126 THE COLLIERY GUARDIAN. July 19, 1918. MEASURING AIR AND GASES WITH THE PITOT TUBE.* By A. H. Anderson. The Pitot tube is now generally understood by engi- neers, but its application has been restricted to research. Perhaps the reason it has not received the attention it deserves in the industrial trades, for metering air and gas, is because recording instruments are available. Recording meters are desirable, but there is still a wide field in which an inexpensive device must be used. For instance, in distributing costs of gas or air among various departments, an indicating Pitot tube may be placed in each branch and practically the same informa- tion obtained as with an elaborate instrument. In the air supply duct to a foundry cupola or a set of forges, the proper quantity of air can be gauged with a Pitot tube and maintained during the period of operation. The Pitot tube is easily changed from place to place, and the pressure or velocity of air may be gauged at different points along an exhaust line where the size of duct changes with the number of branches admitted. The indications of the Pitot tube are made readily available by the use of the tables given below, by means of which calculations are avoided. They apply to air at approximately atmospheric pressure and also to gases of various densities, such as illuminating, coke oven, blast furnace and natural gas. In order that these tables may be applied directly, the coefficient of the Pitot tube must be 1'00. If the coefficient is 0'8, then the tabular value for velocity must be multiplied by 0’8. The liquid used in the gauges is a mineral oil of 0'834 specific gravity, and the colour is red to make it con- spicuous. The scale attached to the gauge is graduated to read in inches of water. The dynamic (or bent) tube is connected to the left-hand end of the upper gauge. The static (or bevelled) tube is placed in communication with the right-hand end of the upper gauge and the left- hand end of the lower gauge. Therefore, the upper gauge indicates the “ velocity pressure ” and the lower gauge indicates the static pressure. The velocity corresponding to the “ velocity pressure ” may now be determined by reference to the tables. Temperature, static pressure and humidity affect the velocity, but within certain limits the static pressure and humidity may be neglected. Table I. shows the velocities of air at 80 degs. Fahr, corresponding to various “velocity pressure ” readings, in inches of water. Table I.—Air Velocities Corresponding to Various Velocity Pressure Readings. Air at 80 degs. Fahr.; 100% humidity; 29'5in. barometer. Vel. Ft. Vel. • Ft. Vel. Ft. Vel. Ft. press. per press. per press. per press. per In. min. In. min. In. min. In. min. 0‘01 ... 409 . .. 0'47 ... . 2,800 . .. 0'93 .. . 3,946 .. . 1'39 .. . 4,822 0'02 ... 579 . .. 0'48 .. . 2,830 . .. 0'94 .. ,. 3,967 .. . 1'40 .. . 4,840 0'03 ... 710 . .. 0'49 .. . 2,865 . .. 0'95 .. .. 3,988 .. . 1'41 .. . 4,857 0'04 ... 818 . .. 0'50 .. . 2,890 . .. 0'96 . .. 4,008 .. . 1'42 .. . 4,874 0'05 ... 914 . .. 0'51 .. . 2,920 . .. 0'97 . ... 4,029 .. . 1'43 .. . 4,891 0'06 ... 1,000 . .. 0'52 .. . 2,950 . .. 0'98 . .. 4,050 .. . 1'44 .. . 4,908 0'07 ... 1,080 . .. 0 53 .. . 2,980 . .. 0'99 . .. 4,070 .. . 1'45 .. . 4,925 0'08 ... 1,155 . .. 0'54 .. . 3,000 . .. 1'00 . .. 4,090 .. . 1'46 .. . 4,912 0'09 ... 1,227 . .. 0'55 .. . 3,930 . .. 1'01 . .. 4,110 .. . 1'47 .. . 4,959 OTO ... 1,295 . .. 0'56 .. . 3,060 . .. 1'02 . .. 4,130 .. . 1'48 .. . 4,976 0'11 ... 1,355 . .. 0'57 .. . 3,090 . .. 1'03 . .. 4,150 .. . 1'49 . 4,993 0'12 ... 1,418 . .. 0'58 .. . 3,115 . .. 1’04 . .. 4,170 .. .. 1'50 .. . 5,010 0'13 ... 1,475 . .. 0'59 .. . 3,140 . .. 1'05 . .. 4,190 .. . 1'60 .. . 5,180 0'14 ... 1,530 . .. 0'60 .. . 3,170 . .. 1'06 . .. 4,210 .. .. 1'70 .. . 5,330 0'15 ... 1,585 . .. 0'61 .. . 3,200 . .. 1'07 . .. 4,230 .. . 1'80 .. . 5,490 0'16 ... 1,635 . .. 0'62 .. . 3,225 . .. 1'08 . .. 4,250 .. . 1'90 .. . 5,640 0'17 ... 1,690 . .. 0'63 .. . 3,250 . .. 1'09 . .. 4,270 .. . 2'00 .. . 5,790 0'18 ... 1,735 . .. 0'64 .. . 3,275 . .. 1'10 . .. 4,290 .. . 2'10 .. . 5,930 0'19 ... 1,785 . .. 0'65 .. . 3,300 . .. 1'11 . .. 4,308 .. . 2'20 .. . 6,080 0'20 ... 1,830 . .. 0'66 .. . 3,325 . .. 1'12 . .. 4,327 .. . 2'30 .. . 6,210 0'21 ... 1,875 . .. 0'67 .. . 3,350 . .. 1'13 4,345 .. . 2'40 .. . 6,330 0'22 ... 1,920 . .. 0'68 .. . 3,375 . .. 1'14 . .. 4,364 .. . 2'50 .. . 6,480 0'23 ... 1,960 . .. 0'69 .. . 3,400 . .. 1'15 . .. 4,382 .. . 2'60 .. . 6,600 0'24 ... 2,005 . .. 0'70 .. . 3,425 . .. 1'16 . .. 4,401 .. . 2'70 .. . 6,720 0'25 ... 2,050 . .. 0'71 .. . 3,450 . .. 1'17 . .. 4,419 .. . 2'80 .. . 6,850 0'26 ... 2,090 . .. 0'72 .. . 3,475 . .. 1'18 . .. 4,438 .. . 2'90 .. . 6,970 0'27 ... 2,125 . .. 0'73 .. . 3,500 . .. 1'19 . .. 4,456... . 3a 0 .. . 7,090 0'28 ... 2,160 . .. 0'74 .. . 3,525 . .. 1'20 . .. 4,475 .. . 3'10 .. . 7,200 0'29 ... 2,200 . .. 0'75 .. . 5,550 . .. 1'21 . .. 4,493 .. . 3'20 .. . 7,320 0'30 ... 2,240 . .. 0'76 .. . 3,570 . .. 1'22 . .. 4,512 .. . 3'30 .. . 7,430 0'31 ... 2,275 . .. 0'77 .. . 2,590 . .. 1'23 . .. 4,530 .. . 3'40 .. . 7,540 0'32 ... 2,310 . .. 0'78 .. . 3,615 . .. 1'24 . .. 4,549 .. . 3'50 .. . 7,660 0'33 ... 2.350 . .. 0'79 .. . 3,640 . .. 1'25 . .. 4,567 .. . 3'60 .. . 7,760 0'34 ... 2,390 . .. 0'80 .. . 3,660 . .. 1'26 . .. 4,586 .. . 3'70 .. . 7,870 0 35 ... 2,420 . .. 0'81 .. . 3,680 . .. 1'27 . .. 4,604 .. . 3'80 .. . 7,970 0'36 ... 2,455 . .. 0'82 .. 3,700 . .. 1'28 . .. 4,623 .. . 3'90 .. . 8,070 0'37 ... 2,485 ... 0'83 ... 3,720 ... 1'29 ...4,641 ... 4'00 ... 8,180 0'38 ... 2,520 ... 0'84 ... 3,740 ... 1'30 ... 4,660 ... 4'10 ... 8,280 0'39 ... 2,550 ... 0'85 .. 3,765 ... 1'31 ...4,678 . 4'20 8,400 0'40 ... 2,590 ... 0'86 ... 3,790 ... 1'32 ... 4,696 ... 4'30 .. 8,480 0'41 ... 2,620 ... 0'87 ... 3,810 ... 1'33 ... 4,714 ... 4'40 ... 8,580 0'42 ... 2,650 ... 0'88 ... 3,835 ... 1'34 ... 4,732 ... 4'50 ... 8 670 0'43 ... 2,680 ... 0 89 ... 3,860 ... 1'35 ... 4,750 ... 4'60 .. 8,770 0‘4i ... 2,710 ... 0'90 ... 3,880 ... 1'36 ... 4,768 ... 4'70 ... 8,870 0'45 ... 2,740 ... 0'91 ... 3,905 ... 1'37 ... 4,786 ... 4'80 8,970 0'46 ... 2,770 ... 0'92 ... 3,925 ... 1'38 ... 4,804 ... 4'90 .. 9,060 5'00 ... 9,140 Table II. shows the coefficient by which the velocities in Table I. may be multiplied when the air is of any other temperature than 80 degs. Fahr. Coefficients are also given in this table for gases of various densities. Table II.—Velocity Coefficients for Various Temperatures. Gas (lb. per cu. ft. at 80 degs.) Degs. Fahr. Air. 0-05 0'055 0'06 0'065 50 ... 0'972 ... 1'160 ... 1'111 ... 1'060 ... 1'020 60 ... 0'980 ... 1'170 ... 1’120 ... 1'070 ... 1'030 70 ... 0'990 ... 1'180 ... 1'130 ... 1-080 ... 1'040 80 ... 1'000 ... 1'200 ... 1'140 ... 1'090 ... 1'050 90 ... 1'010 ... 1'210 ... 1'150 ... 1’100 ... 1'060 100 ... 1'025 ... 1'225 ... 1T70 ... 1’115 ... l'U75 110 ... 1'038 ... 1'245 ... 1T85 ... 1-130 ... 1'090 120 ... 1'049 ... 1'255 ... 1'195 ... 1'143 ... 1'100 130 ... 1'060 ... 1'270 ... 1-210 ... 1-155 ... 1'111 In certain cases where the Pitot tube is placed in a long straight pipe, it may be assumed eddy currents a e absent, and the tip of the tube may be located at a distance from the centre equal to 0'8 of the actual radius, in order to obtain average results. In most cases there are conditions which deflect the air currents in the pipe, and the maximum velocity is found at one side of the centre. Under these conditions, it is necessary for accurate results to determine the average * Compressed Air Magazine. velocity by making a traverse of the pipe with the Pitot tube. This can be done in the following manner:— Determine the actual radius of the round pipe and make a series of Pitot tube observations at intervals from the centre to the wall. The intervals between the readings are determined by the equation:— R ~ 1. where R = distance of various positions from centre of pipe, inches ; r = radius of pipe, inches; N = number of readings desired from wall to wall; and a = order of readings from centre. The Pitot tube is now located in turn at the positions as determined above, first on one side of the centre and then on the other side. The velocities corresponding to the 10 velocity pressures are taken from Table I., and the arithmetical average determined. As an illustration, assume a round duct 10 in. diameter, the number of observations from wall to wall to be 10. In this case r=5in., N =10, a assumes values from 1 to 5. The values of R on one side of the centre resulting from the substitution in the equation are as follow :— • Distance from centre. In. Rx .............................. 1'58 R2 .............................. 2'73 R3 .............................. 3'53 R4 .............................. 4'18 Rs .............................. 4’74 The positions on the other side of the centre are duplicates of those given below. Actual velocity pressures and velocities from a test of a 10 in. pipe are shown in Table III. Table III.—Results of Test of a 10-in. Pipe. Distance Distance Velocity Velocity. from wall. from centre. pressure. Ft. per In. In. In. min. 0 27 4'73 0'105 1,310 0'83 4'17 0'125 1,450 1'50 3'50 0145 1,550 2'27 2'73 0'160 1,630 3'42 1'58 0'165 1,650 6'58 1'58 0'165 1,650 7'73 2'73 0'160 1,630 8'50 3'50 0'145 1,550 9'17 4'17 0'125 1,450 9'73 4'73 0'105 1,310 Average 1,550 The Pitot tube may now be permanently located in the pipe in either of the two ways described:—(1) Locate the tip of the Pitot tube at the proper distance from the centre of the pipe to obtain average reading; or (2) locate the tip of the tube at the centre of the pipe and multiply the indicated velocity by the ratio of the average to the centre velocity. These methods involve preliminary work in order to obtain the proper setting of the tube; but this must be done in any event, whether the apparatus is of the inexpensive type or of the most elaborate design. The method of computing the proper intervals between Pitot tube readings in the average velocity determina- tion may be replaced by the graphical method in which a large circle is drawn to represent the round pipe. Upon the radius is constructed a semicircle. The radius of the large circle is divided into as many equal dividends as there are to be Pitot tube readings. Erect perpendiculars from each point and through the inter- section of each perpendicular and the circumference of the semicircle construct circles with the same centre as the large circle. To obtain a complete traverse of a diameter of the pipe, set the Pitot tube at each of the radii proceeding from one wall to the other. Great stress has been laid on the importance of obtaining accurate values for average velocity. This is necessary because local conditions distort the air currents in a pipe, and each individual application must be examined by itself. The internal diameter of the pipe must be carefully measured, and the cross-sectional area calculated in square feet. The product of the area and the average velocity is the volume of gas or air moved per minute. The Board of Agriculture and Fisheries announces publi- cation of the sixth volume of the special reports on the mineral resources of Great Britain, which have been pre- pared by the Director of the Geological Survey. This volume constitutes the first of a series which will deal with the refractory materials of Great Britain, and is con- cerned chiefly with ganister and silica rocks. Copies, price 7s. 6d. each, may be obtained through any bookseller from Messrs. T. Fisher Unwin Limited, 1, Adelphi- terrace, London, W.C. Briquetting Coal and Coke Dust.—In the new briquetting process introduced by A. and Y. Pidelaserra, the raw material used is coke dust, powder or charcoal powder, cinders, lignite, bituminous coal, and small or dust coal that does not sinter or coke. According to the Boletin de Minas y Metal urgia, the new process consists in mixing the fuel materials in the cold with crushed pitch and sufficient water to give consistency to the mixture. This mass is heated to the coking temperature of the pitch, or even higher, the moisture used in making the mixture workable being thereby evaporated and the pitch (which serves as binding material for the coal dust) coked. The proportions of coal and pitch vary according to the quality of the product. The quantity of water to be used is also variable. The mass may be baked in an ordinary heating furnace, with or without a refractory lining, capable of standing the temperature required for coking the pitch, viz., 400 degs. or 500 degs. Cent., or the operation may be carried on in a retort or open pan. The pitch may be replaced by coal tar, mixed directly with coal, and treated as described until it cokes. The finished mass is consistent, of uniform porous fracture, and when cold can be broken into pieces of convenient size, accord- ing to the grate, etc., on which it is to be burned. The use of water in the mixing process is stated to increase the density of the mass, obviating the loss of either, and helps to distribute the particles of pitch in the baking stage, so that the binding power of the molecules is improved, thus producing a more consistent mass of uniformly porous texture, which is more convenient for use in domestic heating. No special machine is needed for crushing the coal and pitch or for mixing the coal dust, binding material and water. CHECKING LOSS IN FLUE CASES. The accompanying chart has been developed by the Uehling Instrument Company, 71, Broadway, New York, to facilitate estimation of the money lost up almost any chimney through low CO2 content of the flue gases. Simply connect the percentage of CO2 (shown in column C) with the money now being spent per year for coal (shown in column A), and the intersection of the connecting line with column B immediately gives the amount issuing from the chimney in the form of heated gases. The object of the chart is to show that a high per- centage of CO2 is most desirable. Even where the C02 is as high as 21 per cent.—the theoretical maximum— there is a loss because, in the average power plant, the flue gases leaving the boiler have a temperature as high as 500 degs. or 600 degs. Fahr. Loss, therefore, is inevitable unless a blower is used for exhausting the gases and some sort of interchange system is installed for either heating the feed water or preheating air and leading it under the grate. It is significant that most of the large power plants of to-day have adopted’CO2 instruments that record automatically and continuously, keep a constant check on the workers in the boiler room and the efficiency of combustion. The recorder may be placed at any con- venient distance from the boiler in the office of the chief engineer, owner, manager or superintendent, while an auxiliary CO2 indicator is placed on the boiler front in full view of the fireman. The function of the indicator is to keep the fireman constantly informed as to the efficiency of his own work. This feature is most commendable. -7 $2ooo --$3ooo --$4ooo --$5ooo $10,000 --$3oooo - $40,000 -J H --$50,000 --$300 --$4oo --$5oo --$(000 — 2( — 2o --I9 — IS —17 —16 -15 -14 --I3 —(2 —$2ooo < ^•7-$3ooo r o % $10,000 — $2o,ooo E--$3o,ooo 10 9 8 rd O U g--$5o,ooo $100,000 -4- $ 80 OOO Q ' Coal cannot be saved by a CO2 machine alone. If no attention is paid to the indicator or recorder, the installation of such apparatus is useless. The records should be carefully watched and studied, and adjust- ments should constantly be made in firing methods until the best percentage of CO2 is obtained. After the best mark is reached, fluctuation of the CO2 line I elow that mark to any great extent should not be allowed. The chart is based on a flue gas temperature of 600 degs. Fahr, and an outside air temperature of 60 degs. Fahr. Where the flue gas temperature is higher, or the outside air temperature lower, the money loss will be correspondingly increased. On the other hand, with a higher outside air temperature and a louver flue gas temperature, the money loss is proportionately decreased. Further, in the construction of the chart, it has been assumed that the coal has a calorific value of 14,500 B.Th.U. per lb. of combustible. It may also be pointed out that where there is only 3 per cent, of CO2 in the flue gases, 76 per cent, of the heat value of the coal passes up the chimney as waste under the conditions outlined above. It is impossible, however, for these gases to contain as low as 2 per cent., because it would require more than the original quantity of heat in the coal to raise the enormous surplus of air to a temperature of 600 degs. Fahr.—Metallurgical and Chemical Engineering. Partnerships Dissolved.—The London Gazette announces dissolution of the following partnerships: A. G. Thomas (who continues the business) and C. A. Best, trading as the Empire Wire Rope Company, Britannia Works, Clifton Vale, Bristol; T. Prescott (who continues the business) and F. Blackwell, trading as the Cotwell Foundry Com- pany, Victoria Works, Leeds; G. E. Casebourne, A. L. Robinson, and C. G. Casebourne, trading as G. E. Case- bourne and Company, iron and steel merchants, steam- ship charterers, etc., Stockton-on-Tees. Italian Lignite Production.—According to official sta- tistics, about 2,000,000 tons of lignite were extracted in Italy last year, thus saving the importation of about 800,000 tons of English coal. By the recent Decree relating to the direct exploitation of the principal deposits, the Fuel Committee has the faculty of granting concessions to anyone entitled to exploit minerals, and it is antici- pated that the output this year will attain 5,000,000 tons of lignite, which will be equivalent to 2,000,000 tons of English coal, or one-fifth of the total importation.