Function and Operation of Steam Traps
Removal of condensate and air
In a steam system, there is condensate from vapor and air which merge into steam. These materials have to be discharged out of the system immediately because they cause the following problems and oxidation corrosion of machinery.
Decrease in heat transfer efficiency
Condensate and air form a thin film on the heat transfer surface of the heat exchanger, reducing heat transfer efficiency.
Table 2.1 Examples of the heat conductivity of materials
Table 2.1 shows the approximate heat conductivity of air, condensate, steel, and copper. The larger this rate is, the higher the heat conductivity is, and vice versa. The heat conductivity of copper is eight times that of steel. Heat conductivity is quite different depending on whether the heat-transfer plate is made from copper or from steel, and heat conductivity becomes significantly worse if air and condensate are involved. The heat conductivity of air is 1/2000 and the condensate is only 1/100 respectively of steel. If the thickness of condensate film is 0.1mm, the thickness of steel is increased to 10.0mm (0.1mm ×100). If the thickness of air film is 0.1mm, the thickness of steel increased to 200mm (0.1mm × 2,000). Here, if the thickness of the heat-transfer plate is 10.0mm, the heat conductivity is decreased to 50% because of condensate film, but is reduced to only 5% because of air film. If the heat transfer plate is copper, the increase in thickness is extremely significant: 80 mm for the condensate film and 1,600 mm for the air film. You will see how air and condensate reduce heat transfer efficiency.
Reduction of heating temperature
The presence of air decreases the heating temperature as well as the heat transfer efficiency. This can be easily understood by Dalton’s law. Dalton’s law states that the pressure of a gas that contains two or more different gases is equal to the sum of the partial pressure of each gas.
For instance, when 0.4Mpa steam contains 0.1MPa air, the pressure of the steam itself is 0.3MPa. In this case, you are not actually heating at the 0.4MPa saturated temperature of 144℃, but at the 0.3MPa saturated temperature of 134C or lower. However, the pressure gauge indicates 0.4MPa. Thus, machinery users are unaware of this, which causes product issues.
If condensate remains in the system, the destructive phenomenon called “water hammer” may occur on resumption of the system. Water hammer is the phenomenon in which high-speed steam flows with stagnant condensate, and eventually the amount of condensate increases and resulting in a water mass, which hits the bends of the piping and mounting fixtures such as valves.. It is often accompanied by loud impact sound, which can cause damage or destruction of installed equipment.