ASSUMING 5 POUNDS OF COAL PER HORSE POWER_Steam, Its Generation and Use

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| | |______________________________________________|Equiva-|

| meter|Square| 50| 60| 70| 80 | 90 | 100| 110| 125| 150| 175|Square |

|Inches| Feet |___|___|___|____|____|____|____|____|____|____| Stack |

|______|______|______________________________________________|_______|

| | | | | | | | | | | | | |

| 33 | 5.94|106|115|125| 133| 141| 149| | | | | 30 |

| 36 | 7.07|129|141|152| 163| 173| 182| | | | | 32 |

| 39 | 8.30|155|169|183| 196| 208| 219| 229| 245| | | 35 |

| 42 | 9.62|183|200|216| 231| 245| 258| 271| 289| 316| | 38 |

| 48 | 12.57|246|269|290| 311| 330| 348| 365| 389| 426| 460| 43 |

| 54 | 15.90|318|348|376| 402| 427| 449| 472| 503| 551| 595| 48 |

| 60 | 19.64|400|437|473| 505| 536| 565| 593| 632| 692| 748| 54 |

| 66 | 23.76|490|537|580| 620| 658| 694| 728| 776| 849| 918| 59 |

| 72 | 28.27|591|646|698| 747| 792| 835| 876| 934|1023|1105| 64 |

| 78 | 33.18|700|766|828| 885| 939| 990|1038|1107|1212|1310| 70 |

| 84 | 38.48|818|896|968|1035|1098|1157|1214|1294|1418|1531| 75 |

|______|______|___|___|___|____|____|____|____|____|____|____|_______|

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| | |______________________________________________|Equiva-|

| meter|Square| 100| 110 | 125 | 150 | 175 | 200 | 225 | 250 |Square |

|Inches| Feet |____|_____|_____|_____|_____|_____|_____|_____| Stack |

|______|______|______________________________________________|_______|

| | | | | | | | | | | |

| 90 | 44.18|1338| 1403| 1496| 1639| 1770| 1893| 2008| 2116| 80 |

| 96 | 50.27|1532| 1606| 1713| 1876| 2027| 2167| 2298| 2423| 86 |

| 102 | 56.75|1739| 1824| 1944| 2130| 2300| 2459| 2609| 2750| 91 |

| 108 | 63.62|1959| 2054| 2190| 2392| 2592| 2770| 2939| 3098| 98 |

| 114 | 70.88|2192| 2299| 2451| 2685| 2900| 3100| 3288| 3466| 101 |

| 120 | 78.54|2438| 2557| 2726| 2986| 3226| 3448| 3657| 3855| 107 |

| 126 | 86.59|2697| 2829| 3016| 3303| 3568| 3814| 4046| 4265| 112 |

| 132 | 95.03|2970| 3114| 3321| 3637| 3929| 4200| 4455| 4696| 117 |

| 144 |113.10|3554| 3726| 3973| 4352| 4701| 5026| 5331| 5618| 128 |

| 156 |132.73|4190| 4393| 4684| 5131| 5542| 5925| 6285| 6624| 138 |

| 168 |153.94|4878| 5115| 5454| 5974| 6454| 6899| 7318| 7713| 150 |

|______|______|____|_____|_____|_____|_____|_____|_____|_____|_______|

Kent's Stack Tables--Table 55 gives, in convenient form for approximate

work, the sizes of stacks and the horse power of boilers which they will

serve. This table is a modification of Mr. William Kent's stack table

and is calculated from his formula. Provided no unusual conditions are

encountered, it is reliable for the ordinary rates of combustion with

bituminous coals. It is figured on a consumption of 5 pounds of coal

burned per hour per boiler horse power developed, this figure giving a

fairly liberal allowance for the use of poor coal and for a reasonable

overload. When the coal used is a low grade bituminous of the Middle or

Western States, it is strongly recommended that these sizes be increased

materially, such an increase being from 25 to 60 per cent, depending

upon the nature of the coal and the capacity desired. For the coal

burned per hour for any size stack given in the table, the values should

be multiplied by 5.

A convenient rule for large stacks, 200 feet high and over, is to

provide 30 square feet of cross sectional area per 1000 rated horse

power.

Stacks for Oil Fuel--The requirements of stacks connected to boilers

under which oil fuel is burned are entirely different from those where

coal is used. While more attention has been paid to the matter of stack

sizes for oil fuel in recent years, there has not as yet been gathered

the large amount of experimental data available for use in designing

coal stacks.

In the case of oil-fired boilers the loss of draft through the fuel bed

is partially eliminated. While there may be practically no loss through

any checkerwork admitting air to the furnace when a boiler is new, the

areas for the air passage in this checkerwork will in a short time be

decreased, due to the silt which is present in practically all fuel oil.

The loss in draft through the boiler proper at a given rating will be

less than in the case of coal-fired boilers, this being due to a

decrease in the volume of the gases. Further, the action of the oil

burner itself is to a certain extent that of a forced draft. To offset

this decrease in draft requirement, the temperature of the gases

entering the stack will be somewhat lower where oil is used than where

coal is used, and the draft that a stack of a given height would give,

therefore, decreases. The factors as given above, affecting as they do

the intensity of the draft, affect directly the height of the stack to

be used.

As already stated, the volume of gases from oil-fired boilers being less

than in the case of coal, makes it evident that the area of stacks for

oil fuel will be less than for coal. It is assumed that these areas will

vary directly as the volume of the gases to be handled, and this volume

for oil may be taken as approximately 60 per cent of that for coal.

In designing stacks for oil fuel there are two features which must not

be overlooked. In coal-firing practice there is rarely danger of too

much draft. In the burning of oil, however, this may play an important

part in the reduction of plant economy, the influence of excessive draft

being more apparent where the load on the plant may be reduced at

intervals. The reason for this is that, aside from a slight decrease in

temperature at reduced loads, the tendency, due to careless firing, is

toward a constant gas flow through the boiler regardless of the rate of

operation, with the corresponding increase of excess air at light loads.

With excessive stack height, economical operation at varying loads is

almost impossible with hand control. With automatic control, however,

where stacks are necessarily high to take care of known peaks, under

lighter loads this economical operation becomes less difficult. For this

reason the question of designing a stack for a plant where the load is

known to be nearly a constant is easier than for a plant where the load

will vary over a wide range. While great care must be taken to avoid

excessive draft, still more care must be taken to assure a draft suction

within all parts of the setting under any and all conditions of

operation. It is very easily possible to more than offset the economy

gained through low draft, by the losses due to setting deterioration,

resulting from such lack of suction. Under conditions where the suction

is not sufficient to carry off the products of combustion, the action of

the heat on the setting brickwork will cause its rapid failure.

[Illustration: 7800 Horse-power Installation of Babcock & Wilcox

Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the

Metropolitan West Side Elevated Ry. Co., Chicago, Ill.]

It becomes evident, therefore, that the question of stack height for

oil-fired boilers is one which must be considered with the greatest of

care. The designer, on the one hand, must guard against the evils of

excessive draft with the view to plant economy, and, on the other,

against the evils of lack of draft from the viewpoint of upkeep cost.

Stacks for this work should be proportioned to give ample draft for the

maximum overload that a plant will be called upon to carry, all

conditions of overload carefully considered. At the same time, where

this maximum overload is figured liberally enough to insure a draft

suction within the setting under all conditions, care must be taken

against the installation of a stack which would give more than this

maximum draft.