Yoshitake
Heat always moves from high temperature level to low level.
In boiler, heat transfers from a combustion chamber to water through a heat exchanger tube.
Water is heated further, and is conveyed from a boiler header to each system through piping as steam, and the thermal energy of steam is utilized.
Since the temperature of steam which generated in the boiler is higher than ambient air temperature, it radiates heat to the atmosphere through the wall inside piping.
Steam is condensed by this heat loss and changes to condensate.
Therefore, the piping is kept warm in order to make this heat loss into the minimum.
However, since it is important to utilize thermal energy of steam for heat exchanger, air heater, etc., it is necessary to make various consideration so that heat can be transmitted efficiently as long as possible.
In case of indirect heating, or when heating a object indirectly by a heat exchanger etc., steam emits its latent heat and condenses to become condensate.
That is, in case of indirect heating, only latent heat is used.
For saturated vapor, the higher pressure, the less the latent heat of the steam. Conversely, the higher pressure is, the more the sensible heat of saturation water becomes.
This means that, to use for heating, the higher steam pressure is, the more loss of the heat quantity.
For example, 252,000kJ is needed for heating 1,000 kg of water to make it from 20 ℃ to 80 ℃ by a heat exchanger.
When dry saturated vapor of 0.1MPa is used, since the latent heat is 2,210kJ/kg,
252,000kJ/2,210kJ/kg ≒ 114.0kg steam is needed.
Similarly, when dry saturated vapor of 0.7MPa is used, since the latent heat is 2,056 kJ/kg,
252,000kJ/2,056kJ/kg ≒ 122.6kg steam is needed.
Therefore, if using steam of 0.1MPa rather than steam of 0.7MPa, it is possible to save stream of the difference, 122.6-114.0 = 8.6kg steam
Furthermore, in any case, the sensible heat is discharged as condensate by steam trap.
The sensible heat of steam of 0.1MPa is 503kJ/kg, and the sensible heat of steam of 0.7MPa is 719 kJ/kg.
In this case, the difference of the amount of sensible heat become (122.6 kg x 719 kJ/kg)-(114.0 kg x 503 kJ/kg) ≒ 30,807kJ, and this sensible heat will be discharged as condensate.
In case of indirect heating, if using steam of higher pressure than needed, the loss of heat quantity increase very much.
To decide how much pressure is reduced, it is needed to think as precondition that the temperature conditions of the heat exchanger portion and the size of a steam feed opening of the heat exchanger portion are secured, and the heat exchange capability does not decline by pressure reducing.
For steam, the less pressure is, the larger specific volume of steam becomes.
In 0.1MPa, the specific volume of steam is 0.9018 cm3/kg. It is about 3.7 times higher than 0.2448 cm3/kg in 0.7Mpa.
Therefore, when switching to low pressure steam, it is necessary to reexamine nominal size of steam pipe.
In addition, when using low-pressure steam, since the difference of steam temperature between the temperature of heated object becomes small in a process unit, it is necessary to increase a heat transfer area or the number of equipment.
However, it is apparent that the cost for it is temporary, and profits by low-pressure steam are continuous.