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Denbigh's Boiler

The following description of Denbigh's boiler is adapted from
a longer essay by
Denbigh Project volunteer Gene Shimko.

Denbigh was fitted with a single, large, boxlike boiler. This boiler design, very common aboard seagoing steam vessels of the 1850s, was superseded by the familiar cylindrical "Scotch" boiler design, introduced in the 1860s. Denbigh’s boiler operated at a designed pressure of 22 lbs. (about 1.5 atmospheres above ambient), which was near the maximum pressure for this type of boiler construction.

Denbigh’s boiler has not been excavated, but the type is well documented in contemporary engineering texts and journals. The lower part of the boiler probably had three or four coal-fired furnaces opening on its aft face. These opened to a shared combustion chamber at the back (i.e., bow-facing) end of the boiler. From the combustion chamber, hot exhaust gases from the fireboxes were directed through a series of horizontal fire tubes, three to four inches (7.5 to 10cm) in diameter, running aft again through the main body of the boiler, which was filled with water. At the aft end of the fire tubes, the exhaust gases then flowed through a metal casing into another series of tubes, these aligned vertically in a "superheater," and from there exited though the single, narrow funnel of the vessel.

Denbigh’s boiler would have operated with sea water. The salt in sea water would eventually cause scale and corrosion in the boiler and other compartments, but the salt would be retained within the boiler as the steam – pure water – was diverted to the engines. Once spent, this steam would be condensed and returned to the boiler. This "closed loop" system helped to ease the problems of corrosion in the boiler. The salinity of water in the boiler would have been checked frequently, and when it rose above a certain level part of the water in the boiler would have been blown out, and new, clean sea water pumped in to replace it.

Although Denbigh was noted for her high speed in civilian service, by the time she was running the blockade in 1864-65 she was known on both sides as a relatively slow ship, depending on stealth and daring more than speed. In 1864 the acting U.S. consul-general in Havana, Thomas Savage, attributed this to her boiler being "in a bad condition."

The superheater is one of Denbigh’s most interesting features. The superheater on the Denbigh was located just above the boiler and made use of the waste heat in the boiler flue hot exhaust gases. The hot gases were routed through a bundle of vertical tubes around which the boiler steam was circulated. The superheater increased the temperature of the saturated steam that the boiler produced. For a given pressure, the boiler produces steam at a certain temperature. Steam just barely exists at this "saturated" temperature and will readily condense on any surface that is at a lower temperature. Many steam engine designs suffered from this condensation effect; the steam could partially condense in the pipes leading to the engine cylinders if the pipes were not effectively insulated, and could also partially condense in other passages before entering the cylinder. The most troublesome problem was that the steam would condense in the engine cylinders themselves during the expansion as the piston moved. All this premature condensing greatly decreased engine efficiency. Superheated steam prevents premature condensation because more heat has been added to the steam to raise it above its saturation point. Superheat also increases the overall thermodynamic efficiency of the steam engine because there is more heat to convert to work. Some superheaters of the period could raise the temperature of the steam by about 100 F, and create a fuel saving of up to 20%.

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Denbigh's superheater is similar to Parson's superheater, shown here in an article from the British journal The Engineer of April 12, 1861.

The introduction of superheated steam to marine powerplants was very new in 1860, and its use aboard Denbigh is an important indicator of the builder’s willingness to invest in the most advanced, up-to-date technologies of the period. Denbigh’s tubular superheater is broken and partially collapsed into the main body of the boiler, but it appears to be very similar to Parson’s superheater, introduced the year after Denbigh was built. In Parson’s design, two valves on the aft side of the superheater casing allowed steam to pass directly from the upper part of the boiler into the cylindrical superheater. The steam circulated around the superheater tubes and exited the superheater via another valve, from which piping carried the live steam to the engines and other, auxiliary machinery.



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What's New?

new.gif (977 bytes) John Newland Maffitt and the Galveston Blockade | Chasing a Fox new.gif (977 bytes)
new.gif (977 bytes) 2001 Field Crew | In-Kind Contributions  | How Much Coal? new.gif (977 bytes)
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"An Extremely Fast Boat" | The "Mobile Packet" | A "Bold Rascal" | Denbigh Today
Denbigh's Crew | The Erlanger Loan | Birkenhead-Built: An Unrivaled Legacy
Denbigh Primary Source Documents | Galveston During the Civil War | Denbigh, Clwyd, Wales
The U.S. Coast Survey and the Blockade, 1861 | The Ship's Library: Recommended Reading
Running the Blockade Into Galveston: A Personal Narrative | Denbigh Day-by-Day
Denbigh Portrait | Official Number 28,647 | Valve Chest Animation (300kb) | Investors
Links of Interest | Denbigh F.A.Q. | Denbigh's Engines | Denbigh's Boiler
Feathering Sidewheel


April 27-28 Side Scan Survey | May 7-10 Site Mapping
June 16-17 Sub-Bottom Profiling | Site Mapping, July 9-12, 1998 | Dive Trip, October 18-30, 1998
Underwater Images | 1999 Summer Field Season | Denbigh Site Plan
Jerry Williams Speaking Tour | Denbigh Project Benefit Dinner |
Denbigh Artifacts | 2000 Field Crew | 2000 Field Crew Photo Album |
The Denbigh Wreck Site: A Quicktime VR Panaorama
Connecting Rod Recovery, July 22-24, 2000 | Modeling a Shipwreck
Credits & Thank-Yous

J. Barto Arnold et al. 1998-2000, The Denbigh Project, World Wide Web,
Institute of Nautical Archaeology, Texas A&M University, E-mail: (  
Wednesday, January 31, 2001 Revision.

Questions, comments or suggestions about this website? Send them here.

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