Chemical Engineering February 2017 - 32

Facts At Your Fingertips
Steam Concepts
Department Editor: Scott Jenkins
S
team is commonly used in the
chemical process industries
(CPI) for process heating, power
generation, atomization, cleaning
and sterilization, moisturization and
humidification, among other applications.
This one-page reference provides
a refresher for key concepts
related to steam.
Pressure-temperature relation
The properties of steam vary significantly
depending on the temperature
and pressure conditions to which the
steam is subject. Steam produced
under higher pressures is of higher
temperature and carries more energy,
but also requires more heat to generate.
Steam with higher energy per
unit of mass is said to have higher enthalpy,
a thermodynamic property of
a fluid that is defined as the internal
energy of the fluid added to the product
of its pressure and temperature.
Enthalpy is a measure of the available
energy of the fluid, and enthalpies
have been calculated for a wide range
of steam and saturated liquid conditions.
These values may be found in
steam tables, and can be graphically
represented by a temperature-enthalpy
diagram (Figure 1).
Wet steam region
Water can be heated to its boiling
point (point A to point B on the curve),
which depends on the pressure applied.
At its boiling point for a particular
pressure, additional energy input
will not cause a further temperature
change (as long as pressure remains
constant) and the steam moves toward
point C on the diagram. Steam
in this area is termed " wet " since it
contains both water vapor and liquid
water droplets.
At the boiling point for a given pressure,
water changes from a mixture of
100% water/0% steam to a 0% water/100%
steam mixture. Wet steam
contains a certain portion by mass of
liquid water droplets. For example,
steam that contains 8% moisture by
mass has a dryness fraction of 0.92
and is 92% dry.
If sufficient heat is added to convert
32
Water
This line shows
boiling points
at increasing
pressures
Critical point
Lines of
constant
pressure
(Wet steam)
Two phase region
B

C
Superheated steam
A
Enthalpy
FIGURE 1. A temperature-enthalpy phase diagram illustrates the behavior of steam and the relationships
between steam properties
all liquid into vapor at a given pressure,
the steam is said to be saturated
(100% vapor, dry steam; point C).
Saturated steam occurs at pressure
and temperature conditions that allow
water in its vapor and liquid form to
exist simultaneously.
Saturated steam has several properties
that make it desirable for process
heating, including a high heattransfer
coefficient, which allows a
smaller heat-transfer surface area.
The heat-transfer capacity of saturated
steam is much higher than that
of water or superheated steam. Although
the temperature of the boiling
water and saturated steam within the
same system is the same, the heat
energy per unit mass is much greater
in the steam. Further addition of heat
to saturated vapor then increases its
temperatures (superheated region).
Superheated steam is used to turn
turbines in power generation.
Industrial practice
In CPI facilities, it is generally desirable
to use steam with the maximum
possible dryness fraction. However,
perfectly dry (saturated) steam is almost
impossible to produce in industrial
practice. For steam generated
in a boiler, turbulence and splashing
cause a small degree of wetness
from non-vaporized water molecules,
which are carried into the distributed
steam. Radiant heat loss also causes
some of the steam to condense, necessitating
the use of steam traps.
Steam-system users must be
aware of potential problems, such as
corrosion and scale from impurities
in the water, damage to flowmeters
and other equipment, and reduced
heat transfer. Lower heat transfer
rates occur because condensed water
can form a film on heat-transfer
surfaces, which reduces heat transfer
because thermal conductivity
of water is much lower than that of
steam. Other potential problems include
overloading steam traps and
water hammer, if the amount of condensed
water is too high.
Steam grades
Several grades of steam are common
in CPI facilities.
Plant steam. Also known as industrial
steam, plant steam is by far the
most common form used for general
indirect heating in CPI plants.
Filtered
steam.
Also known as
culinary steam, filtered steam is
produced by passing plant steam
through a high-efficiency filter to remove
solid and liquid particles. It also
needs to be free of boiler water-treatment
chemicals.
Clean steam. Clean steam requires
controlled feedwater quality for applications
where contamination is
not acceptable.
Pure steam. Pure steam is similar to
clean steam for use in quality-critical
processes. In pure steam, the resulting
condensate must meet the standards
of the U.S. Pharmacopeia for
water-for-injection (WFI), and contain
no bacteria or pyrogens.
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Chemical Engineering February 2017

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