Overheated Level of Vapors
In general, vapors are transported in a high-temperature and high-pressure state adjusted according to process requirements. To resolve problems, such as occurrence of condensate water caused by reduction in transportation efficiency from the boiler to the point of use and external heat loss in vapor pipelines, vapors need to be at an overheated level. In accordance with process requirements, high-temperature and high-pressure vapors are supplied to processing facilities after lowering the pressure with pressure- reducing valves. Compared with saturated vapors, the pressure-reduced vapors are overheated with higher temperature as shown in the red region of Figure 1. For example, if the 40 barA pressure saturated vapor (saturation temperature 250.3℃ ) is reduced to 5 barA (saturation temperature 158.8℃ ), it becomes 178.1℃ in an overheated vapor state, 19.3℃ higher than the saturation temperature
Characteristics of overheated vapors
‧Higher temperature than saturated vapor
‧Specific volume is larger than saturated vapor
‧Higher energy than saturated vapor's enthalpy
Advantages of overheated vapors
‧Produced generally in a high-pressure state, its specific volume is smaller than low-pressure vapors, so transportation efficiency increases due to reduced pipe diameter.
‧Machinery can be protected against erosion and corrosion caused by condensate water occurring in the transportation of low-overheated-level vapors
‧The heat energy of vapors, i.e. the overheated vapor's high enthalpy, enhances the efficiency of machinery, turbines, etc.
‧If the overheated level at the inlet of low-pressure turbines is raised to reduce wet steam at the outlet of the turbines, the occurrence area of condensate water can be minimized.
Disadvantages of overheated vapors
‧An overheated vapor's total energy consists of the sensible heat in a liquid state and vaporization heat plus the sensible heat of overheated vapor. As the overheated level rises, the sensible heat of the overheated vapor increases compared with the vaporization heat. For example, a 10 barA saturated vapor (saturation temperature 179.9℃ ) is composed of sensible heat of 762.6 kJ/kg and latent heat of 2,013.5 kJ/kg of the saturated water. Compared with this, the enthalpy of a 10 barA/300℃ overheated vapor is 3,051.4 kJ/kg.
‧If overheated vapor is used as the heating source to adjust the temperature of a process media, accurate temperature adjustment and process control will be difficult due to temperature changes in the heating source and occurrence of local temperature deviations.
‧The heat transfer coefficient of the overheated vapor is smaller than that of phase changes in the saturated vapor. Consequently, process facilities need to be larger, while operating and maintenance expenses also increase significantly.
‧If the overheated level is high, high-precision measuring equipment is needed and special considerations also are required in manufacturing process facilities .
How to reduce the overheated level
Reducing the overheated level means converting the temperature of the overheated vapor into a saturated vapor or a vapor at around +10℃.
There are two methods to reduce the overheated level:
Direct contact method:
As shown in Figure 2, one method to control the overheated level is the direct spraying of cooling water into low-pressure overheated vapor, which was produced after pressure reduction of a high-pressure vapor with a pressure-reducing valve. With the direct cooling water spraying method, the composition of the two liquids must be the same and well mixed in order not to cause process problems. The equipment that controls the temperature level of the overheated vapor in the direct contact method is a ”Desuperheater (DSH)”.
This device can measure overheated levels to a minimum saturation temperature of +3.0℃ and is often used for saturation temperature around +6℃.
The flow rate of the supplied cooling water is adjusted with a temperature sensor and a flow control valve installed at the outlet of the DSH in accordance with the temperature level of the overheated vapor. The cooling water is vaporized after being heated by the overheated vapor and the temperature of the overheated vapor is reduced by as much as the energy lost in the vaporization process.
Indirect contact method:
With traditional shell & tube-type heat exchangers, heat exchanges are achieved between two liquids without direct contact. Compared with the direct contact method, heat transfer efficiency is low, but this method can be used effectively when the composition of the two liquids is different. This method is particularly suitable with cooling water, like air or seawater, which cannot be mixed with the overheated vapor.
Desuperheater has applications in steam supply units with limited operating temperatures such as steam tracing system, auxiliary equipment, heat exchangers or dump systems for LNG carriers and power plants. Super heated steam is an excellent method of energy transfer, but certain applications require saturated steam or super heated steam. The most economical way of controlling super heated steam temperature is through direct contact with injected cooling water. The injected cooling water, thoroughly mixed with super heated steam, absorbs energy from the super heated steam and vaporizes. The result is low temperature (approximately saturate temperature or setting temperature) of super heated steam.