Page 287 - Boiler_Operators_Handbook,_Second_Edition

272 Boiler Operator’s Handbook

Since we’ve adopted the label of HTHW any low would require steam or water pressure over 666 psig an-
pressure hot water heating boiler plant is simply called other fluid is used. There are several liquids, mostly hy-
a “hot water” plant with the understanding that it drocarbons, that can be heated to temperatures as high
complies with the code definition of a low pressure hot as 1,000°F without operating at such high pressures. The
water heating boiler. With water heating plants labeled liquids are identified by the trade name given by their
as such we understand a low pressure or high pressure manufacturer and include Dowtherm™ and Paracy-
label to mean a steam generating plant. Don’t ever be mene™ as the more common names. They are supplied
afraid to ask what somebody means. Requirements for in different materials according to the temperatures
licensing of operators frequently depends on whether a required. The common label for boilers that heat these
boiler is a power boiler or heating boiler so you want to liquids is “hot oil” so we call them hot oil boilers. The
get it right. Appendix contains tables, similar to steam tables, for the
more common of those hot oils.
Some of those liquids can be vaporized just like
BOILERS converting water to steam. A common name for them
could be oil vaporizers but it’s far more common for
Boilers do not have to have a burner. All of these the label to use the trade name of the fluid and add the
types can generate hot water or steam by absorbing heat word vaporizer so you’ll normally hear them called
from another fluid. That other fluid can be steam and Dowtherm vaporizers, but there’s no strict rule. Since all
create steam or hot water, it can be HTHW and generate these plants operate at temperatures higher than 250°F
steam, or it can be a hot liquid or gas from some chemi- they require power boilers built in accordance with
cal process that is hot enough to do the job. I imagine I Section I. You could be operating one of these boilers
worked on one of the largest low pressure steam boilers in addition to the steam plant because steam is usually
that was ever built in the late 1960’s and it generated required to quench the fire in the event the hot oil leaks
steam by oxidizing a liquid. The heat source was a large into the furnace to feed the fire.
volume of oil which air was forced through to oxidize Equipment that heats water in an open container
the liquid similar to combustion but at a low tempera- or very small one is not a boiler. Your teapot doesn’t
ture and nowhere near complete combustion. Twenty- have to be constructed in accordance with the code be-
four feet in diameter and ninety feet tall with thousands cause it’s so small. The hot water heater in your home
of square feet of heating surface it made about 25,000 isn’t considered a boiler unless it holds more than 120
pounds per hour. gallons. Another limit on the size of a boiler is an inter-
Other projects included a hot water boiler using nal diameter of 6 inches or less. The exceptions found in
500°F air from a steelmaking operation rated at 100 mil- the code are occasionally stretched to create boilers that,
lion Btuh. Operating that type of equipment to get the by definition, are not.
most steam out of it is wise because you save on fuel Fired air heaters are not boilers unless the air is un-
that would have to be used to generate that steam. These der pressure. Any application that heats air, or any other
boilers can be constructed as unfired pressure vessels in gas for that matter, that doesn’t contain the heated fluid
accordance with Section VIII of the ASME Code, “Rules in an enclosed vessel is normally called a furnace. If the
for Construction of Pressure Vessels.” fluid is air or another gas and it’s under pressure then it
Boilers that are fired must be built to Section I or does meet the definition of a boiler.
Section IV but their construction is limited to materials There are many boilers unique to their respective
that can handle the high rates of heat transfer required industry. You may encounter asphalt heaters, flux heat-
for direct fired equipment. Boilers using waste heat can ers (a raw material that becomes asphalt), many forms
require materials of construction that can’t handle direct of waste heat boilers and equipment like recovery boil-
firing but are essential to prevent corrosion in the waste ers (used in the paper industry) which convert product
heat application. In simpler words, a fired boiler can’t be by burning it. I’ve chosen to limit this book to the more
built in stainless steel, an unfired boiler can be. common types of boilers so you can acquire a basic un-
Since there’s a fixed relationship between pressure derstanding of them. The principles discussed here will
and temperature for steam and water, pressure has to allow you to understand those unique boilers which, by
increase. When we need to heat product or other materi- virtue of their uniqueness, are best understood by read-
als to high temperatures the pressures can get very high. ing the operating and maintenance instruction manuals
To obtain temperatures greater than about 500°F, which for them. This section contains general descriptions of