SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER
GAUGE PRESSURE 180 POUNDS
+-----------+-----------------------------------------+
| Initial | Final Temperature--Degrees Fahrenheit |
|Temperature|-----+-----+-----+-----+-----+-----+-----|
| Fahrenheit| 120 | 140 | 160 | 180 | 200 | 250 | 300 |
+-----------+-----+-----+-----+-----+-----+-----+-----+
| 32 | 7.35| 9.02|10.69|12.36|14.04|18.20|22.38|
| 35 | 7.12| 8.79|10.46|12.14|13.82|18.00|22.18|
| 40 | 6.72| 8.41|10.09|11.77|13.45|17.65|21.86|
| 45 | 6.33| 8.02| 9.71|11.40|13.08|17.30|21.52|
| 50 | 5.93| 7.63| 9.32|11.02|12.72|16.95|21.19|
| 55 | 5.53| 7.24| 8.94|10.64|12.34|16.60|20.86|
| 60 | 5.13| 6.84| 8.55|10.27|11.97|16.24|20.52|
| 65 | 4.72| 6.44| 8.16| 9.87|11.59|15.88|20.18|
| 70 | 4.31| 6.04| 7.77| 9.48|11.21|15.52|19.83|
| 75 | 3.90| 5.64| 7.36| 9.09|10.82|15.16|19.48|
| 80 | 3.48| 5.22| 6.96| 8.70|10.44|14.79|19.13|
| 85 | 3.06| 4.80| 6.55| 8.30|10.05|14.41|18.78|
| 90 | 2.63| 4.39| 6.14| 7.89| 9.65|14.04|18.43|
| 95 | 2.20| 3.97| 5.73| 7.49| 9.25|13.66|18.07|
| 100 | 1.77| 3.54| 5.31| 7.08| 8.85|13.28|17.70|
| 110 | .89| 2.68| 4.47| 6.25| 8.04|12.50|16.97|
| 120 | .00| 1.80| 3.61| 5.41| 7.21|11.71|16.22|
| 130 | | .91| 2.73| 4.55| 6.37|10.91|15.46|
| 140 | | .00| 1.84| 3.67| 5.51|10.09|14.68|
| 150 | | | .93| 2.78| 4.63| 9.26|13.89|
| 160 | | | .00| 1.87| 3.74| 8.41|13.09|
| 170 | | | | .94| 2.83| 7.55|12.27|
| 180 | | | | .00| 1.91| 6.67|11.43|
| 190 | | | | | .96| 5.77|10.58|
| 200 | | | | | .00| 4.86| 9.71|
| 210 | | | | | | 3.92| 8.82|
+-----------+-----+-----+-----+-----+-----+-----+-----+
Besides the saving in fuel effected by the use of feed water heaters,
other advantages are secured. The time required for the conversion of
water into steam is diminished and the steam capacity of the boiler
thereby increased. Further, the feeding of cold water into a boiler has
a tendency toward the setting up of temperature strains, which are
diminished in proportion as the temperature of the feed approaches that
of the steam. An important additional advantage of heating feed water is
that in certain types of heaters a large portion of the scale forming
ingredients are precipitated before entering the boiler, with a
consequent saving in cleaning and losses through decreased efficiency
and capacity.
In general, feed water heaters may be divided into closed heaters, open
heaters and economizers; the first two depend for their heat upon
exhaust, or in some cases live steam, while the last class utilizes the
heat of the waste flue gases to secure the same result. The question of
the type of apparatus to be installed is dependent upon the conditions
attached to each individual case.
In closed heaters the feed water and the exhaust steam do not come into
actual contact with each other. Either the steam or the water passes
through tubes surrounded by the other medium, as the heater is of the
steam-tube or water-tube type. A closed heater is best suited for water
free from scale-forming matter, as such matter soon clogs the passages.
Cleaning such heaters is costly and the efficiency drops off rapidly as
scale forms. A closed heater is not advisable where the engines work
intermittently, as is the case with mine hoisting engines. In this class
of work the frequent coolings between operating periods and the sudden
heatings when operation commences will tend to loosen the tubes or even
pull them apart. For this reason, an open heater, or economizer, will
give more satisfactory service with intermittently operating apparatus.
Open heaters are best suited for waters containing scale-forming matter.
Much of the temporary hardness may be precipitated in the heater and the
sediment easily removed. Such heaters are frequently used with a reagent
for precipitating permanent hardness in the combined heat and chemical
treatment of feed water. The so-called live steam purifiers are open
heaters, the water being raised to the boiling temperature and the
carbonates and a portion of the sulphates being precipitated. The
disadvantage of this class of apparatus is that some of the sulphates
remain in solution to be precipitated as scale when concentrated in the
boiler. Sufficient concentration to have such an effect, however, may
often be prevented by frequent blowing down.
Economizers find their largest field where the design of the boiler is
such that the maximum possible amount of heat is not extracted from the
gases of combustion. The more wasteful the boiler, the greater the
saving effected by the use of the economizer, and it is sometimes
possible to raise the temperature of the feed water to that of high
pressure steam by the installation of such an apparatus, the saving
amounting in some cases to as much as 20 per cent. The fuel used bears
directly on the question of the advisability of an economizer
installation, for when oil is the fuel a boiler efficiency of 80 per
cent or over is frequently realized, an efficiency which would leave a
small opportunity for a commercial gain through the addition of an
economizer.
From the standpoint of space requirements, economizers are at a
disadvantage in that they are bulky and require a considerable increase
over space occupied by a heater of the exhaust type. They also require
additional brickwork or a metal casing, which increases the cost.
Sometimes, too, the frictional resistance of the gases through an
economizer make its adaptability questionable because of the draft
conditions. When figuring the net return on economizer investment, all
of these factors must be considered.
When the feed water is such that scale will quickly encrust the
economizer and throw it out of service for cleaning during an excessive
portion of the time, it will be necessary to purify water before
introducing it into an economizer to make it earn a profit on the
investment.
From the foregoing, it is clearly indicated that it is impossible to
make a definite statement as to the relative saving by heating feed
water in any of the three types. Each case must be worked out
independently and a decision can be reached only after an exhaustive
study of all the conditions affecting the case, including the time the
plant will be in service and probable growth of the plant. When, as a
result of such study, the possible methods for handling the problem have
been determined, the solution of the best apparatus can be made easily
by the balancing of the saving possible by each method against its first
cost, depreciation, maintenance and cost of operation.
Feeding of Water--The choice of methods to be used in introducing feed
water into a boiler lies between an injector and a pump. In most plants,
an injector would not be economical, as the water fed by such means must
be cold, a fact which makes impossible the use of a heater before the
water enters the injector. Such a heater might be installed between the
injector and the boiler but as heat is added to the water in the
injector, the heater could not properly fulfill its function.