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23-12-2010, 01:13 PM
Prepared by::Shashikumar C.M
10TH12F-cryo insulation.pptx (Size: 3.78 MB / Downloads: 118)
The major concern in the selection of the proper insulation for a cryogenic system is the reduction in the heat leak.
The insulation strategy is to minimize the radiative heat transfer, minimize the convective heat transfer, and introduce a minimum of solid conductance media
MODES OF HEAT TRANSFER
Gas convection and conduction
FACTORS CONSIDERED IN THE SELECTION OF THE INSULATION
Thermal Effectiveness ,Cost
Ease Of Fabrication
TYPES OF INSULATION
Foam and fiber insulation
Multi layer insulation
The use of floating shields within the vacuum space between the warm and cold surfaces of a storage dewar or cryogenic engineering container involves one modification in this type of insulation system that has been considered
Another improvement in vacuum insulation has been the search for specialized polymers or paints that can be used to reduce the emissivity that exists between the warm and cold surfaces of these cryogenic systems.
The primary characterization of most powder insulations includes lowthermal conductivity,low density, and particle size distribution to reduce shock and vibrationeffects.
Powders have been used either in an evacuated or nonevacuated mode.
In most cases, the small particle size of the powder limits the gaseous heat transfer in evacuated powders to free molecular conduction.
Often, the heat transfer by the residual gas is further reduced by the lower vapor pressure of the powder itself.
The powders widely used at cryogenic temperatures include
colloidal silica and
Other powders occasionally used are
Plastic Or Glass Microspheres.
Evacuated Powders and Fibrous Insulations
To decrease the thermal conductivity of nonevacuated powders and fibrous insulations is to reduce the residual gas pressure in the insulation space.
The effect of this reduction in gas pressure on the apparent thermal conductivity of several powder insulations
Cryogenic insulation systems that employ glass microspheres in evacuated powder form offer significant advantages over traditional materials for many practical applications
The best insulation material for a cryogenic system is the one that offers the optimal combination of thermal performance, low cost, light weight, durability, and minimal or no maintenance
Cryogenic foam insulations, such as polystyrene and polyurethane, are produced by gaseous expansion of organic and inorganic solids.
The solid–gas mixture creates a low-density solid with many voids.
This relatively lower density, an order of magnitude less than that for powder insulations, results in a smaller heat transfer by solid conduction.
On the other hand, the cellular structure of the foam provides continuous paths by which more heat is conducted through the foam than in the powder insulations, where the inter particle contact areas are very small
Multilayer insulation generally involves 30 to 80 layers of low-emittance radiation shields separated either by low-conductivity spacers or by crinkling or embossing the shields so that, when placed next to each other in the vacuum space, the shields only touch each other at a few discrete locations.
Cryogenic Engineering by Thomas M.Flynn
Insulation Hand book by, Richard T. Bynum, JR
Cryogenic Engineering ,Klaus D. Timmerhaus, Rihard P Reed