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Author Topic: Steam Engine Definitions  (Read 15374 times)

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Steam Engine Definitions
« on: March 05, 2011, 03:48:17 PM »


STEAM ENGINE DEFINITIONS:

Many of the definitions listed below are from "Steam-Engine Principles and Practice", Terrell Croft,
1st Ed., McGraw Hill Book Company, Inc., 1922 (a free public domain book):

1. High speed engine:  An engine that operates at a speed of 200 rpm (revolutions per minute) or more.
2. Medium speed engine:  An engine that operates approximately between 110 rpm and 200 rpm.
3. Low speed engine:  An engine that operates at a speed of 100 rpm or less.  

4. Short-stroke engine:  An engine which has a stroke which is less than the diameter of its cylinder.
5. Long-stroke engine:  An engine which has a stroke which is greater than the diameter of its cylinder.

6. High pressure engine:  An engine that operates at a steam pressure greater than 225 psi (pounds per
square inch).
7. Medium pressure engine:  An engine that operates at a steam pressure between 80 psi and 225 psi.
8. Low pressure engine:  An engine that operates at a steam pressure of less than 80 psi.

9. Condensing engine:  An engine that operates with an exhaust pressure less than atmospheric, and
exhausts into a device with condenses the steam, thus reducing its pressure.
10. Non-condensing engine:  An engine that operates with an exhaust pressure at atmospheric or
greater pressure.

11. Multi-expansion engine:  An engine with multiple cylinders that expands the same steam multiple
times across a number of cylinders.  Also called a compound engine.

12. Superheated steam:  Steam that is heated via a heat exchanger that adds heat to the steam
in addition to the heat normally introduced by the boiler.  Superheaters sometimes consist of coils
of tubing located in the upper flue and exposed to flue gasses.  The intent is to capture heat that
would be wasted in the exhaust gasses in the flue and add this heat to the steam as it travels to
the engine.  Efficiency improvements along the order of 20% or more have been reported with the
addition of superheated steam.
The higher temperature of the superheated steam prevents condenstation within the cylinder that
would normally occur if non-superheated steam was used.
Drawbacks of using superheated steam include the unwanted wear it causes on "D" type slide valves,
and problems with lubrication.
Piston valves are often used successfully with superheated steam.

13. Forward stroke: For a vertical engine, the forward stroke would be completed as the piston moves
from top-dead-center to bottom-dead-center.  Sometimes referred to as the "head end" stroke.
Also referred to as the "out stroke", or stroke towards the crankshaft, and "downstroke" in vertical
engines.

14. Head End:  Refers to the side of the engine that has the plain cylinder head without a gland.

15. Return stroke:  For a vertical engine, the return stroke would be completed as the piston moves
from bottom-dead-center to top-dead-center.  Sometimes referred to as the "crank end" stroke.
Also referred to as the "in stroke".

16. Crank End:  Refers to the side of the engine that has the cylinder head which contains the gland.

17. Angle of advance:  The angle of advance can be determined using the following procedure.
              a. Rotate the crankshaft of the engine until the piston is at top-dead-center.
              b. Rotate the eccentric until the valve is at its midpoint position (position A).
              c. Continue to rotate the eccentric to a new position where the head end steam port
                  just begins to open (the opening should equal the desired lead of the valve).  This
                  second eccentric position will be called position B.
              d. The angle of advance is the difference between the eccentric angle at position A
                  and the eccentric angle at position B.
              e. Typical angles of advance appear to be around 55 degrees.

18. Wire drawing:  Wire drawing is a term used to describe what happens when the steam port is
only partially opened, such as when a variable valve gear linkage is used to limit valve travel.  If the
steam port is not sufficiently opened during the admission phase (approximately the first 1/4 of the
stroke), then the steam flow into the cylinder is restricted, and instead of the piston being acted upon
by almost full boiler pressure, the steam pressure within the clyinder drops sharply as the piston moved
through admission, since the steam port cannot flow enough steam to fully pressurize the cylinder
as the piston moved down the cylinder.
Wire drawing shows up on the indicator card as a sharp drop in the slope of the top pressure line
after admission begins.

19. Receiver:  A receiver was either a chamber or a large pipe that was used to temporarily store
steam as it was moved from the high pressure cylinder, to the intermediate cylinder, and then to
the low pressure cylinder(s) on a compound steam engine.  Most multi-cylinder engine arrangements
do not have to correct timing (crank throws) to allow the steam to be exhausted directly from the high
pressure cylinder into the steam inlet of the intermediate cylinder, and exhausted directly from the
intermediate cylinder directly into the low pressure cylinder(s), so receivers were used to store the
steam until the appropriate time at which it was admitted to the next cylinder.  Multiple low pressure
cylinders were often used since large horsepower engines would require a low pressure cylinder of
prohibitive size.

20. Indicator:  The indicator was a cylindrical instrument that was used with steam engines to record
the variation of pressure at different parts of the engine stroke.  The indicator cylinder was connected
by flexible cord or string to a moving part of the engine such as the crosshead, and the movement of the
crosshead caused the cylinder to rotate.  Small tubes were connected to the head end and crank end
valve cocks, and the pressure in the tubes was used to move a pencil via a diaphram.
The pencil could be elevated as the engine was brought up to temperature, and once the engine was
operating normally, the pencil would be allowed to contact the paper on the drum.  Various springs could
be changed in the indicator to allow the device to operate properly with a a variety of steam pressures.

The shape that was traced on the paper was a diagram of the pressure within the cylinder on the stroke.
If the head end cylinder cock was opened, and the crank end cylinder cock closed, the pressure diagram
would be drawn for the head end, called the "top card".  An opposite arrangement of the steam cocks
would draw the pressure diagram for the crank end of the engine, called the "bottom card".

Points for admission, cutoff, release, and compression could normally be identified on the cards.
The indicator functioned in a mechanical way analogous to the modern day oscilloscope.  The oscilloscope
measures voltage, wheareas the indicator measures pressure.  Voltage in the electrical world is analogous
to pressure in the mechanical world.

(Articles are available online which show a steam engine connected to a dynomometer, using a pressure
transducer and an oscilloscope to draw the pressure diagrams).

Indicator diagrams produced a great deal of information that could be valuable in diagnosing engine
problems, as well as setting valves and balancing power between the two ends of the cylinder.

Often times, indicators were used at large worldwide steam engine expositions, to compare the merits
and efficiencies of numerous engines all operating within the same building.  Some steam engine
manufacturer's refused to have indicator cards made for their engine, in order to hide serious engine
deficiencies.  Most engine exhibitors at exibitions swapped indicator cards much in the fashion of the
modern day trading of baseball cards.

The following information regarding to indicators comes from "Verbal Notes and Sketches for Marine
Engineer Officers", J.W.M Southern, Volume I, publishing date unknown:

Data required in order to use information shown on card diagrams:
(refers to a compound marine steam engine)

-Name of steamer
-Date
-Diameter of cylinder
-Stroke
-Revolutions
-I.P. receiver pressure (intermediate pressure)
-L.P. receiver pressure (low pressure)
-Condenser vacuum
-Cut-off in each cylinder
-Mean effective pressure (for each cylinder)
-Scale of indicator spring (for each cylinder)
-I.H.P. for each cylinder (indicated horsepower)
-Total or collective I.H.P.
-Coal used per twenty-four hours
-Coal used per I.H.P. per hour
-Quality of coal used
-Speed of ship
-Propeller pitch
-Slip percent
-Weather conditions


Information that could be obtained from the indicator cards is as follows:

a. Admission
b. Cutoff
c. Release
d. Compression
e. Effective pressure on the piston (the difference at any given point between the
pressure on the top of the piston and the bottom of the piston)
f. Positive and negative work areas
g. Early and late valve events (lead, compression, exhaust, admission, etc.)
h. High back pressure
i. Valve face leakage
j. Wire drawing
k. Indicator problems (dirty indicator, steam cock partially opened, stretched string,
water in indicator, weak indicator spring)
l.  Piston ring leakage
m. Incorrect valve position
n. Slide valve slack on spindle
o. Excessive compression
p. Reduced engine speed
q. Loss of vacuum
r. Leaking glands
s. Broken cylinder cover
t. Clearance volume
u. Link position
v. Estimated steam consumption
w. Steam efficiency

21. Linking Up: For steam engines with dual eccentrics, and reversing linkage, a position of "full gear"
is one in which the linkage is at the end of travel in forwar or reverse, and the eccentric applies
full travel to the steam valve.  As the reverse linkage is raised up from the full gear position,
the action of the forward and reverse eccentrics act to limit the travel of the steam valve, causing
and earlier cutoff.  If the linkage is placed in the center position, the valve does not move.
Moving the linkage beyond the center position will reverse the engine.
Linking up can be used to improve the efficiency of an operating steam engine, if the engine can
be operated with an earlier cutoff.  Some engines have too much load to allow use of early cutoff.
Eccentric rods for reversing engines can be installed in a open configuration, or can be installed
crossed.  For engines operating linked up, the open rod configuration should be used.

22. Crank Angles: (for multi-cylinder engines)
Twin cylinder steam engines typically had the cranks for the two cylinders set at 90 degrees.  This
arrangement allows the engine to be started from any position, unlike a single-cylinder engine,
which may have to be rotated past top or bottom dead center to be started.
Triple cylinder engines typically had cranks located at 120 degree intervals.  If the power ouput
for the three engines was designed approximately equal, then the torque produced at the
crankshaft of the engine would be constant.

23. Inclined Cylinders:  Inclined cylinders were often seen on the early paddlewheel type vessels.
Two cylinders could be connected to a single open-crank pin, thus avoiding the difficulty of
constructing a center crank.

24. Piston Speed:  Commonly used piston speeds are listed as follows (from "The Elements of Machine
Design, Part II" by W. CAwthorne Unwin, 1891:

Direct-acting pumping engines (without flywheels)---------------80 to 120  fpm (feet per minute)
Beam pumping engines -----------------------------------------90 to 200  fpm
Horizontal Corliss engines ---------------------------------------220 to 400  fpm
Horizontal compound mill engines --------------------------------200 to 400  fpm
Small horizontal engines, ordinary -------------------------------240 to 300  fpm
Short stroke, quick speed ---------------------------------------400 to 550  fpm
Locomotive engines (high speed) --------------------------------400 to 550  fpm
Quick speed, short stroke, single acting engines ------------------625 to 850  fpm
Paddle marine engines -------------------------------------------206  fpm
Screw marine engines -------------------------------------------330  fpm
 
Another source gives the following typical piston speeds, and perhaps reflects the trend toward
using higher speeds in steam engine designs in the very late 1800's.
 
Stationary engine, small        300 to 600 fpm
Stationary engine, medium    600 fpm
Stationary engine, large        750 fpm
Marine engine                      850 to 900 fpm
Locomotive                          600 to 1,200 fpm
 
25. Overhung Crank:  Also referred to as the "side crank", this arrangement uses an open disk
mounted on the end of a straigh shaft, with a pin inserted into the disk to join the connecting rod
to the crankshaft.  Allows the use of a bearing on one side of the crank disk only.  This is a simple
method of making an engine crankshaft, but must be built very rigidly to prevent flexing of the disk
or crank pin.

26. Center-Crank:  This arrangement uses two webs, with one web on either side of the connecting
rod.  A center-crank design requires the use of a bearing adjacent to each crank web.  A center crank
design allows multiple cylinder engine designs.

27. Live Steam:  Refers to steam traveling from the boiler to the steam engine.  This term is used to
distinguish this steam from the low pressure steam in the exhaust piping.

28. Compound steam engine:  Compound engines have two or more cylinders of differing sizes, typically
with a high pressure cylinder, a low pressure cylinder, and sometimes an medium pressure cylinder.
The exhaust from the high pressure cylinder is used to power the medium pressure cylinder, and the
exhaust from the medium pressure cylinder is used to power the low pressure cylinder.

The work produced by the cylinders is designed to be equal.

Often two low pressure cylinders are used on large engines, since the size of a single low pressure
cylinder would be excessive.

An engine with (1) high pressure, (1) medium pressure, and (2) low pressure cylinders is a 4-cylinder
engine, but is called a triple-expansion engine, since the steam expands three times as it passes
through the engine.

29. Cross compound steam engine:  A cross-compound steam engine has two cylinders parallel and
side-by-side (called cross-compound since the steam has to cross over from one cylinder to the other).
A cross compound engine that has cranks at 90 degrees to each other can be self starting in any
position, but a receiver must be used between the cylinders to store the steam that is exhausted
from the high pressure cylinder until the appropriate time to introduce the steam into the low pressure
cylinder.

A cross compound engine with crank pins at 180 degrees to each other, or on the same crank pin does
not require a receiver, and the high pressure cylinder can exhaust directly into the low pressure cylinder.

30. Tandem steam engine:  A tandem steam engine has cylinders connected on the same piston rod
(think tandem bicycle).
The two cylinder tandem engine has dead points, and is not self starting, but does not need a receiver.
The tandem-compound engine has only one set of reciprocating parts.  Tandem engines were typically the
compound type.

31. Double expansion steam engine:  A double-expansion steam engine expands steam twice across
two cylinders prior to the steam being exhausted.

32. Triple expansion steam engine:  A triple-expansion steam engine expands steam three times
across three or more cylinders.

The Titanic engines were triple-expansion, 4-cylinder units, with two low pressure cylinders being
used to prevent the use of an excessively sized low pressure cylinder.  The low pressure cylinders
were exhausted into a 16,000 horsepower steam turbine, thus allowing the energy from the exhaust
of the two 16,000 horsepower vertical reciprocating engines to be captured and converted into useful
work.  The designers of Titanic did not trust the new technology of the steam turbine enough to use
it instead of reciprocating engines.

33. Quadruple (quad) expansion steam engine:  A quad-expansion steam engine expands steam
four times across four or more cylinders.

34. Compound steam engine cylinder ratios:  The cylinder ratios define the proportioning of the sizes
of the cylinders of multi-expansion engines.  The cylinder ratio should be such that nearly equal power
is developed in each cylinder.
The crank throws for multi-cylinder engines were often designed to space the forces from each cylinder
equally around the 360 degrees of the crankshaft.

A common rule for the size of cylinders in compound engines is to make the ratio of the cylinders equal
to the square of the total ratio of expansion.  
A two-cylinder compound engine with an expansion ratio of 9 will have a cylinder volume ratio equal to
the square root of 9 (equal to 3), and the low pressure cylinder volume will be three times the volume
of the high pressure cylinder.

 
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