Alternatives to the One Drum Donkey Boiler

Several alternative schemes have been devised, addressing one or other deficiency of the one drum boiler. Perhaps, the most common is the upright boiler with one or more flues through the middle:

Another scheme is the rocket stove with heat exchanger. The heat exchanger may be constructed from flat plates or from a coiled tube:

All of these, although claiming greater efficiency share several disadvantages:

• The required materials are more expensive and are difficult to source
• Greater skill levels required for construction and maintenance 
• Quickly clog with lime around the flue pipes - must be dismantled to remove the lime
• Require a welder to fabricate and maintain
• Require more frequent feeding
• Many need to be under a roof

These problems arise from a single common source - use of conduction between the rising flue gases and water to effect heat exchange. For this to work, large areas of conductive surface are required and the annulus between the conducting surfaces is consequently reduced.

The one drum donkey boiler relies primarily upon radiative heat transfer from a bed of hot coals to the cooler surface of the drum, with a little assistance from conductive transfer from the hot flue gases. A much smaller surface area is necessary for heat transfer due to the large difference in temperature. Good reason to make improvements upon the one drum boiler rather than going for something far more complex.