Boiler Question

I have a question for the boiler experts.

Is there a rule of thumb for temperature and pressures to shoot for with small two-four cylinder mobile steam engines?
Without multiple expansion or multi-stage turbines there must be an optimum temp/pressure.

For efficiency the largest temperature differential is needed, but for a car type application there is only a certain amount that can reasonably be used and that is what I am looking for. I can use steel pipe for the boiler vs copper so the temps and pressure can be maximized.

Thanks in advance.

edge.

That’s not a boiler question. When you design a steam engine you design it for a given HP at a given pressure and RPM. Expansion and compression and port area and clearance volume, and a lot of other factors.

HP = PLAN = Pressure X Length of stroke in FT X Area of Piston X Number of strokes X RPM divided by 33000 and I take this X 55% for all the frictional losses.

On the boiler side of things you always want 200 degrees superheat over saturated temperature at whatever pressure your operating at.
Rolly

What pressure do you use in that formula?

Sorry, I was not very clear, and steamerandy asked my question ....partially.

One could conceivable plug in almost any numbers. Using your 200 superheat ( I assume F ?? )

If my numbers are correct, which they may not

If I use 1,000 psi then my temperature needs to be about 750 F.
I could plug in 1,500 psi and now I am up to 800.
If I drop to 600 psi then my temp drops to about 690.

For a modern steam vehicle I with a "reasonably sized condenser" I was wondering if there is an optimum temperature/pressure that is efficient for highway usage. Basically I don't have a river to use as a heat sink so trying for extremely high efficiencies may not be worth the effort.

Is my thinking wrong? Don't worry about my feelings, I make 5 mistakes before I get out of bed in the morning

thanks again.

edge.

Coming from you Andy that’s a dumb question.

For the sake of edge if your looking at an existing engine, you need to determine what pressure the cylinder block, crank, rod journals, cross head, was designed to handle.
How many bolts and what size are used in the cylinder heads, Valve chest. How much packing is used in the glands? There are all kinds of clues and there are formulas for calculating every part of the engine.

In my opinion edge its not so much the pressure. You would be surprised at what 300 PSI can do with the right engine and the weight of the car using good superheat.
If you’re looking at a reciprocating engine you need to be concerned with the oil. 750F is the limit. That is unless you are in the same belief as Harry. I am not convinced.
Rolly

edge

Reciprocating steam engines do not optimize on the basis of pressure or temperature, but rather cutoff and clearance. Temperature is a tradeoff with choices for material. Pressure is a tradeoff with the designers choice for displacement.


Rolly,

Tune in! Andy is implying that if you have to correct your calculation by 55%, then something is wrong at the beginning.



jerry

The formula PLAN is for cylinder HP only not engine HP. You have to deduct all frictional loads to get shaft HP. If your building a car you need to know shaft HP. Calculating many engines I have arrived at the 55% level for most conventional steam engines. Of course it varies engine to engine but unless you know the engine it’s a good round number.
Rolly

Gentlemen,

Before unsheathing the swords, in the formula using PLAN, P is the average mean effective pressure, BMEP, NOT the inlet pressure.
This takes some good instrumentation or damn good calculating to establish.

That formula does not include any losses, like friction, ring leakage, port flow problems, heat loss from conduction and radiation, or a host of other irritating things.
It is a simplistic formula that many use; but then are at a loss when the engine won't deliver. Indeed, using 55-60% may be quite close to what the engine is really putting out.
The real way to know the BMEP is by using cylinder pressure transducers and watching the PV curve on the scope. Then you KNOW at all pressures and temperatures you want to examine. Anything else is voodoo.

Rolly, In spite of the fine engines of yesterday, if you don't want to put a strong cap on the efficiency, then you are stuck at MAYBE 750°F
Haughton #80 could be pushed; but not that much.
Don't forget, the superheater outlet may be at 850°F; but by the time it gets into the cylinder, think more in the line of 680-700°F. By measurement in several cars. And yes, the thermocouples were calibrated and had ambient compensation. The only steam temperature that counts is what it is when it enters the cylinder.

Harry is right, injected cylinder oil is bad stuff and has to be eliminated.

Jim



Edited 1 time(s). Last edit at 07/23/2010 07:24PM by Jim Crank.

Reciprocating steam engines do not optimize on the basis of pressure or temperature, but rather cutoff and clearance. Temperature is a tradeoff with choices for material. Pressure is a tradeoff with the designers choice for displacement. Jandp

Jandp
I don’t know how many steam power plants you have built but to me superheat is a plus in all directions. It can be 30% less water needed to run the engine. That’s 30% less water the engine needs to pump into the boiler. And 30% less water the burner has to heat up to steam pressure. And 30% less water that needs to be condensed back to water.
Superheat Superheat Superheat Superheat
Rolly

As I’ve said before Jim, When you run your Doble across the country with no cylinder oil I may think about getting on the bandwagon,
but I doubt it.
Rolly

Your question is more like "how long is a piece of string?"

Most of the issues have been covered above. The design limitations of the engine relating to strength, valve gear and lubrication needs (if any) of the hottest parts will tell you what the boiler conditions need to be. To keep sizes down you need high boiler supply conditions in the superheated range. Each engine design will have an optimum operating range for speed, power and steam economy for each set of supply and exhaust conditions. You need a full set of bench test results to determine this.
There could be a wide band of conditions that give acceptable results and more room for choice. It is likely engine temperature limits will restrict choices for that and power needs will tell you how much pressure is required for the gross output. Small engines have been run at anything from 200psi to over 3200psi. The Carter car engine ran between 300psi and 2500psi at over 1,000 deg F using mid 1970s technology. However high engine temperatures are going to impact on engine life and anything over 750 deg F will be a big technical (and cost) challenge for the designer.

Graeme

Thank you all, and sorry for opening a can of worms here.

edge.

Rolly,

I think if we changed the cylinder liners to one of the new bearing type ceramics and went to poppet valves, plus injecting the water onto the rings at the right place this might work.
But; the whole thing is to restore the Dobles to what they were and not fool around. For that, a brand new car is in order.

Jim

Jim

What? Who said anything about Dobles?

Rolly, I never use the PLAN formula. I use the cycle to figure work output. You did not explain to edge what P is. I understand it to mep a Jim stated. Jerry is right in that with the 55% correction factor it sounds like something is wrong. I haven't built an engine. But I have figured existing engines and come within 1% actual performance figuring 5% losses from the ideal cycle. My cycle includes clearance and compression.

Andy

Hi Jim
I’ve thought of building an engine using one of the ceramic liners. But I’m getting tired of these four and five years’ projects. If I did, I’d stick to piston valves with the same liner. You build an engine then you need a boiler to match and all the support systems to go with it. I’ve still got unfinished projects in house and at 71 things seem to take a little longer. They never taught me the longer you keep metal the heaver it gets. I just finished building my Stanley EX [www.stanleysteamers.com] and need some time to enjoy it. I haven’t used one of the boats in a few years.

Andy, I ribbed Jim about not using oil in his Doble, and supporting Harry, I wish Harry all the best, and your right, some time we assume everyone understands what were talking about.
However we estimate HP or boiler output it’s the testing at the end that is the proof.
I use thermocouples and all the gauges but in the end its how she runs and performes.
A lot of guys are only thinking engine. You need to think power plant. You have to be able to diliver the temperature and pressure to the piston. That can triple the cost of the engine, or more. And like Jim says it’s what at the piston that counts.
Lets keep it fun.
Rolly

Rolly,

I agree that many folks only think of a steam engine as the expander thing that is only a small part of the size, weight and cost of the whole power plant. The problem of the mistaken use of a generic word that means many different things probably goes back to the days when the first power producing machines were built. I had a brief look for authoritative definitions for "steam engine" and suggest the Britannica Concise Encyclopedia definition be used for future reference on this topic, namely:

"Steam Engine - Machine that uses steam power to perform mechanical work through the agency of heat (hence a prime mover). In a steam engine, hot steam, usually supplied by a boiler, expands under pressure, and part of the heat energy is converted into work. The rest of the heat may be allowed to escape, or, for maximum engine efficiency, the steam may be condensed in a separate apparatus, a condenser, at comparatively low temperature and pressure. For high efficiency, the steam must decrease substantially in temperature as it expands within the engine. The most efficient performance (i.e., the greatest output of work in relation to the heat supplied) is obtained by using a low condenser temperature and a high boiler pressure. See also Thomas Newcomen, James Watt.

For more information on steam engine, visit Britannica.com."

Any suggestions that the sub-component of a steam system represents the whole system amounts to misrepresentation and should not be accepted by anyone. This false information has proved costly for some inexperienced inventors or investors who failed to check that published reports of some high specific output steam engines had an error of several hundred percent because the weight of the whole steam system required to give and sustain the claimed power was omitted.

A sobering exercise is calculating the total weight of a non-condensing steam car power plant (including fuel, oils and water) that has a non-stop range of 200 miles. If you assume a bare chassis weight of 600 to 800 lbs for the chassis, body and wheels, and travel at 30mph, have a passenger and driver weight of 400 lb, can make meal or rest stops for driver/passenger benefit only (ie no additional fuel or water can be collected), what could be achieved?

Graeme



Edited 1 time(s). Last edit at 08/01/2010 05:38AM by gvagg2.