Heat energy is transferred on the virtue of temperature difference. So, when there are two bodies, with different temperatures in contact with each other, heat transfer will take place from body at higher temperature to the body at lower temperature until the temperatures of both the bodies become the same.
All the heating processes are based on this fundamental concept of thermodynamics. In a typical heating process, steam loses its latent heat, which is then transferred to the object being heated by the virtue of temperature difference. Heat transfer can take place in two ways, direct and indirect. In direct heating processes, the hot liquid (steam in our case) is mixed with the cold liquid and as a result, temperature of the mixture increases.
Such processes require that steam being used is of such a grade that it can be safely mixed with the product. In many cases, the other way of heat transfer, i.e. indirect heat transfer is used. In indirect heat transfer, the hot and cold fluids are not allowed to mix with each other, but they are separated by conducting material and heat transfer takes place through this conducting separating medium.
When we are using steam as a medium of heat transfer,
we can find some processes where steam is used for direct heating
and also many processes where it is used for indirect heating.
In case of indirect heating, the jacket through which steam is
passed is the medium through which the indirect heating takes
place. In such cases, amount of heat being transferred is
governed by some basic thermodynamic equations. Let us take
an example of a steam carrying pipe, which will have a
cylindrical profile. The thermal resistance offered by this pipe
can be given by the formula-
From the equation mentioned above, it is clear that thermal resistance that will be offered by the pipe is inversely proportional to the thermal conductivity of the pipe material. In an ideal situation, there is only a metal wall in between the steam and the pipe and hence heat transfer depends solely on the conductivity of metal which is used for making the tubes. In reality, there exist many layers on both steam side and the fluid side which affect the heat transfer taking place.
There is likely to be a film of air, condensate and scale on the steam side. 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.
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 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.
Though not as severely as air does, condensate also affects the heat transfer taking place. Condensate is formed as soon as steam loses its latent heat. If this condensate is not removed out of the tube as soon as it is formed, it can affect the heat transfer. As a result of this, it is essential that all the traps are working fine and no water logging takes place.
In short, to achieve efficient heat transfer from steam to the product, proper care should be taken to ensure that condensate is removed as soon as it is formed, air is eliminated via air eliminator and scales are monitored and removed periodically