15. Discussion on Performance of Center-flue Boilers

15. Discussion on Performance of Center-flue Boilers

Commentary on a Magazine Article.

The February 2023 instalment of Model Boats - Boiler Room article focusing on then new "Econo" Steam Plant was very interesting, showing a neat and apparently well engineered package with the boiler burner assembly being an integral part of the boiler base, thus allowing a very compact overall arrangement.
We would, however, like to offer a few comments - but first the details of the artical as best we can reperat it....
Open quote from an unidentified commentator:
"With a small boiler, such as this, one is always struggling to conserve heat so lagging the boiler and the steam pipe would be a practical as well a visual improvement.
There is also an issue to consider with these vertical cross tube boilers - the effective heating area depends to some extent on the water level, so when it gets below about half glass the steam production starts falling off.
However, my main concern would be the practical run time available from this package. The specification says about 15 minutes, but I only get about 25 from my installation, which has the same 11x11mm size engine but with a boiler nearly nine times the capacity. Perhaps I have a very inefficient engine or, as the author suggests, I should have bought this package first to practice getting the best duration and then I could run for over two hours!" End quote.

We see this as very much one of those "I want to talk to the engineer not the oily rag!" scenarios, so it's over to Richard Simpson here... 

Open Quote:
"Many thanks for your comments regarding the February installation of Boiler Room, Tim, to which I would like to add the following observations.
To address your first point, did mention in the article (see p. 62) that would recommend lagging the boiler, for the very reasons you suggest. If I was to use the plant in a model, then that is something that I would always do.
"I understand your comment as regards the available heating surface being reduced as water level reduces with this type of boiler:
I think in all fairness, this occurs with any design of tubed boiler, both model and in real life. This is just as relevant for water tube and smoke tube, cross tubed and linear, so there has to be a point below which the efficiency falls off so much as to cause concern. Actually measuring and evaluating this point would probably be quite subjective with modt boilers, and will, of course, always be a compromise between this and the duration of the model on the water. From a safety perspective I would never suggest that the level is taken below the bottom of the sight glass but, to give a reasonable balance between duration and performance, I usually try to maintain the glass between around 80% to 25%. This is a/so bearing in mind the length of the sight glass fitted and the rate at which the boiler tends to use water.
As regards the duration:
- there are many other factors as we// as the capacity to a/so take into consideration if plants are to be compared. For a given engine bore and stroke steam demand can vary significantly depending on many things, such as hull characteristics and size, propeller size, engine efficiency and even the manner and speed in which the models are operated, so again a very subjective comparison. I would suggest however that,if your boiler is in the region of 675 ml with an oscillating engine of those dimensions, powering a typical sized boat for that plant, then 25 minutes duration is about what I would expect.
I strongly suspect however that your plant is pushing around a model significantly larger than the size of model that the Econo plant is designed for, so your 11mm x 11mm engine would be significantly higher loaded.
Many thanks again for your thoughts and feedback, which are very much appreciated and valued. Hope this explanation has proved of help.Ed"
End quote!

MSM - We hope we have accurately presented the substance of the article and would like to add the following comment on the issue of  boiler performance vs boiler water level.

MSM horizontal and vertical boilers have identical construction apart from the modifications necessary to provide the relevant orientation. 
During the past twenty or so years we have been selling these boilers we have not received any comments to support the argument that the boier performance varies as the water level declines. Actually we would argue that the performance of the attached engine should improve in that the steam will be "wet" and that as the fire tubes are uncovered a measure of "superheating" of the steam would occur passing more power to the engine.  

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