Reference Manual

7−4
After the valve, the steam will have the following
conditions:
Conditions: P
2
= 45 psia
Enthalpy = 1198.9 BTU/LB
Referencing a set of steam tables, we see that at
the above conditions the steam temperature is
328°F giving the impression that it has cooled.
However, from the steam tables we see that the
saturation temperature for 45 psia steam has also
dropped to 274°F. The net result is that our steam
now has 54°F of superheat (328°F - 274°F). Use
of this steam for heat transfer could be
uneconomical and return on investment on a
desuperheater would be most favorable.
Desuperheating
In this section we will briefly discuss the process
of desuperheating. The need to desuperheat is
usually performed simply to control the steam
temperature, or heat content, of the flowing vapor
media. Depending on the process downstream of
the main steam source, a desuperheater will be
utilized to transform the steam into a medium that
is more efficient for heat transfer or just more
conducive for interaction with its surrounding
components. One means of accomplishing this is
with a direct contact heat transfer mechanism.
This can easily be achieved by the use of a single
spray injection nozzle that, when properly placed,
diffuses a calculated quantity of liquid into the
turbulent flow stream. Vaporization of the liquid
phase proceeds while mass, momentum, and
energy transfer occurs, and the resultant vapor
exits the process at the desired temperature or
heat content level.
Desuperheaters
A desuperheater is a device that injects a
controlled amount of cooling water into a
superheated steam flow in an effort to reduce or
control steam temperature (figure 7-3).
Desuperheaters come in various physical
configurations and spray types that optimize
performance within specified control and
installation parameters. Selection should also
always include attention to those details that would
provide the most economic solution without
sacrificing required performance.
Figure 7-3. Insertion style desuperheater injects a
controlled amount of cooling water into super-
heated steam flow.
E0865
The success of a particular desuperheater station
can rest on a number of physical, thermal, and
geometric factors. Some of the factors are quite
obvious and others are more obscure, but they all
have a varying impact on the performance of the
equipment and the system that it is installed in.
Considerable research has been conducted into
the characteristics of desuperheaters and the
transformation of spraywater to vapor. The
findings are of considerable interest to both design
and process engineers. In the next several
sections, we will discuss these findings and how
they relate to the desuperheating system as a
whole.
The most important factor is the selection of the
correct desuperheater type for the respective
application. Units come in all shapes and sizes
and use various energy transfer and mechanical
techniques to achieve the desired performance
criteria and optimize the utilization of the system
environment. These design criteria include:
D Mechanically Atomized − Fixed and Variable
Geometry Spray Orifice
D Geometrically Enhanced
D Externally Energized
The mechanically atomized style of desuperheater
is the most popular and simplistic style that
provides nominal performance over a wide range
of flow and conditions. These models are of the
internally energized variety. The atomization and
injection of the spray water is initiated by the
pressure differential between the spraywater and
the steam. The DMA, fixed geometry spray
orifice, units are the simplest and by design have
a constant area flow path. These units are highly