From: ajw@squid.mel.dit.CSIRO.AU (Andrew Waugh)
Subject: Re: condensing locos
Organization: CSIRO, Division of Information Technology, Melbourne
Date: Sun, 8 Nov 92 22:37:09 GMT

In article <1992Nov6.203025.29051@news.uiowa.edu> jones@pyrite.cs.uiowa.edu (Douglas W. Jones,201H MLH,3193350740,3193382879) writes:
>From article <1992Nov6.164726.16489@ulysses.att.com>,
>by smb@ulysses.att.com (Steven Bellovin):
>> Has anyone ever tried building a locomotive that tried to recondense
>> water from the steam?
>
>The problem is that the surface area of the heat exchanger needed for
>the condenser is huge!  You end up having to build a huge tender with sides
>made of lattices of condenser tubes.

There were a number of advantages to condensing steam engines -- and a number
of disadvantages.

Virtually all stationary and marine steam engines were condensing. One reason
for this is that the power developed by a steam engine is proportional to the
pressure difference between the two ends of the cylinder. If the waste steam
is condensed the presure on the exhaust side of the piston drops to close to
zero (absolute) which gives an extra 15 pounds per square inch of pressure
differential essentially free.

A second major advantage of condensing steam engines was economic. The energy
in steam can be considered to be composed of two parts. The first (usually
the largest) part is the latent heat; this is the energy required to get the
water to boiling point. The second is the excess heat above that required
to get the water to boiling point (this probably has a name, but my references
are at home :-) When the steam is used in the cylinders only this excess
heat can be turned into mechanical energy (as the steam condenses as it reaches
the boiling point and the resulting water still contains the latent heat).
In a condensing steam engine, this hot water was pumped back into the boiler.
The boiler then only had to supply the energy required to reboil the water.
In a non-condensing steam engine, the latent heat was thrown away with the
steam and the boiler had to continually heat cold water to boiling point. This
was one cause of the very low efficiences of locomotive steam engines.
(This is a simplified discussion as it ignores the effects of pressure on
boiling point! This is a very interesting point -- which I won't go into
now -- as it explains why compound locomotives were so attractive.)

Locomotive engineers were well aware of this -- condensing steam engines
predate steam locomotives. Unfortunately, they were working against a number
of limitations.

The first was the difficulty of condensing the water. Doug Jones has noted
the vast amount of radiator surface area required. (Radiators are also
expensive in maintenance as anyone who has an old car will tell you :-)
Mark Brader has noted the alternative of using the tender water to condense
the steam. While simple, the difficulty with that was the injectors would
cease to work if the feed water got too hot. (Injectors work by condensing
the steam, which is more difficult if the feedwater is hot.) This could be a
real problem with the Metropolitan tanks!

The second problem was that the locomotive boiler depended on a forced draft
which was usually supplied by the exhaust steam.

A locomotive boiler had to generate very large quantities of steam from a
boiler which was very small in size. The traditional locomotive boiler was
very good at this; indeed it was far superior (for this application) than
almost any other boiler design.

Using the exhaust steam was the key. It was simple (no moving parts to
maintain and hence cheap to maintain) and it automatically adjusted itself to
the rate of steam production required. A locomotive working hard -- and using
a lot of steam -- generated the strongest draft.

It is interesting to note that the use of the exhaust steam to generate
a forced draft had a serious disadvantage -- apart from the waste of water --
as it increased the back pressure in the cylinders and hence decreased the
amount of power that could be obtained from the engine. Quite significant
performance increases (and an improvement in economy) could be obtained by a
careful tuning of the proportions of the smokebox to generate the required
draft with the lowest possible cylinder backpressure.

Another method of supplying a forced draft had to be supplied in a condensing
locomotive. Although I don't know what the South African locomotives used,
it was probably fans or steam jets. Note that these mechanisms required energy,
possibly quite a lot, and hence reduced the efficiency of the locomotive.
The mechanisms would also have required maintenance and hence increased the
operating cost of the locomotives.

Essentially, the answer was that condensing locomotives had the potential
to use less coal and water, but that the increase in complexity resulted
in an increase in maintenance charges which swallowed the decrease in fuel
and water costs.

andrew waugh
From: wbrill@hpfcso.FC.HP.COM (Warren Brill)
Date: Mon, 9 Nov 1992 17:27:02 GMT
Subject: Re: Misspelling of "Amtrak" (was Re: Metroliners)
Organization: Hewlett-Packard, Fort Collins, CO, USA