TABLE 26_Steam, Its Generation and Use

RESULTS OF "IDALIA" TESTS

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| | | | | | |

|Date 1909 |Oct. 11|Oct. 14|Oct. 14|Oct. 12|Oct. 13|

|_______________________________|_______|_______|_______|_______|_______|

|Degrees of superheat Fahrenheit| 0 | 57 | 88 | 96 | 105 |

|Pressures, pounds per} Throttle| 190 | 196 | 201 | 198 | 203 |

|square inch above } First | | | | | |

|Atmospheric Pressure } Receiver| 68.4 | 66.0 | 64.3 | 61.9 | 63.0 |

| } Second | | | | | |

| } Receiver| 9.7 | 9.2 | 8.7 | 7.8 | 8.4 |

|Vacuum, inches | 25.5 | 25.9 | 25.9 | 25.4 | 25.2 |

| } Feed | 201 | 206 | 205 | 202 | 200 |

| } Hot Well | 116 | 109.5 | 115 | 111.5 | 111 |

| | | | | | |

| {Air Pump | 57 | 56 | 53 | 54 | 45 |

| {Circulating Pump| 196 | 198 | 196 | 198 | 197 |

| {Main Engine | 194.3 | 191.5 | 195.1 | 191.5 | 193.1 |

| Main Engine | 512.3 | 495.2 | 521.1 | 498.3 | 502.2 |

|Water per hour, total pounds |9397 |8430 |8234 |7902 |7790 |

| Horse Power, pounds | 18.3 | 17.0 | 15.8 | 15.8 | 15.5 |

| indicated Horse Power | 314 | 300 | 284 | 286 | 283 |

|Per cent Saving of Steam | ... | 7.1 | 13.7 | 13.7 | 15.3 |

| (computed) | ... | 4.4 | 9.5 | 8.9 | 9.9 |

|_______________________________|_______|_______|_______|_______|_______|

The table shows that the saving in steam consumption with 105 degrees of

superheat was 15.3 per cent and in heat consumption about 10 per cent.

This may be safely stated to be a conservative representation of the

saving that may be accomplished by the use of superheated steam in a

plant as a whole, where superheated steam is furnished not only to the

main engine but also to the auxiliaries. The figures may be taken as

conservative for the reason that in addition to the saving as shown in

the table, there would be in an ordinary plant a saving much greater

than is generally realized in the drips, where the loss with saturated

steam is greatly in excess of that with superheated steam.

The most conclusive and most practical evidence that a saving is

possible through the use of superheated steam is in the fact that in the

largest and most economical plants it is used almost without exception.

Regardless of any such evidence, however, there is a deep rooted

conviction in the minds of certain engineers that the use of superheated

steam will involve operating difficulties which, with additional first

cost, will more than offset any fuel saving. There are, of course,

conditions under which the installation of superheaters would in no way

be advisable. With a poorly designed superheater, no gain would result.

In general, it may be stated that in a new plant, properly designed,

with a boiler and superheater which will have an efficiency at least as

high as a boiler without a superheater, a gain is certain.

Such a gain is dependent upon the class of engine and the power plant

equipment in general. In determining the advisability of making a

superheater installation, all of the factors entering into each

individual case should be considered and balanced, with a view to

determining the saving in relation to cost, maintenance, depreciation

etc.

In highly economical plants, where the water consumption for an

indicated horse power is low, the gain will be less than would result

from the use of superheated steam in less economical plants where the

water consumption is higher. It is impossible to make an accurate

statement as to the saving possible but, broadly, it may vary from 3 to

5 per cent for 100 degrees of superheat in the large and economical

plants using turbines or steam engines, in which there is a large ratio

of expansion, to from 10 to 25 per cent for 100 degrees of superheat for

the less economical steam motors.

Though a properly designed superheater will tend to raise rather than to

decrease the boiler efficiency, it does not follow that all superheaters

are efficient, for if the gases in passing over the superheater do not

follow the path they would ordinarily take in passing over the boiler

heating surface, a loss may result. This is noticeably true where part of

the gases are passed over the superheater and are allowed to pass over

only a part or in some cases none of the boiler heating surface.

With moderate degrees of superheat, from 100 to 200 degrees, where the

piping is properly installed, there will be no greater operating

difficulties than with saturated steam. Engine and turbine builders

guarantee satisfactory operation with superheated steam. With high

degrees of superheat, say, over 250 degrees, apparatus of a special

nature must be used and it is questionable whether the additional care

and liability to operating difficulties will offset any fuel saving

accomplished. It is well established, however, that the operating

difficulties, with the degrees of superheat to which this article is

limited, have been entirely overcome.

The use of cast-iron fittings with superheated steam has been widely

discussed. It is an undoubted fact that while in some instances

superheated steam has caused deterioration of such fittings, in others

cast-iron fittings have been used with 150 degrees of superheat without

the least difficulty. The quality of the cast iron used in such fittings

has doubtless a large bearing on the life of such fittings for this

service. The difficulties that have been encountered are an increase in

the size of the fittings and eventually a deterioration great enough to

lead to serious breakage, the development of cracks, and when flanges

are drawn up too tightly, the breaking of a flange from the body of the

fitting. The latter difficulty is undoubtedly due, in certain instances,

to the form of flange in which the strain of the connecting bolts tended

to distort the metal.

The Babcock & Wilcox Co. have used steel castings in superheated steam

work over a long period and experience has shown that this metal is

suitable for the service. There seems to be a general tendency toward

the use of steel fittings. In European practice, until recently, cast

iron was used with apparently satisfactory results. The claim of

European engineers was to the effect that their cast iron was of better

quality than that found in this country and thus explained the results

secured. Recently, however, certain difficulties have been encountered

with such fittings and European engineers are leaning toward the use of

steel for this work.

The degree of superheat produced by a superheater placed within the

boiler setting will vary according to the class of fuel used, the form

of furnace, the condition of the fire and the rate at which the boiler

is being operated. This is necessarily true of any superheater swept by

the main body of the products of combustion and is a fact that should be

appreciated by the prospective user of superheated steam. With a

properly designed superheater, however, such fluctuations would not be

excessive, provided the boilers are properly operated. As a matter of

fact the point to be guarded against in the use of superheated steam is

that a maximum should not be exceeded. While, as stated, there may be a

considerable fluctuation in the temperature of the steam as delivered

from individual superheaters, where there are a number of boilers on a

line the temperature of the combined flow of steam in the main will be

found to be practically a constant, resulting from the offsetting of

various furnace conditions of one boiler by another.

[Illustration: 8400 Horse-power Installation of Babcock & Wilcox Boilers

and Superheaters at the Butler Street Plant of the Georgia Railway and

Power Co., Atlanta, Ga. This Company Operates a Total of 15,200 Horse

Power of Babcock & Wilcox Boilers]