Small Steel Boilers

Edited By Dave Bond on 18/04/2011 12:48:31

John Olsen How right you are! Heat transfer is the biggest problem.

Steel boilers do not have to be welded! They can be silver brazed or hard brazed using the same methods as as for copper. Hard brazed! I hear the Gnomes of Elfin Safety shrieking, brass looses zinc in hot water. Tripe! de-zincification can be prevented by adding 0.3% arsenic to then brass and any way most brazing rods are guaranteed against this problem.

The steel boilers can be protected against rusting by the use of a good engine cylinder paint (these are proof to 500°F - 343°C). Sacrificial anodes are also very effective providing the boiler is kept full of de-aerated water (done by warming the stuff). De-aeration is why ordinary brass is used in house-hold plumbing

Do not try to galvanise the boiler or use galvanise steel. The surface plating de-laminates at about 200°C and the deposition into the surface of the steel cause’s serious damage to the steel’s structure.

Steel boilers do not have to be welded, they can be brazed. A properly designed and competently made braze has 80 - 90% of the strength of a weld. Anyone who can make a good silver braze can make a good braze (if they have the heat available). There were even low melting brazing rods (old Curly mentions one in Part2 of Roedean one Sifbronze ). Brazing was often used in motor car body repair shops as it did not tend to change the structure of the mother metals. There is a good PDF on this website on the design of brazed joints here. I have shortened the address because I understand long hyperlinks cause trouble. Dezincification occurs because zinc in the presence of water forms Zinc Hydroxide which is soluble. But that is not the whole story. Zinc first has to form Zinc Oxide which it does in air (this protects the brass alloy. But in water especially hot water Zinc Oxide (which is insoluble) reacts with the water and forms the much more soluble Zinc Hydroxide. Remove the oxygen (or air) and all is well. There is an interesting item on this website http://www.hghouston.com/coppers/brass75.htm on the topic.

The practice of brazing car bits was outlawed by the Gnomes of Elfin Safety partly because of poorly designed joints and because of dezincification.

The gnomes of Elfin Safety learned about dezincification or Condenseritis, which was something which plagued ships of the Royal Navy but not the German ships at the turn of the last centenary. The addition of 0.03% Arsenic which occurred naturally in German copper ores was the cure (I wrote 0.3% in my first posting I was wrong). Your local (unfriendly) gnome only knows the FAQS and not the full story.

JasonB. Painting the boiler yes but you paint the insides of the boiler. I have written a ‘Thought Piece’ on boilers and boiler making but I have never submitted it to Model Engineer because it needs a picture of an ‘241-A’ class Locomotives made to M. Chapelon’s design. There is still one preserved at Verine in Switzerland you will see a picture at http://www.241a65.ch/ .

If you want to find out how to paint the inside of a boiler look have a look at the ‘Rotordip system’ which was used on car bodies. It involves degreasing, your ‘treatment’ –phosphoric acid- etc and paint.