PHOTOGRAPH COURTESY OF KEW BRIDGE STEAM MUSEUM, LONDON
Figure 1. An 18th century steam engine
Figure 2. A modern packaged steam heat
exchange system used for producing hot water
Figure 3. Typical two-port control valve with a
pneumatic actuator and positioner
team provides a means of trans-
Sporting controllable amounts of
energy from a central, automated boiler
house, where it can be efficiently and
economically generated, to the point of
use. Therefore, as steam moves around
a plant it can equally be considered to
be the transport and provision of energy.
For many reasons, steam is one of
the most widely used commodities for
conveying heat energy. Its use is popular
throughout industry for a broad range
of tasks from mechanical power production to space heating and process applications.
EFFICIENT AND ECONOMIC
Water is plentiful, inexpensive, non-haz-ardous to health and environmentally
sound. In gaseous form, it also is a safe
and efficient energy carrier. Because
steam can hold five or six times as much
potential energy as an equivalent mass
of water, it is also an efficient way to use
that water.
When water is heated in a boiler, it
begins to absorb energy. The water will
evaporate at certain temperatures
(depending on the pressure in the boiler)
to form steam. This steam contains a
large quantity of stored energy, which
will eventually be transferred to the
process or the space to be heated.
Steam can be generated at high pressures to give high steam temperatures.
The higher the pressure, the higher the
temperature for saturated steam (note
that steam at 2000 psi and 800° F has
less enthalpy than steam at 100 psi and
800° F). Since more heat energy is contained in higher temperatures, steam at
those higher temperatures has the
potential to do more work.
Modern shell boilers are compact
and efficient in their design. They use
multiple passes and efficient burner
technology to transfer a very high proportion of the energy contained in the
fuel to the water, with minimum emissions.
The boiler fuel comes from a variety
of sources, including combustible waste,
which makes the steam boiler an environmentally sound option. In addition,
centralized boiler plants can take
advantage of low interruptible gas tariffs because any suitable standby fuel
can be stored for use when gas supply is
interrupted.
Also, highly effective heat recovery
systems can virtually eliminate blow-down costs, return valuable condensate
to the boiler house and add to the overall efficiency of the steam and condensate loop.
The increasing popularity of Com-
bined Heat and Power (CHP) systems
demonstrates the high regard for steam
systems in today’s environment and
energy-conscious industries.
EASY DISTRIBUTION
Steam is one of the most widely used
media to convey heat over distances,
and because steam flows in response to
the pressure drop along the line, expensive circulating pumps are not needed.
Due to the high heat content of
steam, only relatively small bore
pipework is required to distribute the
steam at high pressure. If needed, the
pressure is then reduced at the point of
use. This arrangement makes installation of steam systems easier and less
expensive than for some other heat
transfer fluid systems.
Overall, the lower capital and running costs of steam generation, distribution and condensate return mean that
many users choose to install new steam
systems over other energy media such as
gas, hot water, electric and thermal oil.
EASY TO CONTROL
Because of the direct relationship
between the pressure and temperature
of saturated steam, the amount of energy input to the process is easy to control. It simply requires control of the
