Quite obviously the function of a boiler is to transfer heat from a burning fuel to water to generate steam under pressure. There are three ways of transferring heat, by conduction, convection and by radiation and all three are involved in a boiler. They all take place through an area of heat transfer surface. As a general rule small boilers have a small surface for heat transfer and large boilers a large surface. This means that small boilers cannot drive large engines even if water is pumped into them to replace steam as it is used. Boilers have to be matched to engines.
The steaming rate that a boiler can achieve is dependent on its design. Loco boilers evolved to have extremely high steaming rates using a water-filled firebox and fire tubes. We have to look at designs that are much more simple and are suitable for miniature steam plant fired by gas.
There are some constraints on the design of a boiler for miniature marine steam plant. I think that an important constraint is its shape. Lots of model boats are very tender because they are narrow and their centres of gravity are too high. This is particularly true of models of naval vessels and paddle steamers. It usually pays to choose steam plant with a low centre of gravity.
If one excludes flash steam boilers and accepts that the best shape for a boiler is cylindrical there are two basic layouts. They are the vertical axis and the horizontal axis boilers. The horizontal boiler is much the best layout but of course when a scale model is to be made the prototype may well have had a vertical boiler and the model must do the same.
Figure 29 shows the arrangement of a vertical boiler by the one-time manufacturer Cheddar Models. The pressure drum extends between two tube plates with space under for a burner and space above for the gases from the vertical tubes to be gathered to leave by the chimney. It cannot avoid being tall if it is to hold a useful quantity of water. If this is fitted with an elaborate chimney complete with a whistle it looks to be totally unsuited to the size of boat that it might be required to drive simply because the centre of gravity is too high. There is an incentive to use a horizontal boiler.
In figure 30 I have drawn the essential features of a horizontal boiler and its flue but omitted cross tubes in the flues and mountings for the sight gauge, safety valve etc. the burner will be fitted in the left hand end of the lower flue and hot gas from the burner flows along the lower flue and up the vertical flue to the upper flue. Here the cooler gases flow towards the chimney base that can be anywhere along the boiler to suit the application. The steam take-off is a pipe that has its open end under a low dome to reduce the amount of water taken out with the steam.
This general arrangement is effectively standard; it is the arrangements for heat transfer to the steam that vary.
In my view all boilers should be fitted with superheaters. We need to understand what is meant by superheating. It is a buzzword like mega- and mach- because it contains a superlative and has become meaningless in normal parlance. It describes a state of the steam. If it is not foggy where you are the air outside contains superheated steam. So superheating is not about high temperatures per se, it is about temperatures higher than the saturation temperature that corresponds to the pressure of the steam. If we are to superheat our steam we must first separate it from the water from which it was raised. That is easy, we simply take it away through a pipe. Then the pipe can be heated and the steam will first dry and then get hotter. We need to loop this pipe back into contact with the hot flue gases to heat it.
Now the simple flue that I have shown in figure 26 has too little surface for successful heat transfer from the hot flue gas to the water so it must be increased. The normal method is to fit cross-tubes in the lower flue. That means that some arrangement for the cross tubes must be chosen. We need to have some ideas about how to choose.
The first thing to recognise is that heat transfer in boilers takes place most successfully by convection that is by movement of the hot gases on one side of the surface and by movement of the water on the other. We need to promote agitation on both sides.
Figure 31 is of water in an open pan being heated with a high gas flame. Those are bubbles of steam on the bottom. They form, break away from the bottom and rise a little way before they disappear. During the disappearance the steam is condensed in the main body of the water which is not yet boiling. As the temperature in the main mass of water rises the bubbles rise further and further until they reach the surface and then the water is boiling vigorously. The same thing must happen in a steam boiler.
In figure 32 I have shown a cross tube in a flue. There will be water surrounding the flue and inside the cross-tube. Hot gases flow along the flue heating both the flue and the tube. Bubbles of steam will form on the outside of the flue and the inside of the tube. The cross-tube is associated with much less water than the flue and the bubbles will form in greater numbers inside the tube to produce a steady and, possibly, quite vigorous flow through the tube. Such a process is good for heat transfer. What would not be so good would be to have a horizontal tube. This tells us that the cross-tubes should be angled.
Inevitably several cross-tubes will have to be fitted so it raises the question of how to arrange angled tubes to get the best heat transfer. For an answer we must look at the flow over these tubes. We want as much turbulence as we can get and a tube in cross flow is very good at breaking up a steady flow, almost as good as a square. So, if we put one tube at 45° to the left followed by more tubes alternately at 45° to right and 45° to left, we shall get as turbulent a flow as possible.
However we have to get a superheater in as well. A superheater must not be exposed directly to the flame because it will burn out. It has to go into the lower flue where the temperature is still high enough to do some good and so it must go in the rear half of the flue. The tube configuration in this region must be changed. If tubes are say at 10° right and 10° left of the vertical this will create a space for the superheater.
In figure 33 I have drawn the arrangement. The burner fits into the open end on the left and the flame that it produces plays directly on the 45° tubes. By the time that the gases from combustion have reached the superheater the temperature will have dropped so that the superheater is not so vulnerable to excessive heating. Even so it pays to use nickel silver in the short vertical links and copper for the rest.
In my second boiler, constructed by Martin Howse and Bayliss, the steam is hot enough to melt plumber’s solder.
If the design is based on carrying water in the boiler there must be some limit to the volume of water that can be used. Presumably there is no question that the boiler cannot be completely filled at the start because some space is needed to permit expansion of the water as it is warmed and to contain the steam when it is generated. There is a limit to the length of the sight gauge set by the practical constraints. Experience suggests that if the water level is visible in the sight gauge there will be adequate steam space.
It is not so easy to set a lower limit. In the presence of any water the boiler will not get excessively hot but if the water is boiled away completely the flame will heat the boiler. Nothing will melt but the lagging and the water gauge seals may suffer. The boiler should not be allowed to run dry. There will be other levels where the lower flue is only partially covered with water. This is not an efficient way of running the boiler but is not harmful to it. So I suppose a lower limit is the top surface of the lower flue and a little below. This gives a margin for misjudgement.