78 THE COLLIERY GUARDIAN. January 9, i914i. for gas engine jacket circulating water, will be found very useful for this purpose. They should be fitted with a long glass tube, similar to a gauge glass as used on steam boilers, but, of course, very much longer. This tube should be fitted into special brass fittings attached to the tank, and a scale should be fixed immediately behind the tube. This scale should be calibrated in pounds, which can be done by placing the tank on a weighbridge and putting in a previously determined amount of water—for example, say 100 lb. The position of the water in the gauge glass should be marked, and then another 100 lb. should be. added, and another point marked on the scale ; this must be continued until the tank is full. If the 100 lb. marks appear to be too far apart, the spaces in between can be divided up equally into fifths or tenths by means of dividers. Fig. 1 shows a tank arranged in this manner. It will be noticed that at the bottom of the tank there is a valve A shown, which should be arranged in such a position that, when it is open, the water in the gauge glass is level with the zero mark of the scales. The water from the jackets should be conveyed to the bottom of the tanks so as not to disturb the surface, as it would cause a fluctuation of the water in the gauge glass if allowed to fall the whole height of the tank. If the air compressor is steam driven, it is very probable that a guarantee has been given of the steam con- sumption. It will therefore be necessary to make arrangements to measure the amount of steam used. By far the most convenient method of taking a con- sumption test is to provide a set of platform scales. Upon this platform a tank is set capable of holding an amount of water equal to that required by the compressor in, say, 40 minutes. An electrical attachment is fitted to the scale beam, which will give warning by ringing a bell whenever the scale beam rises. The method of taking the consumption will be fully described later on. If the compressor is driven by an electric motor, a voltmeter and ammeter will be required. An exceedingly useful and compact instrument can now be obtained, which consists of one meter, the amperes or volts being read by simply throwing over a switch. Methods of Testing Mechanical Efficiency. Having now briefly explained the laws governing the compression of air, the next thing is to make clear the methods of taking a test. The first test to make will be (1) to find the mechanical efficiency of the plant. If it is a steam-driven set, both the steam and air cylinders must be indicated simultaneously. Notes should be taken of the steam pressure at the stop valve, the air pressure in the receiver, the revolutions per minute, and if the set is exhausting to a condenser, the vacuum should also be noted. The indicator cards will then be worked out in exactly the same way as for a steam engine, and the efficiency will be found from the formula— Efficiency — h~ X where A.H.P. is the horse-power indicated in the air cylinder, and I.H.P. is the horse-power indicated in the steam cylinders. If the set is electrically driven, the efficiency will be found as follows :—The volts and amperes will be noted at the same time that the air cylinders are being indicated. From this the electrical horse-power (E.H.P.) can be found. Thus, E.H.P. = where A = amperes, and V = volts, and the combined efficiency will be Volumetric Efficiency. The foregoing are comparatively simple tests. The most important tests are those taken to determine the volumetric efficiency—that is to say, what ratio exists between the volume swept out by the low-pressure piston and that actually delivered by the engine. With a well-designed plant, this efficiency may reach as high a figure as 93 or 94 per cent.; but with leaking valves, contracted and tortuous air passages or poor workman- ship, the efficiency may drop to a very low figure. When only an approximate idea is required of the volumetric efficiency, the following procedure may be adopted. This, although not strictly accurate, has the advantage that it is quickly taken and is quite good enough for a commercial test, and requires a very small amount of apparatus. The necessary arrangement is depicted in fig. 2, where C is a compressor, G- is a pressure gauge, R is a receiver, S is a stop valve, T are thermometers, V is a safety valve. To take this test, the compressor is run up to speed, the stop valve V must be closed tightly, and the safety valve S, which must be of ample size, held as wide open as possible by an assistant. The dimensions of this safety valve must not be stinted, as it is necessary to keep the air pressure in the receiver as near atmo- spheric pressure as possible before the test commences. At a given signal the safety valve is closed, and at the same moment another assistant takes the number of revolutions and the time, with the hand revolution counter already described. Meanwhile the pressure in the receiver is steadily increasing, and when it arrives at a predetermined pressure, which generally will be that at which the machine should work under normal con- ditions, another signal is given, the hand counter is withdrawn and the temperature of the air in the receiver noted, as well as that of the atmosphere. Also the total number of revolutions that the compressor has run, and the time in minutes and seconds should be recorded. The result can now be worked out. Let D = diameter of low-pressure cylinder in feet. L = length of stroke in feet. Vc. = volume swept by piston in cylinder than the volume swept by the piston per stroke is Vc = I D‘2 x L. 4 It is now necessary to find the volume of “ free air ” delivered per stroke. Let P« = pressure of atmosphere, Pr = pressure in receiver, ta =. temperature of atmosphere, tr = „ in receiver, V,. = volume of receiver in cubic feet, V — volume of “free air” which Vr represents. To be absolutely correct, Pa is equal to the height of the barometer, multiplied by 0’492, but for general Fig. 2. II Fig. 3. work it can usually be taken at 14’7, without being very much in error. We have seen that, according to Boyle’s law, the volume of a perfect gas varies inversely as the pressure, and we also know that the volume varies directly as the absolute temperature. Thus we see that V — V x +_461 P7. + 14’7 r tr + 461 1+7 * This gives the equivalent volume of “ free air ” at atmospheric temperature and pressure, which the air under a higher pressure and temperature in the receiver would occupy, but it must not be forgotten that when the safety valve was closed at the commencement of the test, the receiver already contained one volume of air. Therefore the final formula for this is, V —V v + 461 v P,- + 14’7_______v r tr +461 ' 14’7 Let N be the total number of olurvetions required to pump up the receiver to P,. , Vs = volume of free air pumped per stroke. Then the amount of air discharged per stroke will be : For a double-acting compressor— 2N* For a single-acting compressor and the volumetric efficiency = L. X 100. The indicator diagram supplies an approximate method of determining the volumetric efficiency of an air compressor, and is applied as follows. Fig. 3 shows a diagram from a single-cylinder air compressor. The compression of the air starts at D, and discharge commences at E and finishes at F. When the piston reaches the end of its stroke at F, there is still an amount of air at a pressure G, filling the clearance spaces. This air must expand on the return of the piston, until it is at atmospheric pressure, before any more air can be taken into the cylinder. This point is shown at C in the figure, and the position of it will depend upon the amount of clearance, the efficiency of the cooling, and the tightness of the valves and piston rings. From the figure it will be seen that the distance B represents the full stroke of the piston, while the length A shows the portion of the stroke during which ah’ is being drawn into the cylinder ; therefore the approximate volumetric efficiency is 4 x 100. B [To be continued.) SAFETY LAMPS IN 1912.* The following table shows the number of safety lamps of various types in use during 1912 :— 1912. 1911. Total number in use 749,177 .. . 723,934 Number of gauzes :— One 385,245 .. . 384,294 Two 353,150 .. . 335,332 Three 44f.. 7 Not stated 10,7381.. 4,301 Protection:— Shielded 707,607 ... . 684,102 Unshielded 30,825 .. . 35,047 Not stated 10,7451... 4,785 Method of locking Lead rivet 416,715 ... . 405,483 Magnetic 250,095 ... . 237,374 Screw 69,781 67,744 Other 12,586 ... 13,333 Kind of illuminant:— Colza or colza and petroleum 545,198 ... 522,828 Petroleum 59,062 ... 82,942 Volatile spirit 93,404 ... 96,861 Electricity 10,727 ... 4,298 Other illuminant 40,786 ... 17,005 Method of lighting:— By electricity 354,974 ... 343,112 By internal igniters 27,825 ... 12,842 By opening 366,378 ... 367,980 t Including 20 with four gauzes. t Including electric portable lamps. According to district the number of lamps in use was as follows:— Scotland .......... 34,758 Newcastle ......... 61,841 Durham ........ 67,351 York and N. Midland 218,993 Manchester & Irela nd 4 6,943 Liverpool &N. Wales 66,817 South Wales....... 184,066 Midland & Southern 68,408 Classified according to type, the numbers were:— Davy ............... 12,272 | Hepplewhite-Gray ... 713 Clanny ............ 332,339 Routledge & Johnson 3,676 Mueseler ........... 41,986 Electric .......... 10,727 Marsaut ........... 332,180 Miscellaneous ..... 12,334 Wolf .............. 12,950 # From Part II. of the General Report on Mines and Quarries. Northern Mining Industries Exhibitors’ Committee.—- A meeting of the above committee was held at the Midland Hotel, Manchester, on Friday, December 19, the following gentlemen being present:—Messrs. Walter Yates (Messrs. Matthews and Yates Limited), A. E. Mathewson (Tilgh- man’s Patent Sandblast Company Limited), John L. Prest- wich (Protector Lamp and Lighting Company Limited), Eugen Reinecke (Hy. Pels and Co. Limited), Wm. C. Shaw (J. Shaw, Son, and Greenhalgh), R. Wood (John Wood and Sons Limited), F. Friedenthal, John B. Walsh (Walsh and Dick), Joseph Butterworth (Lancaster and Tonge), Charles H Luke (vice-chairman), W. H. Spick, and W. R. Jones (secretary). The secretary, in his report, mentioned that the members of the committee now totalled 62, the following gentlemen having joined ( since the last meeting:—Mr. Becker (Bleicherfs Aerial Transporters Limited), Mr. Eugen Reinecke (Hy. Pels and Co. Limited), Mr. F. J. Stockall and Mr. T. Wilson (Wilson and Stockall), Mr. W. Maurice (Wolf Safety Lamp Company), Mr. R. Cremer (Cremer Lamp and Engi- neering Company), and Mr. R Calrow (Thomas and Bishop). Also that considerably over half of the space avail- able had been booked for the forthcoming Second Northern Colliery and Mining Exhibition, Manchester, June 1914. It was also mentioned that several members of the com- mittee had not yet settled their space, and the committee resolved that the spaces be reserved for these firms until the end of January, after which date any vacant space could be let to firms who aie not members of the committee. The question of tickets and invitations being sent out was discussed, but the committee thought that the system adopted by the management at the previous Colliery and Mining Exhibition in Manchester in 1911 could not be improved upon, and it was left to the organisers to carry on the arrangements on similar lines as previously.