Me wrote:

In article ,
"Arnold Walker" wrote:

Many steamtrains are now ran on air due to boiler code worrys by
insurance companies.

CFR (Call for Reference) on the above. as I believe it to be
Bull****..... the only Steampowered Trains still in existance,
and in commercial service are in third and fouth world countries,
and mostly run on diesel fired boilers. Turning big air compressors
with diesel engines is a very wastefull way to move Railroad Rolling
Stock.


Me
I'm just jumping into this thread, so I haven't seen what set it off, but
here are a few facts...

There are still a lot of steam locomotives in service around the country in
excursion service. They all run in the traditional way - oil or coal or
wood burned to make steam to move a piston. Note the repeated changes in
the state of the energy. Each time you make such a change you lose a lot
of energy - simple thermodynamics.

Reconditioned/restored old steam locos are often run on compressed air for
safety testing. They don't actually go anywhere that way.

Let's look at an energy balance. Energy to heat water from 60 F to 212 F -
152 BTU/lb. Energy to convert water at 212 F to steam at 212 F and 0 psig
- 970 BTU/lb. Energy to take steam at 0 psig to 300 psig - 235 BTU/lb.
Adding these up, the total energy to take water at 60 F to steam at 300
psig is 1357 BTU/lb. The only portion of this that is usable is the energy
in the steam. If the steam enters the cylinder at 300 psig and leaves at 0
psig the actual energy used to do work is 235 BTU/lb, or 17.3% of the
energy added to the water in the tender.

Now add in all the losses involved in converting coal or oil to steam (less
than 50% efficient) and you can see that a steam locomotive is very
inefficient.

So what about compressed air? If you look at the volumes involved you will
see that it is just not practical. 1 cu.ft. of air at 3000 psig is about
200 cu.ft. of air at 0 psig. 1 cu.ft of water is about 1630 cu.ft. of
steam at 0 psig. A UP Big Boy locomotive used about 100 gallons (13.3
cu.ft) of water per mile on flat ground with a full 7000 ton cargo load.
That means that to run on compressed air it would need a storage tank
capable of holding 3000 psig pressures of over 800 gallons to run one mile!
Just how close would the compressed air refueling stations need to be, and
how much would it cost to compress the air?

Sure compressed air is great in the shop, but do you really care if it costs
you 6 cents/hour instead of 2 cents/hour to run your pad sander? By the
same token, why are there no table saws that run on compressed air?
Probably because most of us have neither the money nor the space for a
compressor large enough to do the job.

Note that this is a reply to the whole thread, not to the actual poster the
reply is posted to. In face, he is right. Going from the rotary output of
a deisel engine to compresed air to reciprocating motion of pistons is very
inefficient, and that doesn't even take into account all of the other
problems (non-energy related) of reciprocating piston locomotive drivers.

Peter
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-- PeterZ --