Chemical Engineering May 2020 - 45

1,400
Regions of steam quality
Superheat
1,200
1,000
Wet steam
800
600
400
Unsaturated water
200
Saturated water
Steam latent heat
95% latent heat
90% latent heat
85% latent heat
Saturated steam
5% Wetness
10% Wetness
FIGURE 4. Entrainment
affects the amount of
latent heat per unit of
mass that is provided to
equipment. 10% or 15%
entrainment reflects a
significant decrease in
heat capability, and can
create reliability problems
in certain equipment
100
300
500
700
900
Pressure (psig)
heat. Typical uses are for high process
temperature demand, or to
drive turbines.
Distributed steam is key
When discussing steam distribution,
the topic invariably leads to the importance
of mitigating against water
hammer. While such avoidance is
critical to safe operation, it
is only
part of the story [2].
Water hammer examples norSTEAM
CHARACTERISTICS
VARYING DRYNESS %
Pressure
(psig)
200
200
200
200
150
150
150
150
100
100
100
100
50
50
50
50
15
15
15
15
Temperature
(°F)
388
388
388
388
366
366
366
366
338
338
338
338
298
298
298
298
250
250
250
250
Sensible
heat
(Btu)
362
362
362
362
339
339
339
339
309
309
309
309
267
267
267
267
218
218
218
218
Latent
heat
(Btu)
838
796
754
737
858
815
772
755
881
837
793
775
912
866
821
803
946
899
851
832
Total
heat
(Btu)
1,200
1,158
1,116
1,099
1,196
1,153
1,110
1,093
1,190
1,146
1,102
1,084
1,179
1,133
1,088
1,070
1,164
1,117
1,069
1,050
Dryness
100%
95%
90%
88%
100%
95%
90%
88%
100%
95%
90%
88%
100%
95%
90%
88%
100%
95%
90%
88%
Pressure
(psig)
200
200
200
200
150
150
150
150
100
100
100
100
50
50
50
50
15
15
15
15
1,100
1,300
1,500
mally only show
the liquid condensate
that has
already been disentrained
from
the steam flow.
It rarely considers
the wet steam
droplets that are part of a wet
steam mixture, and how adversely
those droplets can create issues
that affect operational efficiency
and reliability (Figure 3) [5]. A review
of how wet steam reduces
reliability of specific equipment is
provided later, but first additional
information about steam wetness
is provided. The specific enthalpy
of steam can be calculated using
Equation (1):
hg = hf + (D x hfg)
Where:
hg = Steam total heat
hf = Sensible heat
hfg = Latent heat
D = Steam dryness, %
Consider Figure 4, which shows
the regions of steam quality and heat
values. The dark blue line represents
saturated water, with the area under
this line representing the region of
unsaturated water. This water line
demonstrates that as pressure increases
up to 10.35 MPa (1,500
psig), additional heat must be added
to reach boiling.
The red line represents the latent
heat - the heat of steam that
is used for most process applications.
Latent heat decreases in the
graph as the pressure increases.
The higher the pressure, the less
steam heat is available. The mustard-colored
lines under the steam
latent heat line are most important
because they demonstrate the reduction
in latent heat for a given
STEAM CHARACTERISTICS
DRYNESS AFTER 98% MOISTURE REMOVAL
Latent
Sensible
heat (Btu)
362
362
362
362
339
339
339
339
309
309
309
309
267
267
267
267
218
218
218
218
Less
heat (Btu)
838
796
754
737
858
815
772
755
881
837
793
775
912
866
821
803
946
899
851
832
98% H2O
(Btu)
838
837
836
836
858
857
856
855
881
880
879
879
912
911
910
910
946
945
944
943
Total
heat
(Btu)
1,200
1,199
1,198
1,198
1,196
1,195
1,194
1,194
1,190
1,189
1,188
1,188
1,179
1,178
1,177
1,177
1,164
1,163
1,162
1,161
Ending
draness
100%
99.9%
99.8%
99.7%
100%
99.9%
99.8%
99.7%
100%
99.9%
99.8%
99.7%
100%
99.9%
99.8%
99.7%
100%
99.9%
99.8%
99.7%
FIGURE 5. Wet steam can have varying levels of latent heat provided to equipment. Steam that has experienced 98% moisture separation nears true saturation
quality
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45
(1)
Heat (btu)
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Chemical Engineering May 2020

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