Reference Manual

7−5
Figure 7-4. The DMA/AF desuperheater
utilizes variable-geometry, back-pressure
activated spray nozzles.
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dependent on the pressure differential and thus
provide levels of performance that are
commensurate with the magnitude of the
difference. Obviously, the larger the water/steam
differential the better the unit will perform (i.e.,
penetration velocity, flow variation and droplet
size). Since the equipment turndown is usually
limited to 4:1, it is best suited for near steady load
applications.
An upgrade from the fixed geometry unit is the
DMA/AF (figure 7-4) variable geometry nozzle
desuperheater. Here the actual flow geometry of
the unit is varied to maintain an optimum
differential across the discharge orifice. As a
result of this change, the level of flow variation is
greatly enhanced and so is the performance.
Equipment turndowns can exceed 40:1, thus
making this style a good choice for medium to
high load change applications.
Another form of mechanically atomized
desuperheater is the DVI, Geometric Enhanced
style, (figure 7-5). Here, the unit is supplied a high
pressure recovery flow restriction that alters flow
geometry and helps to keep the level of turbulence
and kinetic energy at a high level during all phases
of the units operation due to an increased velocity
at the point of spray water injection. This
increased level of surrounding energy helps to
Figure 7-5. The DVI desuperheater injects
spraywater in the outlet of the venturi section,
assuring excellent mixing and rapid atomization.
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impart energy transfer to the droplets and assists
in break-up, mixing, and vaporization. This style is
best suited for medium turndown applications
typically around 15:1.
The last group of desuperheater units utilizes an
external energy source for the atomization of the
spraywater. The most common medium is a high
pressure steam source. In this case, the high
levels of kinetic energy are provided by a critical
pressure reduction in the desuperheater
sprayhead. The critical drop is used to shear the
water into a fine mist of small droplets, which is
ideal for vaporization, as shown in figure 7-6. This
type of system can provide a very high degree of
flow variation without requiring a high pressure
water supply. Applications requiring turndown
ranges greater then 40:1 utilize this type of
equipment for best performance. In addition to an
external spraywater control valve, the system will
also require an atomizing steam shut-off valve
(figure 7-7).
Other factors that have a large amount of impact
on the performance of a desuperheating system
include:
D Installation Orientation
D Spray Water Temperature
D Spray Water Quantity
D Pipeline Size
D Equipment vs. System Turndown