Compound engine reheater question

[crueby]

Hi all,
I was looking at the web site for the Col. Ward pumping station in Buffalo NY (they are having their annual tour this summer, date TBD still). They have the following animation of how the engines work:

One thing I have never seen (or at least noticed) before on a compound engine is marked R1 and R2 on the drawing, labeled as reheaters for the steam going between the cylinders. Is this a common part on a compound engine? Cannot tell how they reheat, maybe have an interchange of heat from the incoming steam pipe, or maybe a separate flame? Seems like adding heat there could increase back pressure to the previous cylinder? Or is this just to eliminate losses from the resevoir?

Any info most appreciated!Chris

[crueby]

Oh, and here is their website with great photos of the engines:
https://www.buffalohistorygazette.net/2010/09/holly-steam-engines-at-col-francis-g.html

[cnr6400]

Hi Chris, not sure about that specific engine in Buffalo, but many stationary compound engines had "receiver" chambers between the stages to provide storage volume for exhaust steam before the valves on the next cylinder open. As far as I know extra heat was not usually applied to the receivers, but they were insulated to reduce condensation. As usual I am sure there were lots of variations in practice though, and ways to reheat steam in receivers was probably tested thoroughly over the life span of steam power. I'll have to read Dr Eng Andre Chapelon's chapter on compounding again to see what he says on it in his book "La Locomotive A Vapeur" . There wasn't much he didn't know about compound engines, and I'm lucky to have a copy of his book.

[kvom]

Having visited the Henry Ford museum after NAMES, I was wondering how the medium and low pressure cylinders handled the pulsing of the exhaust from the prior cylinder.  I assume that in smaller engines the volume of the steam chest is enough to smooth out the flow.

[crueby]

Maybe they needed to add a little heat given how humongous these engines are (each is 60 feet tall) to keep the steam in those chambers from condensing? The man in charge of the tours kindly sent me a scan of all the blueprints from when they were built, have to see if I can get any more details of the chambers from there...

[crueby]

Took a look through the plans for the pumping station (only 160 pages  ), and did find a couple pages that show the first and second receivers between the three cylinders - they are huge (200 and 300 cubic feet) but show no provision for outside warming, they do show drains for condensate though. The steam feed pipes do not go near them on the way to the first cylinder, but the receivers are between the cylinders and under the lagging, so they would share heat from the cylinders, which would help. I am still trying to get my head fully around how compounds really work and flow from one cylinder to the next, how the timing really works.

[derekwarner]

Chris.......in a still image, the valving Vs = opens to admit steam......and Ve = opens to exhaust steam.......[but in the opposite orientation to each other] ........they are a little clearer here for the lower valve sets

With the intermediate pressure cylinder during the downward piston stroke, the lower Ve raises to admit steam to the LP cylinder...at the same time, the lower Vs of the LP cylinder is in the lower position to seal

Both [sets] of upper and lower valves Vs and Ve....work in opposite directions to each other to open & close

This repeats itself from the HP to the IP and to the LP or 3rd stage......

The Drawings set may show some detail of the eccentrics involved to achieve such valve timing

Thinking about this, diagrammatically  .....the way the valving is shown, it is not unlike the valving of a fuel internal combustion engine   

Derek

[cnr6400]

Hi Chris, One key point about all compounding, and also valve cutoff - the benefit/ increased economy comes from the steam continuing to expand and do work AFTER the valves in the cylinder are shut. The initial boiler pressure of the steam works on the piston to produce a force on the piston rod, and as the piston moves the steam continues to expand and do work. The receiver allows the exhaust steam to continue expansion without restricting its' flow and to keep pressure drop to a minimum. If the exhaust went direct from cylinder to cylinder in small pipes there would be energy loss from pipe friction and there also could be pressure spikes / shock waves which would affect valve operation and cylinder filling. I'm no expert on compounding but I believe this is the gist of it.

[Zephyrin]

Reheating steam would increase back pressure of the exhaust of the preceding cylinder, and seems to me counter productive, as compounding is made to reduce at most the pressure of the exhaust. Or something else I don't get...

[MJM460]

Hi  Chris, a reheat cycle is a relatively simple approach to getting more heat into the steam without exceeding temperature or pressure limits and is relatively normal in power stations with steam turbines.  In turbine plants, steam from the high pressure stage is exhausted to a pipe system that goes back to a separate coil the boiler before returning to the next lower pressure stage of the turbine at a much higher temperature than the hp stage exhaust. 

In this case, I am wondering if the R vessels should actually be labelled Receiver, and the mention of reheat might be an inadvertent error.  The large volume of the receivers, especially without a  visible heat source suggests that they are not reheaters.  It takes a large heating surface and the appropriate temperature difference to add any significant heat to that steam.

Having those large receivers, looking like probably many times the cylinder displacements means a relatively constant pressure is maintained in the receiver while the higher pressure cylinder exhausts into the volume and the lower pressure cylinder draws steam out of it, so makes the timing of the high and low pressure cylinder valving relatively independent of each other.  This would tend to achieve the maximum possible advantage from compounding.  However the large volume required means a large vessel wall surface area, requiring both good insulation and condensate drains particularly for startup.  It is really interesting to see just how large those receivers have to be.

Shared heat from the cylinders would perhaps reduce the overall heat loss from the receiver, but would be at insufficient temperature to contribute reheat.

The alternative compounding scheme (which is the one I have seen on model engine plans) does not have that large receiver, but instead coordinates valve timing so that the hp cylinder exhausts into the lower pressure cylinder with minimal volume in between.  The relative cylinder diameters means that the combined volume of the remaining volume in the hp and the increasing volume of the lap is in total expanding, so still doing work.  A little hard to picture intuitively, and harder to choreograph the dance of the valve timing so it all works.  But it obviously does.  Large receivers make the two cylinders more or less independent, so much easier to visualise and imagine.  And of course in a triple, the whole lot is repeated between the intermediate pressure cylinder and the low pressure cylinder.

You did not mention the steam supply pressure, although it is probably on those drawings.  Looking at the graphic the inlet valve timing seems to cut off at about a third of the stroke.  To make the maths easy, assume the supply pressure is about 300 psig.  Steam inlet for one third of the cylinder volume, then expands to about 100 psig before exhausting to that first receiver.  There is a differential pressure of 200 psi to do work on that hp piston.

Second cylinder receives steam at 100 psig (give or take a little for those pressure fluctuations, pipe loss etc) and expands to about 30 psig into the lap receiver.  About 70 psi differential, but on a much larger piston.  And it obviously is getting work from further expansion of the steam from the hp cylinder, not extra steam from the boiler.

Finally the lp cylinder expands the steam to about 10 psig, which of course is enough vacuum to need that air pump and condensate pump system to drain the condenser.  They probably aim to condense at lower pressure than that, again the design pressure condition is probably on the drawings.  Only 20 psi difference, but that lp cylinder is very large in diameter.

Reheat at the hp receiver has to have enough surface area and temperature to supply heat to that 100 psig steam, and similar for the lp.

It’s a really interesting steam plant, do we sense a new model in the planning?  You could build the complete power station to keep up the need for multiple parts and complexity.  Do I remember 6 engines in that power station?

MJM460

[MJM460]

Hi Zephyrin, your post came in while I was typing, and I decided my post was already more than long enough.

Basically, reheat does add to the energy of the steam, but does not directly add to the pressure, just the specific volume of the steam.  The pressure in the receiver is the final result of two pulses per revolution of steam from the hp cylinder, two pressure ”dips” from the lp steam consumption and the energy input from the reheat source, all acting together in a more or less steady flow, steady state process, despite the instantaneous relatively small fluctuations.  In fact, in a power station there is a pressure loss through the pipe and coils of the reheater system.  But I agree, it is not easy to visualise.  Intuitively, the reheat should increase the pressure of that steam.  It does, but increases the pressure over what it would have been without the energy input, the result of steam exhausting from a hp cylinder and the reheat energy input is not the result of sequential events.

After the inlet valve closes, the hp cylinder expands the steam until the point of release, which, if all is exactly to design, is just enough above the receiver pressure to cause the exhaust flow when the exhaust valve opens, but practically it is normally quite a bit above the receiver pressure at the point of release.  My example pressure in my first post are a bit oversimplified in that regard.

Each cylinder does work by the differential pressure between the two sides of the piston, and the pressure is progressively dropped through the stages of the engine.

I hope that clarifies things a little.

MJM460

[crueby]

The plans, which are from the original build of the engines by Holly and found later in the building, label those chambers as Receiver 1 and 2. The high pressure steam inlet was at 250 psi as I recall.

That plant, with its row of engines, would make an amazing model but is not currently on my build list.