THE VARIOUS RESISTANT FILMS AFFECTING HEAT TRANSFER
The steam system processes are affected by the presence of air, condensate, dust, scale, etc. Heat transfer from steam to secondary fluid is impacted due to the intermediate layer of other fluids. The layers which restrict the rate of heat transfer are termed as ‘Resistant films’. The metal wall consists of resistant films on both sides. As steam condenses it gives off the latent heat of condensation. The resultant if not drained immediately acts as a resistant layer for heat transfer. The other resistant layers are due to air film, scale film, or burnt products. These films reduce the rate of heat transfer. The material of metal also acts as a barrier and temperature drop is across the metal wall is a function of the conductivity of the material and the wall thickness.
The various resistant films are discussed below:
1. Stagnant Film
This film is referred to as fluids with bulk mass and normally having high viscosity. When this product is heated by a steam coil, the adjacent layer is heated. The high viscous fluid cannot flow quickly and gets hotter and hotter. This acts as stagnant film and resistance to heat flow for the subsequent layer. An example of this layer can be tar.
Some fluids form a burnt film on the metal wall when heated, this film should be removed for efficient heat transfer. Eg. a Confectionary or food industry that heats milk in kettles. If high temperature/ pressure steam is supplied a layer of milk in contact with the surface is burnt out and a burnt scale deposits and forms a lining on the inner surface of the kettle.
Heat transfer is enhanced by the circulation of the fluid and the increase of turbulence. The position of the heating coils also decides the rate of heat transfer. Heating coils are preferred to be in a vertical position than that of a horizontal at the bottom for achieving uniform heating.
2. Scale Film
Scales are formed due to the presence of impurities in water or by the calcium and magnesium salts. This film of scale impacts the heat transfer. Accumulation of scale on the metal surface causes a steady fall in performance. It can also cause hot spots which can lead to local heating of the metal surface. Salts of calcium, magnesium, and silica have a low range of conductivity.
The thermal conductivities for some salts are given below:
CaSO4 1-2 kcal/m2.h per degree C (0.20-0.41 Btu/hr.ft2/F)CaCO3 0.5-1 kcal/m2.h per degree C (0.10-0.20 Btu/hr.ft2/F)
SiO2 0.2-0.5 kcal/m2.h per degree C (0.04-0.10 Btu/hr.ft2/F )
Fig. Scaling of boiler tubes
3. Condensate Film
When steam gives its specific enthalpy of evaporation condensate is formed. Condensate (water) has a modest rate of heat transfer and hence as the thickness of the condensate layer increases the heat transfer reduces.
Based on the method of steam flow control and condensate evacuation, condensation can occur in the following two ways:
a) Film condensation- The surface in this type is blanketed by a ‘liquid wall’ of increasing thickness.
b) Drop condensation- the condensation droplets slide down when they reach a particular size. When the droplets slide down the surface becomes clear and is exposed to vapor. The rate of heat transfer is about 10 times greater in dropwise than film condensation.
4. Quality (Dryness Fraction) of steam
Also, check should be kept that the steam reaching the end-use is dry-saturated. The quality of steam used in the process is of utmost importance, Presence of water droplets in steam affects the heat transfer. The use of pressure-reducing valves improves the dryness fraction of steam. A moisture separator should be used upstream of the process equipment to drain moisture carried in steam. Suitable drum level control along with right water chemistry will avoid priming and foaming in boiler drum and hence minimum carryover.
5. Air film
Air is a bad conductor of heat, when air is mixed with steam the heat flow to the process is reduced. The thermal conductivity of air is 0.025 W/m0C (). Air for all practical applications acts as a thermal insulator.
The above table provides a relative comparison of thermal conductive w.r.t air.
Resistance by an air film of 0.025mm (0.0009inch) thickness is equivalent to the resistance of copper plate of 320mm (12.5 inches) thickness.
Steam enters the steam space in various ways. When the plant is shut off the steam is condensed which leads to a partial vacuum, this space is filled by air entering through the valves, valve glands, and pipe joints. When the steam is again turned on the steam space is full of air. Air is also present in boiler water and is carried along with steam from the boiler drum to process equipment along with other non-condensable gases. This air forms an insulating layer in the heat exchange area and should be removed from boiler feedwater and dead ends in steam distribution.
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