Current and Proposed New-Builds

I'm still hoping for revolving nameplates that switch as it crosses the border a la James Bond...

 

It might be different on the main line.
You've also got to considered how many visitors a saturated class 2 tank would attract, compared with something more glamourous.

 

Depends on the particular saturated class 2 tank in question, or rather, how high profile it's TTTE doppelgänger becomes!

I wonder how much current mainline operations skew public perceptions? A screaming whistle and cloud of steam as an express loco unexpectedly thunders close by is something bound to register with just about anyone, however fleeting the moment. Evidently steam still holds the same fascination as ever it did and let's face it ..... even for 99% of us lot on here .... to experience any express loco at speed, doing precisely what it was designed for, is a bloody impressive sight!

 

But then you are keeping two locomotives in traffic instead of one, Two ten year overhauls, two annual boiler inspections, two lots of boiler insurance... I think the savings could be very expensive.

 

Seven letters. My first is in "what" but not "that". And the rest rhymes with anchors....

Tom

 

A practitioner of the Solitary Vice, aka the Sin of Onan?

 

Was he from London........running already, hard hat and extra PPE.

 

Can you share your inputs/assumptions?

 

OK, slightly back of an envelope.

We'll assume a loco running a daily diagram that averages 50 miles per day (that's pretty close to the Bluebell average). We'll assume 100 revenue days per year, i.e. 1000 over a ten year period, or 50,000 miles between overhauls.

The coal used per day will be somewhere well over a ton; however, probably half a ton or so is used lighting up or left on the grate at the end of the day. For equivalent sized locos, those amounts have no difference whether the loco is superheated or not; it is only that burnt on the road that actually can show any saving from superheating. I'd estimate that at about 40lb / mile for a class 2 loco on a hilly line with appropriate load - that is broadly in line with what I estimate our water usage to be for a round trip. (Roughly 30 gallons per mile for a mid-sized engine outside the steam heat season, and assuming a pound of coal evaporates about 7 pounds of water).

That gives a coal usage over ten years of 2,000,000lbs, or just under 900 tons used while running (ignoring that used for lighting up etc). Assume superheating gives a 8% efficiency gain while running (I'm sceptical that you even achieve that for stop-start traffic) means you save about 70 tons of coal in ten years, which at current prices is probably around £12 - £16k. (The 8% efficiency gain comes from figures in Semmens and Goldfinch that take hypothetical superheated and non-superheated locos at 200psi and calculate maximum theoretical efficiencies of 14.8% and 16.0% respectively; in practice the actual gain will be less).

If you look at the component prices for the superheater on the Bluebell Atlantic (chosen just because the figures are to hand) - 28 flues @£220 each; machining the ends @ £100 each gives £9,000 just for the flue tubes. The ends where the elements go into the header are £50-odd each, so there's a another few thousand pounds there, plus the tubing for the elements, and specialist welding the ends to the tubes in the elements (where they bend through 180 degrees). Much of that needs replacing every ten years. At which point, you haven't got any change left from the saving you made on coal...

It's certainly at the very least marginal whether superheating pays for itself over ten years: where it will have most benefit is for locos working hard for sustained periods and pulling heavy loads, since the ten yearly overhaul cost of the superheater will be similar either way, but the more coal you need to burn to run the service, the more potential gain you get. It is probably beneficial on lines like the WSR and NYMR with sustained climbs; but even on the Bluebell, you never have the regulator wide open for more than about 7 minutes at a time. On lines with more modest gradients, you might use less than 40lb / mile, at which point the ten year coal saving is rather less but the maintenance costs just the same.

Obviously, if you have an existing loco that is superheated, it would be folly to remove it and blank off the flues because you would screw up the drafting (as shown by the preserved GNR large Atlantic when it briefly ran in preservation and was reckoned not to steam). But this thread is about potential new builds where you have a free hand, and where I would come back to the point that for most lines, the ideal loco given a blank sheet would not have a superheater.

Tom

 

Thanks Tom. I am not in a position to say that any of that is incorrect, but that 8% advantage of superheating does seem very low. In an ideal theoretical heat engine, my understanding of the (theoretical) maximum efficiency is the relationship of the difference between the inlet and outlet temperatures adjusted to absolute temperatures. So let's say a non-superheated loco uses 100deg of the 473deg "available" to it and a superheated loco say uses 300deg of the 673deg available to it. Which is a big difference. Approaching it another way, if the very best steam locos are 10% efficient overall , then that 8% difference implies that a non-superheated loco could still reach over 9%, which just does not feel right. But that may all be gibberish and my lack of understanding.

 

Without going into a big lecture in thermodynamics, the big heat loss in a steam locomotive is the latent heat of evaporation. Basically, you take feed water in the liquid state, but exhaust it as a gas and throw away the latent heat that was used taking it from water at the boiling point to vapour at the boiling point. Superheating works primarily because you reduce the mass of water you throw away as steam (i.e. a given volume of superheated steam has a lower mass than the same volume of saturated steam, so less mass per stroke is taken through that one-way phase transition) - that is where the efficiency gain is. Having followed the thermodynamic argument, I'm pretty sure the figures are right.

It's why, incidentally, in any steam engine the absolute "killer app" from a thermodynamic point of view is to condense the exhaust and recover that latent heat in a useful fashion. But within the constrained environment of a steam locomotive that introduces all sorts of problems, which was why it was never really successfully achieved.

Edit: Just realised I inadvertantly said "cylinder" efficiency rather than "overall". I've edited my original post.

Tom

 

Good analysis Tom although whilst you don't have to pay for the flues header and elements if you go for a fully saturated loco there is the added costs of the additional small tubes that replace those superheated flues and elements to consider so your sums are not quite right, however whilst you will add some costs for those "additional" small tubes your still in very marginal territory for any potential "savings".

Of course there are other maters to consider if anyone was converting a superheated engine to a saturated one (more likely on a new build with a blank sheet of paper than a rebuild) and that of course is the dryness of the steam especially on a piston valve engine and some modest degree of superheating may be beneficial (like locos from the Gas Works Railway!?) to help in this area.

 

Oh noes!

 

We can probably conjure up all sorts of figures to suit the side of the argument you support. For the NYMR, I reckon on 1.25-1.4 tons for a 36 mile round trip with a 'modern' superheated steam loco and about 2,500 gallons of water, which gives about 9lb water/lb coal. All this generally dragging 7 coaches around. Over 10 years, probably 80,000 miles to knacker it but the trend is now for intermediate overhauls at five years and higher annual mileages (13-15,000). So, coal usage for running trains would be getting on for 3000 tons on my figures. I've no figures to support what the additional coal usage would be for a saturated loco, other than to say that the two saturated locos in use in recent times (LHJC 29 & 65894) use about the same amounts of coal but only drag 5 coaches around. If we take the same 8% saving, we are probably looking at a cost saving of 3000 x 0.08 x £165 (coal cost) = £39,600. A bit more justifiable. Even at 5% it is £24,750. All hypothetical, though as a crap fireman or an S160 can create havoc with the figures.
(and I like S160's.)

If you can get a conventional steam loco to do 10%, you are doing well!

 

If you do not have some superheat, do you get erosion in the steam pipes/valves?

 

I echo fergusmacg's point in post #2193 about the additional small tubes, and would add that there will be more of them than the superheater flues that they replace.

I'd also like to pursue the point that superheating takes a little while to become effective, hence some shunting locos never having it. What is going on when you open the regulator to start away from rest? Why is the steam entering the cylinders no hotter at first than it would be without the superheater? And when it begins to be just bit hotter but not much, isn't even that of some benefit in reducing condensation in the cylinders?

 

A very interesting analysis. I heard that a double chimney Jubilee is worth one coach more than the single chimney version. I don't know whether there are any additional costs in maintaining a double chimney (it sounds unlikely), but it would be interesting to know how many railtours you would need to run to cover the cost of fitting the double chimney. Taking this to it's logical conclusion, there are other options (for example Lempors) which would be worth considering.

 

I've been told that on the KWVR the S160 with the Lempor exhaust used less coal than "Wells" with the Giesl. I know it's not comparing like with like as the S160 will be running closer to the speed it was designed for, than the Bulleid.

 

It is very difficult to compare two smoke box devices unless they are on the same loco to give meaningful results. From memory (I have not got the book to hand) there is a graph in David Whardles (not sure of spelling) book on the SAR's Red Devil which compares the differing devices - the Giesl does show some advantages over a simple blast pipe arrangement but it is not a perfect device (but was well advertised by the late Dr!) - overall a Lempor is a much better alternative.

Having fired Austerity's with and without a Giesl I can confirm the loco equipped with the device does steam better, although lighting up is effected, that's not to say a std. austerity won't steam it's just improved with a Giesl possibly because the std arrangement was more suited to shunting? If I was looking to improve a austerity at modest cost a Lempor is the way to go with of course minimal effect to the external appearance.


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