ACTUAL EVAPORATION FOR DIFFERENT PRESSURES AND TEMPERATURES OF FEED
WATER CORRESPONDING TO ONE HORSE POWER (34½ POUNDS PER HOUR FROM AND AT 212 DEGREES FAHRENHEIT)
-----------------------------------------------------------------------------------------------------------------------------------------
Temperature| |
of | Pressure by Gauge--Pounds per Square Inch |
Feed | |
Degrees | |
Fahrenheit | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | 230 | 240 | 250 |
-----------+-----------------------------------------------------------------------------------------------------------------------------|
32 |28.41|28.36|28.29|28.24|28.20|28.16|28.13|28.09|28.07|28.04|28.02|27.99|27.97|27.95|27.94|27.92|27.90|27.89|27.87|27.86|27.83|
40 |28.61|28.54|28.49|28.44|28.40|28.35|28.32|28.29|28.26|28.23|28.21|28.18|28.16|28.14|28.12|28.11|28.09|28.07|28.06|28.05|28.03|
50 |28.85|28.79|28.73|28.68|28.64|28.60|28.56|28.53|28.50|28.47|28.45|28.43|28.40|28.38|28.36|28.35|28.33|28.31|28.30|28.28|28.27|
60 |29.10|29.04|28.98|28.93|28.88|28.84|28.81|28.77|28.74|28.72|28.69|28.67|28.65|28.62|28.60|28.59|28.57|28.55|28.54|28.52|28.51|
70 |29.36|29.29|29.23|29.18|29.14|29.09|29.06|29.02|28.99|28.96|28.94|28.92|28.89|28.87|28.85|28.83|28.82|28.80|28.78|28.77|28.76|
80 |29.62|29.55|29.49|29.44|29.39|29.35|29.31|29.27|29.24|29.22|29.19|29.17|29.14|29.12|29.10|29.08|29.07|29.05|29.03|29.02|29.00|
90 |29.88|29.81|29.75|29.70|29.65|29.61|29.57|29.53|29.50|29.47|29.45|29.42|29.40|29.38|29.36|29.34|29.32|29.30|29.29|29.27|29.25|
100 |30.15|30.08|30.02|29.96|29.91|29.87|29.83|29.80|29.76|29.73|29.71|29.68|29.66|29.63|29.61|29.60|29.58|29.56|29.54|29.53|29.51|
110 |30.42|30.35|30.29|30.23|30.18|30.14|30.10|30.06|30.03|30.00|29.97|29.95|29.92|29.90|29.88|29.86|29.84|29.82|29.81|29.79|29.77|
120 |30.70|30.63|30.56|30.51|30.46|30.41|30.37|30.33|30.30|30.27|30.24|30.22|30.19|30.17|30.15|30.13|30.11|30.09|30.07|30.06|30.04|
130 |30.99|30.91|30.84|30.79|30.73|30.69|30.65|30.61|30.57|30.54|30.52|30.49|30.47|30.44|30.42|30.40|30.38|30.36|30.35|30.33|30.31|
140 |31.28|31.20|31.13|31.07|31.02|30.97|30.93|30.89|30.86|30.83|30.80|30.77|30.75|30.72|30.70|30.68|30.66|30.64|30.62|30.61|30.59|
150 |31.58|31.49|31.42|31.36|31.31|31.26|31.22|31.18|31.14|31.11|31.08|31.06|31.03|31.01|30.98|30.96|30.94|30.92|30.91|30.89|30.87|
160 |31.87|31.79|31.72|31.66|31.61|31.56|31.51|31.47|31.44|31.40|31.37|31.35|31.32|31.29|31.27|31.25|31.23|31.21|31.19|31.18|31.16|
170 |32.18|32.10|32.02|31.96|31.91|31.86|31.81|31.77|31.73|31.70|31.67|31.64|31.62|31.59|31.57|31.54|31.52|31.50|31.49|31.47|31.46|
180 |32.49|32.41|32.33|32.27|32.22|32.16|32.12|32.08|32.04|32.00|31.97|31.95|31.92|31.89|31.87|31.84|31.82|31.80|31.79|31.77|31.75|
190 |32.81|32.72|32.65|32.59|32.53|32.47|32.43|32.38|32.35|32.32|32.29|32.26|32.23|32.20|32.17|32.15|32.13|32.11|32.09|32.07|32.05|
200 |33.13|33.05|32.97|32.91|32.85|32.79|32.75|32.70|32.66|32.63|32.60|32.57|32.54|32.51|32.49|32.46|32.44|32.42|32.40|32.38|32.36|
210 |33.47|33.38|33.30|33.24|33.18|33.13|33.08|33.03|32.99|32.95|32.92|32.89|32.86|32.83|32.81|32.79|32.76|32.74|32.72|32.70|32.68|
-----------------------------------------------------------------------------------------------------------------------------------------
The proper consideration can be given to the adaptability of any boiler
for the service in view only after a thorough understanding of the
requirements of a good steam boiler, with the application of what has
been said on the proper operation to the special requirements of each
case. Of almost equal importance to the factors mentioned are the
experience, the skill and responsibility of the manufacturer.
With the design of boiler selected that is best adapted to the service
required, the next step is the determination of the boiler power
requirements.
The amount of steam that must be generated is determined from the steam
consumption of the prime movers. It has already been indicated that such
consumption can vary over wide limits with the size and type of the
apparatus used, but fortunately all types have been so tested that
manufacturers are enabled to state within very close limits the actual
consumption under any given set of conditions. It is obvious that
conditions of operation will have a bearing on the steam consumption
that is as important as the type and size of the apparatus itself. This
being the case, any tabular information that can be given on such steam
consumption, unless it be extended to an impracticable size, is only of
use for the most approximate work and more definite figures on this
consumption should in all cases be obtained from the manufacturer of the
apparatus to be used for the conditions under which it will operate.
To the steam consumption of the main prime movers, there is to be added
that of the auxiliaries. Again it is impossible to make a definite
statement of what this allowance should be, the figure depending wholly
upon the type and the number of such auxiliaries. For approximate work,
it is perhaps best to allow 15 or 20 per cent of the steam requirements
of the main engines, for that of auxiliaries. Whatever figure is used
should be taken high enough to be on the conservative side.
When any such figures are based on the actual weight of steam required,
Table 60, which gives the actual evaporation for various pressures and
temperatures of feed corresponding to one boiler horse power (34.5
pounds of water per hour from and at 212 degrees), may be of service.
With the steam requirements known, the next step is the determination of
the number and size of boiler units to be installed. This is directly
affected by the capacity at which a consideration of the economical load
indicates is the best for the operating conditions which will exist. The
other factors entering into such determination are the size of the plant
and the character of the feed water.
The size of the plant has its bearing on the question from the fact that
higher efficiencies are in general obtained from large units, that labor
cost decreases with the number of units, the first cost of brickwork is
lower for large than for small size units, a general decrease in the
complication of piping, etc., and in general the cost per horse power of
any design of boiler decreases with the size of units. To illustrate
this, it is only necessary to consider a plant of, say, 10,000 boiler
horse power, consisting of 40-250 horse-power units or 17-600
horse-power units.
The feed water available has its bearing on the subject from the other
side, for it has already been shown that very large units are not
advisable where the feed water is not of the best.
The character of an installment is also a factor. Where, say, 1000 horse
power is installed in a plant where it is known what the ultimate
capacity is to be, the size of units should be selected with the idea of
this ultimate capacity in mind rather than the amount of the first
installation.
Boiler service, from its nature, is severe. All boilers have to be
cleaned from time to time and certain repairs to settings, etc., are a
necessity. This makes it necessary, in determining the number of boilers
to be installed, to allow a certain number of units or spares to be
operated when any of the regular boilers must be taken off the line.
With the steam requirements determined for a plant of moderate size and
a reasonably constant load, it is highly advisable to install at least
two spare boilers where a continuity of service is essential. This
permits the taking off of one boiler for cleaning or repairs and still
allows a spare boiler in the event of some unforeseen occurrence, such
as the blowing out of a tube or the like. Investment in such spare
apparatus is nothing more nor less than insurance on the necessary
continuity of service. In small plants of, say, 500 or 600 horse power,
two spares are not usually warranted in view of the cost of such
insurance. A large plant is ordinarily laid out in a number of sections
or panels and each section should have its spare boiler or boilers even
though the sections are cross connected. In central station work, where
the peaks are carried on the boilers brought up from the bank, such
spares are, of course, in addition to these banked boilers. From the
aspect of cleaning boilers alone, the number of spare boilers is
determined by the nature of any scale that may be formed. If scale is
formed so rapidly that the boilers cannot be kept clean enough for good
operating results, by cleaning in rotation, one at a time, the number of
spares to take care of such proper cleaning will naturally increase.
In view of the above, it is evident that only a suggestion can be made
as to the number and size of units, as no recommendation will hold for
all cases. In general, it will be found best to install units of the
largest possible size compatible with the size of the plant and
operating conditions, with the total power requirements divided among
such a number of units as will give proper flexibility of load, with
such additional units for spares as conditions of cleaning and insurance
against interruption of service warrant.
In closing the subject of the selection of boilers, it may not be out of
place to refer to the effect of the builder's guarantee upon the
determination of design to be used. Here in one of its most important
aspects appears the responsibility of the manufacturer. Emphasis has
been laid on the difference between test results and those secured in
ordinary operating practice. That such a difference exists is well known
and it is now pretty generally realized that it is the responsible
manufacturer who, where guarantees are necessary, submits the
conservative figures, figures which may readily be exceeded under test
conditions and which may be closely approached under the ordinary plant
conditions that will be met in daily operation.