Feb 10

Introduction to Flat Plate Collectors:

Flat-plate collectors are a very helpful device for low to medium temperature heat collection from the sun. They can be used for a lot of uses including a variety of thermal desalting process from low to medium capacities. Flat-plate collectors enclose simple characteristics: they are simply assembled, and effortlessly operated.

01-Basic flat plate Collector - Parts of Flat Plate Collector

The plan of any system that shifts solar radiant power into thermal energy is the sensible application of the thermal energy itself. Solar Thermal Energy application can be able to direct as heat or indirect by using the heat to force a heat engine to produce useful mechanical energy or to use the thermal energy for electrical energy production. For this cause unique devices are used called solar thermal collectors. The principle of a solar thermal collector is to absorb the radiant energy of the sun and to convey the significant heat to a fluid which in turn relocates the thermal energy to the place of relevance. In familiar, solar collectors are classified according to their thermal output temperature which establishes also the field of their application.

01-flat plate collector parts - flat plate air collectors - flat plate liquid collectors

 

For solar applications to be suitable, this transport headed for a lower temperature stage is important. While high-priced high concentrating solar systems are essential to generate quantities of steam or oil with temperatures of more than a few hundred degree Celsius, temperatures below 150°C can be created by high efficiency standard collectors without or with just low concentration of the solar radiation.

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Flat-plate collectors are planned for applications require low to moderate temperatures in general up to 110°C higher than ambient temperatures. The simplest flat plate collectors are the solar ponds and the solar stills which function by direct utilization of the incident solar radiation acting simultaneously as solar energy converters. The importance of flat-plate collectors is that their thermal performance.

Solar Collector Applications

Solar collectors are very popular for setting up in a region where there are high irradiation intensities and clear sunshine days. They can be installed simply in individual houses for domestic hot water, in hotel buildings and in municipal installations, such as athletic centres for hot water use and for heating swimming pools.

Huge mercantile installations with high capacity are used in industry to produce process water or, in solar desalination plants to produce moderate temperature water, as feed to low or medium temperature distillation plants. In general they are useful tools for hot water production.

01-liquid flat plate collectors - flat plate collector applications - solar water heating

The higher temperatures required which will be achieved by the minimisation of the collector U-value (Heat loss coefficient). It is in the nature of the things that this minimisation of the U-value which increases the stagnation temperature. Solar Thermal Collectors which are used in thermal process heat applications (Range: up to 150°C), must reach stagnation temperatures greater than 300°C. So all the parts of the thermal collector such as selective absorber coatings for the black absorber medium, insulation materials for the backside of the absorber have to be able to endure high temperatures.

In addition to thermal radiation losses the air inside the collector, consistently at atmospheric pressure, is transferring energy primarily by convection as well as by conduction. Hence the collectors itself have to be insulated against the surroundings. Likewise the Back side of the collector also insulated with a variety of temperature and humidity resistant thick materials offered on the market, the front of the collector is more awkward since it is out in the open to solar radiation. Transparent insulation materials in addition high transmittance for solar radiation with low heat conductance are required to achieve more temperature.

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Dec 13

Defects in Welding

1. Porosity: Gas porosity get to be visible as round or stretched insubstantial shady spots, occurring separately or in bunches or scattered all through the casting. This is caused by gas formation during solidification by evaporation of moisture or unstable material from the mold surface. Insufficient core baking, venting or capture of air in the cope surface of the casting before complete solidification could also be the reason. The term ‘gas porosity’ is used to refer to shady dark spots on the radiograph, whose diameters are normally inside 0 to 1 mm.

01-NDT Gas porosity in Welding - Welding defects

01-NDT Gas porosity in Welding - Welding defects-porous holes

2. Slag inclusion and slag lines show up on the radiograph as dark, uneven shapes. These may be isolated, clustered or randomly distributed. A slag line appears as a linear dark shade, continuous or intermittent along the edge of the weld. Slag inclusions happen because of entrapment of foreign materials in the cavities during welding.

01-NDT testing - slag inclusion

3. Incomplete penetration appears on the radiograph as constant or discontinuous dark lines, typically of uniform width, happening in the middle of the weld. This is caused by the failure of the weld metal to fill the root gap.

01-NDT testing - lack of fusion

4. Lack of fusion appears on the radiograph as a thick, dark line. It may also show as diffused and wavy, depending upon the defect introduction with respect to the radiation beam. This flaw is caused by the failure of the weld metal to ruse with the parent metal or previously deposited weld metal. In case of lack of sidewall fusion, the radiographic image illustrates the appearance of a narrow, dark band parallel to the weld bead.

01-NDT Testing - lack of fusion - Welding defects

5. Cracks appear on the radiograph as sharp, straight or jagged dark lines with tapered ends. Cracks may appear in longitudinal or transverse directions. Cracks may occur in the weld and heat affected zones. The effect is caused by the rupture of the weld metal during solidification due to shrinkage or by fracture when cold, because of uneven stresses and poor handling.

01-NDT Testing - cracks in weld - rupture in the weld

6. Undercuts appear on the radiograph as dark lines of uneven width along the edge of the weld. This defect is caused by the formation of a groove or a channel on the surface of the base metal at the toe of the weld bead due to high temperature.

01-NDT Testing - undercuts - weld bead

7. Burn through appears on the radiograph as a dark, round or elongated area surrounded by a lighter ring. This is caused by the melting of metal from the root of the weld or through the backing strip.

8. Icicles appear on the radiograph as secluded, white, rounded indications, irregularly with a small, dark spot in the center. This is caused by fused droplets of weld metal extending beyond the root of the weld.

01-NDT Testing - isicles- fused droplets

9. Tungsten inclusions appear on the radiograph as white areas of round or uneven shape, either segregated or in bunch. This is acquired by the entrapment of tungsten particles in the weld metal. These particles are broken pieces from the tungsten electrode.

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