Chemical Engineering May 2020 - 44

Solid ice
Ice water
at 0oC [32oF]
at 0oC [32oF]
Boiling water
at 100oC [212oF]
Saturated steam
at 100oC [212oF]
100 oC
0 oC
D
S
FIGURE 3. Steam contains condensate, both entrained
in vapor, and disentrained flowing along
the bottom of the pipe as a liquid stream. Only already
disentrained condensate is removed through
condensate discharge locations
Latent Heat (Hs)
Sensible Heat (Hf)
Latent Heat (Hfg)
FIGURE 2. Heat energy is needed to change state and elevate temperature. Phase changes occur when
ice becomes water and water becomes steam. Steam exits boilers with a certain amount of wetness
(3-5%, not saturated)
system: 1) proactively maintain a
minimum threshold of " good " condition
steam traps with high priority
[1, 2]; and 2) implement mechanical
separation equipment for critical processes
as needed.
Look at your heat asset
Plant steam is the major source of
heat for most chemical and refining
processes, yet it often receives
little attention relative to its quality.
Perhaps this is the result of commonly
referring to plant steam as,
" saturated. " Statements such as the
following are typical: " We have 650
psig superheated steam, and three
levels of saturated steam, 150, 50,
and 15 psig. The use of the word
" saturated " is so prevalent, it can
easily become a replacement for
what is actually present in the plant.
That is, plant steam is either superheated,
or wet steam at saturation
temperature. Why is this so important?
If engineers and other plant
personnel believe that most of their
steam is saturated, it might be difficult
to envision the importance of
dealing with normal wetness - and
this can have a profound effect on
plant operations and efficiency.
How is steam used?
Steam has four stages, generation,
distribution, use and return (Figure
1). Once it is in the use stage, it is
important to have the steam as dry
as possible, to drain condensate
quickly after steam's heat has been
transferred to the process, and to
44
recover
the
remaining
heat and
treated water from the process to
return it to the boiler, thereby reducing
the burden on the boiler and its
impact on the cost of heating and
treating raw make-up water.
Why is plant steam wet?
The phase changes that occur with
water are shown in Figure 2 [3].
There are three phases, solid/ice,
liquid/water, vapor/steam. While the
temperature remains the same (0°C,
32°F) at atmospheric condition when
converting ice to water at the point
of freezing, it takes a certain amount
of " latent " heat energy to change
a mass of ice completely to water.
If only some of the required heat is
provided for a phase change, there
would still be bits of ice in the subject
mass - but the temperature
remains the same (amount of added
heat is unclear or hidden).
Once the full ice mass has been
converted to liquid, theoretically it
is now water at freezing temperature,
and it is no longer ice. This
ice-cold water is unsaturated - it
has ability to absorb heat and still
remain in a completely liquid state
up until water's saturation temperature,
also known as its boiling
point (100°C, 212°F). While adding
heat between the point of ice water
and boiling, each increase of heat
raises the water's temperature. It
is why this region's heat is known
as " sensible. " By recording its temperature
and mass, it is possible to
mathematically calculate or sense
the total amount of heat in the notyet-saturated
(unsaturated) water.
Once water is saturated, adding
heat converts some of its molecules
to steam - up until the point
known as " saturated " steam, when
all molecules have been converted
to vapor. However, during this
phase between saturated water
and saturated steam, it is not clear
exactly how much heat has been
added because steam - wet or
saturated, is formed at the same
temperature as saturated water.
The actual heat amount is hidden,
hence the term, " latent heat. " Perhaps
this is why steam is commonly
referred to as saturated, when in
fact, it is wet steam at saturation
temperature - the same saturation
temperature as both boiling
water and steam.
Saturated steam from boilers?
Do boilers produce saturated
steam? The direct answer is, " no. "
Boiler steam has 3-5% wetness
caused by water adhering to the
outside of the steam bubbles as
they break through the surface of
water while exiting the boiler. There
are methods that can be used to
improve the quality of exiting steam,
but even with those, the steam is
not 100% dry and also experiences
some heat loss during the distribution
stage [4].
Once wet steam is
produced,
it can be passed through a superheater,
which removes all wetness
while crossing over the saturated
steam threshold, and then
increases its temperature, volume
and
total
heat
content.
Superheated
steam has a low specific
heat content, so commonly its benefit
is realized when obtaining high
temperature or dryness, not added
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