CHAPTER LXVI

POWER STATIONS

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.

[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._

[Illustrations: FIGS. 2,741 to 2,744.--Installation of forced draft system to old boiler plant. The figures illustrate the simplest method. The fan which is of steel plate with direct connected double cylinder engine, is placed immediately over the end of a brick duct into which the air is discharged. This duct is carried under ground across the front of the boilers, to the ash pits of each of which connection is made through branch ducts. Each branch duct opening is provided with special ash pit damper, operated by notched handle bar, as illustrated in the detail. This method of introduction serves to distribute the air within the ash pit, and to secure even flow through the fuel upon the grate above. Of course, the ash pit doors must remain closed in order to bring about this result. A chimney of sufficient height to merely discharge the gases above objectionable level is all that is absolutely necessary with this arrangement. Although the introduction of a fan in an old plant is usually evidence of the insufficiency of the existing chimney to meet the requirements, such a chimney, will, however, usually serve as a discharge pipe for the gases when the fan is employed. The fan thus becomes more than a mere auxiliary to the chimney; it practically supplants it so far as the method of draught production is concerned.]

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.

Balance pistons as at B, C, F are attached to the rotor, their office being to oppose end thrust upon those blades in corresponding diameter of the rotor. Communication is established through the passage V and pipe M between the eduction pipe and the back of these pistons, thus increasing the efficiency of their balancing and also taking care of any leakage past them.

A small thrust bearing T prevents end play of the rotor, and is adjustable to maintain the proper clearance between the rings of blades; this varies from ⅛ inch at the admission to 1 inch at the exhaust. This bearing also takes up any extra unbalanced thrust. A turbine should operate with a high vacuum, because without this it does not compare favorably with an ordinary reciprocating engine from the point of economy.

_Separate air pumps are provided to create the vacuum._

Where the ordinary type of vertical air pump is employed, a booster or _vacuum increaser_ is added, as nothing below 26 inches is advisable, 28 and 29 inches being always striven for. It is also preferable to use a certain amount of _super-heat_ with steam turbines.

To assist in producing the high vacuum, exhaust passages are made large, the eduction passage E in fig. 2,750 being nearly twenty-three times the area of the steam pipe.

Among other details, a noteworthy feature is a small oil pump K, which circulates oil through bearings of the machinery, the oil being drawn from the tank under the governor shaft and gravitating there after use. No pressure of oil is employed. Stuffing rings prevent leakage; these consist of alternate grooves and collars in shaft and bearing, like the grooves in an indicator piston.

=Ques. Why is a high vacuum desirable?=

Ans. Because the turbine is capable of expanding the steam to a very low terminal pressure, and this is necessary for economy.

=Ques. What may be said of the working pressures for turbines?=

Ans. To meet the varied conditions of service, turbines are designed to operate with: 1, high pressure, 2, low pressure, or 3, mixed pressure.

High pressure turbines operate at about the same initial pressure as triple expansion engines.

Low pressure, as here applied, means the exhaust pressure of the reciprocating engine from which the exhaust steam passes through the turbine before entering the condenser.

Mixed pressure implies that the exhaust steam is supplemented, for heavy loads, by the admission of live steam.

=Ques. What determines the working pressure?=

Ans. When all the power is furnished by the turbine, it is designed for high pressure; when operated in combination with a reciprocating engine, low pressure is used for constant load, and mixed pressure for variable load.

NOTE.--There are logical engineering reasons for the existence of the several types of turbine, viz., single flow, double flow, and semi-double flow. The double flow turbine is not inherently superior to the single flow design, but is used under conditions for which the single flow machine is unsuitable. Similarly, the semi-double flow is recommended only for conditions which it can meet more satisfactorily than either of the other types.

NOTE.--Low pressure turbines use exhaust steam from non-condensing engines and are valuable as an adjunct to existing plants for the purpose of increasing economy and capacity with a minimum outlay for new equipment.

NOTE.--Bleeder turbines are for use in plants which are required to furnish, not only power, but also considerable and varying quantities of low pressure steam for heating purposes. In these turbines a part of the steam after it has done work in the high pressure stages may be diverted to the heating system, and the remainder expanded through the low pressure blading and exhausted into the condenser. In this way none of the energy of the heating steam, due to the difference of pressure between the boiler and the heating system is wasted. On the other hand if no steam is required for heating purposes, the turbine operates just as efficiently as though the bleeder feature were absent.

_The De Laval steam turbine_ is termed by its builders a high speed rotary steam engine. It has but a single wheel, fitted with vanes or buckets of such curvature as has been found to be best adapted for receiving the impulse of the steam jet. There are no stationary or guide blades, the angular position of the nozzles giving direction to the jet. The nozzles are placed at an angle of 20 degrees to the plane of motion of the buckets. The best energy in the steam is practically devoted to the production of velocity in the expanding or divergent nozzle, and the velocity thus attained by the issuing jet of steam is about 4,000 feet per second. To attain the maximum efficiency, the buckets attached to the periphery of the wheel against which this jet impinges should have a speed of about 1,900 feet per second, but, owing to the difficulty of producing a material for the wheel strong enough to withstand the strains induced by such a high speed, it has been found necessary to limit the peripheral speed to 1,200 or 1,300 feet per second.

It is well known that in a correctly designed nozzle the adiabatic expansion of the steam from maximum to minimum pressure will convert the entire static energy of the steam into kinetic energy. Theoretically this is what occurs in the De Laval nozzle. The expanding steam acquires great velocity, and the energy of the jet of steam issuing from the nozzle is equal to the amount of energy that would be developed if an equal volume of steam were allowed to adiabatically expand behind the piston of a reciprocating engine, a condition, however, which for obvious reasons has never yet been attained in practice with the reciprocating engine. But with the divergent nozzle the conditions are different.

_The Curtis turbine_ is built by the General Electric Company at their works in Schenectady, N. Y., and Lynn, Mass. They are of the horizontal and vertical types. In the vertical type the revolving parts are set upon a vertical shaft, the diameter of the shaft corresponding to the size of the machine.

The shaft is supported by and runs upon a step bearing at the bottom. This step bearing consists of two cylindrical cast iron plates bearing upon each other and having a central recess between them into which lubricating oil is forced under pressure by a steam or electrically driven pump, the oil passing up from beneath.

A weighted accumulator is sometimes installed in connection with the oil pipe as a convenient device for governing the step bearing pumps, and also as a safety device in case the pumps should fail, but it is seldom required for the latter purpose, as the step bearing pumps have proven after a long service in a number of cases, to be reliable. The vertical shaft is also held in place and kept steady by three sleeve bearings one just above the step, one between the turbine and generator, and the other near the top.

These guide bearings are lubricated by a standard gravity feed system. It is apparent that the amount of friction in the machine is very small, and as there is no end thrust caused by the action of the steam, the relation between the revolving and stationary blades may be maintained accurately. As a consequence, therefore, the clearances are reduced to the minimum.

The Curtis turbine is divided into two or more stages, and each stage has one, two or more sets of revolving blades bolted upon the peripheries of wheels keyed to the shaft. There are also the corresponding sets of stationary blades bolted to the inner walls of the cylinder or casing.

The governing of speed is accomplished in the first set of nozzles and the control of the admission valves here is effected by means of a centrifugal governor attached to the top end of the shaft. This governor, by a very slight movement, imparts motion to levers, which in turn work the valve mechanism.

The admission of steam to the nozzles is controlled by piston valves which are actuated by steam from small pilot valves which are in turn under the control of the governor.

Speed regulation is effected by varying the number of nozzles in flow, that is, for light loads fewer nozzles are open and a smaller volume of steam is admitted to the turbine wheel, but the steam that is admitted impinges against the moving blades with the same velocity always, no matter whether the volume be large or small. With a full load and all the nozzle sections in flow, the steam passes to the wheel in a broad belt and steady flow.

WEIR TABLE giving cubic feet of water per minute that will flow over a weir one inch wide and from ⅛ to 20⅞ inches deep.

--------+------+------+------+------+------+------+------+------ Depth | | | | | | | | inches | | ⅛ | ¼ | ⅜ | ½ | ⅝ | ¾ | ⅞ --------+------+------+------+------+------+------+------+------ =0= | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32 =1= | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02 =2= | 1.13 | 1.23 | 1.35 | 1.36 | 1.58 | 1.70 | 1.82 | 1.95 =3= | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05 =4= | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30 =5= | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69 =6= | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21 =7= | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83 =8= | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 |10.13 |10.35 |10.57 =9= |10.80 |11.02 |11.25 |11.48 |11.71 |11.94 |12.17 |12.41 =10= |12.64 |12.88 |13.12 |13.36 |13.60 |13.85 |14.09 |14.34 =11= |14.59 |14.84 |15.09 |15.34 |15.59 |15.85 |16.11 |16.36 =12= |16.62 |16.88 |17.15 |17.41 |17.67 |17.94 |18.21 |18.47 =13= |18.74 |19.01 |19.29 |19.56 |19.84 |20.11 |20.39 |20.67 =14= |20.95 |21.23 |21.51 |21.80 |22.08 |22.37 |22.65 |22.94 =15= |23.23 |23.52 |23.82 |24.11 |24.40 |24.70 |25.00 |25.30 =16= |25.60 |25.90 |26.20 |26.50 |26.80 |27.11 |27.42 |27.72 =17= |28.03 |28.34 |28.65 |28.97 |29.28 |29.59 |29.91 |30.22 =18= |30.54 |30.86 |31.18 |31.50 |31.82 |32.15 |32.47 |32.80 =19= |33.12 |33.45 |33.78 |34.11 |34.44 |34.77 |35.10 |35.44 =20= |35.77 |36.11 |36.45 |36.78 |37.12 |37.46 |37.80 |38.15 --------+------+------+------+------+------+------+------+------

NOTE.--The weir table on this page contains figures 1, 2, 3, etc., in the first vertical column which indicates the inches depth of water running over weir board notches. Frequently the depths measured represent also fractional inches, between 1 and 2, 2 and 3, etc. The horizontal line of fraction at the top represents these fractional parts, and can be applied between any of the numbers of inches depth, from 1 to 25. The body of the table shows the cubic feet, and the fractional parts of a cubic foot, which will pass each minute for each inch in depth, and for each fractional part of an inch by eighths for all depths from 1 to 25 inches. Each of these results is for only one inch width of weir. To estimate for any width of weir the result obtained for one inch width must be multiplied by the number of inches constituting the whole horizontal length of weir.

=Hydro-Electric Plants.=--The economy with which electricity can be transmitted long distances by high tension alternating currents, has led to the development of a large number of water powers in more or less remote regions.

This economy is possible by the facility with which alternating current can be transformed up and down. Thus at the hydro-electro plant, the current generated by the water wheel driven alternator is transformed to very high pressure and transmitted with economy a long distance to the distributing point where it is transformed down to the proper pressure for distribution.

A water wheel or turbine is a machine in which a rotary motion is obtained by transference of the momentum of water; broadly speaking, the fluid is guided by fixed blades, attached with a casing, and impinging on other blades mounted on a drum or shaft, causing the latter to revolve.

There are two general classes of turbine: 1. Impulse turbines; 2. Reaction turbines.

=Ques. What is an impulse turbine?=

Ans. One in which the fluid is directed by means of a series of nozzles against vanes which it drives.

=Ques. What is a reaction turbine?=

Ans. One in which the pressure or head of the water is employed rather than its velocity. The current is deflected upon the wheel by the action of suitably disposed guide blades, the passages being full of water. Rotary motion is obtained by the change in the direction and momentum of the fluid.

=Ques. Name three classes of reaction turbines.=

Ans. Parallel flow, inward flow, and outward flow.

Parallel flow turbines have an efficiency of about 70% and are suited for low falls not over 30 feet. Inward and outward flow turbines have an efficiency of about 85%. Impulse turbines are suitable for high heads.

=Isolated Plants.=--When electric power transmission from central stations first came into commercial use, the distance from the station at which current could be obtained at a reasonable cost was exceedingly limited.

Consequently, persons desiring electrical power were in the majority of cases forced to install their own apparatus for producing it, this being the origin of isolated plants.

From the nature of the case it is evident that an isolated plant is as a rule smaller and more simple in construction than a central station, and in consequence much more readily operated and managed. It is generally owned by a private individual or a corporation and operated in conjunction with other affairs of a similar character. A basement or other portion of a building is usually set aside in which the necessary apparatus is installed.

Although electricity is now transmitted economically to great distances from central stations, there is still a field for the isolated plant.

The average type of isolated plant has enlarged from a small dynamo driven by a little slide valve engine located in an out of the way corner to direct connected generators and engines of hundreds and even thousands of horse power assembled in a large room specially adapted to the purpose.

In the more modern of these, the electrical outputs are each frequently equal to that of a town central station of respectable size, and the auxiliary equipments are similar in every particular. As a matter of fact, in certain modern isolated plants the only feature that distinguishes them from central stations is that in the former case the owner of the plant represents the sole consumer and conducts other business in connection with it, whereas in the latter case there are a large number of consumers uninterested financially in the enterprise, which is itself generally owned and operated by a company conducting no other business.

=Sub-Stations.=--According to the usual meaning of the term, a sub-station is a building provided with apparatus for changing high pressure alternating current received from the central station into direct current of the requisite pressure, which in the case of railways is 550 to 600 volts.

Where traffic is heavy and the railway system of considerable distance, sub-stations are provided at intervals along the line, each receiving high pressure current from one large central station and converting it into moderate pressure direct current for their districts.

=Ques. Upon what does the arrangement of the sub-station depend?=

Ans. Upon the character of the work and the type of apparatus employed for converting the high pressure alternating current into direct current.

In general it should be substantial, convenient to install or replace the heavy machines, and the layout arranged so that the apparatus can be readily operated by those in attendance.

An overhead traveling crane is the most convenient method of handling the heavy machinery, and is frequently used in large sub-stations.

Fig. 2,776 shows a sectional view, and fig. 2,777, a plan for a small sub-station containing two rotary converters and two banks of three single phase static transformers operating on a three phase system at 11,000 or 13,200 volts, together with the auxiliary apparatus.

=Ques. For three phase installations, what are the merits of separate and combined transformers?=

Ans. With separate transformer for each phase, repairs are more readily made in case of accident or burnouts in the coils. The three phase units have the advantage of low first cost.

Sub-station transformers produce considerable heat, due to the hysteresis and eddy currents, and it is necessary to get rid of it.

Small transformers radiate the heat from the shell and the medium sizes have corrugated shells which increase the surface and provide more rapid radiation.

Large transformers are cooled by an air blast supplied by motor driven blowers or by water pumped through a coil of pipe which is immersed in the insulating oil of the transformer. The large size oil insulated, water cooled transformers are used on circuits of 33,000 volts or more. In water turbine plants, the water may be piped to the transformer under pressure and the pump omitted which cuts down the cost of operating. Air blast transformers usually have a damper or shutter for air control.

=Ques. Explain the use of reactance coils in sub-stations.=

Ans. In order that the direct current voltage of the ordinary rotary may be regulated by a field rheostat, which calls for a corresponding change in the alternating current voltage, a reactance coil is provided between the low tension winding and the converter.

Without such a reactance the maintenance of the same voltage at full load as at no load involves excessive leading and lagging currents and consequently excessive heating in the armature inductors, unless the resistance drop from the source of constant pressure is small, or the natural reactance of the circuit high.

=Ques. What is the effect of weakening the converter field?=

Ans. A lagging current is set up which causes a drop in the reactance coil.

=Ques. State the effect of strengthening converter field.=

Ans. A leading current is set up which gives a rise of voltage in the reactance coil.

Hence when a heavy current passes through the series coil of a compound wound converter and tends to produce a leading current, the reactance coil will balance it, and improve the power factor of the whole line.

=Portable Sub-Stations.=--A portable sub-station constitutes a spare equipment for practically any number of permanent sub-stations and renders unnecessary the installation of spare equipment in each.

It can be used to increase the capacity of a permanent sub-station when the load is unusually heavy, or to provide service while a permanent sub-station is being overhauled or rebuilt.

The transformer can be used for emergency lighting, the primary being connected to a high pressure line and the secondary to the load, if special provision be made at the time the transformer is built to adapt it for these applications.

When an electric railway has a portable sub-station, direct current can be provided at any point on the system where there is track at the high pressure line. The direct current can be made available very quickly as its production involves only the transferring of the sub-station, and its connection to the high pressure line.

Portable sub-stations range in capacity from 200 to 500 kw., and for all alternating current voltages up to 66,000, and frequencies of 25 and 60 cycles.

Although portable sub-stations usually must be of more or less special design to adapt them to the conditions under which they must operate, there are certain general features that are common to all. All members are readily accessible and there are no unnecessary parts. The weight and dimensions are a minimum insuring ease of transportation. Live parts are so protected that the danger of accidental contact with them is minimized.

=Ques. What are the advantages of using outdoor transformers on portable sub-stations?=

Ans. All high pressure wiring is kept out of the car. The transformer is more effectively cooled and the heat dissipated by the transformer does not warm the interior of the cab. The transformer is much more accessible. The car can be run under a crane and the transformer coils pulled out with a hoist.

Taps for different high and low pressure voltages can be readily provided at the time the transformer is being built.