Chapter LXVI: Power Stations (1)

The term _power station_ is usually applied to any building containing an installation of machinery for the conversion of energy from one form into another form. There are three general classes of station:

1. Central stations;
2. Sub-stations;
3. Isolated plants.

These may also be classified with respect to their function as

1. Generating stations;
2. Distributing stations;
3. Converting stations.

and with respect to the form of power used in generating the electric current, generating stations may be classed as

1. Steam electric;
2. Hydro-electric;
3. Gas electric, etc.

=Central Stations.=--It must be evident that the general type of central station to be adapted to a given case, that is to say, the general character of the machinery to be installed depends upon the kind of natural energy available for conversion into electrical energy, and the character of the electrical energy required by the consumers.

This gives rise to a further classification, as

1. Alternating current stations;
2. Direct current stations;
3. Alternating and direct current stations.

The alternators or dynamos may be driven by steam or water turbines, reciprocating engines, or gas engines, according to the character of the natural energy available.

=Ques. Why is the reciprocating engine being largely replaced by the steam turbine, especially for large units?=

Ans. Because of its higher rotative speed, and absence of a multiplicity of bearings which in the case of a high speed, reciprocating engine must be maintained in close adjustment for the proper operation of the engine.

The higher speed of rotation results in a more compact unit,
desirable for driving high frequency alternators.

=Ques. Is the steam turbine more economical than a high duty reciprocating engine?=

Ans. No.

=Location of Central Stations.=--As a rule, central stations should be so located that the average loss of voltage in overcoming the resistance of the lines is a minimum, and this point is located at the center of gravity of the system. In fig. 2,706 is shown a graphical method of locating this important spot.

Suppose a rough canvass of prospective consumers in a district
to be supplied with electric light or power shows the principal
loads to be located at A, B, C, D, E, etc., and for simplicity
assume that these loads will be approximately equal, so that each
may be denoted by 1 for example:

The relative locations of A, B, C, D, E, etc., should be drawn to scale (say 1 inch to the 1,000 feet) after which the problem resolves itself into finding the location of the station with respect to this scale.

The solution consists in first finding the center of gravity
of any two of the loads, such as those at A and B. Since each of
these is 1, they will together have the same effect on the system
as the resultant load of 1 and 1, or 2, located at their center of
gravity, this point being so chosen that the product of the loads
by their respective distances from this point will in both cases
be equal.

The loads being equal in this case the distances must be equal
in order that the products be the same, so that the center of
gravity of A + B is at G, which point is midway between A and B.

Considering, next, the resultant load of 2 at G and the load of
1 at C, the resultant load at the center or gravity of these will
be 3, and this must be situated at a distance of two units from C
and one unit from G so that the distance 2 times the load 1 at C
equals the distance 1 times the load 2 at G. Having thus located
the load 3 at H, the same method is followed in finding the load 4
at I. Then in like manner the resultant load 4 and the load 1 at E
gives a load 5 at S.

The point S being the last to be determined represents,
therefore, the position of the center of gravity of the entire
system, and consequently the proper position of the plant in order
to give the minimum loss of voltage on the lines.

=Ques. Is the center of gravity of the system, as obtained in fig. 2,706, the proper location for the central station?=

Ans. It is very rarely the best location.

=Ques. Why?=

Ans. Other conditions, such as the price of land, difficulty of obtaining water, facilities for delivery of coal and removal of ashes, etc., may more than offset the minimum line losses and copper cost due to locating the station at the center of gravity of the system.

=Ques. How then should the station be located?=

Ans. The more practical experience the designer has had, and the more common sense he possesses, the better is he equipped to handle the problem, as the solution is generally such that it cannot be worked out by any rule of thumb method.

=Ques. What are the general considerations with respect to the price of land?=

Ans. The cost for the station site may be so high as to necessitate building or renting room at a considerable distance from the district to be supplied.

If the price of land selected for the station be high, the
running expenses will be similarly affected, inasmuch as more
interest must then be paid on the capital invested.

The price or rent of real estate might also in certain
instances alter the proposed interior arrangement of the station,
particularly so in the case of a company with small capital
operating in a city where high prices prevail. In general,
however, it may be stated that whatever effect the price of real
estate would have upon the arrangement, operation and location
of a central station it can quite readily and accurately be
determined in advance.

=Ques. With respect to the cost of the land what should be especially considered?=

Ans. Room for the future extension of the plant.

Although such additional space need not be purchased at the time
of the original installation it is well, if possible, to make
provision whereby it can be obtained at a reasonable figure when
desired. The preliminary canvass of consumers will aid in deciding
the amount of space advisable to allow for future extensions; as a
rule, however, it is wise to count on the plant enlarging to not
less than twice its original size, as often the dimensions have to
be increased four and even six times those found sufficient at the
beginning.

=Ques. What trouble is likely to be encountered with an illy located plant after it is in operation?=

Ans. It may be considered a nuisance by those residing in the vicinity, occasioning many complaints.

Thus, if the plant be placed in a residential section of the
community the smoke, noise and vibration of the machines may
become a nuisance to the surrounding inhabitants, and eventually
end in suits for damage against the company responsible for the
same. For these and the other reasons just given a company is
sometimes forced to disregard entirely the location of a central
station near the center of gravity of the system, and build at a
considerable distance; such a proceeding would, if the distance be
great, necessitate the installation of a high pressure system.

There might, however, be certain local laws in force restricting
the use of high pressure currents on account of the danger
resulting to life, that would prevent this solution of the
problem. In such cases there could undoubtedly be found some site
where the objections previously noted would be tolerated; thus,
there would naturally be little objection to locating next to a
stable, a brewery, or a factory of any description.

=Ques. Why is the matter of water supply important for a central station?=

Ans. Because, in a steam driven plant, water is used in the boilers for the production of steam by boiling, and if the engines be of the condensing type it is also used in them for creating a vacuum into which the exhaust steam passes so as to increase the efficiency of the engine above what it would be if the exhaust steam were obliged to discharge into the comparatively high pressure of the atmosphere.

The force of this will be apparent by considering that the
water consumption of the engine ordinarily is from 15 to 25 lbs.
of "feed water" per horse power per hour, and the amount of
"circulating water" required to maintain the vacuum is about 25 to
30 times the feed water, and in the case of turbines with their 28
or 29 inch vacuum, much more. For instance, a 1,000 horse power
plant running on 15 lbs. of feed water and 30 to 1 circulating
water would require (1,000 × 15) × (30 + 1) = 465,000 lbs. or
55,822 gals. per hour at full capacity.

=Ques. Besides price what other considerations are important with respect to water?=

Ans. Its quality and the possibility of a scarcity of supply.

It is quite necessary that the water used in the boilers should be as free as possible from impurities, so as to prevent the deposition within them of any scale or sediments. The quality of the water used for condensing purposes, however, is not quite so important, although the purer it is the better.

If the plant is to be located in a city, the matter of water supply need not generally be considered, because, as a rule, it can be obtained from the waterworks; there will then, of course, be a water tax to consider and this, if large, may warrant an effort being made to obtain the water in some other way. In any event, however, the possibility of a scarcity in the supply should be reduced to a minimum.

If the plant be located in the country, some natural source of water would be utilized unless the place be supplied with waterworks, which is not generally the case. It is usual, however, to find a stream, lake or pond in the vicinity, but if none such be conveniently near, an artesian or other form of well must be sunk.

If abundance of water exist in the vicinity of the proposed installation, not only would the location of the plant be governed thereby, but the kind of power to be used for its operation would depend thereon. Thus, if the quantity of the water were sufficient throughout the entire year to supply the necessary power, water wheels might be installed and used in place of boilers and steam engines for driving the generators. The station would then, of course, be situated close to the waterfall, regardless of the center of gravity of the system.

=Ques. What should be noted with respect to the coal supply?=

Ans. The facility for transporting the coal from the supply point to the boiler room.

In this connection, an admirable location, other conditions
permitting, is adjacent to a railway line or water front so that
coal delivered by car or boat may be unloaded directly into the
bins supplying the boilers.

If the coal be brought by train, a side or branch track will
usually be found convenient, and this will usually render any
carting of the fuel entirely unnecessary.

In whatever way the coal is to be supplied, the liability of a
shortage due to traffic or navigation being closed at any time of
the year should be well looked into, as should also the facility
for the removal of ashes, before deciding upon the final location
for the plant.

=Choice of System.=--The chief considerations in the design of a central station are economy and capacity. When the current has to be transmitted long distances for either lighting or power purposes, economy is attainable only by reducing the weight of the copper conductors. This can be accomplished only by the use of the high voltage currents obtainable from alternators.

Again, where the consumers are located within a radius of two miles from the central station, thereby requiring a transmission voltage of 550 volts or less, dynamos may be employed with greater economy.

Alternating current possesses serious disadvantages for certain important applications.

For instance, in operating electric railways and for lighting it is often necessary to transmit direct current at 500 volts a distance of five or ten miles. In such cases, the excessive drop cannot be economically reduced by increasing the sizes of the line wire, while a sufficient increase of the voltage would cause serious variations under changes of load. Hence, it is common practice to employ some form of auxiliary generator or booster, which when connected in series with the feeder, automatically maintains the required pressure in the most remote districts so long as the main generators continue to furnish the normal or working voltage.

The advantage of a direct current installation in such cases over a similar plant supplying alternating current line is the fact that a storage battery may be used in connection with the former for taking up the fluctuations of the current, thereby permitting the dynamo to run with a less variable load, and consequently at higher efficiency.

=Ques. Name some services requiring direct current.=

Ans. Direct current is required for certain kinds of electrolytic work, such as electro-plating, the electrical separation of metals, etc., also the charging of storage batteries for electric automobiles.

=Ques. How is direct current supplied?=

Ans. Sometimes the central station is equipped with suitable apparatus for supplying both direct and alternating current. This may be accomplished in several different ways: By installing both direct and alternating current generators in the central station; by the use of double current generators or dynamotors, from which direct current may be taken from one side and alternating current from the other side; or by installing, in the sub-station of an alternating current central station, in addition to the transformers usually placed therein, a rotary converter for changing or converting alternating current into direct current.

Thus, it is evident that the character of a central station
will be governed to a great extent by the class of services to be
supplied.

An exception to this is where the entire output has to be
transmitted a long distance to the point of utilization.

In such cases a copper economy demands the use of high tension
alternating current, and its distribution to consumers may be made
directly by means of step down transformers mounted near by or
within the consumers' premises, or it may be transformed into low
voltage alternating current by a conveniently located sub-station.

Where the current is to be used chiefly for lighting and there
are only a few or no motors to be supplied, the choice between
direct current and alternating current will depend greatly upon
the size of the installation, direct current being preferable
for small installations and alternating current for large
installations.

If the current is to be used primarily for operating machinery,
such as elevators, travelling cranes, machine tools and other
devices of a similar character, which have to be operated
intermittently and at varying speeds and loads, direct current is
the more suitable; but if the motors performing such work can be
operated continuously for many hours at a time under practically
constant loads, as, for instance in the general work of a pumping
station, alternating current may be employed with advantage.

of the customer's group of lamps is 1.5 ÷ 2.5 = .6. In the diagram the ordinates of the curves show the ratio _maximum demand_ to _connected load_ for various kinds of electric lighting service in Chicago.]

[A] NOTE.--The diversity factor of a customer's group of lamps, namely, the ratio of maximum demand to connected load is usually called the _demand factor_ of the customer.

=Size of Plant.=--Before any definite calculation can be made, or plans drawn, the engineer must determine the probable load. This is usually ascertained in terms of the number and distances of lamps that will be required, by making a thorough canvass of the city or town, or that portion for which electrical energy is to be supplied. The probable load that the station is to carry when it begins operation, the nature of this load, and the probable rate of increase are matters upon which the design and construction chiefly depend.

=Ques. What is the nature of the load carried by a central station?=

Ans. It fluctuates with the time of day and also with the time of year.

=Ques. How is a fluctuating load best represented?=

Ans. Graphically, that is to say by means of a curve plotted on coordinate paper of which ordinates represent load values and the corresponding abscissæ time values, as in the accompanying curves.

=What is the nature of a power load?=

Ans. Where electricity is supplied for power purposes to a number of factories, the load is fairly steady, dropping, of course, during meal hours. In the case of traction, the average value of the load is fairly steady but there are momentarily violent fluctuations due to starting cars or trains.

=Ques. What is the peak load?=

Ans. The maximum load which has to be carried by the station at any time of day or night as shown by the highest point of the load curve.

=Ques. Define the load factor.=

Ans. The machinery of the station evidently must be large enough to carry the peak load, and therefore considerably in excess of that required for the average demand. The ratio of the average to the maximum load is called the load factor.

There are two kinds of load factor: the annual, and the daily.

The annual load factor is obtained as a percentage by
multiplying the number of units sold (per year) by 100, and
dividing by the product of the maximum load and the number of
hours in the year. The daily load factor is obtained by taking the
figures for 24 hours instead of a year.

=Ques. What must be provided in addition to the machinery required to supply the peak load?=

Ans. Additional units must be installed for use in case of repairs or break down of some of the other units.

EXAMPLE.--What would be the boiler horse power required to
generate 5,000 kw. under the following conditions: Efficiency of
generators 85%; efficiency of engines 90%; feed water of engines
and auxiliaries 15 lbs. per I. H. P.; boiler pressure 175 lbs.;
temperature of feed water 150° Fahr? With a rate of combustion of
15 lbs. of coal per sq. foot of grate per hour and an evaporation
(from and at 212°) of 8 lbs. of water per lb. of coal, what area
of grate would be required and how much heating surface?

5,000 kw. = 5,000 ÷ .746 = 6,702= electrical horse power

To obtain this electrical horse power with alternators whose
efficiency is 85% requires

6,702 ÷ .85 = 7,885 brake horse power at the engine

This, with mechanical efficiency of 90% is equivalent to

7,885 ÷ .9 = 8,761 indicated horse power

Since 15 lbs. of feed water are required for the engines and
auxiliaries per indicated horse power per hour, the total feed
water or evaporation required to generate 5,000 kw. is

15 × 8,761 = 131,415 lbs. per hour.

that is to say, the boilers must be of sufficient capacity
to generate 131,415 lbs. of steam per hour from water at a
temperature of 150° Fahr. This must be multiplied by the _factor
of evaporation_ for steam at 175 lbs. pressure from feed water at
a temperature of 150°, in order to get the equivalent evaporation
"_from and at 212_°."

The formula for the factor of evaporation is

H - _h_
factor of evaporation = ------- (1)
965.7

in which

H = total heat of steam at the observed pressure;
_h_ = total heat of feed water of the observed temperature;
965.7 = latent heat, of steam at atmospheric pressure.

Substituting in (1) values for the observed pressure and
temperature as obtained from the steam table

1,197 - 118
factor of evaporation = ------------ = 1.117
965.7

for which the equivalent evaporation "_from and at 212_°" is

131,415 × 1.117 = 146,791 lbs.= per hour

=FACTORS OF EVAPORATION=

-------------+-----------------------------------------------------+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.214|1.216|1.220|1.222|1.225|1.227|1.229|1.231|1.232|
40 |1.206|1.209|1.212|1.214|1.216|1.219|1.220|1.222|1.224|
50 |1.195|1.197|1.201|1.204|1.206|1.208|1.210|1.212|1.214|
60 |1.185|1.188|1.191|1.193|1.196|1.198|1.200|1.202|1.203|
70 |1.175|1.178|1.180|1.183|1.185|1.187|1.189|1.191|1.193|
80 |1.164|1.167|1.170|1.173|1.175|1.177|1.179|1.181|1.183|
90 |1.154|1.157|1.160|1.162|1.165|1.167|1.169|1.170|1.172|
100 |1.144|1.147|1.150|1.152|1.154|1.156|1.158|1.160|1.162|
110 |1.133|1.136|1.139|1.142|1.144|1.146|1.148|1.150|1.152|
120 |1.123|1.126|1.129|1.131|1.133|1.136|1.138|1.140|1.141|
130 |1.113|1.116|1.118|1.121|1.123|1.125|1.127|1.129|1.130|
140 |1.102|1.105|1.108|1.110|1.113|1.115|1.117|1.119|1.120|
150 |1.091|1.095|1.098|1.100|1.102|1.104|1.106|1.108|1.110|
160 |1.081|1.084|1.087|1.090|1.092|1.094|1.096|1.098|1.100|
170 |1.070|1.074|1.077|1.079|1.081|1.083|1.085|1.087|1.089|
180 |1.060|1.063|1.066|1.069|1.071|1.073|1.075|1.077|1.079|
190 |1.050|1.053|1.056|1.058|1.060|1.063|1.065|1.066|1.068|
200 |1.039|1.043|1.045|1.048|1.050|1.052|1.054|1.056|1.058|
210 |1.029|1.032|1.035|1.037|1.040|1.042|1.044|1.046|1.047|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.234|1.236|1.237|1.239|1.240|1.241|1.243|1.244|1.245|
40 |1.226|1.227|1.229|1.230|1.232|1.233|1.234|1.236|1.237|
50 |1.215|1.217|1.218|1.220|1.221|1.223|1.224|1.225|1.226|
60 |1.205|1.207|1.208|1.210|1.211|1.212|1.214|1.215|1.216|
70 |1.194|1.196|1.197|1.199|1.200|1.202|1.203|1.205|1.206|
80 |1.184|1.186|1.187|1.189|1.190|1.192|1.193|1.194|1.195|
90 |1.174|1.176|1.177|1.179|1.180|1.181|1.183|1.184|1.185|
100 |1.164|1.165|1.167|1.168|1.170|1.171|1.172|1.174|1.175|
110 |1.153|1.155|1.156|1.158|1.159|1.160|1.162|1.163|1.164|
120 |1.143|1.145|1.146|1.147|1.149|1.150|1.151|1.153|1.154|
130 |1.132|1.134|1.136|1.137|1.138|1.140|1.141|1.142|1.144|
140 |1.122|1.124|1.125|1.127|1.128|1.129|1.131|1.132|1.133|
150 |1.111|1.113|1.115|1.116|1.118|1.119|1.120|1.121|1.123|
160 |1.101|1.103|1.104|1.106|1.107|1.108|1.110|1.111|1.112|
170 |1.091|1.092|1.094|1.095|1.097|1.098|1.099|1.101|1.102|
180 |1.080|1.082|1.083|1.085|1.086|1.088|1.089|1.090|1.091|
190 |1.070|1.071|1.073|1.074|1.076|1.077|1.078|1.080|1.081|
200 |1.059|1.061|1.063|1.064|1.065|1.067|1.068|1.069|1.071|
210 |1.049|1.051|1.052|1.053|1.055|1.056|1.057|1.059|1.060|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp. of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 | |
-------------+-----+-----+-----+-----+-----+-----+-----+----+------+
32 |1.246|1.247|1.248|1.250|1.251|1.252|1.253|1.254|
40 |1.238|1.239|1.240|1.241|1.242|1.243|1.244|1.245|
50 |1.228|1.229|1.230|1.231|1.232|1.233|1.234|1.235|
60 |1.217|1.218|1.219|1.220|1.221|1.222|1.223|1.224|
70 |1.207|1.208|1.209|1.210|1.211|1.212|1.213|1.214|
80 |1.196|1.198|1.199|1.200|1.201|1.202|1.203|1.204|
90 |1.186|1.187|1.188|1.189|1.190|1.191|1.192|1.193|
100 |1.176|1.177|1.178|1.179|1.180|1.181|1.182|1.183|
110 |1.166|1.167|1.168|1.169|1.170|1.171|1.172|1.173|
120 |1.155|1.156|1.157|1.158|1.159|1.160|1.161|1.162|
130 |1.145|1.146|1.147|1.148|1.149|1.150|1.151|1.152|
140 |1.134|1.135|1.136|1.137|1.138|1.139|1.140|1.141|
150 |1.124|1.125|1.126|1.127|1.128|1.129|1.130|1.131|
160 |1.113|1.115|1.116|1.117|1.118|1.119|1.120|1.121|
170 |1.103|1.104|1.105|1.106|1.107|1.108|1.109|1.110|
180 |1.093|1.094|1.095|1.096|1.097|1.098|1.099|1.100|
190 |1.082|1.083|1.084|1.085|1.086|1.087|1.088|1.089|
200 |1.072|1.073|1.074|1.075|1.076|1.077|1.078|1.079|
210 |1.061|1.062|1.063|1.064|1.065|1.066|1.067|1.068|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+

One boiler horse power being equal to _an evaporation of_ 34½
_lbs. of water from a feed water temperature of 212° Fahr., into
steam at the same temperature_, the boiler capacity is accordingly

148,105 ÷ 34.5 = 4,293 boiler horse power.

The rate of evaporation is given at 8 lbs. of water (from and at
212° Fahr.), for which the fuel required is

148,105 ÷ 8 = 18,513 lbs. of coal per hour.

For a rate of combustion of 15 lbs. of coal per hour per square
foot of grate,

grate area = 18,513 ÷ 15 = 1,234 sq. ft.

For stationary boilers the usual ratio of heating surface to
grate area is 35:1, accordingly the heating surface corresponding
to this ratio is

1,234 × 35 = 43,190 sq.ft.

The above calculation is based on a rate of evaporation of 8
lbs. of water per lb. of coal and a rate of combustion of 15 lbs.
of coal per sq. ft. of grate. For other rates the required grate
area may be obtained from the following table:

----------------------------------------------------------------------
GRATE SURFACE PER HORSE POWER (KENT)
------------+------+-----+--------------------------------------------
|Pounds| |
| of | Lbs.| Pounds of coal burned per square foot of
|water | of | grate per hour
| from | coal+----+----+----+----+----+----+----+----+----
|and at| per | | | | | | | | |
| 212° | h.p.| 8 | 10 | 12 | 15 | 20 | 25 | 30 | 35 | 40
| per | per | | | | | | | | |
|pound | hour+----+----+----+----+----+----+----+----+----
| of | | Square feet grate per horse power
| coal | |
------------+------+-----+----+----+----+----+----+----+----+----+----
Good coal | }10 | 3.45| .43|.35 | .28| .23| .17| .14| .11| .10| .09
and boiler | } 9 | 3.83| .48| .38| .32| .25| .19| .15| .13| .11| .10
| | | | | | | | | | |
Fair coal |{8.61 | 4. | .50| .40| .33| .26| .20| .16| .13| .12| .10
or boiler |{8 | 4.31| .54| .43| .36| .29| .22| .17| .14| .13| .11
|{7 | 4.93| .62| .49| .41| .33| .24| .20| .17| .14| .12
| | | | | | | | | | |
Poor coal |{6.9 | 5. | .63| .50| .42| .34| .25| .20| .17| .15| .13
or boiler |{6 | 5.75| .72| .58| .48| .38| .29| .23| .19| .17| .14
|{5 | 6.9 | .86| .69| .58| .46| .35| .28| .23| .22| .17
| | | | | | | | | | |
Lignite and|}3.45 |10. |1.25|1.00| .83| .67| .50| .40| .33| .29| .25
poor boiler|} | | | | | | | | | |
------------+------+-----+----+----+----+----+----+----+----+----+----

=General Arrangement of Station.=--In designing an electrical station, it is preferable that whatever rooms or divisions of the interior space are desired should determine the total outside dimensions of the plant in the original plans of the building than that these latter dimensions be fixed and the rooms, etc., be fitted in afterward.

=SAVING DUE TO HEATING THE FEED WATER=

Table showing the percentage of saving for each degree of increase in
temperature of feed water heated by waste steam.

----------------------------------------------------------------------
Init| Pressure of steam in boiler, lbs. per sq. inch above atmosphere
temp|-----------------------------------------------------------------
of |
feed| 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
32°|.0872|.0861|.0855|.0851|.0847|.0844|.0841|.0839|.0837|.0835|.0833
40 |.0878|.0867|.0861|.0856|.0853|.0850|.0847|.0845|.0843|.0841|.0839
50 |.0886|.0875|.0868|.0864|.0860|.0857|.0854|.0852|.0850|.0848|.0846
60 |.0894|.0883|.0876|.0872|.0867|.0864|.0862|.0859|.0856|.0855|.0853
70 |.0902|.0890|.0884|.0879|.0875|.0872|.0869|.0867|.0864|.0862|.0860
80 |.0910|.0898|.0891|.0887|.0883|.0879|.0877|.0874|.0872|.0870|.0868
90 |.0919|.0907|.0900|.0895|.0888|.0887|.0884|.0883|.0879|.0877|.0875
100 |.0927|.0915|.0908|.0903|.0899|.0895|.0892|.0890|.0887|.0885|.0883
110 |.0936|.0923|.0916|.0911|.0907|.0903|.0900|.0898|.0895|.0893|.0891
120 |.0945|.0932|.0925|.0919|.0915|.0911|.0908|.0906|.0903|.0901|.0899
130 |.0954|.0941|.0934|.0928|.0924|.0920|.0917|.0914|.0912|.0909|.0907
140 |.0963|.0950|.0943|.0937|.0932|.0929|.0925|.0923|.0920|.0918|.0916
150 |.0973|.0959|.0951|.0946|.0941|.0937|.0934|.0931|.0929|.0926|.0924
160 |.0982|.0968|.0961|.0955|.0950|.0946|.0943|.0940|.0937|.0935|.0933
170 |.0992|.0978|.0970|.0964|.0959|.0955|.0952|.0949|.0946|.0944|.0941
180 |.1002|.0988|.0981|.0973|.0969|.0965|.0961|.0958|.0955|.0953|.0951
190 |.1012|.0998|.0989|.0983|.0978|.0974|.0971|.0968|.0964|.0062|.0960
200 |.1022|.1008|.0999|.0993|.0988|.0984|.0980|.0977|.0974|.0972|.0969
210 |.1033|.1018|.1010|.1003|.0998|.0994|.0990|.0987|.0984|.0981|.0979
220 | -- |.1029|.1019|.1013|.1008|.1004|.1000|.0997|.0994|.0991|.0989
230 | -- |.1039|.1031|.1024|.1018|.1012|.1010|.1007|.1003|.1001|.0999
240 | -- |.1050|.1041|.1034|.1029|.1024|.1020|.1017|.1014|.1011|.1009
250 | -- |.1062|.1052|.1045|.1040|.1035|.1031|.1027|.1025|.1022|.1019
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

NOTE.--An approximate rule for the conditions of ordinary
practice is a saving of 1 per cent. made by each increase of
11° in the temperature of the feed water. This corresponds to
.0909 per cent. per degree. The calculation of saving is made as
follows: Boiler pressure, 100 lbs. gauge; total heat in steam
above 32° = 1,185 B.T.U. feed water, original temperature 60°,
final temperature 209°F. Increase in heat units, 150. Heat units
above 32° in feed water of original temperature = 28. Heat units
in steam above that in cold feed water, 1,185-28 = 1,157. Saving
by the feed water heater = 150 ÷ 1,157 = 12.96 per cent. The
same result is obtained by the use of the table. Increase in
temperature 150° × tabular figure .0864 = 12.96 per cent. Let
total heat of 1 lb. of steam at the boiler pressure = H; total
heat of 1 lb. of feed water before entering the heater = _h'_, and
after passing through the heater = _h''_; then the saving made by
the heater is (_h''_-_h'_) ÷ (H-_h'_).

Under usual conditions the plans of an electrical station are readily drawn, as they are generally of a simple nature. The engines and generators will occupy the majority of the space, and these are usually placed in one large room; in some stations, however, they are located respectively in two adjacent rooms. The boilers are generally located in a room apart from the engines and dynamos, and in some cases a separate building is provided for them; the pumps, etc., must be installed not far from the boilers, and space must also be allowed near the boilers for coal and ashes.

Fig. 2,720 shows the floor plan of an electrical station, in
which a countershaft and belted connections are used between
the engines and generators. Referring first to the plan of the
building itself, A represents the engine and dynamo room, B
denotes the boiler room, C the office, D the store room, and E the
chimney connected with the boilers by means of the uptake _w_.
Referring next to the apparatus installed, S, S, S, S represents
a battery of four boilers; these are connected by steam piping
VV to the two steam engines, M and M, which are belted to the
countershaft O. Belted to the countershaft are the generators,
T, T, T, T, the circuits from which are controlled on the
switchboard, H.

=Ques. What are the objections to the arrangement shown in fig. 2,720.=?

Ans. The large space required by the belt drive especially in locations where land is expensive. Another objection is the frictional loss due to the belt drive with its countershaft, etc.

=Ques. What are the desirable features of the belt drive?=

Ans. High speed generators may be used, thus reducing the first cost, and the multiplicity of speeds and flexibility of the system resulting from the use of a friction clutch.

Thus in fig. 2,720, each pulley may be mounted on the counter
shaft O with a friction clutch. A jaw clutch may also be provided
at Z, thus permitting the shaft O to be divided into two sections.
It is therefore possible by this arrangement to cause either of
the engines to drive any one of the generators, or all of them, or
both of the engines to drive all of the generators simultaneously.

=Ques. Under what condition is the counter shaft belt drive particularly valuable?=

Ans. In case of a break down of any one of the engines or generators, and also when it becomes necessary to clean them without interrupting the service.

=Ques. How may the design in fig. 2,720 be modified for the installation of a storage battery?=

Ans. If a storage battery be necessary, a partition may be constructed across the room A, as indicated by the dotted lines, and the battery installed in the room thus formed.

=Ques. Mention a few details in the general arrangement of the building fig. 2,720.=

Ans. Two doors to the room A may conveniently be provided at K and L, the former connecting with the boiler room B, and the latter serving as the main entrance to the station. There is little that need be added to what has already been stated regarding the boiler room B. The door at F provides for the entrance of coal and the removal of ashes, while at P, the pump and heaters may conveniently be located. In the office C, visitors may be received, the station reports made out, bulletins issued from time to time, and whatever engineering problems arise may here be solved on paper by the engineer in charge of the plant. The store room D will be found convenient for various supplies, tools and appliances needed in the operation of the station. These may here be kept under lock and key and the daily waste and loss resulting from carelessness avoided.

=Ques. What important point should be noted in locating the engines and boilers?=

Ans. They should be so placed that the piping between them will be as short and direct as possible.

=Ques. Why?=

Ans. The steam pipe should be short to reduce the loss of heat between engine and boiler to a minimum, and both short and direct to avoid undue friction and consequent drop in pressure of the steam in passing through the pipe to the engine.

Entirely too little attention is given to this matter on the
part of designers and it cannot be too strongly emphasized that,
for economy, the steam pipe between an engine and boiler should
be as short and direct as possible, having regard of course, for
proper piping methods.

=Ques. What should be provided for the steam pipe?=

Ans. A heavy covering of approved material should be placed around the pipe to reduce the loss of heat by radiation. For this purpose hair felt, mineral wool and asbestos are used.

=Ques. How should the piping be arranged between the engine and condenser, and why?=

Ans. It should be as short and direct as possible; especially should elbows be avoided so that the back pressure on the engine piston will be reduced as near as can be to that of the condenser.

That is to say, in order to get nearly the full effect of the
vacuum in the condenser the frictional resistance of the piping
should be reduced to a minimum.

Where 90° turns are necessary, easy bends should be used instead
of sharp elbows. The force of this argument must be apparent by
noting the practice of steam turbine builders of placing the
turbine right up against the condenser, and remembering that a
high vacuum is necessary to the economical working of a turbine.
See fig. 1,445, page 1,182.

=Ques. What are the considerations respecting the number and type of engine to be used?=

Ans. In the illustration fig. 2,720, two engines M and M' are employed, one belted to each end of the countershaft O. These engines should be of similar or identical pattern; for a small output they may be either simple or compound, as the conditions of fuel expenditure may dictate, but if the output be large, triple expansion engines or turbines are advisable.

Corliss or similar slow speed engines may advantageously be used
in either case. In all cases the engine should be run condensing
unless the cost for circulating water is prohibitive; even in such
cases cooling towers may be installed and effect a saving.

In operation, during the greater part of the day, one engine
running two or perhaps three of the generators, will carry the
load, but when the load is particularly heavy, as in the morning
and evening, both engines and all the generators may be required
to meet the demands.

By exercising a little ingenuity in shifting the load on
different machines at different times, both engines and dynamos,
may readily be cleaned and repaired without interrupting the
service.

=Ques. For economy what kind of steam should be used?=

Ans. Super-heated steam.

The saving due to the use of superheated steam is about 1% for
every ten degrees Fahr. of super-heat. It should be used in all
cases.

=Ques. How should the machines be located?=

Ans. Sufficient space should be allowed between them that cleaning and repairing may be done easily, quickly and effectually.

=Ques. How should the switchboard be located?=

Ans. In fig. 2,720, the switchboard H is mounted against the wall dividing the room A from the room B, and is in line with the machines.

The advantages arising from a switchboard thus installed are,
that the switchboard attendant working thereon can obtain at any
time an unobstructed view of the performance of each individual
machine, and he has in consequence a much better control of them;
then, too, while he is engaged at the engines or generators he
can also see the measuring instruments on the switchboard, and
ascertain approximately the readings upon them.

In cases of emergency it is sometimes necessary for the
engineer in charge of a plant to be in several places at the same
time in order to prevent an accident, and that this seemingly
impossibility may be approximated as nearly as possible, it is
essential that the controlling devices be located as closely
together as is consistent, and that no moving belt or pulley
intervene between them.

These conditions are well satisfied in fig. 2,720, and owing to
the short distances between the generators and the switchboard the
drop of voltage in each of the conducting wires between them will
be low.

This latter advantage is worthy of notice in a station
generating large currents at a low pressure. To offset the
advantages just mentioned, the location of the switchboard in
line with the machines introduces an element of danger to the
switchboard, its apparatus, and the attendant, on account of the
possible bursting of a flywheel or other parts of the machines
from centrifugal force.

If the switchboard be placed in the dotted position at H', or,
in fact, at the opposite end of the room A, the damage to life
and property that might result from the effects of centrifugal
force would be eliminated, but in place thereof would be the
disadvantages of an obstructed view of the machines from the
switchboard, an obstructed view of the switchboard from the
machines, inaccessibility between these two, and a greater drop
of voltage in the majority of the conducting wires between the
generators and the switchboard.

=Ques. Describe a second arrangement of station with belt drive and compare it with the design shown in fig. 2,720.=

Ans. A floor plan somewhat different from that presented in fig. 2,720 is shown in fig. 2,731. Here a belt drive is employed, but no countershaft is used. Each generator, therefore, is dependent upon its respective engine, and in consequence the flexibility obtained by the use of a countershaft is lost. On the other hand, there is less loss of mechanical power between the engines and generators in the driving of the latter, and less floor space is necessary in the room A. If, however, the floor area of this room be made the same as in the previous arrangement and the same number of machines are to be installed, they may be spaced further apart, affording in consequence considerably more room for cleaning and repairing them.

In operation, the normal conditions should be such that any two
of the engine and generator sets may readily carry the average
load, the third set to be used only as a reserve either to aid the
other two when the load is unusually heavy or to replace one of
the other sets when it becomes necessary to clean or repair the
latter.

The switchboard may perhaps be best located at H, as a similar
position on the opposite side of the room A would bring it beneath
one or more of the steam pipes and thus endanger it should a
possible leakage occur from these pipes. If located at H, however,
it will be in line with the machines, and therefore will be
subject to the disadvantages previously mentioned for such cases;
consequently it might be as well to place it at the further end
of the room, either against the partition (shown dotted) of the
storage battery room if this be built, or else (if no storage
battery is to be installed), against the end wall itself. The
nearer end of the room A would not be very desirable for the
switchboard installation on account of being so far removed from
the machines, and therefore more or less inaccessible from them.
Outside of what has now been mentioned, the division of the floor
plan and the arrangement therein is practically the same as in
fig. 2,720, accordingly what has already been stated regarding the
former installation applies, therefore, with equal force to the
present installation.

=Ques. Describe a plant with direct drive.=

Ans. This type of drive is shown in fig. 2,732. Each engine is directly connected to a generator, that is, the main shafts of both are joined together in line so that the generator is driven without the aid of a belt.

=Ques. What is the advantage of direct drive?=

Ans. The great saving in floor space, which is plainly shown in fig. 2,732, the portion A' representing the saving which results over the installations previously illustrated in figs. 2,720 and 2,731.

=Ques. How could the floor space be further reduced?=

Ans. By employing vertical instead of horizontal engines.

=Ques. What should be done before drawing the plans for the station?=

Ans. The types of the various machines and apparatus to be installed should, as nearly as possible, be selected in advance so that their approximate dimensions may serve as a guide in drawing up the plans of the building.

Owing to the great difference in these dimensions for the
various types, and in fact for the same types as manufactured by
different concerns, no definite rules regarding the necessary
space required can here be given. In a general way, however, the
author has endeavoured to indicate by the drawings the relative
amounts of space that ordinarily would be considered sufficient.

=Ques. What is the disadvantage of direct drive?=

Ans. A more expensive generator is required because it must run at the same speed as the engine, which is relatively low as compared with that of a belted generator.

=Station Construction.=--The construction or rearrangement of the building intended for the plant is a problem that under ordinary conditions would be solved by an architect, or at least by an architect with the assistance of an electrical or mechanical engineer, still there are many installations where the electrical engineer has been compelled to design the building.

In such instances he should be equipped with a general knowledge of the construction of buildings.

=Foundations.=--The foundation may be either natural or artificial; that is, it may be composed of rock or soil sufficiently solid to serve the purpose unaided, or it may be such as to require strengthening by means of wood or iron beams, etc. In either case any tendency toward a considerable settling or shifting of the foundation due to the action of water, frost, etc., after the station has been completed must be well guarded against. To this end special attention should be given to the matter of drainage.

=Ques. How should the foundation be constructed for the machines?=

Ans. The foundations constructed for the machines should be entirely separate from that built for the walls of the building, so that the vibrations of the former will not affect the latter.

If there be several engines and dynamos to be installed, it is
best to construct two foundations, one for the engines and one for
the dynamos. If, however, there be considerable distance between
the units, it may be advisable to build a separate foundation
for each engine and for each dynamo. The material of which these
foundations are composed should if the machines be of 20 horse
power or over, possess considerable strength and be impervious to
moisture. Brick, stone and concrete are desirable for the purpose,
and only the best quality of cement mortar should be employed.
Care must be taken that lime mortar is not used in place of
cement mortar, as the former is not well adapted to withstand the
vibrations of the machines without crumbling.

=Ques. Describe a method of constructing foundations.=

Ans. An excavation is made to the desired depth and a form inserted corresponding to the desired dimensions for the foundation. A template is placed on top locating all the centers, with iron pipes suspended from these centers, two or three sizes larger than the anchor bolts. At the lower end of the pipes are core boxes. Concrete is poured into the mould thus formed, and when hard, the forms are removed thus leaving the solid foundation. The anchor bolts are inserted through the pipes and passed through iron plates at the lower end as shown in fig. 2,734, being secured by nuts. By using pipe of two or three bolt diameters a margin is provided for adjustment so the bolts will pass through the holes in the frame of the machine thus allowing for any slight errors in laying out the centers on the template.

=Ques. What is the object of the openings in the bottom of the foundation?=

Ans. In case of a defective bolt, it may be replaced by a new one without injury to the foundation.

=Walls.=--Regarding the material for the walls of the station iron, stone, brick and wood may be considered. Of these, iron in the form of sheets or plates would be entirely fireproof, but being itself a conductor would introduce difficulties in maintaining a high insulation resistance of the current carrying circuits; it would also make the building difficult to heat in winter and to keep cool in summer. Stone in the form of limestone, granite or sandstone, as a building material is desirable for solidity and attractiveness; it is also fireproof and an insulator, but the high cost of such a structure for an electrical station usually prohibits its use except in private plants or in electrical stations located in large cities.

Brick is a good material and is readily obtained in nearly all parts of the country; it is comparatively cheap, and is also an insulating and fireproof material. The bricks selected for this purpose should possess true sharp edges, and be hard burned.

=Ques. What are the features of wood?=

Ans. Wood forms the cheapest material that can be used for the walls of electrical stations, and it usually affords satisfaction, but has the disadvantage of high fire risk.

=Roofs=.--In fig. 2,736 is shown one form of construction for the roof of an electrical station. The end view here presented shows the upper portion of the walls at B and D; these support the iron trusses C, and the roof proper MN. In many stations there is provided throughout the length of the building, a monitor or raised structure on the peak of the roof for ventilation and light. The end view of the monitor is shown at S in the figure; its sides should be fitted with windows adjustable from the floor.

=Floors.=--The floor of the station should be so designed that it will be capable of supporting a reasonable weight, but as the weights of the machines are borne entirely by their respective foundations the normal weight upon the floor will not be great; for short periods, however, it may be called upon to support one or two machines while they are being placed in position or interchanged, and due allowance must be made for such occurrences.

Station floors for engine and dynamo rooms are, as a rule, constructed of wood. Where very high currents are generated, however, insulated floors of special construction mounted on glass are necessary as a protection from injurious shocks. Brick, concrete, cement, and other substances of a similar nature are objectionable as a floor material for engine and dynamo rooms on account of the grit from them, caused by constant wear, being liable to get into the bearings of the machines.

Where there are no moving parts, however, as in the boiler room, the materials just mentioned possess no disadvantages and are preferable to wood on account of being fireproof.

=THEORETICAL DRAFT PRESSURE IN INCHES OF WATER IN=
=A CHIMNEY 100 FEET HIGH=

(For other heights the draft varies directly as the height)

Temp. in TEMP. OF EXTERNAL AIR. (BAROMETER 30 INCHES)
Chimney, °F. 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
200° .453 .419 .384 .353 .321 .292 .263 .234 .209 .182 .157
220 .488 .453 .419 .388 .355 .326 .298 .269 .244 .217 .192
240 .520 .488 .451 .421 .388 .359 .330 .301 .276 .250 .225
260 .555 .528 .484 .453 .420 .392 .363 .334 .309 .282 .257
280 .584 .549 .515 .482 .451 .422 .394 .365 .340 .313 .288
300 .611 .576 .541 .511 .478 .449 .420 .392 .367 .340 .315
320 .637 .603 .568 .538 .505 .476 .447 .419 .394 .367 .342
340 .662 .638 .593 .563 .530 .501 .472 .443 .419 .392 .367
360 .687 .653 .618 .588 .555 .526 .497 .468 .444 .417 .392
380 .710 .676 .641 .611 .578 .549 .520 .492 .467 .440 .415
400 .732 .697 .662 .632 .598 .570 .541 .513 .488 .461 .436
420 .753 .718 .684 .653 .620 .591 .563 .534 .509 .482 .457
440 .774 .739 .705 .674 .641 .612 .584 .555 .530 .503 .478
460 .793 .758 .724 .694 .660 .632 .603 .574 .549 .522 .497
480 .810 .776 .741 .710 .678 .649 .620 .591 .566 .540 .515
500 .829 .791 .760 .730 .697 .669 .639 .610 .586 .559 .534

=Chimneys.=--These are generally constructed of brick and iron, sometimes of concrete. Iron chimneys cost less than brick chimneys, necessitate less substantial foundations, and are free from the liability of cracking. They must be painted to prevent corrosion, are less substantial, and lose considerably more heat by radiation than do brick chimneys.

Both brick and iron chimneys, require an inner wall or lining of brick, which forms the flue proper, and in order that this wall be not cracked by sudden cooling an air space is left between it and the outer wall. In a brick chimney the inner wall need not extend much beyond half the height of the chimney, but when iron is used it should reach to the top.

=Ques. Upon what does the force of natural draught in a chimney depend?=

Ans. It depends upon the difference between the weight of the column of hot gases inside the chimney and the weight of a like column of the cold external air.

=Ques. How is the intensity of the draught expressed?=

Ans. In terms of the number of inches of a water column sustained by the pressure produced.

=Ques. Are high chimneys necessary?=

Ans. No.

_Chimneys above 150 feet in height are very costly, and their
increased cost is not justified by increased efficiency._

The latest chimney practice is to build two or more small
chimneys instead of one large one. A notable example is the
Spreckels Sugar Refinery in Philadelphia, where three separate
chimneys are used for one boiler plant of 7,500 horse power. The
three chimneys are said to have cost several thousand dollars less
than an equivalent single chimney.

=Very tall chimneys= have been characterized by one writer as
"_monuments to the folly of their builders._"

=Ques. How is mechanical draft secured?=

Ans. In two ways, known respectively as _induced draught_ and _forced draught_.

=Ques. Describe the method of induced draft.=

Ans. A fan is located in the smoke flue, and which in operation draws the gases through the furnace and discharges them into a _short_ chimney.

=Ques. Describe the method of forced draft.=

Ans. In this method, air is forced into the furnace underneath the grate bars by means of a fan or a steam jet blower.

=Ques. What is the application of the two systems?=

Ans. Induced draft is installed mostly in new plants, while forced draft is better adapted to old plants.

=Steam Turbines=.--It is not the author's intention to discuss at length the steam end of the electric plant, because too much space would be required, and also because the subject belongs properly to the field of mechanical engineering rather than electrical engineering. However, because of the recent introduction of the steam turbine for the direct driving of large generators, and the fact that it is now almost universally used in large central stations, a detailed explanation of its principles and construction may not be out of place.

A turbine is a machine in which a rotary motion is obtained by transference of the _momentum_ of a fluid or gas. In general the fluid is guided by fixed blades, attached to a casing, and, impinging on other blades mounted on a drum or shaft, causing the latter to revolve.

Turbines are classed in various ways as: 1, _radial flow_, when the steam enters near the center and escapes toward the circumference; and 2, _parallel flow_, when the steam travels _axially_ or parallel to the length of the turning body.

Turbines are commonly, yet erroneously classed as:

1. Impulse;
2. Reaction.

=Ques. What is the distinction between these two types?=

Ans. In the so called impulse type, _steam enters and leaves the passages between the vanes at the same pressure_. In the so called reaction type, _the pressure is less on the exit side of the vanes than on the entrance side_.

Fig. 2,750 is a sectional view of the Parsons-Westinghouse
parallel flow turbine. Steam from the boiler enters first a
receiver in which are the governor controlled admission valves.
These valves are actuated by a centrifugal governor.

_Steam does not enter the turbine in a continuous blast, but
intermittently, or in puffs._ The speed regulation is therefore
accomplished by proportioning the duration of these puffs to the
load of the engine, this being effected by the governor, fig.
2,752.

The governor of the turbine has only to move a small pilot
valve, or slide, E, which admits steam under the piston F, and
lifts the throttle valve proper off its seat.

As soon as the pilot valve closes, the spring shifts the main
throttle valve. Thus, at light loads, the main throttle or
admission valve is continually opening and shutting at uniform
intervals, the length of time during which it remains open
depending upon the load.

As the load increases, the duration of the valve opening also
increases, until at full load the valve does not reach its seat at
all and the steam flows steadily through the turbine. The steam
thus admitted flows into the annular passage A, fig. 2,750, by the
opening S, and then past the blades, revolving the rotor.

When the load increases above the normal rated amount a
secondary pilot valve is moved by the same means, this in turn
admitting steam to a piston, similar to F, which lifts another
throttle valve. This admits steam into the annular space I, so
that it acts upon the larger diameter of the drum or rotor, giving
largely increased power for the time being.

The levers or arms of the governor are mounted upon knife edges
instead of pins, making it extremely sensitive. The tension spring
may be adjusted by hand while the turbine is running.

The governor does not actually move the pilot valve, but shifts
the point L in fig. 2,752. A reciprocating motion is given to the
rod I by a small eccentric on the governor shaft; this is driven
by worm gearing shown near O in fig. 2,750, so that the eccentric
makes one revolution to about eight of the turbine. Thus, with a
turbine running 1,200 revolutions, the rod I would be moved up and
down 150 times per minute. As the points A and H are fixed, the
motion is conveyed to the small pilot valve E, thus giving 150
puffs a minute. The governor in shifting the point L brings the
edge of the pilot valve nearer the port and so cuts off the steam
earlier.

The annular diameter or space between the rotor and the stator
is gradually increased from inlet to exhaust, the blades being
made longer in each ring. When the mechanical limit is reached,
the diameter of the rotor is increased as at I and D so as to
keep the length of blade within bound.