Reference Manual

7−3
production of condensate liberates the enthalpy of
evaporation, the major component of the total
thermal energy content. The temperature-
enthalpy diagram in figure 7-2 is generalized to
show the thermodynamic relationship at various
pressures.
The graph in figure 7-2 illustrates three distinct
phases (i.e., liquid, vapor, and liquid-vapor) 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 two-phase, liquid-vapor
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 two-phase
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; Liquid-Vapor.
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.
Why Desuperheat?
Desuperheating, or attemperation as it is
sometimes called, is most often used to:
D Improve the efficiency of thermal transfer in
heat exchangers
D Reduce or control superheated steam
temperatures that might otherwise be harmful to
equipment, process or product
D Control temperature and flow with load
variation
Dry superheated steam is ideally suited for
mechanical work. It can be readily converted to
kinetic energy to drive turbines, compressors and
fans. However, as the steep temperature-
enthalpy line slope would indicate, the amount of
heat output per unit of temperature drop is very
small. A heat exchanger using superheated
steam would have to be extremely large, use great
quantities of steam, or take tremendous
temperature drops. A 10°F drop in temperature
liberates only 4.7 BTU per pound.
If this same steam had been desuperheated to
near saturation the thermal capabilities would be
greatly enhanced. The same 10°F drop in
temperature would result in the release of over
976 BTU of heat. This illustrates the obvious
advantages of desuperheating when thermal
processes are involved. Only by desuperheating
the superheated steam is it possible to
economically retrieve the energy associated with
vaporization. By changing steam pressure, the
saturation temperature can be moved to match the
temperature needs of the process and still gain
the thermal benefits of operating near saturation.
The previous discussion centered on why we
superheat steam (to do mechanical work) and
when it should be desuperheated back to
saturation (to heat). There are many situations
when saturated steam suddenly and
unintentionally acquires more superheat than the
downstream process was designed to
accommodate. This “unintentional” superheat
produces the same thermal inefficiencies
mentioned previously. In this case, we are talking
about the sudden expansion and temperature
change associated with a pressure reducing valve.
Take the following steam header conditions for
example:
Conditions: P
1
= 165 psia
T
1
= 370°F
Enthalpy = 1198.9 BTU/LB
Saturation temperature at 165 psia is 366°F.
Therefore, the steam has only 4°F of superheat
and would be excellent for heat transfer. Assume
that another thermal process requires some
steam, but at 45 psia rather than 165 psia. The
simple solution is to install a pressure reducing
valve. Since throttling devices, such as valves
and orifices, are isenthalpic (constant enthalpy
processes) the total heat content of the steam will
not change as flow passes through the restriction.