Steam, Its Generation and Use Part 35

The heat losses are:

(A) Loss due to moisture in coal,

= .01831 ((212-81)+970.4+.47(480-212)) = 22. B. t. u., = 0.15 per cent.

(B) The loss due to the burning of hydrogen:

= 9.0560((212-81)+970.4+.47(480-212)) = 618 B. t. u., = 4.34 per cent.

(C) To compute the loss in the heat carried away by dry chimney gases per pound of coal the weight of such gases must be first determined.

This weight per pound of coal is:

(11CO_{2}+8O+7(CO+N)) (-------------------)C ( 3(CO_{2}+CO) )

where CO_{2}, O, CO and H are the percentage by volume as determined by the flue gas a.n.a.lysis and C is the percentage by weight of carbon in the dry fuel. Hence the weight of gas per pound of coal will be,

(1114.33+84.54+7(0.11+81.02)) (-----------------------------)78.57 = 13.7 pounds.

( 3(14.33+0.11) )

Therefore the loss of heat in the dry gases carried up the chimney =

13.70.24(480-81) = 1311 B. t. u., = 9.22 per cent.

(D) The loss due to incomplete combustion as evidenced by the presence of CO in the flue gas a.n.a.lysis is:

0.11 ----------.785710,150 = 61. B. t. u., 14.33+0.11 = .43 per cent.

(E) The loss due to unconsumed carbon in the ash:

The a.n.a.lysis of the ash showed 17.9 per cent to be combustible matter, all of which is a.s.sumed to be carbon. The test showed 10.00 of the total dry fuel fired to be ash. Hence 10.00.179 = 1.79 per cent of the total fuel represents the proportion of this total unconsumed in the ash and the loss due to this cause is

1.79 per cent 14,600 = 261 B. t. u., = 1.83 per cent.

The heat absorbed by the boilers per pound of dry fuel is 11.71970.4 = 11,363 B. t. u. This quant.i.ty plus losses (A), (B), (C), (D) and (E), or 11,363+22+618+1311+61+261 = 13,636 B. t. u. accounted for. The heat value of the coal, 14,225 B. t. u., less 13,636 B. t. u., leaves 589 B. t. u., unaccounted for losses, or 4.15 per cent.

The heat balance should be arranged in the form indicated by Table 58.

TABLE 58

HEAT BALANCE

B. T. U. PER POUND DRY COAL 14,225

+----------------------------------------------------------------------+ |+--------------------------------------------------------------------+| || |B. t. u.|Per Cent|| |+--------------------------------------------------+--------+--------+| ||Heat absorbed by Boiler | 11,363 | 79.88 || ||Loss due to Evaporation of Moisture in Fuel | 22 | 0.15 || ||Loss due to Moisture formed by Burning of Hydrogen| 618 | 4.34 || ||Loss due to Heat carried away in Dry Chimney Gases| 1311 | 9.22 || ||Loss due to Incomplete Combustion of Carbon | 61 | 0.43 || ||Loss due to Unconsumed Carbon in the Ash | 261 | 1.83 || ||Loss due to Radiation and Unaccounted Losses | 589 | 4.15 || |+--------------------------------------------------+--------+--------+| ||Total | 14,225 | 100.00 || |+--------------------------------------------------+--------+--------+| +----------------------------------------------------------------------+

Application of Heat Balance--A heat balance should be made in connection with any boiler trial on which sufficient data for its computation has been obtained. This is particularly true where the boiler performance has been considered unsatisfactory. The distribution of the heat is thus determined and any extraordinary loss may be detected. Where accurate data for computing such a heat balance is not available, such a calculation based on certain a.s.sumptions is sometimes sufficient to indicate unusual losses.

The largest loss is ordinarily due to the chimney gases, which depends directly upon the weight of the gas and its temperature leaving the boiler. As pointed out in the chapter on flue gas a.n.a.lysis, the lower limit of the weight of gas is fixed by the minimum air supplied with which complete combustion may be obtained. As shown, where this supply is unduly small, the loss caused by burning the carbon to CO instead of to CO_{2} more than offsets the gain in decreasing the weight of gas.

The lower limit of the stack temperature, as has been shown in the chapter on draft, is more or less fixed by the temperature necessary to create sufficient draft suction for good combustion. With natural draft, this lower limit is probably between 400 and 450 degrees.

Capacity--Before the capacity of a boiler is considered, it is necessary to define the basis to which such a term may be referred. Such a basis is the so-called boiler horse power.

The unit of motive power in general use among steam engineers is the "horse power" which is equivalent to 33,000 foot pounds per minute.

Stationary boilers are at the present time rated in horse power, though such a basis of rating may lead and has often led to a misunderstanding.

_Work_, as the term is used in mechanics, is the overcoming of resistance through s.p.a.ce, while _power_ is the _rate_ of work or the amount done per unit of time. As the operation of a boiler in service implies no motion, it can produce no power in the sense of the term as understood in mechanics. Its operation is the generation of steam, which acts as a medium to convey the energy of the fuel which is in the form of heat to a prime mover in which that heat energy is converted into energy of motion or work, and power is developed.

If all engines developed the same amount of power from an equal amount of heat, a boiler might be designated as one having a definite horse power, dependent upon the amount of engine horse power its steam would develop. Such a statement of the rating of boilers, though it would still be inaccurate, if the term is considered in its mechanical sense, could, through custom, be interpreted to indicate that a boiler was of the exact capacity required to generate the steam necessary to develop a definite amount of horse power in an engine. Such a basis of rating, however, is obviously impossible when the fact is considered that the amount of steam necessary to produce the same power in prime movers of different types and sizes varies over very wide limits.

To do away with the confusion resulting from an indefinite meaning of the term boiler horse power, the Committee of Judges in charge of the boiler trials at the Centennial Exposition, 1876, at Philadelphia, ascertained that a good engine of the type prevailing at the time required approximately 30 pounds of steam per hour per horse power developed. In order to establish a relation between the engine power and the size of a boiler required to develop that power, they recommended that an evaporation of 30 pounds of water from an initial temperature of 100 degrees Fahrenheit to steam at 70 pounds gauge pressure be considered as _one boiler horse power_. This recommendation has been generally accepted by American engineers as a standard, and when the term boiler horse power is used in connection with stationary boilers[58] throughout this country,[59] without special definition, it is understood to have this meaning.

Inasmuch as an equivalent evaporation from and at 212 degrees Fahrenheit is the generally accepted basis of comparison[60], it is now customary to consider the standard boiler horse power as recommended by the Centennial Exposition Committee, in terms of equivalent evaporation from and at 212 degrees. This will be 30 pounds multiplied by the factor of evaporation for 70 pounds gauge pressure and 100 degrees feed temperature, or 1.1494. 30 1.1494 = 34.482, or approximately 34.5 pounds. Hence, _one boiler horse power is equal to an evaporation of 34.5 pounds of water per hour from and at 212 degrees Fahrenheit_. The term boiler horse power, therefore, is clearly a measure of evaporation and not of power.

A method of basing the horse power rating of a boiler adopted by boiler manufacturers is that of heating surfaces. Such a method is absolutely arbitrary and changes in no way the definition of a boiler horse power just given. It is simply a statement by the manufacturer that his product, under ordinary operating conditions or conditions which may be specified, will evaporate 34.5 pounds of water from and at 212 degrees per definite amount of heating surface provided. The amount of heating surface that has been considered by manufacturers capable of evaporating 34.5 pounds from and at 212 degrees per hour has changed from time to time as the art has progressed. At the present time 10 square feet of heating surface is ordinarily considered the equivalent of one boiler horse power among manufacturers of stationary boilers. In view of the arbitrary nature of such rating and of the widely varying rates of evaporation possible per square foot of heating surface with different boilers and different operating conditions, such a basis of rating has in reality no particular bearing on the question of horse power and should be considered merely as a convenience.

The whole question of a unit of boiler capacity has been widely discussed with a view to the adoption of a standard to which there would appear to be a more rational and definite basis. Many suggestions have been offered as to such a basis but up to the present time there has been none which has met with universal approval or which would appear likely to be generally adopted.

With the meaning of boiler horse power as given above, that is, a measure of evaporation, it is evident that the capacity of a boiler is a measure of the power it can develop expressed in boiler horse power.

Since it is necessary, as stated, for boiler manufacturers to adopt a standard for reasons of convenience in selling, the horse power for which a boiler is sold is known as its normal rated capacity.

The efficiency of a boiler and the maximum capacity it will develop can be determined accurately only by a boiler test. The standard methods of conducting such tests are given on the following pages, these methods being the recommendations of the Power Test Committee of the American Society of Mechanical Engineers brought out in 1913.[61] Certain changes have been made to incorporate in the boiler code such portions of the "Instructions Regarding Tests in General" as apply to boiler testing.

Methods of calculation and such matter as are treated in other portions of the book have been omitted from the code as noted.

[Ill.u.s.tration: Portion of 2600 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers, Equipped with Babc.o.c.k & Wilc.o.x Chain Grate Stokers at the Peter Schoenhofen Brewing Co., Chicago, Ill.]

1. OBJECT

Ascertain the specific object of the test, and keep this in view not only in the work of preparation, but also during the progress of the test, and do not let it be obscured by devoting too close attention to matters of minor importance. Whatever the object of the test may be, accuracy and reliability must underlie the work from beginning to end.

If questions of fulfillment of contract are involved, there should be a clear understanding between all the parties, preferably in writing, as to the operating conditions which should obtain during the trial, and as to the methods of testing to be followed, unless these are already expressed in the contract itself.

Among the many objects of performance tests, the following may be noted:

Determination of capacity and efficiency, and how these compare with standard or guaranteed results.

Comparison of different conditions or methods of operation.

Determination of the cause of either inferior or superior results.

Comparison of different kinds of fuel.

Determination of the effect of changes of design or proportion upon capacity or efficiency, etc.

2. PREPARATIONS