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18 MARINE REVIEW. [January 3, eee MECHANICAL DRAFT. ITS APPLICATION TO STATIONARY BOILERS—A SUMMARY OF ITS ADVANTAGES AS AGAINST THE OHIMNEY. Abstract of lecture by Walter B. Snow, Boston. In any consideration of mechanical draft, comparison must naturally be made between the chimney and the fan blower as a means of draft pro- duction. Inasmuch as the steam jet usually requires for its operation from five to ten times as much steam as does the fan, this method of pro- ducing draft does not properly enter the discussion. The chimney, be- cause of its method of operation, is of necessity an exceedingly inefficient device for moving air, for a relatively high temperature of the flue gases is absolutely essential to its action. As a result, from 20 to 40 per cent. of the heat of the fuel is dissipated in the atmosphere without further useful effect. This is by far the greatest waste occurring in modern boiler practice. Any attempt to utilize a portion of this waste heat necessarily reduces the temperature and lessens the draft, which, in the case of a chimney, can only be made good by increasing its height. A chimney 100 ft. high with external air at 60° and internal gases at 500° will pro- duce an unbalanced pressure of about 0.65 in. If the temperature of the gases be lowered to 300° a chimney about 150 ft. in height will be required to produce the same intensity, while for a temperature as low as 200° the chimney. will have to be about 240 ft. high. With the low chimney effective utilization of the heat of the waste gases is therefore impossible, for in order to maintain the draft they must ( Wily Seu Paeeemenns doen ‘Cabs A Si res LLL LL Ll. ni ARLINGTON MILLS, LAWRENOE, MASS. of necessity pass away at relatively high temperature. A high chimney on the other hand entails considerable expense and even then continues to wastefully employ the heat. As compared with this wasteful process of air movement a fan calls for an expenditure of only about 1-75 of the heat required by the ordinary chimney to produce the same results. Or in other words, practically all of the heat of the waste gases is rendered available for utilization and it only remains to provide the necessary means for abstracting the heat. For effective working the initial temperature of these gases must be approximately 75° above the temperature of the steam within the boiler. Between this point and the atmospheric tempera- ture is marked the range through which the heat abstractor may be effec- tive. The fuel economizer is now well established as an effective means of transferring the heat from the gases to the water fed to the boiler. It naturally shows the highest efficiency when the initial temperature of the gases is excessive, that of the water entering the economizer is low and the - fuel is high priced. The temperature of the gases necessary to produce the requisite draft with a chimney, evidently marks the limit to which they may be cooled by the economizer. With a fan, however, the intensity of the draft is practically independent of the temperature of the gases, and in point of fact when a fan is applied upon the induced principle, a decrease in the temperature actually increases the intensity of the draft produced at a given speed. It is here therefore that the fan shows its fundamental advantage over the chimney, in that it renders available for utilization practically all of the waste heat. Reports of tests made by Mr. William R. Roney in plants equipped with fuel economizers and mechan- ic apparatus in place of a chimney show an average saving Ol po ect dea ceaiing from the introduction of this combination. Another means of utilizing the heat of the flue gases is by air heaters or heat abstractors. In 1881-82 Mr. John C. Hoadley carried on at the Pacific mills, Lawrence, Mass., an exhaustive series of tests of the. Mar- land apparatus so arranged above the boiler that the gases drawn there- from by means of a blower served to heat the incoming air before it en- tered the ash pit. The result was an increase of about 300° in the tem- perature of the air so admitted and an opportunity for perfect control over its volume and pressure independently of atmospheric conditions. A maximum boiler efficiency of nearly 90 per cent. was thus secured while the steam required to drive the blower with a reasonably good engine was stated to be only 1 per cent. of that generated by its use. Although cer- tain features of this particular apparatus militated against its general use other devices such as those applied to marine boilers by Ellis & Eaves and by Howden have shown satisfactory economical results. Evidently there are limits to such attempts at saving, for they can hardly be seri- ously considered in small plants, but the field is invitingly open for further economical advancement along these lines. : In the primary introduction of mechanical draft as a substitute for the chimney, first consideration is of course to be given to the relative costs of the two devices and to the expense of their maintenance and operation. From comparison of a considerable number of plants differing widely in character and capacity it appears that under ordinary conditions a single forced draft fan with direct connected engine and short stack can be installed for less than 20 per cent. of the cost of a brick chimney; a single induced draft fan with direct connected engine and short stack will cost less than 30 per cent. of a chimney; and that a duplex induced draft apparatus consisting of two fans with direct connected engines, inlet and outlet connections and short stack will cost but little more than 40 per cent. of a chimney. The latter arrangement is only employed where, as in the case of an electric lighting plant, it is the practice to introduce relay units. Of course the height of the delivery stack must depend upon the elevation of the surrounding buildings. Where tall buildings are not ad- jacent and the boiler house is of moderate height, the smoke pipe need pass only just above the roof. This is in fact the customary practice. But where the smoke must be discharged near to other buildings it is ob- vious that the delivery pipe must be carried above their roof line. Owing, however, to the positive action of such a fan, the area of the stack may be materially less than would be required with natural draft. The cost for maintaining a mechanical draft apparatus depends to a certain extent upon the method of application. If applied to force air to the ash pits it will operate at moderate speed, will handle only cool air and will certainly have as long a life of service as the usual boiler fixtures and attachments. If employed to exhaust the gases from the boilers, it will be subjected to temperatures ranging up to perhaps 600°. Disregard- ing leakage the capacity will have to be greater in proportion to the in- creased temperature and decreased density of the gases, and the circum- ferential speed will have to be higher to produce the same intensity of draft. Within the ordinary range of temperatures the constituent gases, which are of necessity dry, have no deleterious effect upon the steel plate of which the fan is constructed. Overheating of shaft bearings is avoided by passing water through chambers provided in the boxes. The con- ditions, so far as they affect the life of the fan, are therefore practically no more adverse than in the case of a forced draft installation. As evidencing the durability of a fan operated under these conditions it may be stated that in 1892 ‘Cheney Bros. of South Manchester, Conn., in- stalled in connection with a plant of 1,000 H.P.; an induced draft fan for taking care of additional boilers and economizers, the chimney being of insufficient capacity to do the work. A recent report indicates that this fan sooNe pe effects of wear and that there is no evidence but that it will last for.a long time. The expense of operation depends of course upon the disposition of the exhaust from the fan engine. If this is utilized as it should be in any well planned plant the cost of draft production becomes almost infini- tesimal. If, however, the exhaust must be discharged to the atmosphere the actual expense for operation, as measured by the proportion of steam so used, compared with that generated, ranges from 4 to 2 per cent. in plants of reasonable size up to perhaps 3 or 4 per cent. in small plants. It may be readily shown that with the smaller amount the annual fuel expense for operating the fan will be materially less than the fixed charges on the addi- tional cost of a brick chimney to produce the same results. Any great expenditure for operation is almost universally offset by increased effi- ciency of combustion and decreased cost of fuel. It is evident that mechanical draft may be introduced either as a substitute for, or as an auxiliary to, a chimney and furthermore that it may be installed for operation on either the forced or the induced principle as may be desired. An interesting comparison between a chimney and an induced drait fan standing side by side, is presented in the accompanying illustration, which shows the arrangement at the Arlington ‘Mills, Lawrence, ‘Mass., as installed by the B. F. Sturtevant Co., Boston, Mass. The chimney serves an old battery of boilers while the fan produces draft for a new battery of nearly 100 per cent. greater capacity. It will be noted that the area of the fan outlet is less than that of the chimney, notwithstanding that it serves nearly double the boiler horse power. The fan wheel is overhung within the casing, the shaft being supported by external bearings. The gases passing through the inlet A are discharged through the pipe B which extends only a short distance above the roof. The fan is driven by belt from a horizontal engine which is placed below. Otherwise than this no valuable floor space is occupied. All types of engine-driven fans can be readily controlled by the steam pressure through the medium of a special regulating valve, or a combina- tion of damper regulator and lever valve. A very slight rise or fall in the steam pressure actuates the valve and greatly decreases or increases the amount of steam admitted to the engine. The steam pressure can thus be automatically maintained within a pound above or below the normal while the fan instantly responds to changes in conditions and particularly to sudden demands for more steam. This feature of flexibility and of ready response is almost invaluable in' plants where there are great fluctu- ations in the steam requirements, such for instance as in dye houses and bleacheries. Higher fan speed not only increases the air volume but coincidently increases the intensity of the draft, which of course facilitates the ready passage of larger volumes at higher velocity. Surplus capacity