insulation is required for both spherical and flat bottomed cylindrical refrigerated tanks. Two types of insulation systems are commonly used for low temperature service — single wall and double wall.
In the single wall system, the vessel wall is designed to withstand the design service conditions of the liquid to be stored. The outer surface of this wall is then covered with a suitable insulating material such as rigid polyurethane foam. An aluminum jacket is then installed to provide protection against the elements and physical damage. It is extremely important that the insulation be sealed with a good vapor barrier to minimize air leakage and thereby reduce the quantity of water that may migrate into the insulation. Such moisture migration can ultimately damage the insulation.
The welded steel plate outer shell of a double wall system provides containment and vapor protection for the insulation material, generally perlite. The outer wall also provides protection against fires at temperatures up to 600°F. Double wall tanks are considered in storing products at temperatures below –28°F. This system minimizes heat leak which generally means lower operating and maintenance costs. As an added safety feature, the outer wall is completely sealed and therefore permits the insulation space to be continually purged with an appropriate inert gas, which keeps the insulation isolated from outside humid air. Figs. 6-15 and 6-17 provide a range of typical thermal conductivities for various types of insulating and tank shell materials.