Water Tube Boilers

I am currently reading a book called 'Model Locomotive and Marine Boilers' by Martin Evans. A chapter towards the back of the book shows a very simple 'water tube' gas fired design for a 2 1/2" gauge locomotive and the book suggests that similar designs could be applied to larger scales - say 3 1/2" gauge. The design appears to be far easier to build than the traditional coal fired version and seems to use a much smaller proportion of very valuable copper/ silver solder.

Is anybody familiar with the performance of these types of boiler and why with such high material costs is this type of boiler not gaining in popularity e.g., is the performance simply not good enough. Any views?

PhilH

do you have a sketch/picture? I had a go at water tube boilers some time ago, it all depends on what type of water tube boiler you mean?

Good idea Fizzy so here it is. You might not be able to see all the detail but it is 3 1/4" diameter and 10" long. Its firebox section is obviously narrow enough for 2 1/2" gauge but this could surely be made wider. It is not far off the size of a boiler that would fit Rob Roy - maybe. However, I am still reading the book and the next chapter might hold the key. Chapter 8 is about the testing of boilers and it might give an insight into performance.

I suspect that for a larger loco expected to do some work i.e pull people around, you won't get enough heating surface to produce the steam output required. Probably be ok for a loco pulling a few wagons or coaches around?

John

John,

Yes, that is my suspicion but there is a suggestion from the book that this is not necessarily the case. I guess without doing some sums - it is difficult to see why the heating surface is lower because heat from the burner is able to heat the small bore water tubes, the bottom and sides of the inner vessel and the exhaust fumes are able to heat its entire length all the way to the smokebox. Without sums, instinctively, the area seems much higher???!!!

PhilH

Without sums, instinctively, the area seems much higher???!!!

and therein lies the problem. There is a very finite amount of boiler space (for water) which scaled up to a bigger gauge would mean a very fast turn around, thus making heating difficult. Think of a little mamod boiler, small amount of water and runs for ages, but step this up a few fold and you cant heat the replacement water fast enough.

Fizzy - I am definitely not with this (by the way - Im sure it is me not understanding it - not your explanation) because If you took this to its extreme i.e., kept reducing the water volume but increased the heating area - the likely result is flash steam isn't it?

I am going to try a few estimates/ calculations to understand what is going on. I am quite sure that something is wrong with the idea or everyone would be doing it (unless people simply with to stay with tradition of course).

Thanks so far guys.

Philh

I also built a 7 1/4 Brotan boiler, have a search for that one.

At one time there was a interest in stainless steel monotube boiler design that would fit inside a normal boiler shell. As I recall the safe working pressure was about 1000psi and that combined with the very small water volume made it exceptionally safe, although it wasn't popular with testers who didn't know how to treat it. It may have been Geoff Sheppard or another ME editor who was involved.

Neil

Michael,

Thanks for the answer and interest in my question. I think the section above is the crucial bit because as you point out in your answer - basically, all boilers have heat exchange surfaces with water on one side and a hot, heating gas flowing past the other.

I guess my question should be modified to ask - is anyone aware of any ME articles where a set of different designs have been tested against eachother. I would imagine that such tests might well have been carried out many years ago but as Neil suggests - there might be a more recent examples.

I understand that a well designed water tube might not be as good as a typical Stephenson type design but is this really so significant e.g., a large Stephenson might be capable of pulling 12 passengers comfortably but a different design might only be able to pull 8 comfortably??

Along with different boiler designs, there is also the build quality of the engine chassis, cylinders and valve gear to take into account and of course - engines also vary in size from say 0-4-0 to 4-6-2.

Do we actually know that an alternative design will not be good enough or do we only suspect (maybe with good reason) that an alternative is not practicle?

PhilH

I built a small (1" scale) water tube boiler for a traction engine for a grandson in Australia. It was spirit fired and roared away at a tremendous rate. So much so that it outpaced me and the inevitable happened.......it upended itself in the shrubbery and the spilt meths caught a bush alight. This caused a bit of a rethink, in view of the wildfires that occur in Oz.......only recently to thisevent, a fire originating from a spark from an angle grinder had destroyed 30 homes. I therefore converted it to run on butane. In theory it had a far greater heat input but it never had the same performance as on spirit. Whichever the fuel, it was quite greedy and it was also very heavy on water consumption.

Mick

Guys,

As suggested by Mike, I did an initial search and found two articles. One called 'Battle of the Boilers' and a Wikipedia article on the same subject.

The first article was inconclusive. It said that Bassett & Lowke (spirit fired water tube) versus LBSC (fire tubed) both built a test locomotive and both were successful. The larger B&L engine ran longer but the LBSC ran with a heavier load.

The second article suggested that the fire tube engine could pull about 200lb load (at small scale) whereas the typical spirit fired engine could manage about 30lb.

PhilH