The small size of the fan makes it very convenient "The general arrangement of the draft system includes a steel stack designed on the ejector principle. Near the bottom of the stack a section formed like an inverted frustum of cone contains the nozzle through which the pressure air is discharged; the next section of the stack is the throat, which is cylindrical in form, and from that point up the stack flares outward, acting as a diffuser. The draft is formed in the stack by the discharge of the 'motive gas' at relatively high velocity. "There are two general arrangements of the Prat system in use, the first employing atmospheric air for the motive gas, and the second taking a small portion of the stack gases for this purpose. The latter scheme gives higher efficiency, as there is no chilling of the stack gases and consequent reduction of the natural draft available; the total amount of gas to be handled through the stack is also kept at a minimum. Construction "The stack is made of heavy sheet steel; although there has been some prejudice against steel stacks on account of their high depreciation it is possible by proper design and care to obtain a life of from fifteen to forty years. The greatest care has been put into the design of the fan, which is simple and robust. Special external blading is used for cooling the bearings, this taking the place of water cooling. Arrangement of Emergency Steam Jet System "A disadvantage of the direct induced draft system is that in case of fan or motor failure the entire unit is shut down unless a spare fan is provided,-which involves high expense and great difficulty in installation. In the Prat system this reserve takes the form of a steam jet inserted in the nozzle, and this can be installed simply and cheaply. It can also be used in conjunction with the normal Prat draft at times of emergency. Characteristics of Prat System "The Prat system makes the best possible use of natural draft as derived from the heat in the chimney gases, and the introduction of the 'pressure transformer' reduces this natural draft by a very small amount-as stated hereinbefore. "Comparing the Prat system with induced draft, the latter has to furnish enough additional power to overcome the extra resistance caused by the fan itself in the circuit and to compensate for the relatively low draft efficiency of the fan, due to improper relations of fan size, discharge orifice, etc. The Prat draft, for ratings from that possible by natural draft up to moderate overloads, requires therefore a considerably smaller power input to the fan. Practical Advantages of Prat Draft "Mechanical draft, although fifteen years ago meeting with a great deal of opposition, has now become standard for new installations and for additions to old. In 1901 the Prat draft systems installed totaled about 15,000 bhp; installations increased rapidly until in 1911 the total was 245,000 bhp. The largest plant using the Prat draft is that of the Paris Company for Electrical Distribution, which has that system installed on 180,000 bhp of its boilers. The principal advantages are flexibility of steam production, economy in combustion made possible by the ease of air control, and reliability of the draft produced. Independent Unit Installations "The Prat system of draft, as compared with large stacks for natural draft, permits the making of one large boiler with economizer and draft system, a single unit, with the consequent tendency to reduce service losses due to equipment out of operation. Reduction in Investment Cost "As compared with natural draft systems, the Prat system permits using smaller or fewer boilers, with a consequently smaller plant and resultant lower cost. As an instance, a certain French plant was estimated to cost about $330,000 using natural draft, and about $260,000 with the Prat system. The Prat Tuyere in Brick Stacks "It is entirely possible to install the Prat system in large existing stacks designed for natural draft. This was done in the case of a 55,000 bhp central station owned by the Electrical Society of Paris (SaintDenis). Each of the chimneys was 55 m high and served six boilers, each with 420 m2 of heating surface, and with economizer. Before the installation of mechanical draft the stacks produced natural draft of from 15 to 22 mm, according to the season, and each of the boilers could turn out only from 7,500 to 8,000 kg steam. After the installation of the Prat draft the steam rate was increased to 14,000 kg, with a draft of 36 mm water at the base of the stack. "The results obtained in trial, with six boilers in service and one stack, were as follows: "By the use of the Prat draft it is possible to obtain higher economy than with other systems. The quantity of air can be more closely proportioned to rate of coal consumption for all rates of forcing, with consequent control of the percentage of CO2. It is also possible to use low grade fuels such as sub-bituminous coals, poor anthracite and coke breeze, these often carrying a price low in proportion to the heating value. The furnace efficiency may also be improved by carrying relatively high furnace temperatures due to the higher rates of combustion. Low Power Requirements "An ordinary induced draft equipment with fan handling the entire volume of flue gas requires at least the equivalent of 2% of the entire steam production of the boiler served; 3% is not uncommon. Prat draft, on the other hand, with a fan taking atmospheric air, requires only 0.5 to 1.0% of the steam produced, and with the fan ‘in circuit' only about 0.3%. As an example, a prominent central station in Paris, using the Prat system of draft, may be cited. A comparison was made of the requirements of a battery of ten boilers, each with a normal output of 10,000 kg steam per hour and served by a single stack. It was desired to increase the total steam production of these boilers from a normal of 100,000 kg steam per hour up to 150,000. Careful analysis of the power absorbed by direct induced draft and by the Prat system showed that at the normal rating of 100,000 kg, direct induced draft required 95 kw as fan input, and the Prat draft 25 kw. The power required by the Prat system remained below that called for by direct induced draft up to a little over 150,000 kg steam production per hour, where the power requirements became the same (155 kw). Simplicity of Installation "The Prat system requiring only a light, low chimney, a small fan, and no pressure conduits such as needed by the forced draft system, is simpler and cheaper to install than either the forced draft or the direct induced draft. The Prat system also means the greatest simplicity, the most robust equipment, and the least expense in operation and maintenance. Mechanical Draft for Kilns and Furnaces "The Prat system is also conveniently adaptable to the uses of large cement kilns, brick and porcelain kilns, furnaces for melting metals and other materials, and re-heating furnaces." In addition to the foregoing abstract of the pamphlet by Louis Prat there is given herewith a translation of sections of an article which appeared in the March 1920 issue of Chaleur & Industrie. The article which deals with natural and mechanical draft was written by M. Lebrasseur, Engineer-Director of the French Sturtevant Company. "The system of induced draft by the ejector principle had for its object, in the beginning, the handling of hot gases without the necessity of passing them through a fan. It was patented in 1878 by Friedmann, and later described and computed by Ser in his treatise on Industrial Physics in 1882. "The arrangement employed is in principle as follows: "Into the base of a stack forming an inverted truncated cone and having a rather small angle at the top-about 7°-atmospheric air is blown, with a velocity sufficient to carry along at the same time the gas used as the motive agent and the gas to be ejected. The stack, being in this form of truncated cone, serves to reduce progressively the velocity of the gas, and hence to diminish the energy losses which would result from discharging_it_into the atmosphere at high velocity, as shown in Fig. 99. "It is necessary, with the system (in this form), to put in motion not only the products of combustion. but also a volume of air in inverse proportion to the velocity used. In addition to this, the chimney gases are cooled off and the natural draft reduced. The scheme has accordingly been abandoned, with the exception that it is sometimes used to handle gases which would cause corrosion in a fan, or of a temperature higher than 450 to 500 degrees Centigrade. "In order to reduce the total mass of gas which is necessary to put into motion, a portion of the products of combustion may be used as the motive gas for the process. This gas is taken from the side of the stack, by a fan which raises its pressure and blows it at high speed into the central portion of the ejector, as previously indicated. "Fig. 592 shows the principle of the arrangement used. "The theory and calculation of draft by ejectors of this kind have been published by one of my old engineers in 'Modern Technology' (Technique Moderne) of February 1, 1914. As I have not the time to develop this sufficiently 'dry theory, I will confine myself to showing, in a purely qualitative and very simple way, why the efficiency of ejectors is lower than that of equipment acting directly on the gas (forced or induced draft). Referring to Fig. 99, a weight of gas 'p' is to be lifted to the height 'h,' the work done would be p h. Let us compare this weight p to a train, which has to climb the grade A B. Our first idea would be to do this work by attaching a locomotive to one end of the train, making it climb the grade drawing the train (induced draft) or pushing it (forced draft). Would we consider detaching the locomotive, making it climb the opposite grade C to the height H-which is greater than h-in order to let it drop and strike the train, forcing it up from A to B? "This is nevertheless the principle of draft by the ejector method, in which one takes a volume of gas and raises it by means of a compressor to a high level of pressure, in order to discharge it through the intermediary of the ejector, an equipment of very low mechanical efficiency, into the remainder of the gas, with a speed such that the combined volume is forced up to B. Draft by, ejector is of interest, in my opinion, only to produce very low intensities, and in such a case to do away with the complication of a fan; on the contrary, there is required the use of an arrangement to reduce gradually the velocity of the gas, this requiring a considerable expenditure. Direct induced draft (by fan) permits reducing the size of chimney to the minimum, as much in height as in section, and makes unnecessary any considerable length for the purpose of gradually reducing gas velocities. "Ejectors employing as a motive fluid either steam or compressed air have, like those discussed above, serious drawbacks from the point of view of mechanical efficiency. They have besides the disadvantage of consuming, at high cost, quantities of steam or compressed air. They should be used only for reserve purposes, and in exceptional cases." The Schutte & Koerting Company have this to say regarding the Prat System of Induced Draft: "Our Company has discontinued quoting on such suction draft equipments, as they think that the mechanical induced draft or forced draft under all conditions is preferable in this country. "They further state that the only reason they know of why this suction draft outfit is extensively used in Europe is that the feature of only handling cold, clean air appeals to them, and sheet metal work used to be very inexpensive, while brick and concrete work were just the opposite. Here in the States where a concrete stack costs less money than one of the suction draft outfits, we found it almost impossible to sell them." If the boilers in the Paris plant cited in the Prat pamphlets were actually working at what we call normal rating in turning out 100,000 kg steam per hour, it would appear that under European conditions at the time the pamphlet was written the Prat system would be advisable up to 150% of rating; this was the point at which the curves of power use of Prat system and direct induced draft fan crossed. With modern requirements, however, involving high boiler ratings and with the material improvement in induced draft fans, conditions have altered widely during the past few years. It is possible that if the Prat equipment could be supplied cheaply in this country it would under some conditions be indicated for low boiler ratings with coal firing, or for oil firing, especially in situations where tall stacks would be expensive. In general, however, with moderate ratings for which the Prat system seems suitable, conditions in this country make it possible, as stated in the Schutte & Koerting letter, to provide draft by means of a stack more cheaply. For high forcing rates the information at hand indicates that the direct induced draft fan is more suitable than the Prat draft. The Prat system is, however, being installed in largé European plants, and no definite statement can be made at this time. Induced Draft Fans Induced draft fans are available today to suit various conditions encountered for steam drive and electric drive, direct current and alternating current, constant speed and variable speed. A careful selection of fans for the particular type of drive will give relatively low power consumption per unit of steam delivery, particularly where variable speed is used in connection with fans of proper characteristics. As indicating ordinary present-day efficiency of induced draft fans, one of the member companies has just bought equipments for eight 10,000 sq. ft. double Stirling boilers, the fan motor input being about 35 kw at 175% boiler rating and 135 kw at 275% rating. The boiler uses forced draft. traveling grates and is equipped with cast iron economizers having about 35% of the heating surface of the boiler. The stack is 158 ft. high above the grates. It is approximately true that at 175% rating the fan motor required about 1.0% of the steam turned out by the boiler (used through the main turbo-generator and auxilaries), and at 275% rating about 2.5%. Boiler Feed Pumps The Committee has made a definite recommendation that as far as possible all pumps of this character, together with driving turbines and auxiliary pumps, for other duties be equipped. with horizontally split cases. Manufacturers generally are doing this now, with the exception of small sizes where some objection on their part exists, due to difficulties of construction. Small centrifugal boiler feed pumps for installations of 1,000 H. P. and below have proved generally inefficient and more or less unreliable and the development today for this purpose will probably depend on the possibilities of single stage, high pressure, pumps. Pumps have been developed in England in single stages for duty and pressures up to 400 lbs. The American practice is considerably below this, the average manufacturer using a limit of 100 lbs. or below per stage. A report was obtained from two different sources in England on the single stage, boiler feed pump mentioned above. One manufacturer, Messrs. G. & J. Weir, Ltd., Glasgow, has been building a single stage unit for several years which seems to be giving good satisfaction. Their development of this type of pump covers capacity ranges from 7,000 to 35,000 G. P. H., with limited deliveries of even larger capacities. The drive is usually by turbine, due to the high speeds required, ordinarily 5,000 to 6,000 R. P. M. No considerable erosion is reported and units seem to handle hot water satisfactorily, provided the pump is properly flooded. The majority of the pumps are operating at a pressure in the neighborhood of 250 lbs. per sq. in. One other manufacturer has made single stage units which did not, however, prove successful. Most of the manufacturers in England still prefer the multistage units on account of decreased speeds and probably reduced leakage and erosion. A London correspondent advises us that the Bethlehem Shipbuilding Corporation is in a position to furnish the Weir pumps in the United States. Fig. 593 which shows a sectional view of the Weir Turbo Feed pump will give an idea of the general construction of the unit, which is arranged on two bearings, the pump runner overhanging on the turbine shaft. Performance data on this pump are included in the statements of the Bethlehem Shipbuilding Corporation and of Merz & McLelland. A number of companies have reported trouble due to failure of boiler feed pumps to handle hot water satisfactorily at full rating and pressure, due probably to improper design and lack of facilities for testing these pumps under actual working conditions at the factories. Some of the manufacturers have installed special equipment permitting test of such pumps to be conducted with hot water under practical working conditions. A brief description of the test plant of the Lea-Courtney Company is included in their state ment. Some of the pump manufacturers have formed an association called the Hydraulic Society, 450 Fourth Avenue, New York City, for the purpose of standardizing and co-ordinating the development of pumps for all purposes. Turbines for Auxiliary Drive The recommendation of the Committee covering horizontally split cases for auxiliary drive turbines was submitted to the manufacturers and most of them have adopted the suggestion except on turbines of small size. Investigation was made to determine the progress accomplished in design of such units to increase the efficiency and eliminate superheat from the exhaust steam. It is the general opinion that considerable changes to accomplish this feature would reduce the reliability of the turbine without effecting any very great saving in overall plant economy. These opinions are based on the idea that such steam will be absorbed in the heaters. It is true, nevertheless, that in modern plants the economy of the auxiliary drive is an item not to be overlooked and efficiency of the auxiliary driving units should have proportionately the same consideration as that given to the main units. The manufacturers should advance their design so as to be able to take full advantage of the expansion available in high pressure and high superheat stations. Conversion of old auxiliary equipment designed for saturated steam, for operation with superheat, generally is one where each individual case must be studied on its merits. In most cases considerable change of parts, including steam chest, steam cylinder, governor, operating gear, etc., will be required where temperatures exceed 500 to 550° F. Saddle supported small turbines and units built integral with boiler feed pumps and other auxiliaries which were mentioned in last year's report seem to have solved a large percentage of the troubles due to mis-alignment. Manufacturers' Statements Your Committee is in receipt of statements from the following manufacturers covering developments in auxiliary equipment during the past year: American Blower Company "Generally speaking we have three separate, distinct types of fans available for this service: The Sirocco Fan, The Steel Plate Fan, and The High Speed Fan. The first two types are suitable either for forced or induced draft, while the last named is usually applied to forced draft only. "The Steel Plate Fan of the Paddle Wheel Type is the oldest type in use and is well suited because of its pressure characteristics, and is usually used on installations, where direct connected engine drive is desired. For a given duty it requires greater space, operates at a tip speed of approximately mid-way etween Sirocco and High Speed Fans, and has a mechanical efficiency between 60 and 65%. "The Sirocco Fan is also applicable to the same class of service as is the Steel Plate Fan but possess certain other marked advantages. For a given capacity it occupies less space than any other type of fan, it gives the highest pressure of any fan for a given peripheral velocity of wheel, and its mechanical efficiency is unusually high, usually 70 to 75%. "The High Speed Fan in space requirements comes about mid-way between the Steel Plate and Sirocco Fan. This is also true of the mechanical efficiency. The ratio of static pressure to peripheral velocity pressure is low, requiring high speeds, substantially double the tip speeds of the Sirocco fans. Because of the extreme speed at which they operate, particular emphasis is placed upon the wheel design, and shaft and bearing proportions. As sizes and speeds increase certain manufacturers increase the weight of the blades to withstand the increasing stresses due to the transverse loading of the blades. That naturally requires heavier center plates and rings because of the increased centrifugal force. Heavier blades and rims involve larger diameter shafts and hubs, all of which tend to great weight in the revolving mass. "The striking feature of our construction of high speed wheels has been to provide greater strength as increased speeds and widths of wheels are encountered, by the substitution of additional annular rings limiting the width of the blade. In this way any degree of blade strength desired can be obtained without increasing the weight of the blades (except for the additional flanges required at the edge of the blade for riveting purposes). This gives us tremendous strength with a minimum of weight, a condition which is most desirable on rotating masses at high speeds. This construction also makes possible the use of driving rings adjacent to the outer end of the blade, the function of which is not only to absorb inertia stresses due to acceleration but keep the wheel permanently concentric. The center plate, the annular rings, and driving rings are made from single sheets of steel accurately turned to size. "The hubs are finished all over, inside and out, to assist in more accurately balancing. We use no welded construction whatever on our high speed wheels. "It is because of these constructional features that it is possible to operate our high speed fans at peripheral velocities as high as 18,000 feet per minute if "Generally speaking, we make no distinction in the type of fan used where variable speed is permissible, such as with steam engines, steam turbines, or variable speed motors, either A. C. or D. C., as it is possible to obtain efficient speed control with any of these methods of drive. This leaves us free to select the type of fan best suited for the particular installation. "It is difficult also to make a statement other than in a general way as to the best type for a given installation, as there are so many variables and conditions which must be taken care of and which is the province of the fan engineer. "As a rule we consider it preferable to design the fan for its required duty and then adapt the drive to the fan, rather than to select a fan to suit the drive and at the same time expect it to be most efficient for the service required of it. This means that it is of advantage to use reduction gearing, unless in the case of turbine driven fans the steam consumption is of small moment." The Green Fuel Economizer Company which show that there is no overload on the motor at any point within the whole range of operation. This is a very important feature as there is no danger of motor burnouts." Bethlehem Shipbuilding Corporation, Ltd. "The Bethlehem-Weir single stage turbine feed pumps are designed for a speed which is suitable for direct connection to a small high speed, efficient steam turbine. This affords a simple and compact arrangement and reduces to a minimum the space occupied. The pump is fitted with a special design of labyrinth packing on both suction and discharge which has enabled us to secure in our designs, a pump as efficient as and more reliable than the multi-stage pump. The pump casing is of simple construction and at the same time insures safety and reliability at the highest boiler pressures. It is not liable to expansion troubles when dealing with very hot water such as may be experienced with a multi-stage pump. The number of wearing parts is reduced to a minimum and comparatively a small number of spare parts are required. "We make a pump capable of delivering 34,400 U. S. gallons of water per hour against a total pressure of 428 lbs. per square inch when supplied with steam at a pressure of 265 lbs. per square inch gauge, 100° Fahr. superheat and exhausting against a back pressure of 25 lbs. per square inch absolute. . turbine wheel and pump impeller are mounted |