Reference Manual

7−6
Figure 7-6. The DSA desuperheater uses
high-pressure steam for rapid and complete
atomization of spraywater in low-velocity steam
lines.
W6311-2
Installation orientation is often overlooked, but a
critical factor in the performance of the system.
Correct placement of the desuperheater can have
more impact on the operation than the style of the
unit itself. For most units, the optimum orientation
is in a vertical pipeline with the flow direction up.
This flow direction is ideal, as the natural flow
direction of the injected water tends to be in the
counter direction due to effect of gravity. The role
of gravity in this orientation will suspend the
droplets in the flow longer while they are being
evaporated, thus shortening the required
downstream distance or efficient mixing.
Other orientation factors that are of concern
include downstream pipefittings, elbows, and any
other type of pipeline obstruction that can provide
a point for water impingement or fallout.
Spraywater temperature can have an great impact
on the desuperheater performance. While it goes
against logical convention, hotter water is better
for cooling. As the temperature increases and
moves closer to saturation, its flow and thermal
characteristics are improved and impact most
significantly the following:
D Surface Tension
D Drop Size Distribution
D Latent Heat of Vaporization
D Vaporization Rate
Improvement in all these areas will act to improve
the overall performance of the system, as the
spraywater will evaporate and mix with the steam
at a faster rate.
The quantity of water to be injected will, as with
any mass flow calculation, have a directly
proportionate affect on the time for vaporization.
The heat transfer process is time dependent; thus,
the quantity of spray water will increase the time
for complete vaporization and thermal stability.
Another concern for proper system performance is
pipeline size. Pipe size should be determined in an
effort to balance the velocity of the steam flow.
Steam traveling at a fast rate will require longer
distances to effectively cool, as heat transfer is a
function of time. Steam traveling at low velocity
will not have enough momentum to suspend water
droplets long enough for evaporation. As a result,
water will fall out of the steam flow to collect along
the bottom of the pipe, and it will not cool the
steam effectively. Ideal velocity is typically in the
range of 250 ft/sec to 300 ft/sec.
As the pipeline size increases to limit steam
velocity, more attention must be paid to the
penetration velocity of the spray and the coverage
in the flow stream. Experience shows that single
point injection type desuperheaters will have
insufficient nozzle energy to disperse throughout
the entire cross-sectional flow area of the pipeline.
As a result, the spray pattern collapses and
thermal stratification occurs (i.e., sub-cooled
center core within a superheated outer jacket.)
This condition normally is eliminated after the flow
stream undergoes several direction changes,
although this is not always possible within the
limits of the control system or process. Proper
placement of high-energy TBX-T (figure 7-8)
multi-nozzle steam coolers in the larger pipelines
will normally prevent thermal stratification.
The most over used and misunderstood word in
the field of desuperheating is “turndown.” When
applied to a final control element, such as a valve,
it is a simple ratio of the maximum to minimum
controllable flow rate. Turndown is sometimes
used interchangeably with rangeability; however,
the exact meaning differs considerably when it