A BRIEF EXPLANATORY NOTE REGARDING TUBE ROUGHENING AS APPLIED TO MODERN STEAM LOCOMOTIVE ENGINEERING

INTRODUCTION

The subject of heat flow, heat transmission and heat absorption has, for many years, been the subject of much misinformation, misguidance and misunderstanding within the steam locomotive engineering community. From the writers experience this is most notable in English speaking parts of the world. This is not the case within power generation or marine engineering circles. The blocking off of firetubes in steam locomotive boilers in order to gain better steaming allied with a decrease in fuel and water consumption has been viewed as something of a “black art” by past and present steam locomotive engineers. This document makes an attempt to summarise the situation in relation to the steam locomotive so as to advance the understanding, and hence the practical application, of heat absorbing devices such as roughened tubes in the case of steam locomotive firetube boilers.

HEAT FLOW

There are three mechanisms involved in heat flow processes, these being conduction, convection and radiation.

CONDUCTION

This is apparently the most simple and straightforward method of heat transfer. A practical example being as follows: If a metal rod (use copper for demonstration purposes in this case) has one end exposed to a flame then a rise in temperature over time will be observed at the opposing end. In the case of a relatively small model steam locomotive boiler these rods (which are brazed in position so as to pass from the fireside to the water side of the boiler shell) can be exposed to the fire thus receiving heat which is conducted up and into the water space of the boiler shell thus heating and subsequently boiling the water which eventually produces steam pressure. Conduction is a relatively slow process which is dependant upon the vibration of the molecules exciting nearby molecules until the water which surrounds the rods is excited in a corresponding manner.

CONVECTION

By virtue of this mechanism hot fluid molecules approach the metal surface, however their effectiveness is obstructed by the universal formation of a film that occurs when a fluid flows over a solid surface. Heat transfer through the film to the metal is in the form of conduction and is therefore slow (as outlined above.) Exactly the same thing occurs when the flow of heat attempts to leave the metal and is transferred to any fluid beyond i.e. hot gas flow transfer to boiler firetube inner wall and subsequent heat transfer from firetube outer wall to surrounding boiler water (firetube boiler example). Film resistance is diminished if the fluid flow is turbulent, the turbulence having a “scouring” effect upon the obstructive film. The film resistance to heat flow can be greater than the resistance of the metal tube walls on which the film is formed. For an adequate heat flow to take place the hot gas mass flow should be turbulent, if this is not the case then larger (thus inefficient!) heating surfaces are required in order to produce adequate heat absorption area.

RADIATION

This is the term given to the transmission of energy through the medium of space, until it comes into contact with an absorbing surface upon which it can be manifested by a consequent rise in temperature. The fact that few people fully understand this mechanism does not prevent the phenomenon being used to its full extent. A special and familiar everyday case is that of sunlight; some objects are transparent to it, some reflect it and others absorb it! Radiation travels in straight lines (at least relatively straight relative to our scale of things as everything, including time and light, travel in an elliptic manner) and the energy flux is dependant on the fourth power of the absolute temperature of the source. A boiler engineer can therefore benefit from an intense fire which can “see” (flame footprint) the bulk of the heat absorbing surface. For a long time the area of heating surface of a boiler was believed to be a measure of its output, in modern steam engineering (and thanks to many years of theoretical and practical research and development work in this particular field) we now know that at best this represents less than half of the truth! Surfaces exposed to a radiant fire are much more effective than those exposed to hot gases alone. One hundred years ago eminent professors of engineering considered that concentrating attention on area alone was delaying modern boiler development. Even in those days experiments carried out had proved that a locomotive firebox having only one tenth of the heating surface was responsible for half of the corresponding evaporation.  

Corresponding anomalies arose in the area of convected heat flow before the doctrine of the surface film or boundary layer was appreciated. During experiments on firetube boilers, some eighty years ago, which involved a number of tubes being blanked off an improvement in performance was noted. Nowadays this can readily be explained. The higher velocities in the remaining “free” (unblanked) tubes of the boiler in question were rendered to a much more turbulent flow and hence the boundary film, that would normally restrict heat flow (heat absorption), was being scoured away – a similar action to that which occurs when steam locomotive boiler firetubes/superheater flues are sanded on the run in order to remove any layers of soot build up! Experiments which were intended to compare the relative effectiveness of iron and copper tubes could be inconclusive, the reason for this was that the better conductivity of copper could be masked by small differences in the fluid flow conditions.

Twisted sheets of metal (swirl inducers) were often placed in firetubes. This practice imparted a swirling and hence more turbulent fluid flow. Although they were more often than not referred to as “retarders” in marine practice their intended purpose was exactly the opposite! “Turbulence stimulators” might well have been a better name to adopt at the time. Unlike the marine engineering world, steam locomotive engineers were very slow to pick up on this idea thus allowing “good enough” practices to creep into their everyday engineering theory and practice.

S.T. McMahon
O.P.I.M.
(9407) RIO TURBIO
Santa Cruz
Argentina
4th September 2007.