length is 8.4" The boiler capacity is approx 2.2ltr and is noted as 12bar litres. The smoke-box as a volume of 56cu.in.( ie. 4.375"dia. x 3.75"long). From the boiler to the top of the chimney the height is .883", the chimney opening is .875"dia. The total length including pettycoat pipe is 2.195" The pettycoat opening is 1.3125" dia and the choke dia. is .75". The choke height is .75" from the bottom of the pettycoat or approx. 1/3 the total height of the chimney

regards, Boldminer

[julian atkinsParticipant @julianatkins58923]

soz meant to type 61962 instead of boldminer. martin evans falways followed the Greenly formlae as set out by 61962. you should be ok with design as drawn, though check with the 1:3 and 1:6 templates.

[BoldminerParticipant @boldminer]

thank you Julian, I'm just about to check my actual measurements using auto-cad.

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