Wednesday 9 March 2011

MATERIAL OF CONSTRUCTION


In the fabrication of Shell and Spiral Coil Heat Exchanger we use following material

·        Acrylic
·        Copper
·        Stainless Steel

Acrylic: -
                   Acrylic is a useful, clear plastic that resembles glass, but has properties that make it superior to glass in many ways. Common brands of high-grade acrylic include Polycast, Lucite and Plexiglass.

There are two basic types of acrylic:

Extruded Acrylic: -
Extruded or "continuous cast" acrylic is made by a less expensive process, is softer, can scratch easier and may contain impurities.

Cell cast: -
Cell cast acrylic is a higher quality acrylic and U.S. domestic cell cast is a good choice for applications that require the best. Imported cell cast acrylic is often manufactured to lesser standards.


Applications: -


Acrylic is used to make various products, such as shell, cylinders, shower doors, bath enclosures, windows and skylights. It is chosen over glass for many reasons. It is many times stronger than glass, making it much more impact resistant and therefore safer. Acrylic also insulates better than glass, potentially saving on heating bills.
Another great advantage of acrylic is that it is only half as heavy as glass. This makes working with acrylic much easier. It can also be sawed, whereas glass must be scored.
Adding to this favorable array of properties, a transparency rate of 93% makes acrylic the clearest material known. Very thick glass will have a green tint, while acrylic remains clear. A unique property of plastic is its ability to be shaped. Bow-front aquariums are beautiful examples of acrylic's wonderful properties. There are also no seams in acrylic structures, as chemical welding at the molecular level actually "melts" seams into one piece of solid material. Seams that are welded and polished are invisible.
There are some misconceptions about acrylic, namely that it yellows, turns brittle and cracks over time. Though this might be true of very cheap forms of plastic, it is not so with acrylic. For example, the fighter planes of WWII have acrylic bubble-tops. Airplane windows are also acrylic. If taken care of, acrylic remains new looking regardless of age or exposure to sun. Some people worry that acrylic s Today acrylic is used more than ever. Virtually all major public aquariums now build display tanks out of acrylic. You will also find acrylic in malls, institutions, prisons, hospitals and commercial buildings. Acrylic just over one inch thick (32mm) is bullet resistant. The Presidential motorcade, the Pope's booth-vehicle, teller enclosures and drive-through window enclosures all feature bullet-resistant acrylic.
If upgrading the windows in your house, remodeling your bathroom, or adding a beautiful aquarium, consider acrylic. It may cost a little more than glass, but its sheer clarity, light weight and insulating properties make it a superior choice for many applications.cratches too easily, but unlike glass, scratches can be easily buffed out of acrylic.




Characteristics : -

Acrylic plastic sheet is produced in several formulations to provide specific physical properties required for various types of applications. Generally speaking, however, the physical characteristics of acrylics are:


Transparency In colorless form acrylic plastic is as transparent as the finest optical glass. Its total white light transmittance is 92%, the highest transmittance physically possible of any material.


Breakage Resistance - Acrylic sheet has from 6 to 17 times greater impact resistance than ordinary glass in thicknesses of .125" to .250". When subjected to blows beyond its resistance, acrylic sheet reduces the hazard of injury because it breaks into large relatively dull edged pieces, which disperse at low velocity, due to the light weight of the material.


Weather Resistance - The many years of actual outdoor exposure of acrylics in a wide variety of applications, proving its weather resistance, cannot be matched by any other transparent plastic material.


Chemical Resistance - Acrylic plastic has excellent resistance to most chemicals, including solutions of inorganic alkalis and acids such as ammonia and sulfuric acid, and aliphatic hydrocarbons such as hexane, octane and VM&P naphtha.
 It is attacked by the following chemicals:
1.Gasoline
2.Chlorinated hydrocarbons such as methylene chloride, a solvent cement widely used, and carbon tetrachloride.
3.Aromatic solvents such as turpentine, benzene, and toluene.
4.Ethyl and methyl alcohol.
5.Organic acids such as acetic acid, phenols, and lysol.
6.Lacquer thinners and other esters, ketones, and ethers.


Light Weight - Acrylic sheet is less than half as heavy as glass: it is 43% as heavy as aluminum and 70% as heavy as magnesium.


Dimensional Stability - Acrylic sheet is notable for its freedom from shrinking and deterioration through long periods of use. Many drawing instruments requiring exact dimensional stability have been fabricated from Plexiglas.


Combustibility - Acrylic sheet is a combustible thermoplastic and should be treated as an ordinary combustible material such as wood. The self ignition temperature (spontaneous combustion) of acrylic is between 850 degree F and 869 degree F. The temperature at which the material will ignite in the presence of a flame is between 550 degree F and 570 degree F. While the ignition temperatures of acrylic are higher than that of most woods, it burns vigorously and generates heat rapidly when involved in fire. The primary products of acrylic combustion are carbon monoxide and carbon dioxide; however, burning acrylic plastic does not produce either excessive quantities of smoke or gasses more toxic than those produced by burning wood or paper. Observe fire precautions appropriate for comparable forms of wood and paper products.


Heat Resistance - The maximum "continuous service temperature" of acrylic is between 180 degrees F and 200 degrees F depending on the particular use. This means that while the material can withstand higher temperatures for very short periods of time, it will soften and lose its form or shape if subjected to these higher temperatures for any period of time. While acrylics respond to heat, they are not affected by cold, and will not become cracked or brittle in cold weather.


Electrical Properties - Acrylic plastics are affected only a minor degree by weathering or moisture. Its surface resistivity is higher than that of most other materials, and makes an ideal insulator.


Thermal Insulation- heat transfer through .187" thick acrylic is approximately 20% less than through equivalent thickness of glass.
(0.187" acrylic = 1.09 BTU's/hr./sq. ft./ degree F with 15 MPH and 0 degree F air on one side and 0 MPH and 70 degree F air on the other. 0.187" Glass = 1.23 BTU's/hr./sq. ft./degree F under the same conditions).


Ease of Fabrication - It can be sawed, drilled, and machined like wood or soft metals. When heated to a pliable state, Plexiglas can be formed to almost any shape.



Disadvantages
For all of its advantages, there are two disadvantages of acrylic: it is more expensive than glass, and if exposed to a direct flame it will melt and eventually burn.


Physical Properties


Density
0.0415-0.043 lb/in3
Water Absorption
.3-2 %
Moisture Absorption at Equilibrium
0.3-0.33 %
Linear Mold Shrinkage
0.003-0.0065 in/in
Melt Flow
0.9-27 g/10 min

Thermal Properties


Heat Capacity
0.349-0.351 BTU/lb-°F
Thermal Conductivity
1.32-1.67 BTU-in/hr-ft²-°F
Melting Point
266°F
Maximum Service Temperature, Air
106-217°F

Copper: -
                   Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with excellent electrical  and thermal conductivity, and finds extensive use as an electrical conductor, heat conductor, as a building material, and as a component of various alloys
Copper is a reddish-coloured metal; it has its characteristic color because of its band structure. In its liquefied state, a pure copper surface without ambient light appears somewhat greenish, a characteristic shared with gold. When liquid copper is in bright ambient light, it retains some of its pinkish luster.
Copper exists as a metallically bonded substance, allowing it to have a wide variety of metallic properties.
Copper has a high electrical and thermal conductivity, second only to silver among pure metals at room temperature.
                                    

Physical Properties
Like gold and silver, copper is malleable. That is, it can be bent and shaped without cracking, when either hot or cold. It can be rolled into sheets as thin as 1/500 of an inch.
Copper also is ductile, that is, it can be drawn out into thin wire. A copper bar 4 inches thick can be heated, rolled, then drawn into a round wire so thin that it is thinner than a human hair. This wire is 20 million times longer than the original bar!
Industry valued copper for these properties. Copper is second only to silver in its ability to conduct electricity, but silver is too expensive for this sort of use. Bronze and brass, however, do not conduct electricity as well as pure copper.
Besides electricity, copper also is an excellent conductor of heat, making it an important metal in cookware, refrigerators, and radiators.
Copper is resistant to corrosion, that is, it will not rust. If the air around it often is damp, it will change from its usual reddish orange color to reddish-brown. Eventually, it is coated with a green film called a "patina" that stops all further corrosion.

 

Mechanical Properties

A single crystal copper consists of a few microns of small crystals. In this form of crystal (c), the yield stress is high and crystal undergoes a large amount of elastic deformation before going into the plastic deformation region. The plastic deformation region has an unpredictable outcome. The stress level decreases significantly as necking begins to occur. Polycrystal copper has many crystal of different geometries combined. The plastic deformation of polycrystal is similar to mild steel. Copper has a high ductility and will continue to elongate as stress is applied. It is very useful in copper wire drawing.
Numerous copper alloys exist, many with important historical and contemporary uses. Speculum metal and bronze are alloys of copper and tin. Brass is an alloy of copper and zinc. Monel metal, also called cupronickel, is an alloy of copper and nickel. While the metal "bronze" usually refers to copper-tin alloys, it also is a generic term for any alloy of copper, such as aluminium bronze, silicon bronze, and manganese bronze.


Corrosion Of Copper: -


Pure water and air: -              Copper is a metal that does not react with water (H2O), but the oxygen of the air will react slowly at room temperature to form a layer of copper oxide on copper metal.Coppr in "pure" water is more noble than hydrogen. As a result it does not corrode in oxygen free water and the corrosion rate in oxygenated water is low. It is important to note that in contrast to the oxidation of iron by wet air that the layer formed by the reaction of air with copper has a protective effect against further corrosion.


Sulfide media: -          Copper metal does react with hydrogen sulfide and sulfide containing solutions. A series of different copper sulfides can form on the surface of the copper metal.It is clear that the copper is now able to corrode even without the need for oxygen as the copper is now less noble than hydrogen. This can be observed in every day life when copper metal surfaces tarnish after exposure to air which contains sulfur compounds.


Ammonia media:-       Copper does react with oxygen-containing ammonia solutions because the ammonia forms water-soluble copper complexes. The formation of these complexes causes the corrosion to become more thermodynamically favored than the corrosion of copper in an identical solution that does not contain the ammonia.


Chloride media: -       Copper does react with a combination of oxygen and hydrochloric acid to form a series of copper chlorides. It is interesting to note that if copper(II) chloride (green/blue) is boiled with copper metal (with little or no oxygen present) then white copper(I) chloride will be formed.


Stainless Steel
Stainless steel is defined as an iron-carbon alloy with a minimum of 10.5% chromium content. The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel .This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type and grade, particularly in the aviation industry. As such, there are now different and easily accessible grades and surface finishes of stainless steel, to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are everyday cutlery and watch straps.
Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments; however, it is important to select the correct type and grade of stainless steel for the particular application.
High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments. The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, which means that the metal remains lustrous. It is, however, impervious to water and air, protecting the metal beneath. Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.
             Nickel also contributes to passivation, as do other less commonly used ingredients such 
               as molybdenum and vanadium.


 
             

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