STEAM PIPES.
Inlet pipes should have an area of not less than that of the inlet ports.
Too small a pipe will reduce the effective pressure in the cylinders.

Exhaust pipes should always be larger than the inlet pipes. For small low-pressure engines, about double the inlet pipe area usual.
An exhaust pipe that is too small will create back pressure.

For a locomotive or other multi-cylinder engine, the various pipes need to be of different sizes.
As an example, in a two cylinder locomotive where the cylinders are 1.5" x 2", the pipe sizes are:
Superheater to junction (where the steam is split to go to the individual cylinders) 1/2"
Junction to cylinders 3/8"
Cylinders to exhaust junction, 1/2"
Exhaust junction to blast nozzle 5/8"

The speed of steam in pipes should not exceed 5,000 feet per minute.

BLAST PIPE SIZES.
The exact size is not important at the design stage as it is easy to alter afterwards.

The table will give a starting point for one of the computer programmes, and will usually be close enough to give reasonable  performance.

It can easily be changed at a later date, which is why full-size designers did not bother too much with exactitude, and many  locomotive drivers used to make the occasional unofficial modification, usually with an iron bar lodged in the blast nozzle to reduce the area.

(Errum that should have been designers of full size engines, not,... oh, well.)


The size is always a compromise between having sufficient draught to give the airflow that the fire requires but not too much which would burn more coal than is required, and on the other hand, making the nozzle too small which creates back pressure on the pistons.

If the engine dies not "steam" then a smaller hole in the nozzle may be the answer.

When making the nozzle it only takes a few moments to make a couple of spares, without holes, which can be drilled and fitted if required.

BLAST NOZZLE
PROPORTIONS.

The cone of steam emerging from the blast nozzle must completely fill the chimney.
The blast nozzle should also be
centred under the chimney.
A template cut from cardboard, as below, is useful to check the line-up and proportions of the blast nozzle.

A good starting point for the proportions and locations of the chimney, and blastpipe inside the smokebox is to divide the latter into three, with the chimney (or any extension) going down to the 1/3rd mark, and the blastpipe at the 1/3rd-from-bottom point.

Although the "traditional" blast nozzle sizes shown above will  work, there has been a lot of work to
improve the efficiency of steam locomotives.
Some of the most recent work is by  Jos Koopmans from the Netherlands, who has kindly  written an article detailing his work.

It is worth noting that Jos's formula has been tried on models and on full size engines and has been found to improve the draughting and efficiency.