A Text-book of Coal-mining: For the Use of Colliery Managers and Others by Herbert William Hughes

Author:Herbert William Hughes
Language: eng
Format: epub
Publisher: Charles Griffin & Co
Published: 1904-03-25T05:00:00+00:00

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This leaves a margin of 16*32 seconds under the minute allowed. As the four decks of the cage will be changed simultaneously in 10 seconds, 6*32 seconds per wind are available for contingencies, equal in 480 winds to 50 minutes during the complete working day.

The steam pressure in boilers is 150 lbs., but a drop of 10 lbs. should be allowed between boilers and stop valve on engine making the absolute* pressure there 155 lbs. a p\ If it is decided to use a pair of single-cylinder direct-acting engines and cut off steam at 80 per cent, of the length of the stroke during the five revolutions in which acceleration is taking place, the theoretical number of expansions will

be -^ » 1*25, but the actual expansions will be less, owing to the

effect of steam left in the clearance spaces of the cylinder. In this class of engine a factor of 0*85 may be taken, and, consequently, the actual number of expansions is 1*25 x 0*85 « i*o6, say I'l « r. The theoretical mean steam pressure (jd***)

^ ,i_+Jog^ r

= .55 4^ = .S4-a lb-.

but the actual mean pressure is always lower on account of steam condensing on the cylinder walls, &c., and the figure so obtained must be reduced by multiplying by 0*85, which s;ives the actual mean pressure = 154*2 x 0*85 » 131 lbs. The effective pressure on the piston is the mean pressure less back pressure, aud if the engine works non-condensing this can be taken as 5 lbs. above the atmosphere — i.e., 20 lbs. absolute—but condensing it will only be 5 lbs. absolute. These engines will be connected to a condenser, and, therefore, the effective pressure will be 131 -5= i26»Pin formula.

The two cases for consideration are when the engine runs with and without the load being balanced.

I. When the load is balanced by a rope beneath the cages:—

* Boiler pressure plus that of the atmosphere.

t The hyperbolic or Naperian logarithm of the ratio of expansion.

The total length of rope to be set in motion = twice the depth of shaft (600 yds.) + height of headgear (35 yds.) + distance to drum (35 yds.) = 2 X 670 s= 1340 + the rope coiled on drum, which is equal to the depth of the shaft, or a total of 1940 yards. At 16 lbs. to the yard « 31,040 lbs.

The total weight, W, to be set in motion—

Rope,*

2 cages, hooks, ftc., So cwts. each, 16 tubs, full and empty ones, 8 cwts. each. Mineral (6 tubs coal and 2 of dirt), 144 cwts Wood laggings on drum, 80 cwts., . . . Half-weight of drum (26 tons) and half-weight of 2 pulleys (8 tons) = 340 cwts., .

126,464

9$

The stroke of the engine may be taken at 5^^ feet, one-third the diameter of the drum, and as the maximum velocity is attained in five revolutions, S = 5x2x5'5 = 55 feet. The unbalanced load in this case is minerals only.

W = 126,464. Ls 16,128.

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