STEAM ANSWERS

[SandyC]

Hi Guy's,

Further to my posting of some simplified steam notes in a thread being posted by Rich (‘Firebird’), This being: - ‘A SMALL STEAM ENGINE’, on the ‘WORK IN PROGRESS BOARD’ some additional questions have been raised.

In order that Rich's build thread does not get taken too far off topic I have chosen to start this new thread topic in order to answer them.

The attached document is in answer to some questions from NickG.

If anyone has any other general steam questions, then I will be more than happy to answer them for you, however, I do suggest that you ask them under this new thread... that way we keep 'firebird's (Rich) build thread clear... unless, of course the question is directly related to Rich's build.

Happy reading :big: :big: :big:

I would like to take this opportunity to also thank those of you who have responded so positively to the original notes I posted. Your kind comments are most appreciated.

Best regards.

SandyC ;D ;D ;D

View attachment STEAM ANSWERS.pdf

 

[Maryak]

Hi All,

I discussed this topic with Sandy and together we hope to give relevant information on the fascinating subject of Steam.

Attached is the first in a series of modules covering combustion, steam generation/distribution, reciprocating steam engines and pumps.

Sit back enjoy the ride and ask any questions you may have. If we don't know the answer we probably have a reasonable idea of where to find it.

Best Regards
Bob

View attachment Combustion.pdf

 

[firebird]

Hi Sandy & Bob

I've just read the 2 pdf's. Very good. I've no questions at the moment, I'll have to read them several times yet before some of it will sink in. Many thanks.
Cheers

Rich

 

[black85vette]

I would like to know a little more about timing and port sizes on small steam engines. I have been doing it by the seat of the pants method. Any info you have on that would be helpful.

Thanks for the first topics. Enjoyed both of them. Thm:

 

[Maryak]

B85V,

I have copied some of Sandy's previous comments on port sizing for steam engines.

The crank throw only dictates the stroke length of the cylinder and the only possible bearing this would have on the valve gear would be in defining the correct cross sectional area required for the steam/exhaust ports for a given maximum speed of rotation. In this case the stroke length would dictate the maximum velocity of the piston. I.E. a long stroke engine rotating at the same RPM as a short stroke engine would have a much higher piston velocity and this in turn would effect the size of the required steam ports.

The actual formula for port cross sectional area is: -

a = Av/V

Where A = the area of the piston in sq ins.;
v = piston speed, in feet per minute;
V = velocity of flow of steam, in feet per minute.
a = port cross sectional area, in sq. ins.

The values of V commonly used for the steam velocity are 4000 for the EXHAUST and 6000 for the INLET, however, in the case of a slide valve or piston valve, where the same port is used for both Inlet and Exhaust the lower figure of 4000 should be used.
This will determine the required (minimum) cross sectional area for the ports.

Having calculated the required cross sectional area then the actual shape/dimensions of the ports can be determined…. In your case this is a 1/8” dia hole….. in most slide valve engines it would be a more rectangular shape and generally would have a length of between 0.6 and 0.8 x the bore of the cylinder and having a width large enough to achieve the cross sectional are required.

Once this width is determined then the required valve travel can be defined and it is THIS that dictates the THROW required from the eccentric.

To sum this up then, the eccentric throw required for a directly connected valve drive (such as you are using) is dictated solely by the valve travel required to fully open the cylinder port to either STEAM or EXHAUST, which in turn is directly related to the port width in the steam chest.
Sadly this is not the case with the more complex valve gears such as ‘Stephensons’ link etc where several other factors such as die slip and other geometric angularity losses would need to be taken into account, however, they are for another time.



As Sandy says this is only the start and we will get to the timing in greater detail. As a basis, the valve is ahead of the crank, (leads), by 900 for outside admission, (usually a common D valve), and is behind the crank, (lags), by 900 for inside admission, (usually a piston valve).

To try and keep my own series in some form of order - We have dealt with combustion of fuels and next is what this fuel does to the water and steam in "Properties of Steam"

Best Regards
Bob

View attachment Steam props.pdf

View attachment Steam props.pdf

 

[deverett]

Steam powered engines frequently had lap and/or lead in the valves. If running a 'steam' engine on compressed air (only), is it better to have lap/lead built in, or leave the valve without these efficiencies?

Dave
The Emerald Island

 

[Maryak]

Dave,

My personal opinion is a small lead and a small exhaust lap as a cushion. Because compressed air does not expand very much there is little point in steam lap to allow for this expansion.

Hope this helps.

Best Regards
Bob

 

[Maryak]

Well,

Here is episode 3, "Steam Distribution" which was where Sandy and Rich were going with their discussion about pipes, runs, sizes etc.

Best Regards
Bob

View attachment Steam distribution.pdf

 

[4wheels]

I have not read the pdf's yet so I apologise in advance if my point is covered in there.

I read somewhere (probably ME mag from UK) that leads and laps are great for engine efficiency in full size where it really counts. However it is more important with model engines that they run very smoothly at low revs and this is best achived with no lead or lap.

Anything which may cause the engine to slow up at any point during each rotation is a negative with model engines. Often one sees little models running fast for half a stroke and slow for the other half. Not a good look.

Happy running and keep those flywheels reving smoothly.

Brian

 

[steamer]

In small engines like models up to say 5 HP launch, exhaust lap is not necessary as the inertia loads are low. It can be fatal to small models as they condense quite a bit already, and trying to compress water makes small engines stop very quickly.

Running steam lap is to be recommended though, unless you have a big boiler!
But cut-offs below say 70% in full gear in models should probably be avoided too.
This equates to some but not a lot of steam lap.

Dave

 

[4wheels]

Hi Dave,

I guess I was caught out on the 'line in the sand' between model and full size. My idea of this is that models are less than say half to one HP and are for display purposes and are either not driving a load or driving a very small demo type load. There I believe no lead and no lap are idea to show off the engine running slowly perfectly. Here economy of steam is not important and is likly to be run on air.

On the other hand engines of one HP and up are likely to be in proper work driving a boat or stationary load. As such I view these as full size and should have lead and lap to suit the engine application as a workhorse.

Although the engine size can and does jump my imaginary boundaries I still hold the general premise that there are two types of engines - demo's & workhorses. IMHO.

 

[deverett]

Thanks to Bob, Brian and Dave for their explanation for the need of lap and lead in working engines, and in our sizes the need not to have any for best slow running performance of models on compressed air.

Something else to store in the grey matter. Just hope the senior moments don't come too soon!

Dave
The Emerald Isle

 

[steamer]

No worries Brian...to each their own.

For models for demo, your right, as long as steam consumption isn't an issue

Air powering aside...

Duplex steam pumps run without lap all the time and need to to work properly

It is sometimes a problem on a model boat where the boiler size is critical to keep your consumption down...

Horses for courses

Dave

 

[Maryak]

Hi,

My forum time is very limited due to moving and working on the accessories for the new house but just to keep things rolling along attached is Episode 4 "Steam Systems."

I hope these are of use to you.

Best Regards
Bob

View attachment Steam Systems.pdf

 

[Maryak]

At last the bits to make shake and placate steam are out of the way and we are at the nitty gritty of HMEM. "The Reciprocating Steam Engine"

(I like this bit too).

Best Regards
Bob

View attachment Recip Engine.pdf

 

[Philjoe5]

Sandy, Bob, Dave(s), Brian and others,

I thank you for posting this information. Many members of this board encouraged me to try steam when I first posted some videos and pictures of my engines running on air over a year ago. I have since become a volunteer at a local engine museum that has a central boiler supplying steam to the model engine building. This makes it easy to try the steam experience. There’s a lot of expertise at the museum to draw from for setting up and plumbing in the engine. Just last weekend I ran my latest engine build on steam. It really is different and introduces several new variables that you’ve pointed out. I went from being a model engine builder addict to a steam engine addict in one weekend! ;D I'm reading and rereading your pdf's - good stuff :bow:

Thanks again,

Cheers,
Phil

 

[steamer]

It's a fatal disease ...there is no known cure... ;D

 

[Maryak]

Phil,

Thanks for following along and thank you for the positive feedback. :bow: :bow:

Best Regards
Bob

 

[Maryak]

Almost there

A little about Condensers.

Best Regards
Bob

View attachment Condensers.pdf

 

[Maryak]

Hi All,

I will be going off the air for a few days whilst we complete the move and get our phone and internet connections sorted at our new home.

Before I go it seemed to me like a good idea to finish off my series of presentations so here is lucky last "Pumps."

I hope those of you who have taken the trouble to download the PDF's have found the information useful.

Best Regards
Bob

View attachment Pumps.pdf