Eliminate air steam system to ensure effective heating and thus fuel savings. Air can be between 1500 and 3000 times more resistant to heat flow than steel, and 8000 to 16000 more resistant than copper.
A film of air only 0.025 mm thick may resist as much heat transfer as a wall of copper 400 mm thick! Of course all of these comparisons depend on the temperature profiles across each layer.
During indirect heat exchange, metal walls are not the only barriers to heat transfer. There is also likely to be a film of air, condensate and scale on the steam side. Proper cleaning on the steam side can eliminate or minimize scales. But air and condensate films are of greater concern.
Air enters the system when a plant starts after a halt or after a batch. It can be removed via air eliminators thereby ensuring effective heating and savings in fuel.
The above graph represents the temperature gradient across a steam pipe carrying steam at 3 bar g with saturation temperature of 145°C. In between the steam and product are a 0.2 mm air film , 1 mm condensate film and 6 mm stainless steel pipe wall.
As seen in the illustration a drop of 40°C occurs across an air film of just 0.2mm thickness.