The
following description of Denbigh's engines is condensed (pun intended) from
a longer essay by Denbigh Project volunteer Gene Shimko.
Denbigh’s two steam engines are categorized as
"diagonal" engines, based on the way they are mounted in the hull. The axis of
each engine’s cylinder is aligned fore and aft. The two engines are aft of and below
the paddlewheel shafts, so that the cylinders’ axes lie at a shallow angle, about 15
degrees from horizontal. The engines, which were built at Lairds’, are a matched
pair, with each being a mirror image of its twin across the ship’s centerline. The
top of the valve chest on each engine is about level with the upper surface of the deck
beam amidships, and the engines themselves sit relatively high up in the hull.
The specifications of Denbigh’s engines, taken
from both historical documentation and examination of the site, are as follows:
|
Nominal power: |
100 horses (total for both
engines) |
|
|
Indicated power: |
500 horses (total for both
engines) |
|
|
Cylinder internal
diameter: |
40 inches (102cm) |
|
|
Cylinder external
diameter: |
48 inches (122cm) |
|
|
Length of stroke: |
3 feet 6 inches (107cm) |
|
|
Steam pressure: |
22 lbs (about 1.5 atmospheres
above ambient) |
|
|
Engine speed: |
38 to 40 rpm max. |
|
Each engine consists of a single cylinder. They are
double-acting, that is, steam from the boiler is released
alternately into each end of the cylinder, so that actual power is applied to the piston
moving in both directions. At the end of each stroke, waste steam is vented directly into
a condenser. The purpose of the condenser is to convert the spent steam back into liquid
water that can be returned to the boiler for recirculation. Although the condensers on Denbigh
have not yet been excavated, it was common at the time for marine condensers to use an
injection of relatively cold seawater to quickly condense the spent steam. This was an
important part of the engine’s operation, as the partial vacuum that resulted in the
condenser helped to "pull" the piston in the cylinder, and produce substantially
more force than the engine’s low operating pressure would suggest. Indeed, early
steam engines were often referred to as "vaccuum" engines, because it was this
effect that provided the majority of force working on the piston.
Normal operation would be to keep both engines and drive
trains mechanically connected so that the paddlewheels turned at the same speed. This
eliminated much needless minor adjustment of each engine in order to match paddlewheel
speeds exactly. However, each engine could be run independently, in both forward and
reverse, and turn the paddlewheel on that side of the ship independent of the engine and
paddlewheel on the opposite side of the ship.
Click
here for a large (600kb) animated GIF image of Denbigh's engines.
The injection of steam into the cylinder and the exhaust
of spent steam into the condenser is controlled by a rectangular valve chest atop each
cylinder. Although the valve chest on the port side engine, the only one excavated,
appears to have been partially disassembled by salvors in the 19th century, it is possible
to describe its functioning in general terms, based on contemporary engineering texts.
A concept model showing a possible arrangement of one of Denbigh's engine
cylinders. The boxlike valve chest, at top, feeds steam alternately into one of two
channels leading to either end of the cylinder. This steam pushes the piston back and
forth, giving the engine its basic motion. At the same time steam is circulated through
cavities in the cylinder walls. This "jacketing" improves engine efficiency by
keeping the cylinder warm and reducing condensation within the cylinder itself.
There are two valves in the
chest, a cutoff (or expansion) valve and the main slide valve. The main slide valve
assembly sits directly on top of the cylinder and slides on a machined face. The cutoff
valve consists of two blocks connected by a double reverse screw. The cutoff valve slides
within the main slide valve assembly, sitting directly on the top of the main slide
valve’s ports. In effect, the cutoff valve regulates live steam to the main valve
ports, which in turn passes it into alternate ends of the engine cylinder, pushing the
piston back and forth. At the same time, the spent steam from the previous stroke is
exhausted out of the cylinder to the condenser, where it will be converted back to liquid
water and returned to the boiler. The cutoff and main valves are themselves powered by the
rotating paddlewheel shaft via eccentrics (see below).
An overhead view of Denbigh's engines and
paddlewheel shafts. Red arrow
indicates the connecting rod recovered at the end of the 2000 field season.
The motion of each engine’s piston rod was
transferred to the paddlewheels via a connecting rod. This iron rod, which measured about
eight feet (2.44m) in overall length, changed the engine’s fore-and-aft motion to a
rotary motion at the crank on each paddlewheel shaft. The connecting rod of Denbigh’s
port engine was recovered during the 2000 field season. As
detailed drawings of the area were being prepared, it became clear that the ends of the
connecting rod had been detached from the piston rod and paddlewheel crank, probably in an
effort to salvage the engine's bronze fittings shortly after the ship grounded and burned.
This 19th century salvage attempt greatly simplified the recovery of the connecting rod
135 years later. The connecting rod, as recovered, was estimated to weigh 1,150 pounds
(520kg).
What appears to be a single shaft running across the
center of the ship with a paddlewheel on either end is actually three shafts – one
for each wheel and a so-called "intermediate shaft," that was used to connect
the two to allow them to operate in unison. A pair of eccentrics – disks mounted
off-center on each paddlewheel shaft – were linked to the cutoff and main slide
valves in the valve chest to synchronize the flow of steam in and out of the engine with
the rotation of the main shaft. |
