Part 32 (1/2)

Losses in Flues--The loss of draft in straight flues due to friction and inertia can be calculated approxiiven for loss in stacks It is to be borne in mind that C in this formula is the actual perimeter of the flue and is least, relative to the cross sectional area, when the section is a circle, is greater for a square section, and greatest for a rectangular section The retarding effect of a square flue is 12 per cent greater than that of a circular flue of the saular with sides as 1 and 1, 15 per cent greater The greater resistance of the more or less uneven brick or concrete flue is provided for in the value of the constants given for formula (26) Both steel and brick flues should be short and should have as near a circular or square cross section as possible

Abrupt turns are to be avoided, but as long easy sweeps require valuable space, it is often desirable to increase the height of the stack rather than to take up added space in the boiler roole turns reduce the draft by an ahly approximated as equal to 005 inch for each turn The turns which the gasesa horizontal flue and in turning up into a stack should always be considered The cross sectional areas of the passages leading from the boilers to the stack should be of aainst undue frictional loss It is poor economy to restrict the size of the flue and thus ht necessary to overcoeneral practice is to er than that of the stack; these should be, preferably, at least 20 per cent greater, and a safe rule to follow in figuring flue areas is to allow 35 square feet per 1000 horse power It is unnecessary to maintain the same size of flue the entire distance behind a row of boilers, and the areas at any point ases that will pass that point That is, the areas may be reduced as connections to various boilers are passed

[Illustration: 6000 Horse-power Installation of Babcock & Wilcox Boilers at the United States Navy Yard, Washi+ngton, D C]

With circular steel flues of approximately the same size as the stacks, or reduced proportionally to the voluases they will handle, a convenient rule is to allow 01 inch draft loss per 100 feet of flue length and 005 inch for each right-angle turn These figures are also good for square or rectangular steel flues with areas sufficiently large to provide against excessive frictional loss For losses in brick or concrete flues, these figures should be doubled

Underground flues are less desirable than overhead or rear flues for the reason that in ases will have to round flues are used and because the cross sectional area of such flues will oftentimes be decreased on account of an accumulation of dirt or water which it s, such as office buildings, it is frequently necessary in order to carry spent gases above the roofs, to install a stack the height of which is out of all proportion to the requirements of the boilers In such cases it is permissible to decrease the diameter of a stack, but care must be taken that this decrease is not sufficient to cause a frictional loss in the stack as great as the added draft intensity due to the increase in height, which local conditions make necessary

In such cases also the fact that the stack diameter is per to the stack should be decreased These should still be figured in proportion to the area of the stack that would be furnished under ordinary conditions or with an allowance of 35 square feet per 1000 horse power, even though the cross sectional area appears out of proportion to the stack area

Loss in Boiler--In calculating the available draft of a chiases per boiler horse power This covers an overload of the boiler to an extent of 50 per cent and provides for the use of poor coal The loss in draft through a boiler proper will depend upon its type and baffling and will increase with the per cent of rating at which it is run No figures can be given which will cover all conditions, but for approxi the available draft necessary it h a boiler will be 025 inch where the boiler is run at rating, 040 inch where it is run at 150 per cent of its rated capacity, and 070 inch where it is run at 200 per cent of its rated capacity

Loss in Furnace--The draft loss in the furnace or through the fuel bed varies betide lih the interstices of the coal on the grate Where these are large, as is the case with broken coal, but little pressure is required to force the air through the bed; but if they are small, as with bitureater pressure is needed

If the draft is insufficient the coal will accurates and a dead s poor coreat, the coal rate, leaving the fire thin in spots and a portion of the grates uncovered with the resulting losses due to an excessive amount of air

[Graph: Force of Draft between Furnace and Ash Pit--Inches of Water against Pounds of Coal burned per Square Foot of Grate Surface per Hour

Fig 34 Draft Required at Different Combustion Rates for Various Kinds of Coal]

Draft Required for Different Fuels--For every kind of fuel and rate of coeneral results are obtained A co bitue of volatile hest for the small sizes of anthracites Numerous other factors such as the thickness of fires, the percentage of ash and the air spaces in the grates bear directly on this question of the draft best suited to a given combustion rate The effect of these factors can only be found by experiment It is almost impossible to show by one set of curves the furnace draft required at various rates of combustion for all of the different conditions of fuel, etc, that ive the furnace draft necessary to burn various kinds of coal at the coeneral set of conditions These curves have been plotted froin for econo coals of the kinds noted

Rate of Combustion--The amount of coal which can be burned per hour per square foot of grate surface is governed by the character of the coal and the draft available When the boiler and grate are properly proportioned, the efficiency will be practically the same, within reasonable lirate, and the ratio of this area to the boiler heating surface will depend upon the nature of the fuel to be burned, and the stack should be so designed as to give a draft sufficient to burn the rate surface corresponding to the maximum evaporative requirements of the boiler

Solution of a Proble 33 The height can be deterh the boiler and flues, and coive this draft

Example: Proportion a stack for boilers rated at 2000 horse power, equipped with stokers, and burning bituminous coal that will evaporate 8 pounds of water frorees Fahrenheit per pound of fuel; the ratio of boiler heating surface to grate surface being 50:1; the flues being 100 feet long and containing two right-angle turns; the stack to be able to handle overloads of 50 per cent; and the rated horse power of the boilers based on 10 square feet of heating surface per horse power

The atrees Fahrenheit and the flue terees Fahrenheit

The grate surface equals 400 square feet

2000 34 The total coal burned at rating = ---------- = 8624 pounds

8

The coal per square foot of grate surface per hour at rating =

8624 ---- = 22 pounds

400

For 50 per cent overload the coreater than this or 160 22 = 35 pounds per square foot of grate surface per hour The furnace draft required for the co 34, is 06 inch The loss in the boiler will be 04 inch, in the flue 01 inch, and in the turns 2 005 = 01 inch

The available draft required at the base of the stack is, therefore,