Although there are a number of possible ways to construct a spark arrester, the following principles may be considered when designing from scratch.

1) An external spark arrester will definitely detract from the appearance of a small steam locomotive, so where possible one should be fitted internally. A possible exception may be a temporary arrangement for a locomotive that is rarely steamed, and is unlikely to be rebuilt.

2) Any arrangement that restricts the throat area of the petticoat pipe / chimney is likely to increase back pressure. This will reduce the smokebox vacuum and 'draw' on the fire, and may cause poor steaming. The goal should be to have little if any reduction in draught efficiency.

3) Any device that is placed in the path of the exhaust steam will become sticky from the oil in the exhaust, and clog up quicker. It is better not to restrict the blast exhaust path.

4) Any device that encircles the blast pipe, be it a mesh cage, tube or circular baffle should be as large a diameter (and hence surface area) as reasonably possible, whilst allowing for removal. This is to minimize the restriction of the exhaust gas flow, whilst also allowing normal operation of 'blower jets'. It has been noted that a area of 16 holes per square inch mesh effectively reduces the flow area by up to 43 %, hence the emphasis in this article to use the largest spark arrester possible.

5) Any device is likely to become clogged with cinders, so must be easily accessible for removal and cleaning, and where possible designed so that cinders 'fall off' into the smokebox.

6) It should be of simple design to enable it to be easily (and inexpensively) renewed if damaged.

Please select the next page of this article for further thoughts on designing.