STEAM, THE ENERGY FLUID
saturated steam pressure. In addition,
modern steam controls are designed to
respond very rapidly to process changes.
The item shown in Figure 3 is a typical, two-port control valve and pneumatic actuator assembly designed for use on
steam. Its accuracy is enhanced by the
use of a pneumatic valve positioner.
The use of two-port valves to control
temperature via pressure, rather than
the three-port valves to bypass flow
around the heat exchanger (often necessary for liquid systems), simplifies control and installation in steam systems,
which may reduce equipment costs.
Steam
High heat content
Inexpensive; some water
treatment costs
Good heat transfer
coefficients
High pressure required for
high temperatures
No circulating pumps
required; small pipes
Hot water
Moderate heat content
Inexpensive; only
occasional dosing
High temperature oils
Poor heat content
Expensive
Moderate coefficients Relatively poor coefficients
Easy to control with two-way valves
EASY TO TRANSFER
Steam provides an excellent way to
transfer heat. What’s more, when the
steam reaches the plant, the condensation process efficiently transfers that
heat to the product.
Steam can surround or be injected
into the product being heated. It can fill
any space at a uniform temperature and
will supply heat by condensing at a constant temperature. This eliminates temperature gradients that can be found
along heat transfer surfaces—a problem often found with high-temperature
oils or hot water heating. As a result,
quality problems, such as distortion of
materials being dried, are reduced.
Because the heat transfer properties
of steam are so high, the required heat
transfer area is relatively small. This
enables the use of more compact heat
exchangers, which are easier to install
and take up less space in the plant. A
modern packaged unit for steam-heated
hot water, rated to 4M btu/hr (1200
k W) and incorporating a steam plate
heat exchanger and all the controls,
requires only 24. 7 ft2 (0.7 m2) floor
space. In comparison, a packaged unit
incorporating a shell and tube heat
exchanger would typically cover an area
two to three times that size.
High pressure needed for Low pressures only to get
high temperatures high temperatures
Circulating pumps required; Circulating pumps required;
large pipes even larger pipes
More complex to control; More complex to control;
three-way valves or three-way valves or
differential pressure valves differential pressure valves
may be required may be required
Temperature breakdown is
easy through a reducing
valve
Steam traps required
Condensate to be handled
Flash steam available
Boiler blowdown necessary
Water treatment required
to prevent corrosion
Temperature breakdown
more difficult
Temperature breakdown
more difficult
No steam traps required
No condensate handling
No flash steam
No blowdown necessary
No steam traps required
No condensate handling
No flash steam
No blowdown necessary
Less corrosion
Negligible corrosion
Reasonable pipework
required
Searching medium, welded
or flanged joints usual
No fire risk
System very flexible
No fire risk
System less flexible
Very searching medium,
welded or flanged joints
usual
Fire risk
System inflexible
Table 1. Comparison of heating media
EASY TO MANAGE
Increasingly, industrial energy users are
looking to maximize energy efficiency
and minimize production costs and overhead. The Kyoto Agreement for climate
protection, a major external driver of
today’s energy efficiency trend, has led
to various conservation measures
around the globe. But also, in today’s
competitive markets, the organizations
with the lowest costs can often achieve
important advantages over rivals. Production costs can mean the difference
between survival and failure in this marketplace.
Methods for increasing energy efficiency used today include monitoring
and charging energy consumption to relevant departments. These actions build
awareness of where costs really are and
focus management on meeting targets.
Planned, systematic maintenance also
minimizes overhead, while maximizing
process efficiency, improving quality
and cutting downtime.
Most steam controls today can inter-
face with modern networked instrumentation and systems to allow centralized
control, such as a SCADA system or a
Building/Energy Management System.
If a user wishes, the components of the
steam system can also operate independently (stand-alone).
With proper maintenance, a steam
plant will last for many years. The condition of the system also is easy to monitor automatically. When compared with
other systems, the management and
monitoring of steam traps is easy to
achieve with a trap monitoring system,
which automatically pinpoints leaks or
blockages and immediately brings them
to the attention of the engineer.
In contrast, costly equipment is
required for gas leak monitoring and much
