Highly superheated steam, (i.e. 900 Ć 1100°F) is usually generated to do mechanical work such as drive turbines. As the dry steam is expanded through each turbine stage, increasing amounts of thermal energy is transformed into kinetic energy and turns the turbine rotor at the specified speed. In the process, heat is transferred and work is accomplished. The spent steam exits the turbine at greatly reduced pressure and temperature in accordance with the first law of thermodynamics. This extremely hot vapor may appear to be an excellent source for heat transfer, but in reality it is just the opposite. Utilization of superheated steam for heat transfer processes is very inefficient. It is only when superheated steam temperatures are lowered to values closer to saturation that its heat transfer properties are significantly improved. Analysis has shown that the resultant increase in efficiency will very quickly pay for the additional desuperheating equipment that is required. In order to understand why desuperheating is so essential for optimization of heat transfer and efficiency, we must examine the thermodynamic relationship of temperature and the enthalpy of water. Figure 7Ć1 illustrates the changes of state that occur in water over a range of temperatures, at constant pressure, and relates them to the enthalpy or thermal energy of the fluid.
Figure 7Ć1. Temperature enthalpy diagram for water. Note that the greatest amount of thermal energy input is used to vaporize the water. Maximum efficiency in heat transfer requires operation at near saturation temperature to recover this energy. E0117 300 200 100 32 0 0 500 1000
Btu added to 1 pound of water
Temperataure, def F
1/2 Btu per degree Ice heating at about 144 Btu to melt ice
Water heating at 1 Btu per degree Atmospheric pressure Evaporation at 14.7 psi Evaporation at more than 14.7 psi
All data for 1 lb. water which water cannot
exist as a liquid
These lines curve and meet at 705.4 deg F the critical temperature, above
Steam superheating at about 0.4 Btu per degree 212 deg F
970 Btu to boil water
In the lower left portion of the graph, the water is frozen, at atmospheric pressure and below 32°F. At this point, heat is being rejected from the water as it maintains its solid state. As heat is gradually added, the ice begins to change. Addition of heat to the ice raises the temperature and slows the rate of heat rejection. It requires approximately 1/2 BTU of thermal energy to be added to a pound of ice to raise its temperature 1°F. Upon reaching 32°F, the addition of more heat does not
immediately result in an increase in temperature. Additional heat at this point begins to melt the ice and results in a transformation of state from a solid to a liquid. A total of 144 BTUs is required to melt one pound of ice and change it to water at 32°F.
Once the phase change from a solid to a liquid is complete, the addition of more heat energy to the water will again raise its temperature. One BTU of heat is required to raise the temperature of one pound of water by 1°F. This relationship remains proportionate until the boiling point (212°F) is reached. At this point, the further addition of heat energy will not increase the temperature of the water. This is called the saturated liquid stage.
Figure 7Ć2. Temperature enthalpy diagram for water showing that saturation temperature varies with pressure. By choosing an appropriate pressure, both correct system temperature and thermal efficiency can be accommodated.
Te mperataure, def F TĆH DIAGRAM WATER LIQUID 800 PSIA 14.7 PSIA LIQUIDĆVAPOR VAPOR ENTHALPY, BTU/LBM E0118
The water begins once again to change state, in this case from water to steam. The complete evaporation of the water requires the addition of 970 BTUs per pound. This is referred to as the latent heat of vaporization and is different at each individual pressure level. During the vaporization process, the liquid and vapor states coĆexist at constant temperature and pressure. Once all the water, or liquid phase, has been eliminated, we now have one pound of steam at 212°F. This is called the saturated vapor stage. The addition of further thermal energy to the steam will now again increase the temperature. This process is known as superheating. To superheat one pound of steam 1°F requires the addition of approximately .4 BTUs of thermal energy.
The potential thermal energy release resulting from a steam temperature change differs significantly depending on temperature and superheat condition. It is much more efficient, on a mass basis, to cool by addition of ice rather than by the addition of cold fluids. Similarly, it is more efficient to heat with steam at temperatures near the saturation temperature rather than in the superheated region. In the saturated region, much more heat is liberated per degree of temperature change than in the superheated
7-3 range because production of condensate liberates
the enthalpy of evaporation, the major component of the total thermal energy content. The
temperatureĆenthalpy diagram in Figure 7Ć2 is generalized to show the thermodynamic relationship at various pressures.
The graph in Figure 7Ć2 illustrates three distinct phases (i.e., liquid, vapor, and liquidĆvapor) and how enthalpy relates to temperature in each phase at constant pressure. The rounded section in the middle of the graph is called the "steam dome" and encompasses the twoĆphase, liquidĆvapor region. The left boundary of the steam dome is called the saturated liquid line. The right boundary line is the saturated vapor line. The two
boundaries meet at a point at the top of the dome called the critical point. Above this point, 3206 psi and 705°F, liquid water will flash directly to dry steam without undergoing a twoĆphase
coexistence. When conditions exceed this critical point they are considered to be existing in the supercritical state.
In the lower left side of the graph, the saturated liquid line intersects the temperature axis at 32°F. At this point we have water and a defined enthalpy of 0 BTU/LB. As heat is added to the system, the temperature and enthalpy rise and we progress up the saturated liquid line. Water boils at 212°F at 14.7 psia. Thus, at 212°F and 180 BTU/LB, we note a deviation from the saturated liquid line. The water has begun to boil and enter a new phase; LiquidĆVapor.
As long as the liquid stays in contact with the vapor, the temperature will remain constant as more heat is added. At 1150 BTU/LB (at 14.7 psi) we break through to the saturated vapor line. Thus, after inputting 970 BTU/LB all of the water has been vaporized and enters the pure vapor state. As more heat is added, the temperature rises very quickly as the steam becomes superheated.