Steam & Excursion > Compound Steam Engines Questions

I understand the concept of compound engines using the steam twice. Most commonly this was in articulated double engine locomotives. I'm interested in the performance and characteristics of single engine locomotives such as 4-6-0 where the right side cylinder was high pressure and the left side large low pressure cylinders. They must not have been terribly effective because most were rebuilt into simple engines in a few years. What was the general problem?

Tom

An arrangement with the high pressure on one side and a low pressure cylinder on the other is rare. It would be difficult to balance. A more common arrangement would be to have high and low pressure on the same side linked to the same cross head. The biggest problem with compounding is in starting. A complex mechanism is required to add steam to the low pressure side only when starting. Most of the patents covering compound locomotives deal with the starting mechanism. Compounding can be made more efficient by reheating the steam before it is applied to the low pressure cylinder. This also adds to the complexity of the engine, if used. More pieces = more maintenance = higher cost. Most compounds were rebuilt as simple expansion engines simply to reduce maintenance costs.

There were many types of compounds. What you describe would be called a cross compound. There were also 3 cylinder compounds with a central HP cylinder and 2 LP cylinders in the normal places, 4 cylinder balanced compounds with 2 HP cyls between drivers and LP in the normal places, tandem compounds with HP and LP cyinders in line with a very long piston shaft with two pistons on it, and the Vauclain compounds with HP and LP atop each other and yoked together at the crossheads. Naturally the only design that really caught on was the Mallet compound articulated.

filmteknik Wrote:
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> Naturally the only design that really caught on
> was the Mallet compound articulated.


In the US, yes, but internationally (and especially in Europe), other compounding arrangements seemed to be much more popular. Perhaps because of less expensive labor and/or more expensive fuel.

- Arved

What really killed compounds was the development of effective superheating.

-Larry Doyle

What REALLY killed the compound?

The discriminating factor was the temperature of the steam arriving at the cylinders.
Steam that is heated to the highest possible temperature in the boiler is "heat-saturated" ..meaning that the steam is at the same temperature as the boiling water in the vessel. Engines that run on heat-saturated steam, all suffer the same fate: as hot water vapor moves further from the heat source, some of the hot water precipitates out of the 'steam' and forms warm water! Study all the engines that you can find that are running "saturated", and marvel at the the fact that they work at ALL -- a saving grace was the use of the outside admission slide valves -- under great hydraulic lock pressure from the pistons, the slide valves could be forced from their 'seats' and relieve the hydraulic lock before damage could be done .. but I digress..

Baldwin and others, before the perfection of a workable superheater, tried many ways to economize the way 'saturated' engines pumped water out the stack (albeit, the water vapor in voluminous clouds is NOT 'steam', simply water vapor. Pure steam is a GAS of water molecules and is invisible to human vision). Compounding was an attempt to increase the work performed --a futile exercise. It made for great advertising, but colder and colder water vapor does less and less work.

With the perfection of the effective Schmidt Superheater the cold-steam problem was solved!
Superheating removes the water gas from the boiling water of the vessel, conducts it through the path of the firebox/flue gasses and the steam's temperature is raised! The rise in steam temperature is really an indicator of the heated water molecules increased space between the bouncing atoms.
In a hot superheater-- more steam comes out of the units than goes in! It is hotter, does NOT condense as readily as saturated steam and superheat can be measured leaving the exhaust stack!

NOW! if you combine compounding with the superheater, you're really rolling! Bring back the N&W Y6bs!
The compounds that were developed late in the life of locomotives were much better machines that the models of the late 19th century!

Compound marine engines are common, but all use superheating. Steam Turbines have stages of the rotor that are of increasing diameters in order to get the torque effect to the shaft -- compounding of a different sort.

The Superheater is the REAL Hero in this saga.

Wes Camp

PS. If you want to know the Real Story about Super Power, explained in an understandable way, simply ask!

A compound without some measure of superheating between the high and low cylinders suffers from the
same malady as any compressed air system and that's as mentioned, as the temperature drops the water
comes out of suspension. That nice big pipe between the cylinders on early examples makes a great heat
exchanger to get rid of that wanted heat required to make the thing work. As mentioned, it's surprising that
they didn't give up on the spot.

Re: rehunn--

I'm not aware of 're-heating apparatus' used on superheated compounds.
The steam exhausted from the high pressure cylinders is still at a comparatively high pressure and retains a good portion of its superheat as it proceeds to perform more work in the low pressure cylinders.

I'll have to look in Ralph Johnson's BLW book to see what he says about this.

Samuel Vauclain, of Baldwin, was a large promoter, designer and technician of many types of pre-1900 compounds.

Thanks for your added comment.

Wes.

The last Y6b's had a booster valve that would put a
little high pressure steam to the low pressure steam
line. Controlled by engineer. (can't find my book
that has info about it)

hollywood
NS B-Line MP 74

Here's a good question: How does what comes out of the HP cylinder(s) on a compound compare with the exhaust of a simple engine, in terms of temp and pressure? If they are similar then I'll agree with the old adage about using steam twice. But I've always thought of compounding not as using steam twice (as though exhaust steam is a free resource that simple engines are wasting) but rather that the steam is partly expanded in one cylinder (or set of them) and expanded the rest of the way in cylinder(s) designed expressly for LP steam, which a simple cylinder at the extreme end of its stroke is not. I suppose maybe it's just semantics but there must be a difference in design if you are trying to fully expand the steam in one cylinder vs. knowing it's going on to another stage.

Let me back up a bit, compounding without some reheat on the high pressure exhaust was certainly
commonly used. The insulation, joints, piping runs, etc certainly improved as the design was made more
practical but compounding without reheat leads to the adaptation of simple locomotives. The equation
relative to the amount of energy which can be transferred to the low pressure cylinders always must
contend with various losses and without the reintroduction of some energy into the downstream
application you'll always reach a point where the losses kill you. Hence simple or single expansion.
Compounding works and works well for certain applications but trying to equalize the force generated
by the two cylinder sets and considering the expansion issues is not an easy issue. I won;t say this
never works in modern day pneumatic applications but life's too short to even try and that's without the
problems of heat, expansion, etc.

The Santa Fe flexible boiler mallets also had a reheater. The locomotive was made by placing two boilers together, connected by a series of metal rings. The rear ``boiler'' was the actual boiler, the front an extended smoke box. The water was forced into the front unit by an injector where it was preheated. After boiling in the rear unit the steam went back into the forward unit for superheating. Then it went into the second unit's cylinders.
After working it went through the reheater to regain some heat then went into the forward set of cylinders.
The engine was prone to back pressure in the rear set of cylinders. That was fixed by setting the valve gear on the front set to cut off more into the corner than the valve gear on the rear set. One engine even had two Johnson bars.
Reheaters were rare in railroad applications but frequently used on ships.

wcamp1472 Wrote:
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> I'm not aware of 're-heating apparatus' used on
> superheated compounds.
> The steam exhausted from the high pressure
> cylinders is still at a comparatively high
> pressure and retains a good portion of its
> superheat as it proceeds to perform more work in
> the low pressure cylinders.

Standard practice on Baldwin separable boilers.

-Larry Doyle

Most of my experience with compound engines has been working at power plants, where there are two and sometime three stages of turbines powering direct-connected generators. Power plants (which my company calls "generating stations") have the advantages of A) not going anywhere, that is, they don't have to put up with the effects of running along a track, coupling to trains, etc., and B) they don't have to fit through tunnels, so they can be built in the most efficient configuration. The boilers are usually "water tube", rather than "fire tube", a type of construction that was tried in locomotives, but never really succeeded on on the railways. Because of the stationary nature of power plants, steam pressure can be much higher--up to 4,000 psi in some cases. Steamship engines are between locomotives and power plants: there's more room, and the components can be arranged to minimize piping runs, but on seagoing ships, the design has to anticipate foul weather and the vessel being subject to the wrath of King Neptune.

Y6bs, cont'd..
The use of the 'reducing' valve to admit live steam at a lower pressure dates from the earliest days of experimenting with compound locomotives.

A reducing valve is necessitated by the larger diameter of the LP pistons --- steam at full pressure would terrible strains and in the case of articulated compounds, the LP drivers would spin uncontrollably!

It is necessitated by the fact that the low pressure cylinders receive NO steam until the high pressure cylinders make enough revolutions to pressurize the 'receiver' pipe that connects the low pressure cylinders to a steam source. Obviously, trying to start a heavy train on half an engine soon ended in a stalled condition!
Not enough revolutions could be made to pressurize the receiver! Especially trying to start on an upgrade!

Link's first released recording has a wonderful 'cut' of a Y class leaving Waynesboro, Va and shifting to full compound as it passes the mike! MY GAWD!
You can hear the reducing valve venting the high pressure steam from the receiver pipe as a loud wooshing sound and when the reducing valve closes, only the four exhaust beats of the LP cylinders is heard...listen at FULL VOLUME for a thrill of what could almost be the sound level of true Y class!


Extra Credit:
Note also, the common use of the Cross-Compound Air compressors and their great success.
What role does the low pressure air play in the successful operation of the Cross-Compound Compressors?

Wes.

Grande473..

I suspect that your explanation of the routing of the steam is not entirely accurate....
The Schmidt Superheater only works by conveying the heat-saturated steam directly into the path of the still incandescent flue gasses.
The residual heat of the flue gasses passing further down the flues is readily absorbed by the 'cold' water surrounding the flues and tubes of the boiling-unit, per se.

There is NO excess heat in the chilled flue gasses to both preheat the injector water and provide any beneficial superheating (from the front water heater of the early Santa Fes) to produce any measurable superheat that benefits the LP cylinders.

I'll check your explanation of the routing of the steam on these type ATSF locos.
Also, the explanation of the different cut-offs doesn't seem plausible. (However, The only PRR Q1 had engine-synch and wheel slip problems and one solution never made it off the drawing boards: the use of 2 separate power reverse 'gears'.... some ideas often get recycled).

Wes.