16. Thermocouples. State of aggregation

The most widely used thermometer in the chemical process industries is the thermocouple. Its popularity is due to its simplicity, low cost, swift response, and reliability over virtually the whole range of temperatures of industrial interest (from -300 to 3100°F).

The thermocouple is a heat engine consisting of two wires of dissimilar materials joined at their ends . If the connections at the two ends are at different temperatures, a current will flow in the circuit commensurate with the net beat exchange at the Junctions.

Industrial thermocouples should be characterized by large thermal emf for easy measurement: by a linear relation between temperature difference and emf for easy recording; by resistance to corrosive attack for long life, etc.

There are four couples in common use. The noble-metal couples resist corrosion and oxidation best, and they are excellently reproducible, but their sensitivity is relatively low. Iron-constantan couples are probably the most widely used.

For increased sensitivity, a number of thermocouples may be connected in series. The resulting element is called a multijunction thermocouple, or a thermopile, end the signal is multiplied by the number of couples so connected.

Thermocouple outputs are detected by a deflectional instrument (millivoltmeter) or by a balancing instrument (potentiometer). The former is simple and inexpensive, but it is subject to error because it requires that a current flow in the circuit. With the balancing instrument, the system constitutes в null circuit, i.e., no current flow, and consequently, the various resistances in the thermocouple circuit cannot distort the emf generated by the couple.

Measuring. State of Aggregation

The melting points of various solids afford a means of measuring temperatures. In ceramics-manufacture, for example.

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relatively high temperatures are indicated by the fusion of Seger cones. Marking crayons are available which indicate the approximate temperature of surfaces when they ■• are rubbed on the surfaces.

Although this class of methods of measuring temperature is important industrially, it is not useful for automatic control.

Commentary

1. Its popularity is due to ее /термопары/обусловлено .. .

2. Seger cone - конус Зегера

3. emf = electromotive force

широкое распространение І

электродвижущая сила

Words and word combinations

1. thermocouple - термопара

2. response - отклик, реакция

3. range - ряд, диапазон

4. wire - провод, проволока

5. junction - стык, место соединения

6. recording - запись, регистрация

7. sensitivity - чувствительность

8. thermopile - термоэлемент, термоэлектрическая батарея '

9. output - зд.: результаты измерения '

10. to detect - обнаруживать, регистрировать

11. deflectional instrument - измерительный прибор с отклоняющейся стрелкой

12. error - ошибка

13. current - ток

14. circuit - цепь /эл./

15. to distort - искажать

16. fusion - плавление 17. crayon - цветной мелок

Answer the following questions :

1. What thermometer is widely used in the chemical

process

industries? 2. Why is the thermocouple so popular?

3, What does a' thermocouple consist of? How does it

operate? 4. By what should industrial thermocouples be

characterized?

5. What is a thermopile?

6. By what instruments are thermocouple outputs detected?

7. How are high temperatures indicated in ceramics manufacture?

Exercises:

I. Translate the following sentences paying attention to the meaning of the word "but":

1. Peace means life, happiness and progress, but war brings nothing but death and ruin.

2. One cannot but mention here that the first work on electricity published in Russia was written by Lomonosov.

3. When we came to the station the train had left but a few minutes before.

4. Dickens was but a child of eight when he began to earn hie living.

5. But for the air, no life on earth could have developed. -

6. He came last-but one.

7. There is but one way for solving the problem but it will take too much time.

8. The engineer saw that all the devices but one were opera- ting in the proper way.

II. Translate the following sentences paying attention to the meaning of "due to" :

1. Due to Russian scientist D.I.Mendeleyev elements have been arranged in the order of their atomic weights.

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2. 12

   13

   14

   The change of colour of this mixture is due to heating.

3. Due to its greater energy content ozone is more reactive than oxygen.

4. The popularity of an thermocouple is due to its simplicity low cost and swift-response.

III. Translate the sentences. Mind the functions of the; Infinitives:

To separate iron from sulphur is an easy task. To understand this phenomenon well one must know the structure of the material.

The earth is known to have a magnetic field. 4. Experiments proved carbon to occur in many compounds.

Crystalline silicon which is said to have a structure resembling that of, diamond is very bard. To produce artificial diamonds from carbon high pressure and high temperature are required. Gold was probably one of the first metals to attract the attention of man.

S.V.Lebedev was the first to study synthetic rubber like compounds.

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This compound seams to possess some valuable properties» , It is often necessary for the chemist to measure

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the volume of liquid to be used in his work.

The instrument to be described here was developed

several years ago.

Engineers consider thermocouple to be the most

widely used thermometer in the chemical process

industries.

The new power station is reported to have gone into

operation.

All progressive mankind wants atomic energy to be

used in peaceful purposes only.

Supplementary Reading

MATERIALS SELECTION FOR CORROSION CONTROL

From a purely technical standpoint, an obvious answer to corrosion problems is to use more-resistant materials» In many cases, this approach is an economical alternative to other corrosion-control methods.

Corrosion resistance is not the only property to be con-sidered in making material selections but it is of major importance in the chemical process industries. Eventual choice of a material is the result of several compromises. For exam-pie, the technical appraisal of an alloy will generally be a compromise between corrosion resistance and some other proper-ties ouch as strength and weldability. And the final selection will be a compromise between technical competence and economic factors.

For now, we shall discuss in general terms the various metals and alloys used in the chemical process industries.

Iron and Steel. Iron and steel, the most commonly used metals, corrode in many media including most outdoor atmosphere . Usually they are selected not for their corrosion resis-tance but for such properties as strength, ease, of fabrication and cost.

In some circumstances, the addition of 0,3% copper to carbon steel can reduce the rate of rusting by one-quarter or even by one-half. The elements copper, phosphorus, chromium and nickel improve resistance to atmospheric corrosion. l?or~ mation of a dense, tightly adhering rust scale is a factor in lowering the rate of attack.

Stainless Steels. Stainless steels always contain about 10 to 30% chromium, and may also have nickel, molybdentim and copper added for increased corrosion resistance (and for other reasons). The higher the chromium content, the more re-sistant the steel is to oxidizing media and high-temperature

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oxidation. Nickel (up to 35%), copper (2 to 3%), and molybdenum (1 to 4%) are added to improve resistance in less-oxidizing media. In particular," these elements increase resistance to sulfuric and a variety of organic acids.

There are more than 60 types of stainless steel. Many posses similar corrosion resistance but are designed for some other specific property (e.g. high strength), or fabrication characteristic (e.g., machinability).

Nickel and Alloys. Nickel and its alloys have good re-sistaoce to many of the chloride-bearing and reducing media that attack stainless steels. The resistance of nickel to reducing media is further enhanced by molybdenum and copper» Alloy В (Ni - 27Mo) is resistant to hydrochloric acid. Alloy 400 (Ni - 30Cu) is widely used in natural waters and in heat-exchanger applications.

Titanium. Following its commercial introduction in the 1950s, titanium has become an established corrosion-resistant material. In the chemical industry, the grade most used is commercial-purity titanium. Like the stainless steels, it is dependent upon an oxide film for its corrosion resistance. Therefore,it performs best in oxidizing media such as hot nitric acid. The oxide film formed on titanium is more protective than that on stainless steel, and it often performs well in media that cause pitting and crevice corrosion in the latter (e.g., seawater, wet chlorine, organic chlorides).

Copper Alloys. There are 300 or more commercial copper alloys. Zinc is added to copper in amounts ranging from about 5 to 45%. As a general rule, corrosion resistance decreases as zinc content increases. It is customary to distinguish between those alloys containing leas than 15% zinc (better corrosion resistance), and those with higher amounts.

The cupro-nickels (10 to 30% Ni) are the most corrosion-resistant copper alloys. They are used in a variety of water applications, heat exchangers, etc.

Aluminium. As a general rule, the alloys of aluminium (particularly the Al-Cu 2000 series) can be less corrosian resistant than the commercial-purity metal. Alloy additions are made primarily to increase strength. Corrosion resistance of alumunium is dependent upon a 'protective oxide film. This film is stable in aqueous media. Aluminium is used in high-purity-water systems, and to hold and transfer a variety of organic solutions.

Other Materials. Lead has been used for thousands of years as a pipe material for water. It also resists some in-organic acids, such as sulfurous, chromic, phosphoric, cold hydro-fluoric and sulfuric acids. Of the refractory metals, tantalum has the widest use in the chemical process industries. Most applications involve acid solutions that cannot be handled with iron or nickel-base alloys. Zirconium and its al-loys are used in nuclear applications that require good resis-bunco to high-temperature water and steam.

2. FANS AND BLOWERS

Few pieces of equipment have as wide a range of application in the chemical process industries as do fans and blowers. Considering that they have such diverse uses as exhausting or introducing air or other gases into process reactors, dryers, cooling towers; assisying combustion in furnaces; conveying pneumatically; or simply ventilating for safety and comfort, these machines can well be regarded as basic pieces of equipment.

In the last few years, fan-assisted, air-cooled heat ex-changers also have made considerable inroads into the CM, as engineers have sought to solve thermal water-pollution problems.

Because of an increasing demand for smaller, more-reliable fans and blowers, and due to the new impetus on occupational health and Safety, these machines are now receiving in-

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creasing attention. At the same time that user requirements have forced manufacturers to build fans for higher pressures -with resulting higher speeds-environmental considerations have pressed for lower noise levels and shorter noise-exposure times.

Because fan manufacturers are supplying machines at higher compression ratios and at lower and higher flow-rates than ever before, an in-depth engineering evaluation of fans or blowers may be justified before selecting one or the other. For this, a basic knowledge of what the various types of fans and blowers can and cannot do is essential.

Classification of Fans and Blowers. The word fan is or­dinarily used to describe machines with pressure rises up to about 2 psig. Between this pressure and approximately 10 psig, the name applied to the machine is blower. For higher discharge pressures, the term used is compressor.

Fans are normally classified as axial (where air or gas moves parallel to the axis of rotation) or centrifugal (air or gas moves perpendicular to the axis). There are two general categories of axial-flow (AF) fans: tube-axial and vane-axial.

AF units are usually considered for low-resistance applications because of their ability to move large quantities of air at low pressure.

Centrifugal-flow (CF) fans are used for jobs requiring a greater head, where moving air encounters high frictional resistance. CF fans are classified by blade configurations radial, forward-curved, backward-curved or inclined, and airfoil.

Blowers are generally single-stage, high-speed machines, or multi-stage units that operate at pressures close to, or in the range of, compressors. The term blower is also applied to rotary (positive-displacement) compressors that can handle relatively low flows at high compression ratios.'

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3. FILTRATION - EQUIPMENT DESIGN

There is an endless variety of filters - constantly be- ing modified - the majority Of which fall under the general classifscations of batcn filtration under pressure, or conti-nuous vcuum filtration.

Continuous filters can be devided into rotary-drum, disk, table, and horizontal-bolt. Drum flitres are further divided according to the method used for cake removal. Almost invari-ibiy, continuous filters use vacuum, and are best suited for materials that permit a reasonably font rate of cake-thick-buildup. With the exception of ptecoat filters, they do not ordinarily produce absolutely clear filters.

Continuous filters work best on medium-sized particles in the range of 5 - 50 m. Since slurries consisting of such particles usually settle reasonably well under gravity, they are frequently thickened. The larger particles generally encountered exert minor capillary force; therefore, cake drying can be accomplished by sucking air through the cakes under vacuum.

Batch filters consist principally of tank-types filled with leaves, or presses with plates and frames. They are further divided according to horizontal or vertical positioning of the filter surface. Methods for discharging cakes vary, and further affect classification.

In batch filters, cakes are normally built under pressure. If filtration resistance is large, the particles will be email and drying cannot be done by blowing or sucking air through the cake, as is commonly done with continuous filters. Moisture reduction is usually performed by . increasing the pressure, or by squeezing mechanically.

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4. CHEMICAL PUMPS

[Designing With Special Materials)

As described earlier, a number of, low-mechanical-strength, materials have been used extensively in chemical-pump const-ruction. While breakage problems are inherently associated with these materials, their excellent corrosion resistance has allowed them to remain competitive with higher-strength alloys. Of course, their low tensile strength and brittleneas makes them sensitive to tensile or bending stresses, requiring special pump designs. Che parts are held together by outside clamping means, and braced to prevent bending. The unit must also be protected from sudden temperature changes and from mechanical impact from outside sources.

Although produced by very few manufacturers, high-silicon iron is the most universally corrosion-resistant metallic material' available at an economic price. This resistance, coup-led with a hardness of approximately Brinell 520, provides an excellent material for handling abrasive chemical slurries. The material's hardness, however, precludes normal machining operations, and the parts must be designed for machine grinding. The hardness also eliminates the possibility of using drilled or tapped holes for connecting piping to the pump parts. Therefore, special designs are required for process piping, stuffing-box lubrication, and drain connections.

Ceramics and glass are similar to high-silicon iron in regard to hardness, brittleness and susceptibility to thermal or mechanical shock. Pump designs must, therefore, incorporate the same special considerations.

Glass linings or coatings on iron or steel parts are sometimes used to eliminate some of the undesirable characteristics of solid glass. While this usage provides for connecting process piping, the dissimilar expansion characteristics of the two materials generate small cracks in the glass, al-lowing corrosive attack.

Thermosetting and thermoplastic materials are used ex­tensively in "services where chlorides are present. Their pri-

mary disadvantage is loss in strength, at higher pumping temperatures. Phenolic and epoxy parts are subject to gradual loss of dimensional integrity because of the material's creep characteristics. The low tensile strength of the unfilled re-sins again dictates a design that will place these parts in compression, and eliminate bending stresses.

Polytetrafluoroethylene and hexafluoropropylene possess excellent corrosion resistance. These resins have been used for gaskets, packing, mechanical-seal parts, and flexible-piping connectors. Several pumps made of these materials have reached the market in recent years. Problems associated have centered around these materials tendency to cold flow under pressure, and their high coefficient of expansion compared to the metallic components of the unit. Pumps may be made of heavy solid sections, or may use more-conventional metallic components lined with the fluoro-carbon material.

5. IMPRERVIOUS GRAPHITE FOR PROCESS EQUIPMENT

Nature of Graphite. Just what is graphite and how is it distinguished from carbon; Actually graphite is carbons it la one of the three crystalline forms of carbon foij^nd in nature. (The other two are diamond and charcoal). Graphite is hexagonal crystal structure, whereas diamond is. cubic, and charco-al is amorphous. The amorphous form is the most common, and the term carbon generally is applied to this form; examples are coal, coke and lampblack.

Carbon and graphite have distinct, and in some cases diametrically opposed, phisical properties. The reason for this variation of properties Is understandable if one traces the atomic ordering process by which carbon transforms to graphi-te.

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How Graphite Is Manufactured. Although graphite is naturally occurring mineral mined commercially, almost all in-dustrlal graphite is manufactured, and is properly called electrographite. It is made from two raw materials, petroleum coke and coal-tar pitch. The coke is calcined at 1,300°C (2,400°F) to drive off volatiles; it is then crushed, combined with coal-tar pitch, and the resulting mix is shaped by extrusion or molding* The shape is slowly baked at 750 to 1,000°C (1,382 to 1,832°F) for 20 to 40 days to pyrolisia the coal-tar binder and produce a rigid porous composite of coke particles bonded by carbonized pitch. The structure, comprises randomly-oriented hexagonal rings of carbon atoms in a disor-dared carbon matrix.

The product is then converted to graphite by subjecting it to temperatures of 2,500°C (4,500°F) for 10 to 20 days. The high temperature purifies the carbon and causes the' crystallites to grow and orient themselves in parallel planes, producing the three-dimensional stacking order. This transfor-mation from randomly-oriented amorphous,, carbon to an ordered graphite structure results In major changes' in the physical and chemical properties of the material.

Typical Properties. Graphite comprises about 85 to 90% of the weight of impervious graphite; the balance is the re--sin. Properties of the impervious graphite are essentially those of graphite although, in some cases, the resin does exert a significant influence. As an example, the mechanical properties of graphite are beneficially affected by the presence of the resin. In fact, strength is almost doubled.

Thermal conductivity is unaffected by the presence of a resin impregnant. In other words, impervious graphite possesses the same outstanding thermal conductivity as graphite, and this property makes it ideal for heat-transfer equipment. It is interesting to note the wide variation between car-bon and graphite . This bears out an earlier statement regar-ding the influence of atomic arrangement on the property dif-ferences between ,amorphous carbon and its crystalline forms.

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