There are a lot of different furnaces out there, each useful for different kinds of foundry work. As we have previously discussed, the induction furnace has become increasingly popular with iron foundries as they are simple to operate, generally safer, and more efficient than older styles of a furnace. However, there are certain applications that just call for a different approach. In the iron foundry, as in most places in this world, one size doesn’t really fit all. That’s where the Water-Walled Cupola furnace comes in.
Constructed much like a smokestack, the cupola style furnace has been around for a very long time. Examples of this kind of furnace have been found in China dating as far back as 400-221 B.C. Now, I could walk you through the operation of the water-walled cupola furnace but it would all seem very familiar. That’s because the cupola furnace is actually just a blast furnace, which we recently described in some detail. However, why ‘water-walled’? Why is this particular modification to the blast furnace important? The biggest reason is that water has amazing thermal properties. Properties that make water a fantastic insulator.
Water is not very conductive. So while it’s likely it would be surrounded by an inner and an outer metal wall, it will still keep the heat of the furnace from radiating out uselessly. After all, if you can keep more of the furnace’s heat inside, then you’ll need to use less fuel to keep it going.
Water also has a very high specific heat capacity. Specific heat capacity is a measure of how much heat a given unit of a substance can absorb before changing the temperature one degree, and thanks to the molecular bonds that hold the hydrogen and the oxygen together, water can absorb a ton of heat.
One might ask if there is a danger of the water boiling. In an open system, yes. However, in a water-walled cupola iron furnace, the water can be kept at higher than atmospheric pressure. This matters because the higher the pressure the water is under, the higher the boiling point. If you’ve ever heard that it takes less time to boil water in Denver, CO than at sea-level (all other things being equal) it’s because Denver is at a higher altitude, meaning that there is less atmosphere, meaning that there is less atmospheric pressure. The lower atmospheric pressure is what causes the water to boil sooner, i.e. at a lower temperature.
Temperature can be controlled in a couple of different ways. If you have a small and simple system, excess heat can simply be allowed to radiate through the outer metal wall. If you have a larger system, you’ll need a heat exchanger. There are of course lots of different variations on this setup. One can simply allow the water to circulate through a radiator and back to the furnace. Another would be to use a true heat exchanger with a parallel, cross flow or multi-pass shell-and-tube construction depending on how much cooling you need. A pump and a couple of throttle valves allow for fine temperature control, allowing you to closely govern the conditions inside the furnace.
Another benefit of using water as an insulator is that it also works as a coolant. This is particularly useful if all the power should go out and you don’t have another way to cool the blast furnace. As the water heats, it will rise (or move to the highest spot in the system) and the cooler waterfall, creating a natural circulation that will help dissipate the heat coming from the furnace. It is also handy if there should be a breach in the system somehow. If the inner wall is breached, the water can cool the iron and slag in the furnace. If the outer wall should be damaged, the water could be dangerous briefly but even water at hundreds of degrees is less dangerous than the super-heated steam that could be escaping otherwise.
Why would a modern iron foundry still be making use of such an old technology as a blast furnace? Sometimes, it’s a simple matter of cost. Getting rid of it might cost almost as much money as a new unit. In that case, it makes sense to keep it and get some use out of it. The best reason though is that it is great for getting used out of iron that would otherwise be scrapped. Leftover material from other processes that might be highly contaminated can be smelted in the water-walled cupola better than in any other kind of industrial furnace. That makes this simple style of furnace ideal for minimizing waste and getting the most out of every ounce of raw material.