I built a steel boiler for my 3"scale traction engine, made from a no longer required CO2 cylinder rated at 3,000 bar! At 10" diameter it was ideal and with the use of the dip tubes for the fire tubes it was a very useful supplier for materials.The cylinder walls are 6mm thick and with the help of these 1mm cutting blades in the grinder its shape was carved out ready for welding.(cuts like butter but very noisy for the neighbours)With the help and supervision of a coded welder the firebox.back plate and boiler sides were cut from 6mm mild steel plate.I purchase a set of cast iron fire bars from an E-bay supplier and eventually the completed boiler was ready for firing.
After the usual pressure tests with water and pump it was tested to 150psi as a starting point and passed with flying colours. The steam test however was a different story! The boiler holds about 8 liters of water and took approximately one hour to reach boiling point with a very hot coal fire. However I tried stoking the fire and with a 240volt 2" bore smoke fan inducing the draught it burned brilliantly.Now I might be telling you steam buffs how to suck eggs but I could not raise the steam pressure above 40psi. I tried steam coal, wood fires, charcoal etc but all to no avail. What was the problem? I noticed that the ash pan and fire grate were glowing bright red from the fire? So what was the problem? It became revealed through experimenting with the position of the fire grate. Following recomendations from several well known books about steam boilers the firegrate was positioned level with the foundation ring of the boiler originally. Because of all the wasted heat reaching the ashpan I decided to raise its position within the firebox by 10mm at a time. The best results came when the firegrate reached a position 40mm inside the firebox. Using the same fuel ( steam coal) the pressure reached an amazing 80psi immediately at the first firing at this new position.I can only assume that the wasted heat was now reaching the boiler sides instead of the ashpan!This enabled me to set the safety valves at 60psi for the initial trial run. I have now steamed the boiler many times and from cold the safety valves blow off within 35 -40 minutes.The added bonus if you like is that the coal fire now provides steam approx twice as long before burning to ash.5 shovels full will steam the boiler for approximately one hour without the blower fan and using the exhaust steam for draught.Lesson is that if you have a poor steamer of a boiler then try raising the firegrate, I am not saying that it will cure the problem but it certainly did with my steel monster!!I have now built a copper silver soldered boiler for my latest project a Sweat Pea steam train and with a marine type boiler the fire is surrounded by the boiler walls, So it will be intesting to observe the results as the designer reckons that it is a brilliant steamer! More results to follow shortly