Insulation practices

Figure 31.5(a) Typical insulating practice to isolate enclosure of a discontinuous IPB system (Courtesy Best Crompton)... 

Insulating materials and their properties Ageing of insulation Practices of insulation systems Procedure for vacuum pressure impregnation Maintenance of insulation Monitoring the quality of insulation of HT formed coils during manufacturing... 

The optimum thickness of insulation can be established by economic analysis when all of the cost data are available, but in practice a rather limited range of thicknesses is employed. Table 8.22 of piping insulation practice in one instance is an example. 

InaU cases, the induced recirculating downward mass flow has been discovered to carry heat down towards the liquid/vapour interface. In flie case of good insulation practice, this residual convective heat flow contributes to the boil-off, and may become the major source of heat entering the liquid. (The term residual is used to distinguish this heat flow from all other sources.)... 

With a batch process, such as hot isostatic compaction (HIP), heat exchange as used in a continuous reactor is not possible, and it is common practice to provide a furnace within the pressure vessel which is thermally insulated to ensure that the temperature of the vessel does not rise above 300°C. Most HIP operations involve gas pressures in the range 70—200 MPa (10—29,000 psi) and temperatures of 1250—2000°C, occasionally 2250°C (74). The pressure vessel may have a bore diameter from 27 to 1524 mm (75) and is nearly always provided with threaded closures sealed with O-rings made of elastomer provided the temperature is low enough. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

F. Clark, Insulating Materials for Design and Engineering Practice, John Wiley Sons, Inc., New York, 1962. 

R. V. Latham, ed.. High Voltage Vacuum Insulation Basic Concepts and Technological Practice, Academic Press, Inc., New York, 1995. 

Loose asbestos fibers, or formulations containing asbestos fibers for spray coatings, have been widely used in the building industry for fire protection and heat or sound insulation. Such applications used mainly chrysotile or amosite but, because of health concerns, this practice has been discontinued. 

The low melting point of caprolactam and its stabiUty and low viscosity form the basis for commercial transportation practice caprolactam is handled as a Hquid in insulated tank cats or tmcks. 

The liquefied gas must be maintained at or below its boiling point. Refrigeration can be used, but the usual practice is to cool by evaporation. The quantity of liquid evaporated is minimized by insulation. The vapor may be vented to the atmosphere (wasteful), it may be compressed and reliquefied, or it may be used. 

The effective therm conductivity values generally obtained in practice are at least a factor of two greater than the one-dimensional thermal conductivity values measured in the laboratoiy with carefully controlled techniques. This degradation in insulation thermal performance is caused by the combined presence of edge exposure to isothermal boundaries, gaps, joints, or penetrations in the insulation blanket required for structure supports, fill and vent hnes, and high lateral thermal conductivity of these insulation systems. 

The choice of materials from which to make the holder is important. Materials must be durable enough to ensure satisfactory completion of the test. It is good practice to select very resistant materials for the test assembly. Insulating materials used are plastics, porcelain, Teflon, and glass. A phenohc plastic answers most purposes its principal hmitations are unsuitability for use at temperatures over I50°C (300°F) and lack of adequate resistance to concentrated alkalies. 

Interna] Insulation The practice of insulating within the vessel (as opposed to applying insulating materials on the equipment exterior) is accomplished by the use of fiber blankets and hghtweight aggregates in ceramic cements. Such construction frequently incorporates a thin, high-alloy shroud (with slip joints to allow for thermal expansion) to protect the ceramic from erosion. In many cases this design is more economical than externally insulated equipment because it allows use of less expensive lower-alloy structural materials. 

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