Inlet and Oulet Tube sizes

Greetings

Is there a standard reference chart showing what size inlet/ out tubes you need for various piston sizes

or is it as simple as the largest you can get away with and still remain "in scale"

I believe the general rule is the exhaust should be double the inlet

Found one for boilers but cant find anything re pipes

cheers

Joe

The general rule for port sizing is exhaust should be twice that of the inlet.

When it comes to pipework there are other factors that come into play such as stroke of piston eg a long 2" stroke x 1" bore will have twice the capacity of a 1" x 1" engine so you can't just go by piston size. Then there is speed, a slow reving beam engine will not need to flow the same volume through the pipes as a high speed enclosed type steam engine.

There is a table in "Model Engineers Handbook" which lists the suggested pipe sizes for stuart's engines.

J

Edited By JasonB on 17/01/2014 08:42:29

It's simple enough to calculate. If you know the cylinder diameter, stroke, maximum engine rpm and cutoff percentage one can calculate the volumetric flow rate. Assuming that the flow speed is around 4000-6000 ft/min (well below the speed of sound, so no compressibility effects) one can then calculate the pipe area, and thus diameter, needed. If you know the boiler pressure, and degree of any superheat, one can also calculate the mass flow rate.

If one really wants to get into detail one can also calculate the Reynolds number of the flow to ensure that the flow is turbulent rather than laminar, as that should result in a lower pressure drop.

In practice, for a small model that isn't likely to be doing any significant work, it probably doesn't matter. The engine will run even if a significant proportion of the boiler pressure is lost on the way to the valve chest. Just use the biggest practical pipe. Of the two the exhaust is probably the more important, as if one can't get the steam out, it's going to be more difficult to get it in on the other side of the piston. Clearly the exhaust passages need to be bigger, as the pressure is lower, assuming some expansion in the cylinder. As stated, times two is a good starting point. The exhaust is particularly important if it is also used to provide a draught for the fire; one needs to ensure that a draught is created in the chimney, without causing backpressure in the cylinder.

Regards,

Andrew

hi andrew,

your post was superb and sums up succinctly and thoroughly all the factors in these things. very impressive!

worthy of careful consideration by anyone building a steam loco or any steam engine.

cheers,

julian

thanks for the replies

A copy of Model Engineers on its way

cheers

Joe

That's very interesting; I am currently looking at the maths of compounding, and then intend to move onto steam flows to ensure that the passage ways in my traction engine are not overly constrictive. The basic steam passages are about 15mm diameter, at around 10 bar. With those numbers, assuming saturated steam, I expect a Reynolds number on the order of 140000. That should put the flow comfortably into the turbulent regime. I'd be interested to understand better what happens with relatively short pipes. Does the flow start out laminar and stay laminar, or degenerate into a turbulent flow if the Reynolds number is large enough? If so what would you estimate the pipe length to be to ensure a transition to turbulent flow?

Regards,

Andrew

hi michael,

could you possibly summarise the above so that those of us without a detailed engineering background can understand, and put it in simple language for miniature loco builders please? what does the above mean for miniature locos? the late jim ewins was of the view that so long as our locos were superheated the steam was (until expansion) much more fluid than fullsize and that flow problems encountered in fullsize werent such a problem.

cheers,

julian

> Disturbed flow will often trip laminar flow into turbulent .

I'm reminded of 'turbulators' - lengths of cotton glued along the wings of model gliders.

Neil

that's the sort of terminology that started me scratching my hair out!

Standard on full size sailplanes too, except we don't use cotton. The turbulator is normally a zigzag plastic strip running along the wing at about 2/3 to 3/4 chord. The plastic is hard, and surprisingly sharp, it'll shred your fingers if you're not careful. And oddly enough pilots get sulky if you get blood all over their nice polished wing.

Regards,

Andrew

> pilots get sulky if you get blood all over their nice polished wing.

Sounds like a good way to appease the spirits of the air for trespassing on their clouds!

Neil

Some interesting points raised by MikeW. The steam flow from boiler to HP valve chest in my traction engine is fairly convoluted, with several changes of section and bends. The steam is saturated, and most likely wet, which complicates the calculations. I doubt that the flow will be laminar. I am going to base my port size calculations on a flow speed of about 5000 ft/min, so well subsonic, but still a high enough Reynolds number to ensure, in theory, turbulent flow. When I finish the engines it will be interesting to measure the pressure drop from boiler to HP valve chest under varying conditions. When the regulator is partially closed there is a sharp edged transition into the safety valve chest. I am assuming that this will approximate an isenthalpic expansion, with the benefit that it will dry the steam.

I've found Reynolds numbers to be fairly useful in aerodynamics, at least for ensuring that experimental results are comparing like for like, at least for subsonic flows. Supersonic is a whole different ball game. Boundary layer control is of particular interest for sailplanes. Most of the modern wing sections control laminar flow boundary layers by controlling pressure gradients. Although in the past holes for blowing/sucking air have been tried to ensure that the boundary layer stays attached.

Andrew

A very interesting discussion, but I suspect way beyond what we actually need in order to make some sensible calculations looking at port sizes and the steam flow from the boiler to HP valve chest and beyond to check that the passages are of a sensible size.

Regards,

Andrew

The level and depth of knowledge and experience available via forums such as this one never ceases to amaze and instruct me. Thank you for the lessons!