Ever wonder why your to-go coffee cup is made of shiny stainless steel? It’s not just for sanitary reasons. The low emissivity surface that is provided by a polished stainless cup helps to maintain the temperature of your hot or cold beverage.
Low Emissivity Surfaces
A simple method to help illustrate this is to use a stainless steel bowl. Fill the bowl with hot tap water and cup your hand about a centimeter away from the outside of the bowl. Your hand may be able to detect a small amount of heat, but by moving closer and touching the bowl you will see that it is much hotter than your hand may have detected. This is because of the low thermal emissivity of the shiny stainless steel surface. Polished stainless steel has an emissivity coefficient of about 0.08.
Highly Emissive Surfaces
Now, try the same experiment with a highly emissive container like a Pyrex bowl. Pyrex has an emissivity coefficient of about 0.92. When holding your hand a centimeter away from the Pyrex bowl full of hot tap water you should feel much more radiant heat coming from the bowl. In this simple comparison the low emissivity surface (stainless steel) emits a small amount of thermal radiation (heat) and the highly emissive surface (Pyrex) emits a large amount of heat.
Over time the water in the Pyrex bowl should cool quicker than the water in the stainless bowl because the surface area of the Pyrex bowl is throwing out much more heat from its surface. Other practical uses for the “insulating” quality that a low emissive surface provides would be stainless steel insulated stove pipes, metallic heating ducts and vehicle exhaust systems.