Paper Technology International 2024 - Journal - Page 100
PAPERTECHNOLOGYINTERNATIONAL
then be applied to determine the appropriate sizing for the throat and
motive steam nozzle. The throat size determines the dimension and
other aspects of the body, and it is normally sized to handle the large
昀氀ow volume at the lowest dryer pressure. The steam nozzle is sized
to provide enough energy for the maximum work of compression at
the highest dryer pressure. As paper machines typically run with a
wide range of operating conditions, designing a thermocompressor
can be somewhat con昀氀icting, generating most design challenges
for process engineers. The throat is normally no larger than about
47% of the discharge pipe bore. During the most ef昀椀cient mode of
operation, the steam jet entrains suction steam at the suction 昀氀ange,
and the 昀氀ow volume of the mixture exactly 昀椀lls the throat. One
problem we typically see with poorly performing thermocompressors
is that, if the steam jet 昀氀ow is increased further at this point, the
volume 昀氀ow of the mixture becomes greater than what can pass
through the throat and a reduction in suction 昀氀ow occurs. This effect
is called “choking” and can severely complicate control of dryer
differential pressures. Another challenge for this application is that
when working at the highest dryer pressure, the density and velocity
of the steam are at a maximum, such that the 昀氀ow volume does not
昀椀ll the throat. Accordingly, the 昀氀ow is subject to turbulent shock loss
that reduces performance. Our proprietary software accounts for
this shock loss and provides an accurate sizing of the steam nozzle.
However, we have seen situations where the design of the nozzle is
oversized, resulting in high gain in control loops and other problems
such as the need for larger safety relief valves.
Operating Characteristics
Increasing energy costs and price volatility are key factors
for the pro昀椀tability of most paper mills. However, a common
misconception about motive steam requirements is that by using
lower motive steam pressure, the mill will save energy. The theory
behind this incorrect assumption is that if motive steam is extracted
at 8 bars instead of 20 bars, the difference will automatically be
converted into more generated power. In a typical case where
dryers operate within the 3.5 bar range, utilizing 20 bar of steam
results in approximately 40% lower consumption. Therefore, the net
energy cost to the mill decreases, or at worst remains unchanged,
compared to using 8 bar of motive steam. In the case where 8 bars
are used as a motive steam, there is little more than 50% higher
than the dryer steam pressure. At this level, motive steam required
rises to more than double the amount of steam being recirculated,
and it may exceed the amount needed by the dryer section, resulting
in major steam losses and dif昀椀culty in controlling pressures.
Some managers fear that super heat in the motive steam
will in inhibit drying or wear steam joint seals. The fact is that even
starting with high levels of super heat, the conversion of energy
to the work of compression results in supersaturation in the throat
(it’s wet), and this stuff mixes with recirculated steam that has
two or three percent carryover condensate. The discharge after
recompression rarely has more than 10 degrees super heat.
Moreover, normally, a 6” or 8” pipe dumps blow-through
steam carrying three- or four-hundred percent condensate in the
form of a heavy fog into a separator tank at a velocity of much over
900 m/min. There are still plenty of inef昀椀cient, obsolete, separators
in the market, which lack effective baf昀氀es (often eroded out) and
the blow-through steam passes right on through without dropping
out much of the condensate. From the separator it goes up to the
thermocompressor, accelerates to very high velocity in the throat,
and discharges back into the dryers. The wet steam erodes the
throat of the thermocompressor, increases compression work
by adding pipe transport and lift losses to the pressure rise and
impairs compression ef昀椀ciency. No separator is 100% ef昀椀cient but
high separation ef昀椀ciency is also important to thermocompressor
operations.
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Solving the Problem
A still common method of controlling dryer drainage with
thermocompressors is the recirculation system with DP control
illustrated in (Figure 2:). Ideally, the DPIC (differential pressure
controller) modulates the thermocompressor within the lower half
range of its output signal. If the thermocompressor fails to raise
the DP to setpoint after fully opening, the signal continues to
rise, opening the dump valve (to waste) in the upper half range.
In the event of overpressure in the dryers, the LSR (low signal
selector relay) allows the PIC (pressure controller) to throttle the
thermocompressor after 昀椀rst closing the makeup valve.
DP control systems are not optimal for coping with wide
changes in condensing load. If designed to handle the maximum
condensing load at maximum pressure, the exponential effects of
increasing blow-through 昀氀ow and pipeline pressure losses overload
the thermocompressor as pressure and condensing load decrease,
leading to excess blow-through steam waste. Typically, this type
of system cannot operate below 1.5 bar of dryer pressure without
dumping, hindering performance in paper machines. Additionally,
web breaks tend to double the 昀氀ow of blow-through steam,
overwhelming the condenser system and 昀氀ooding dryers.
The combination of blow-through control and the
thermocompressor offers an effective solution. The system
con昀椀guration in Figure 3 is similar to the recirculation system, but
with DP control maintained across an ori昀椀ce plate in the blowthrough line instead of between dryer inlet and outlet headers.
Under blow-through control, the range of action required of the
thermocompressor is greatly reduced, as blow-through 昀氀ow
automatically adjusts with steam pressure and condensing rate.
This results in a smaller thermocompressor throat size despite
the increase in steam volume. Recent cases have shown
successful operation with dryer pressures as low as 0.5 bar using
thermocompressors to recycle blow-through steam. It’s crucial to
distinguish blow-through control from 昀氀ow control. Blow-through
control manages the velocity head of the blow-through steam, while
昀氀ow control regulates 昀氀ow in kg/hr, which may not ef昀椀ciently drain
dryers in various scenarios.
