Part 50

For stacks operating at altitude it is necessary not only to increase the height but also the diameter, as there is an added resistance within the stack due to the added friction from the additional height. This frictional loss can be compensated by a suitable increase in the diameter and when so compensated, it is evident that on the assumptions as given, the chimney height would have to be increased at a ratio inversely proportional to the square of the normal barometric pressure.

In designing a boiler for high altitudes, as already stated, the assumption is usually made that a given grade of fuel will require the same draft measured in inches of water at the boiler damper as at sea level, and this leads to making the stack height inversely as the barometric pressures, instead of inversely as the square of the barometric pressures. The correct height, no doubt, falls somewhere between the two values as larger flues are usually used at the higher altitudes, whereas to obtain the ratio of the squares, the flues must be the same size in each case, and again the effect of an increased velocity of a given weight of air through the fire at a high altitude, on the combustion, must be neglected. In making capacity tests with coal fuel, no difference has been noted in the rates of combustion for a given draft suction measured by a water column at high and low altitudes, and this would make it appear that the correct height to use is more nearly that obtained by the inverse ratio of the barometric readings than by the inverse ratio of the squares of the barometric readings. If the assumption is made that the value falls midway between the two formulae, the error in using a stack figured in the ordinary way by making the height inversely proportional to the barometric readings would differ about 10 per cent in capacity at an altitude of 10,000 feet, which difference is well within the probable variation of the size determined by different methods. It would, therefore, appear that ample accuracy is obtained in all cases by simply making the height inversely proportional to the barometric readings and increasing the diameter so that the stacks used at high altitudes have the same frictional resistance as those used at low altitudes, although, if desired, the stack may be made somewhat higher at high altitudes than this rule calls for in order to be on the safe side.

The increase of stack diameter necessary to maintain the same friction loss is inversely as the two-fifths power of the barometric pressure.

Table 54 gives the ratio of barometric readings of various altitudes to sea level, values for the square of this ratio and values of the two-fifths power of this ratio.

TABLE 54

STACK CAPACITIES, CORRECTION FACTORS FOR ALTITUDES

______________________________________________________________________ | | | | | | | Altitude | | R | | R^{2/5} | | Height in Feet | Normal | Ratio Barometer | | Ratio Increase | | Above | Barometer | Reading | R² | in Stack | | Sea Level | | Sea Level to | | Diameter | | | | Altitude | | | |______________|_________|_______________|_____|______________| | | | | | | | 0 | 30.00 | 1.000 | 1.000 | 1.000 | | 1000 | 28.88 | 1.039 | 1.079 | 1.015 | | 2000 | 27.80 | 1.079 | 1.064 | 1.030 | | 3000 | 26.76 | 1.121 | 1.257 | 1.047 | | 4000 | 25.76 | 1.165 | 1.356 | 1.063 | | 5000 | 24.79 | 1.210 | 1.464 | 1.079 | | 6000 | 23.87 | 1.257 | 1.580 | 1.096 | | 7000 | 22.97 | 1.306 | 1.706 | 1.113 | | 8000 | 22.11 | 1.357 | 1.841 | 1.130 | | 9000 | 21.28 | 1.410 | 1.988 | 1.147 | | 10000 | 20.49 | 1.464 | 2.144 | 1.165 | |______________|_________|_______________|_____|_______________|

These figures show that the altitude affects the height to a much greater extent than the diameter and that practically no increase in diameter is necessary for altitudes up to 3000 feet.

For high altitudes the increase in stack height necessary is, in some cases, such as to make the proportion of height to diameter impracticable. The method to be recommended in overcoming, at least partially, the great increase in height necessary at high altitudes is an increase in the grate surface of the boilers which the stack serves, in this way reducing the combustion rate necessary to develop a given power and hence the draft required for such combustion rate.

TABLE 55

STACK SIZES BY KENT'S FORMULA

ASSUMING 5 POUNDS OF COAL PER HORSE POWER

___________________________________________________________________ | | | | | | | | Height of Stack in Feet |Side of| | | |____________________________________________|Equiva-| | Dia- | Area | | | | | | | | | | | lent | | meter|Square| 50| 60| 70| 80 | 90 | 100| 110| 125| 150| 175|Square | |Inches| Feet |_|_|_|__|__|__|__|__|__|__| Stack | | | | |Inches | | | | Commercial Horse Power | | |____|____|____________________________________________|_____| | | | | | | | | | | | | | | | 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 | |____|____|_|_|_|__|__|__|__|__|__|__|_____| | | | | | | | | Height of Stack in Feet |Side of| | | |____________________________________________|Equiva-| | Dia- | Area | | | | | | | | | lent | | meter|Square| 100| 110 | 125 | 150 | 175 | 200 | 225 | 250 |Square | |Inches| Feet |__|___|___|___|___|___|___|___| Stack | | | | |Inches | | | | Commercial Horse Power | | |____|____|____________________________________________|_____| | | | | | | | | | | | | | 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.

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.

TABLE 56

STACK SIZES FOR OIL FUEL

ADAPTED FROM C. R. WEYMOUTH'S TABLE (TRANS. A. S. M. E. VOL. 34)

+----------------------------------------------------+ |+--------+-----------------------------------------+| || | Height in Feet Above Boiler Room Floor || ||Diameter+------+------+------+-----+--------------+| || Inches | 80 | 90 | 100 | 120 | 140 | 160 || |+--------+------+------+------+------+------+------+| || 33 | 161 | 206 | 233 | 270 | 306 | 315 || || 36 | 208 | 253 | 295 | 331 | 363 | 387 || || 39 | 251 | 303 | 343 | 399 | 488 | 467 || || 42 | 295 | 359 | 403 | 474 | 521 | 557 || || 48 | 399 | 486 | 551 | 645 | 713 | 760 || || 54 | 519 | 634 | 720 | 847 | 933 | 1000 || || 60 | 657 | 800 | 913 | 1073 | 1193 | 1280 || || 66 | 813 | 993 | 1133 | 1333 | 1480 | 1593 || || 72 | 980 | 1206 | 1373 | 1620 | 1807 | 1940 || || 84 | 1373 | 1587 | 1933 | 2293 | 2560 | 2767 || || 96 | 1833 | 2260 | 2587 | 3087 | 3453 | 3740 || || 108 | 2367 | 2920 | 3347 | 4000 | 4483 | 4867 || || 120 | 3060 | 3660 | 4207 | 5040 | 5660 | 6160 || |+--------+------+------+------+------+------+------+| +----------------------------------------------------+

Figures represent nominal rated horse power. Sizes as given good for 50 per cent overloads.

Based on centrally located stacks, short direct flues and ordinary operating efficiencies.

Table 56 gives the sizes of stacks, and horse power which they will serve for oil fuel. This table is, in modified form, one calculated by Mr. C. R. Weymouth after an exhaustive study of data pertaining to the subject, and will ordinarily give satisfactory results.

Stacks for Blast Furnace Gas Work--For boilers burning blast furnace gas, as in the case of oil-fired boilers, stack sizes as suited for coal firing will have to be modified. The diameter of stacks for this work should be approximately the same as for coal-fired boilers. The volume of gases would be slightly greater than from a coal fire and would decrease the draft with a given stack, but such a decrease due to volume is about offset by an increase due to somewhat higher temperatures in the case of the blast furnace gases.