[modeleng] Re: Question - maybe off topic- certainly academic
- From: "alanjstepney" <alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Thu, 14 Oct 2004 10:44:36 +0100
In one of my books I have a diagram showing the heat losses through the
water leg on a full size loco.
From what I recall, the theory is / was that the water in contact with the
wall would bubble as it boiled, and the circulation of the water would
replace it by some more.
Some of the boiler design software, particularly that of Bill Hall, gives
the rate of heat transfer.
I am sure that in most cases, once a boiler has seen some use, scale is a
bigger hindrance to it than is vapour.
Must admit that I have never heard of Nagaoka curves, but anything with a
name like that deserves to be right.
(I dont feel that it is off-topic, but it certainly is academic).
alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
www.alanstepney.info
Model Engineering, Steam Engine, and Railway technical pages.
----- Original Message -----
From: <peter.chadwick@xxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Wednesday, October 13, 2004 3:51 PM
Subject: [modeleng] Question - maybe off topic- certainly academic
If you look at the Nagaoka curves for the quantity of heat transferred
through a material such as a metal sheet or shape (eg firebox) to a
liquid, you can see that as the liquid boils, a sheath of vapour forms
around the metal surface, which tends to insulate the metal from the
liquid. Thus for a given heat input, the heat transfer goes down and the
temperature of the metal wall goes up. This can go so far as to lead to
melting of the metal.
The Nagaoka curves show this by the temperature v. heat transfer graph
increasing to a point of inflexion and then the gradient goes negative -
it's basically a straight line to that point as I remember.
I can see that items such as plate and rod stays will modify this, but has
anyone any information as to how much heat transfer can be achieved
through a firebox before boiling becomes so intense as to cause trouble?
can it make the inside of the firebox more likely to suffer cinder cutting
or other ills by tending to soften it?
An extension of this, is of course, the effects of the build up scale ,
which is a more extreme case.
Peter Chadwick Stockholm
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