New Hope for Steam Power: Flash Boilers

The boiler for this project posed an interesting problem. It reminded me of words I'd once heard, a phrase that went something like "The enemy of engineering is the lack of constraints", or alternately "The enemy of art is complete freedom".

Without some sense of what's available, it was almost impossible to solve a problem with this many variables. To guide my decision-making process, the only thing I knew for sure was that this boiler had to use a monotube flash-boiler design. These boilers consist of a single coiled tube that is wound above a heat source with water constantly flowing through. I chose this style for its simplicity and inherent safety.

Cast Iron and Steel

In antiquity, the most common flash boilers were massive cast-iron pressure vessels that held hundreds of gallons of superheated water. They had certain advantages, such as the ability to instantly provide peak power over short periods of time. Additionally, these boilers were difficult to overheat because the water acted as a heat sink and kept the iron relatively cool. Unfortunately, like any large-diameter pressure vessel, any crack could result in a catastrophic and fatal explosion.

Flash boilers heat only the quantity of water the system needs at a given time rather than all at once. Water is pumped constantly through the pipe and heated to steam. This style of boiler can quickly overheat if the water flow is stopped, but there are only several cubic inches of water being heated at any given time (depending upon the boiler size). Because of its significantly lower explosive potential, this quantity can be easily contained with a steel shell.

Test and Re-Test

During prototype testing, my first boiler suffered a pump failure and promptly overheated and melted. Even though that particular model didn't have a shroud protecting it, the result was simply to release steam and water, which doused the fire. No explosions, no shrapnel, and minimal danger (although a ruined boiler core).

With propane as my fuel and an old turkey fryer as my burner, I constructed several models from cast iron fittings and quarter-inch copper pipe courtesy of the Home Depot plumbing department. Then I surrounded the assembly with an insulated steel pipe to contain any potential failures and increase the efficiency, connected everything to the engine with flexible high-temperature hose, and fired it up. But I'll wait until my next blog entry to post the results.