CHAPTER2
Boilers,
as
a part
of
a hot water
syste~
require
proper
circulation
and the
system
must be
operated
as
intended by its designer
to
avoid
shock
or
severe, possibly damaging, stresses
oc-
curring to the pressure vessel.
B.
CONSTRUCTION
Those steam boilers designed for
15
psig and hot
water boilers designed for
zso•p
at 125 psi
or
less
are constructed in accordance with Section
IV,
Power Boilers,
of
the
ASME
Code.
Those steam boilers designed for operation
ex-
ceeding
15
psig are constructed in accordance
with Section
I,
Power Boilers,
of
the
ASME
Code. Hot water boilers for operation over
zso•p
or 125 psi are likewise built
to
this Code.
C.
WATER
REQUIREMENTS
1.
Hot Water Boiler
Air
Removal
The hot water outlet includes a dip tube which
extends
2 to 3 inches into the boiler. This dip tube
reduces the possibility
of
any air which may
be
trapped at the top
of
the shell from entering into
the system.
Any oxygen
or
air which is released in the boiler
will collect or be trapped at the top
of
the boiler
shell.
The air vent tapping
on
the top center line
of
the
boiler should be piped into the expansion
or
compression tank. Any air that
is
trapped at the
top
of
the boiler will fmd its way out
of
the boiler
through this tapping.
Minimum Boiler Water
Temperature-
The
minimum recommended operating boiler water
temperature is
170°F. When water temperatures
lower than
170oF are used, the combustion gases
are reduced
in
temperature
to
a point where the
water vapor condenses. The net result
is
that
corrosion occurs in the boiler and breeching.
CHAPTER 2·2
THE
PRESSURE VESSEL
This condensation problem
is
more severe on a
unit that operates intermittently and that is great-
ly
oversized for the actual load. This is not a
matter which can be controlled by boiler design,
since
an
efficient boiler extracts all the possible
heat
from the combustion gases. However, this
problem can
be
minimized by maintaining boiler
water temperatures above
170•F.
Another reason for maintaining boiler water
temperature above
170°F
is
to
provide a suffi-
cient temperature "head" when No. 6 fuel oil
is
to
be heated to the proper atomizing temperature
by the boiler water in a safety
type oil preheater.
(The electric preheateron the boiler must provide
additional heat
to
the oil
if
boiler water tempera-
ture is not maintained above
zoo·F.)
CAUTION!
If
the
operating
water
tempera-
ture
going
to
the
system
must
be
lower
than
170°F,
the
operat-
Ing
boiler
water
temperature
should
be a
minimum
of
170°F
(200°F
if
used
to
preheat
No. 6
oil)
and
mixing
valves
should
be
used
to
avoid
damage
to
the
equipment.
Rapid Replacement
of
Boiler
Water-
The
system layout and controls should
be
arranged to
prevent the possibility
of
pumping large quan-
tities
of
cold water into a hot boiler, thus causing
shock
or
thermal stresses. A formula,
or
"magic
number," cannot
be
given, but it should be borne
in
mind that
200oF
or
240oF
water
in
a boiler
cannot be completely replaced with
so•p
water
in
a few minutes time without causing thermal
stress. This applies to periods
of
"normal opera-
tion,"
as
well
as
during initial stan-up.
75()..150
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