Advances in Materials. Proceedings of a Symposium Organised by the North Western Branch of the Institution of Chemical Engineers Held at Manchester, 6–9 April, 1964

251 Pages·1966·21.149 MB·English

ORGANISING COMMITTEE
Mr. L. Holliday, M.A., B.Sc., F.R.I.C., C.Eng., M.I. Chem.E.
Mr. B. F. Street, B.Sc., C. Eng., A.M.I. Chem.E.
Dr. P. E. Evans, M.A., Ph.D.
Mr. J. M. Hutcheon, B.Sc, A.R.I.C.
Mr. M. Shaw, B.Sc, D.L.C., A.R.I.C, C.Eng., A.M.I. Chem.E.
Honorary Editor of these Proceedings
Mr. P. A. Rottenburg, M.A., C.Eng., A.M.I.Chem.E.
ADVANCES IN
MATERIALS
Proceedings of a Symposium organised by the
North Western Branch of the
The Institution of Chemical Engineers
held at
Manchester, 6-9 April, 1964
Published for
THE INSTITUTION OF CHEMICAL ENGINEERS
by
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PREFACE
THE symposium on "Advances in Materials" was held in the Renold
Building, Manchester College of Science and Technology, Altrincham
Street, Manchester 1. The organisers and hosts were the North Western
Branch of The Institution of Chemical Engineers.
The object of the symposium was to survey the latest developments
in materials science and technology. The limitations imposed by mater­
ials on the progress of modern technology were examined, together with
developments to meet the increasingly exacting requirements of industry.
The meeting considered the subject on a broad front and was not re­
stricted to chemical engineering.
CHALLENGES TO MATERIALS SCIENCE IN THE
CHEMICAL INDUSTRY: INDUSTRIAL NEEDS AND
AN INFLUENCE ON DESIGN
By C. EDELEANU, M.A., Ph.D.*
SYNOPSIS
During recent years very much effort has been devoted to studies of materials largely as a result of defence
requirements. Progress has been extremely fast in certain fields, and perhaps the largest current problem is
the assimilation of new information.
In a new project there are three district stages at which questions of materials arise. The earliest stage is
that represented by laboratory work. At this stage various process possibilities may be discounted as liable to
present insurmountable materials difficulties. Frequently these decisions are made almost subconsciously and
tend to reduce both the challenge to the materials specialist and the scope of the project.
The next stage is the preparation of the flowsheet when the physical shape of the plant is visualized. During
this stage materials " problems " can be avoided but it is equally easy to miss opportunities which progress in
materials have made possible.
The final and most common stage at which materials are considered is the detail or functional stage of design.
By this time practically all the major decisions have already been made, but clear thinking on materials is still
important. Attention to detail will make all the difference between a plant which works satisfactorily and one
which gives a great deal of trouble during start-up. Frequently this is the only stage at which materials are
considered seriously because it is the only one during which the result of not doing so is obvious.
Introduction Not so obvious, time is of the utmost importance not only
because of the cost of idle capital which is very high if con­
Not all of us who are interested in materials are motivated
struction is slow but because, in a competitive world, being
by the same things, but the final expression of our work is a
ready just at the right time may make all the difference between
piece of hardware. Expressing ourselves through the
winning or losing a market. Being late, after having sold the
designers involves establishing understanding and this presents
product of a yet unfinished plant, can be a disaster. These
some of the most challenging problems which have to be
are among the things which have to be considered by anyone
solved if anything is to happen. Important as the designer
expecting to have his recommendations taken seriously.
may be to us there are others who are more directly involved
The problem is one of communication and it is worth
in creating the ever-pressing technical challenge in the field
considering some of the difficulties which are found in
of materials. The purpose of this paper is to examine just
practice. Two types of people are involved in the exercise:
how problems arise and how we can resolve them so that our
the engineers and the materials specialist are the technical
work is eventually used rather than to dwell on actual present
parties while the buyer and salesman are concerned with the
problems. Naturally many people in the materials field are
commercial aspects of the negotiations. Roughly the techni­
not greatly concerned with practical issues and, to some, the
cal people decide what is to be bought whilst the commercial
study of materials and their properties is an end itself. No
people decide how it is bought. Difficulties arise when either
one must underestimate the contribution made by such people,
the buyer or the salesman does not know what is being
but when they feel let down by the practical people who are
bought, and unfortunately this is not unknown. Perhaps the
slow to " appreciate " and use their work they might like to
task is made even more difficult when the materials expert has
know some of the difficulties involved. If they then still feel
no idea of the commercial implications of his recommendations
strongly perhaps they might even consider giving a helping
and has no appreciation of the engineer's problems. Equally
hand to the rest of us, but not necessarily at the expense of
frustrating is the situation when the engineer is so engrossed
their own task.The presentation of their work in a suitable
in his own problems that he does not stop to think just what
" language " would help a great deal.
are his problems, and asks for answers to meaningless
questions. In practice, difficulties arise just as frequently
because of the shortcomings of the technical people as those
of their commercial colleagues. The matter is even further
Final Selection of a Material complicated when for either good or bad reasons secrecy on
The critical step in the selection of a material is an engin­ either commercial or technical matters is involved.
eering decision and it is important to realize what is involved It is perhaps worth illustrating the problem with a few
in it. Basically this is a marriage between the technicalities examples. New materials and some which are not at all
and economics, and both words are used in the broadest new are sold under trade names and there are no national
possible sense. The economic decision does, of course, take specifications, standards, etc. The technical information is
account of capital costs but this is only one of the many available in trade publications and, as often as not, gives
factors such as process and maintenance costs, probable " typical " properties, and is very uninformative about
market fluctuation, transport, storage and other such costs. actual minimum properties. Recommendations on design
stress or on similarly useful matters are not given and even
* Imperial Chemical Industries Ltd., Billingham. greater coyness is displayed on costs, availability or anything
3
4 EDELEANU. CHALLENGES TO MATERIALS SCIENCE
else that may help anyone to decide whether it is even worth system operating at say 900°C which would be highly
shortlisting the material. All this is done presumably for economic. It could well be that he would get no further since
good commercial reasons and the relevant information can the consequential container problem might label the whole
eventually be extracted, normally after spending time first with idea as unrealistic. This is a case which actually arose at
a commercial man who cannot help with the technical side but Billingham a few years ago but fortunately the project was not
agrees to send along his technical people who are then floored killed even though it was fully appreciated that the conventional
by any questions of price, delivery, etc. Commercial contact power industry was having difficulties in going even to 650°C.
is then made again in the form of a firm enquiry and as sure Chances had to be taken, but in a matter of some three years
as fate the preliminary information on cost, delivery, etc., is the first pressure steam reforming plant for light distillate was
utterly wrong, and if it is not, it turns out that say Grade B on line, and now some two years later there are at least 33
strongly recommended by the technical man as slightly such plants on line or in various states of completion.
superior and worth having every time is twice the price of Assuming that an idea is not killed from the start, and
Grade A, or it is an experimental grade which is on a year's laboratory work proceeds, various decisions on materials are
delivery. Naturally there are alternatives to this but it is no made again sometimes almost subconsciously during the
wonder that a project engineer faced with numerous routine research. For instance, everyone knows that hydrochloric
materials decisions, possibly hundreds, and with a year to acid is a difficult acid and no matter what other merits it has
complete detailed design, clear a site and erect a plant will it could well be excluded in an investigation simply on this
not be delighted to consider what he thinks, and rightly so basis. It may also be that artificial limits on pressure,
under the circumstances, are crackpot ideas. The meals that temperature or other such factors may be imposed just in
have to be consumed in order to arrive at an understanding are order to avoid potential materials problems. Naturally this
in themselves a reason to avoid novelty. In most projects may be justified and certainly it will reduce the eventual
there are only one, two, or at the most three really important demands on materials, but it is also likely to reduce the
novel decisions to make, but even so the work which has to be chances of a really spectacular new project. The nitric acid
done in connection with the remainder does tend to make life process universally used now is a good example, also from
difficult. Billingham, of a process which would not have seen the light
The most valuable contribution a materials man working of day when it did had it not occurred to someone at the time
for a supplier can make is to be sure that the trade literature that there was what then was a novel material—stainless steel
means something and fulfils a purpose. This exercise will —and that it was not merely a laboratory curiosity and that
teach him a few lessons and he must not forget that it is no it could be considered seriously for a major engineering
use him recommending, which is the polite word for selling, a construction. The realization, of course, not only caused
material on the basis of its physical or chemical properties progress in chemical engineering, but brought forward the
if he ignores the price (which includes delivery, fabrication, development of the stainless steel industry which in turn
etc.) and which is also a property. Furthermore, when he transformed chemical engineering.
selects topics for development or research work and looks at The second stage in the project comes when the laboratory
the various properties of materials he must look realistically work has been finished and it has been established that some­
at the one universally important property which is cost to the thing can be done. During this stage it is necessary to
buyer. This property is at least as complicated as any others decide exactly what shall be done and finish up with a flow­
the material has and it is sometimes governed by what at first sheet. This stage is very similar for a really new or for an
appear completely illogical laws. The relationship with only slightly modified process, and again a variety of decisions
production costs is sometimes statistically non-existent, and are made which, consciously or subconsciously, involve
whilst sometimes there are good commercial reasons for this assumptions about materials. For instance, important
there are situations in which a good product which may cost factors are the size of plant, the operating pressures, tempera­
the same or even less to manufacture than a bad one does not tures, compositions of various streams, etc., and the limits
require extra manufacturing capital or cause any other in­ are imposed by what are commonly called engineering factors.
convenience is priced out. It may not be the recognized A typical such limit may be the size of the pressure vessel, but
function of a materials man to concern himself with prices is this really a true engineering limitation ? The basic physics
but it is the thesis of this paper that he should, and it is of a pressure vessel are not affected appreciably by size, and if
suggested that he will find at least as much challenge in there are limits to the size of pressure vessels these are imposed
investigating the interesting laws governing accountants and by metallurgical factors such as the metallurgy of thick plates,
bodies such as the Iron and Steel Board as the physical laws on being able to weld and inspect the vessel, and other factors
of nature. which are in the materials field. Naturally there are also
Price as stated above means price of the final item, and this practical matters such as the transport of the vessel to a
in turn involves a knowledge of methods of construction and particular site which may impose limits, but the metallurgist
their costs. It is no use for instance specifying a PTFE lining can overcome sometimes even these by allowing site welding.
and stating the price of PTFE without also considering whether By the time this second stage is reached generally time is
and how such a construction can be made and the final cost. becoming short, and unless the necessary homework was done
during the first stage, conventional materials and modes of
construction are the order of the day. Savings, even if
considerable, which might be possible by adopting completely
Stages in a Project at which Material Decisions are Taken
novel methods have become less attractive especially since it
Thinking in terms of a chemical project it is possible to has already been established that the exercise as a whole is
divide the decisions made on materials into three stages. economically rewarding and there is already a " bird in the
Initially there is a stage during which an idea is conceived and hand ".
laboratory work on the chemistry of the various reactions is Finally, when it has been established what shall be done,
done. During this stage a great many materials decisions can design starts which involves decisions on how it is to be done.
be made, either consciously or subconsciously. Indeed even By now things are moving at all possible speed, capital has
before laboratory work is sanctioned the matter of materials been committed and every single day may cost £1000 or more
may influence the project. To give an example, a chemist depending on the size of the project. The project has
may say that he can visualize a process involving a pressure splintered into various design tasks and many designers are
EDELEANU. CHALLENGES TO MATERIALS SCIENCE 5
involved. It is perhaps regrettable, but by far the greatest well known to the engineer and easy to use since there are
number of decisions on materials have to be taken during this many codes, national specifications, standards, etc., and a
stage. Even if the major decisions have been taken, there are well-organized and established engineering industry to back
numerous minor ones and these involve, amongst other them up. Strangely these are also their handicaps because
things, the laying down of detailed specifications, establish­ many of the standards and established engineering practices
ment of standards, instruction on welding, inspection, and the are out of date and greatly restrictive. For instance,
unforeseen difficulties which crop up during detail design. B.S. 1500 lays down that pressure vessels must be designed to
The odd joint, gland, inspection point or delay in delivery a quarter of the U.T.S.—a standard laid down countless years
of a preferred material are typical details which all cause ago which admittedly is being revised, but only after it has
hold-ups, and equally important, if not considered adequately, been shown by many years experience in Germany that much
endless start-up troubles. At this stage, like at all others, higher stresses are safe in practice. There are countless other
decisions must be correct, but one thing is never permitted examples—to quote one other it is traditional to use forged
now and that is to take time to make them. These two steel for critical items, but why ? The economics of the steam
requirements may appear contradictory but in practice they reforming process already mentioned were greatly affected by
frequently are not and the way out may be to do something using centrispun tubes of high alloy steel at roughly a third the
which appears quite stupid economically or clumsy from a cost of the extruded tubes, and on a six-week delivery instead
design point of view but which is bound to work. When it of one year. As it happens they have also proved technically
has worked unfortunately it then becomes established and the better, but in order to obtain these advantages one has to fight
mistakes, because this is what these compromises are, are then tradition. Those who feel safe that they have a good quality
repeated when new plants are built. product unlikely to be replaceable by others may be fortunate,
To sum up, the really big challenge arises at the conception but are more likely to be mistaken, and they have to consider
of a novel idea for a process. There is then freedom of exactly what the customer buys and not assume that this is
action and time. Unfortunately decisions at that stage are the same thing as the manufacturing description. A manu­
often made subconsciously and even at the next stage, the facturer may think he is selling forgings but in fact the
flowsheet stage, when worthwhile things can still be done, it is customer buys vessels, tubes, etc., and provided they are
not always appreciated just where materials stand. The final technically sound he does not care if they are knitted. An
stage is a race against time, the decisions are still important example is provided by the railways which would not be in
because they make all the difference between a plant with or trouble now if from the start they had realized they were in the
without start-up difficulties. That engineering is applied transport business and not in the railway business.
material science amongst other things is appreciated at this Many of the people in the traditional metal industry would
stage especially when things go wrong, but it is too late to do do well to spend more effort on the deadly boring subject of
anything really big. codes, standards, etc. If they feel that, as highly trained
people, it would be a waste of time to do so, they have to
remember that ultimately the man at the drawing board can
only make use of developments when these have been in­
Present Challenging Problems
corporated in such codes or at least in trade publications.
This is a most difficult topic and each person is bound to be Papers in the various metallurgical journals are no use at all for
influenced by his immediate experience. As I see it, the most this particular purpose and there are obvious disadvantages in
interesting feature at present is the hotting up of the com­ sending the second fifteen to B.S. committees.
petition between metallic and non-metallic materials, and this Technically it is hardly worth saying that most of the
is becoming quite real. It is not confined to the exotic novel challenge comes from the desire to go even to higher tempera­
ceramics, cermets, etc., which allow one to do things tures, the wish to build even larger cold items, the use of
undreamt of only a few years ago. A simple thing like the higher pressures, and from the numerous compatibility
tile lining of a vessel was a very adequate way of dealing with a problems {i.e. corrosion, product purity, catalyst performance,
reasonably innocuous acid, but now it is possible to build a etc.). There are numerous challenges in every field, but a
prestressed tile-lined vessel and consider for instance holding practical hint may amuse some. Pumps, fans and other such
hyrdochloric acid in it, or building a pressure vessel in this items sometimes suffer rapid deterioration in service. They
way. The first French reactors showed how quite large are then buttered up with various hard metals, etc., and are
pressure vessels can be built in concrete whilst the Dracone still in trouble, and then someone bodges them up with an
and the various smaller rubber-cum-plastic transport vessels epoxy resin or some similar compound, and that often is the
are finding many uses. end of the problem. It is these small things which are often
In this competition metals have a great advantage in being so very important.
DESIGN FOR DIFFICULT ENVIRONMENTS
By D. R. LOVELL, D.L.C., M.Inst.F., A.M.I.Chem.E.,* and A. E. S. WHITE, A.R.C.S., B.Sc*
SYNOPSIS
Difficult environments are defined for the purposes of this paper as those in which mechanical stress, chemical
attack, and thermal shock occur at high temperatures (above about 1200 C).
Existing or new designs may be combined with existing or new materials to form four possible methods of
combating these conditions. Examples of each method are discussed with reference to problems of kiln-roof
design; tube-furnace design; liquid-metal temperature measurement; reactive-metal containers: high-
temperature gas-turbine blades and rocket nozzles.
Two cases are mentioned where design and materials have advanced independently with the result that progress
has been very limited.
It is emphasised that the design must suit the properties of the material and that when new materials are
developed the application of theoretical principles of, for example, thermodynamics and solid-state physics,
can be of great value in guiding the practical investigations and in predicting new compounds and properties.
Introduction challenging and, if successful, the most satisfying to a designer,
since it requires all his ingenuity and inspiration to devise
In the design of chemical-engineering plant it is necessary
a solution which in effect avoids the difficulties.
to select materials of construction that will withstand the
The third route—existing design, new material·—is en­
operating conditions and to produce the component in an
countered in three forms:
appropriate shape to suit both the available methods of
manufacture for the chosen material and the application. (1) The development which occurs in a family of
Where the difficult environments considered in this paper materials. The heat-resisting alloys are an example where
occur, consisting of chemical attack, erosion, mechanical improvements in materials allow furnace components of
stress, and thermal shock at temperatures above about 1200°C, the same design to withstand more difficult environments—
the material selected is frequently a ceramic and design higher temperature and larger stresses in corrosive atmo­
problems are increased by the special properties of this type spheres—and successive alloys of a series are suitable for
of material and the variability in properties from piece to progressively more severe conditions.
piece.
(2) When a material of the same family with the neces­
Sometimes it is possible to avoid the difficulties by modi­
sary properties is not available, a change of material can
fying the design and using ordinary materials, but as environ­
sometimes be made without affecting the design. In the
ments become more difficult new materials have to be de­
construction of muffle tubes for furnaces it is often possible
veloped, and these often require a different design because of
to use a similar design, although a ceramic is substituted
limitations imposed by the material.
for a metal casting.
There are four possible routes to the solution of a problem.
In order of complexity, cost, time required, and effort involved (3) Where neither of these approaches provides a satis­
these are: factory material an attempt has to be made to produce
a " tailor-made ", entirely new material to meet the appli­
(1) The use of an existing design using an existing
cation. This is a challenge to the materials scientist and
material.
it becomes increasingly necessary as environments become
(2) The production of a new design using an existing
more difficult. To carry out such a development success­
material.
fully, the thermodynamics and solid-state properties of the
(3) The use of an existing design using a new material. system must be considered and this makes possible the
(4) The production of a new design using a new material. prediction of properties and of possible new compounds.
For the designer to be able to consider all these routes it is
The fourth route—new design, new material—has to be
essential that the problem is clearly understood and stated
used if the available form of the new material prevents the
in the broadest terms. For example, by stating a problem as
existing design being used, e.g. the new material is available
" Design a plant to melt titanium " rather than restricting it
only as a casting when the previous design was formed from
to " Provide a crucible to contain molten titanium " it is
wrought materials, or if the properties of the material impose
possible to avoid a difficult materials problem.
some restriction on the shape or size of the article, as fre­
The first route is worth some study because a knowledge
quently occurs with ceramics. In this case especially, the
of the present " state of the art ", especially under operating
designer must consider both the material and the application,
conditions approaching the limits which the materials will and the interrelation between them.1
withstand, provides a basis on which to develop new designs.
Sometimes the designer and the materials scientist find
The second route-—new design, existing material—has been
themselves out of step so that materials are available that the
illustrated in the example on melting titanium and other
designer can find very little use for, or the materials scientist
examples will be mentioned later. This route is the most
is unable to provide a material meeting the designer's needs
and the designer cannot devise an economic way of avoiding
* Morganite Research and Development Ltd., Battersea Church
Road, London, S.W.I 1. the problem.
7
8 LOVELL & WHITE. DESIGN FOR DIFFICULT ENVIRONMENTS
of hot-face insulating bricks their mechanical strength is low.
These same properties, however, mean that they can be cut
easily to shape and rubbed to make mating joints which
require only a very thin layer of cement.
s mi CAS/MS. TABLE I.—Comparison of Some Properties of Firebrick and Hoi-face
Insulation Brick
Firebrick H.F.I, brick
Refractoriness:
Seger cone 32(1710°C) 32(1710°C)
Porosity (%) 23 72
Bulk density (lb/ft3) 125 47-5
Thermal conductivity at
mean temperature of 800°C
(Btu/h ft2 degF in.) 10 3
Cold crushing strength (lb/in2) 2500-5500 140
We have had in use for many years a series of round down-
draught kilns working up to 1450°C on a five-day cycle and
some of these have been constructed or reconstructed using
hot-face insulating bricks. The general arrangement of this
type of kiln is shown in Fig. 1 and the roof illustrates the
effect of the properties of the material on design.
This roof weighs only one-third as much as it would if
constructed in firebrick and is best built by laying the bricks
in circular courses, each brick being rubbed down from a
standard rectangular shape to match the curvature required
and the dome being developed by a rotating jig (Fig. 2). It
is necessary to cut bricks occasionally to prevent radial joints
in a number of courses coming into line, because this would
provide a path for a crack to develop. Fig. 3 shows a well-
constructed roof and Fig. 4 a poorly-constructed one in
which staggering of the radial joints has not been carried out
and the rotating jig has moved during the construction. It
Fig. I.—Generai arrangement of round downdraught kiln
should be noted that the use of cement between the bricks is
theoretically unnecessary as the bricks are tapered and should
Examples of each of the above routes to a solution have support each other. Although cement is in fact used to
occurred in our experience and a description of some of the enable the rings to be built, the movement of the roof during
more interesting ones will illustrate the close interrelationship firing produces a number of radial and circumferential cracks
between design and materials which is essential when dealing between courses which separate the dome into a series of
with difficult environments. mutually supporting panels. Because the expansion cracks
open and close as the roof rises and falls on heating and
cooling, it is not desirable to cover it with powder insulation
Existing Design Using Existing Materials
since this percolates into the cracks and prevents them closing.
The use of hot-face insulating bricks has become widespread However, a diatomaceous-brick insulation or a slightly
in the last 10 to 15 years because of their two great advantages, flexible asbestos-sodium silicate paste layer can be used.
low-heat storage capacity and low thermal conductivity, One disadvantage of hot-face insulation bricks has recently
compared with ordinary firebrick. Table I provides com­ been overcome by mechanically keying a thin layer of dense
parative figures. Because of the low density and high porosity castable refractory to the hot face. This provides an erosion
Fig. 2.—Method of constructing domed roof
LOVELL & WHITE. DESIGN FOR DIFFICULT ENVIRONMENTS 9
and abrasion-resistant surface without sacrificing much ther­
mal efficiency. A thin firebrick lining to achieve the same
purpose would be difficult to bond into the brickwork. In
some cases a washcoating can be used.
Later in this paper we refer to metal-ceramic thermocouple
sheaths for temperature measurement in liquid metals. The
design of the furnaces in which these pieces are fired is another
interesting example of the effect of the use of refractory
materials on design. The problem is essentially that of
heating a tube of the largest economic diameter to a tem­
perature of 1600-1850°C over a length of about 18 in.
uniformly while maintaining an atmosphere which may be
slightly oxidising, inert or reducing, inside it. The heating
and cooling rates are not critical but the selected temperature
must be controlled to within 5°C (Le. i% of the operating
temperature). The cost of kilning the product must be kept
reasonably low.
The problem is about as easy, in general terms, as it is
possible to imagine—merely to heat repeatedly the central
portion of a tube to a given temperature—but because the
temperature is close to the melting-point of the ceramic
material, and the atmosphere must be closely controlled,
necessitating an impermeable tube, it is in fact difficult to
achieve an economical solution.
Nine major issues must be considered: these are detailed
below (see also Fig. 5).
Material
The most readily available material capable of with­
standing 1850°C and providing an impermeable tube about
48 in. long (to give an 18-in. uniform hot zone) is recrystallised
Fig. 3.—Well-constructed domed roof
alumina. It can be obtained in tubes with diameters up to
about 4 in. O/D from stock and larger as special orders.
Heating method
The cheapest way of heating 30 in. of tube (to give 18 in.
uniform) to 1850°C is to wind a molybdenum wire on the
tube and use it as an electrical element in a hydrogen atmo­
sphere to prevent oxidization.
Size
The larger the diameter of the tube the more expensive it
is and the lower its resistance to thermal gradients, shock,
and mechanical stresses. It is therefore most economic to
use the smallest permissible diameter.
Temperature control
For the accuracy stated a total-radiation pyrometer sighted
on a closed sheath touching the heating element is now used.
This controls the power to the winding by on/off switching.
The performance of tungsten/rhenium thermocouples is now
being investigated.
Temperature gradients
Longitudinal gradients must be restricted to about
125°C/in. for alumina, and with the available length of tube
being 48 in. the 15 in. at each end just accommodates such
a gradient, provided the distribution is a uniform rate of fall
from the end of the 18-in. hot zone.
Insulation
The insulating material must be of equal purity to the
alumina tube to prevent contaminants migrating, and alumina
powder is used. To reduce the quantity of expensive powder
the lower-temperature outer areas are constructed in insulating
brick. The whole is cased in steel to provide a gas-tight
container for the hydrogen used to protect the heating
element. Fig. 4.—Poorly-constructed domed roof