Selected Answers

From William Slama of International Paint/Ceilcote Products on August 14, 2014:
The term “cold wall effect” is used frequently to ...read more The term “cold wall effect” is used frequently to refer to an important challenge to linings and coatings used to protect steel surfaces (tanks, piping) that are exposed to aqueous solutions at elevated temperatures, generally above 100  F. The following is a brief explanation of this phenomenon to better understand it and the implications on correct choice of a protective lining or coating for a given application. Basics: In addition to being resistant to aggressive chemicals, immersion linings in aqueous solutions must also resist the permeation of water vapor, especially important at temperatures above 100  F. The polymer components of linings and coatings are non-porous but are all measurably permeable to water vapor. In the past, before highly permeation-resistant linings became standard to protect against corrosive solutions at elevated temperatures, one method included use of relative thin coatings combined with thermal insulation on the exterior of the tank or pipe. However, experience showed that wall areas that had metal brackets or stiffening members attached to the exterior were subject to premature failure with loss of adhesion and blistering. Realizing that this problem was caused by the external heat transfer lowering the steel wall temperature, it became common to simply refer to it as the cold wall effect. The term is now used for the permeation vapor pressure differential on any elevated temperature uninsulated vessel. Heat Transfer: We know that thermal energy (heat) can be transferred from one body to another by several methods – conduction (through materials or by direct contact), convection (air currents moving past a warmer or colder surface), and radiation (from hot elements to cooler surrounding surfaces). Radiation is significant at very high temperatures, but for simplicity, will be ignored in this discussion. So, in the case of a vertical steel (or other metal) tank surface, wherein the (aqueous) content is significantly hotter than the surrounding exterior air, the interior heat “wants” to transfer to the cooler ambient air. Somewhat simplified, the heat (transfer) path is first through the interior tank lining, then through the steel wall (both by conduction), then to the air by convective cooling. The conduction rate and resulting temperature drop through the lining and steel wall are directly related to the thermal conductivity factor of each material. This is very basic, just like the passage of electricity through resistors. Convective cooling is a bit more complex because it depends on the velocity of the air adjacent to the tank exterior. In the absence of forced external velocity (such as wind), a natural air velocity pattern develops due to the difference in density of hotter air adjacent to the steel wall. That mechanism carries the heat away from the steel wall. PREVENTION: There are many coatings and linings that are designed to perform even with the “cold wall  effect.” For coatings and linings that require insulation, care should be taken to insulate external stiffeners or other metallic attachments to the shell that could transmit heat away from the wall.