[Illustration: Fig. 97.]
There are two forms of feed water heaters: (1) _The closed heater_,
where the feed water passes through tubes, which are enclosed in a
shell, through which the exhaust steam passes.(2) _The open heater_, in
which the steam and water come into contact. In the latter the water is
sprayed into a space, through which the exhaust steam passes, or is run
over a number of inclined perforated copper plates, mingled with the
exhaust steam.
The original feed water heater called a “pot heater,” consisted of
a vessel so constructed that the feed water was sprayed through the
exhaust steam into a globe formed tank, from the bottom of which the
heated water was pumped into the boiler; its name was originally the
“pot heater,” but as it was open to the air through the exhaust pipe,
it was, with its successively improved forms called the open heater.
All the heat imparted to the feed water, before it enters the boiler,
is so much saved, not only in the cost of fuel, but by the increased
capacity of the boiler, as the fuel in the furnace will not have this
duty to perform. There are two sources of waste heat which can be
utilized for this purpose: the chimney gases and the exhaust steam.
The gases escaping to the chimney after being reduced to the lowest
possible temperature contain a considerable quantity of heat. This
waste of heat energy may be largely saved by the device illustrated on
page 186.
[Illustration: Fig. 98.]
How much saving is obtained under any given condition is a question
requiring for its solution a careful calculation of all of the
conditions which have a bearing on the subject. Exhaust steam under
atmospheric pressure only has a sensible temperature of 212 degrees,
but exhaust steam contains also a large number of heat units which are
given up when the steam is condensed into water; for this reason it
might be thought possible to raise the temperature of the feed water a
few degrees higher even than the sensible temperature of the exhaust
steam. But this should not be expected, on account of the radiation of
heat that would occur above that of the steam.
The steam which escapes from the exhaust pipe dissipates into the
atmosphere or discharges into the condenser over nine tenths of the
heat it contained when leaving the boiler. This can be best utilized
by _exhaust feed water heaters_, for the use of live steam heaters
represents no saving in fuel, as all the heat imparted to the feed
water by their use comes directly from the boiler. The purpose for
which they are used is to elevate the temperature of the feed water
above the boiling point, so as to precipitate the sulphate of lime
and other scale forming substances, and prevent them from entering
the boiler. Neither does the heat in the feed water introduced by
an injector represent saving, as it comes from the boiler and was
generated by the fuel.
It is important to note these two statements: 1, That neither live
steam feed water heaters, nor 2, injectors save the heat from the
escaping steam.
It is also well to remember that it requires _a pound of water_ to
absorb 1.146 heat units, and that this quantity of heat is distributed
through the whole quantity of water, and _as a pound of steam is the
same as a pound of water_, it may be understood that at 212° each pound
of exhaust steam contains 1,146 heat units; ten pounds of steam contain
11,460 heat units distributed through the mass, etc.: thus, to explain
still further:
To evaporate water into steam, it must first be heated to the boiling
point, and then sufficient heat still further added to change it from
the liquid to the gaseous state, or steam. Take one pound of water at
32 degrees and heat it to the boiling point, it will have received 212°
- 32° = 180 heat units. A heat unit being the amount of heat necessary
to raise one pound of water through one degree at its greatest density.
To convert it into steam after it has been raised to the boiling point,
requires the addition of 966 heat units, which are called latent, as
they cannot be detected by the thermometer. This makes 180 + 966 =
1146 heat units, which is the total heat contained _in one pound of
water_ made into steam at the atmospheric pressure. And at atmospheric
density the volume of this steam is equal to 26.36 cubic feet, and this
amount of steam contains 1,146 units of heat, distributed throughout
the whole quantity, while the temperature at any given point at which
the thermometer may be inserted is 212 degrees. If two pounds of water
be evaporated, making a volume of 52.72 cubic feet, then the number of
heat units present would be doubled, while the temperature would still
remain at 212, the same as with one pound.
If by utilizing the heat that would otherwise go to waste, the
temperature of the feed water is raised 125 degrees, the saving would
be 125/1146 of the total amount of heat required for its evaporation,
or about 11 per cent. Thus it can be seen the percentage of saving
depends upon the initial temperature of the feed water, and the
pressure at which it is evaporated.
For example, a boiler carrying steam at 100 pounds pressure has the
temperature of the feed water raised from 60 to 200 degrees, what is
the percentage of gain?
By referring to a table pressure of “saturated steam,” it will be seen
that the total heat in steam at 100 pounds pressure is 1185 heat units.
These calculations are from 32 degrees above zero, consequently the
feed must be computed likewise.
In the first case, the heat to be supplied by the furnace is the total
heat, less that which the feed water contains, or 1185 - 28 = 1157
heat units. In the second case it is 1185 - 168 = 1017 heat units,
the difference being 1157 - 1017 = 140, which represents a saving of
140/1157 or about 12 per cent.
Where feed water is heated no more than 20 degrees above its normal
temperature the gain effected cannot amount to more than 2%, not
sufficient to pay for the introduction and maintenance of a feed water
heating device, no matter how simple, but if the temperature of the
water can be increased 60 degrees the gain will be in the neighborhood
of 5%. To make feed water heating practical and economical it would be
necessary to increase the temperature of the water about 180 degrees at
least, and to do this, using the exhaust from a non-condensing engine
without back pressure, would require such a capacity of heater as would
give fully 10 square feet of heating surface to each horse power of
work developed, and to raise the temperature above this would require a
certain amount of back pressure or an increased capacity of heater, so
that the subject resolves itself into a question of large capacity of
heater, or a higher temperature of the exhaust steam, which could only
be obtained through a given amount of back pressure.
In the same way has been calculated the following table, showing
percentages of saving of fuel by heating feed-water to various
temperatures by exhaust steam, otherwise waste:
_Percentage of saving._ (_Steam at 60 pounds gauge pressure._)
-----+--------------------------------------------------------------
Final| Initial Temperature of Water (Fahrenheit).
Temp.+--------+--------+--------+--------+--------+--------+--------
Fahr.| 32 Deg.| 40 Deg.| 50 Deg.| 60 Deg.| 70 Deg.| 80 Deg.| 90 Deg.
-----+--------+--------+--------+--------+--------+--------+--------
60 | 2.39 | 1.71 | 9.86 | … | … | … | …
80 | 4.09 | 3.43 | 2.59 | 1.74 | 0.88 | … | …
100 | 5.79 | 5.14 | 4.32 | 3.49 | 2.64 | 1.77 | .90
120 | 7.50 | 6.85 | 6.05 | 5.23 | 4.40 | 3.55 | 2.68
140 | 9.20 | 8.57 | 7.77 | 6.97 | 6.15 | 5.32 | 4.47
160 | 10.90 | 10.28 | 9.50 | 8.72 | 7.91 | 7.09 | 6.26
180 | 12.60 | 12.00 | 11.23 | 10.46 | 9.68 | 8.87 | 8.06
200 | 14.36 | 13.71 | 13.00 | 12.20 | 11.43 | 10.65 | 9.85
220 | 16.00 | 15.42 | 14.70 | 14.00 | 13.19 | 12.33 | 11.64
-----+--------+--------+--------+--------+--------+--------+--------
|100 Deg.|120 Deg.|140 Deg.|160 Deg.|180 Deg.|200 Deg.|
-----+--------+--------+--------+--------+--------+--------+--------
60 | … | … | … | … | … | … |
80 | … | … | … | … | … | … |
100 | … | … | … | … | … | … |
120 | 1.80 | … | … | … | … | … |
140 | 3.61 | 1.84 | … | … | … | … |
160 | 5.42 | 3.67 | 1.87 | … | … | … |
180 | 7.23 | 5.52 | 3.75 | 1.91 | … | … |
200 | 9.03 | 7.36 | 5.62 | 3.82 | 1.96 | … |
220 | 10.84 | 9.20 | 7.50 | 5.73 | 3.93 | 1.98 |
-----+--------+--------+--------+--------+--------+--------+--------
A good feed-water heater of adequate proportions should readily raise
the temperature of feed-water up to 200° Fahr., and, as is seen by
inspection of the table, thus effect a saving of fuel, ranging from
14.3 per cent. to 9.03 per cent., according as the atmospheric or
normal temperature of the water varies from 32° Fahr. in the height of
winter, to 100° Fahr. in the height of summer.
The percentage of saving which may be obtained from the use of exhaust
steam for heating the feed water, with which the boiler is supplied,
will depend upon the temperature to which the water is raised, and
this, in turn, will depend upon the length of time that the water
remains under the influence of the exhaust steam. This should be as
long as possible, and unless a sufficient amount of heating surface is
employed in the heater best results cannot be expected.
It does not necessarily require all the exhaust steam—or the whole
volume of waste steam passing from the engine to bring the feed water
up to the temperature desired, and the larger the heating appliance the
smaller proportion is needed—hence heaters are best made with two exits
nicely proportioned to avoid back pressure and at the same time utilize
enough of the exhaust to heat the feed water.
An impression prevails among many who are running a condenser on their
engine that a feed water heater can not be used in connection with it;
large numbers of heaters running on condensing engines with results as
follows: the feed water is delivered to the boiler at a temperature of
150° to 160° Fahr., depending on the vacuum: the higher the vacuum the
less the heat in the feed water.
A heater applied to a condensing engine generally increases the vacuum
one to two inches.
When cold water is used for the feed water, the saving in fuel by the
use of the heater is from 7 to 14 per cent.
When feed water is taken from the hot well, it will save 7 to 8 per
cent.
Where all the steam generated by a boiler is used in the engine and the
exhaust passed through a heater it is found by actual experiment, where
iron tubes are used in the heater, that approximately ten square feet
of heating surface will be required for each 30 lbs. of water supplied
to the boiler at a temperature of 200 degrees Fahr.
Ten square feet of heating surface in the feed water heater also
represents one horse power.