SOUTH BULL! AND BELLAMBI COLLIERIES. 11)1 



valve, unloading device, and an air filter to clarify 

 the air from dust. This air filter consists of a 

 <;ast-iroii box, in which are placed two gauze trays of 

 very fine mesh, the space between being filled with cotton 

 wool. A branch pipe taken from the water main to the lo- 

 cality of the compressor has a spray attached to it ; in this way 

 the air drawn into the compressor is kept moist and cool, so 

 .as to lessen any chance of an explosion taking place, due to 

 the heat developed by compression, in the presence of coal dust. 

 The Reavell compressor is a different type to that generally 

 seen at mines, and much more compact, as will be noticed by 

 referring to Fig. 119. It has four cylinders (A) arranged radi- 

 ally in a circular shaped casing (B), each cylinder has a pis- 

 ton (C), and the connecting rods (D^ are all driven by a crank- 

 pin in common. Each cylinder is a separate single acting 

 compressor, and as they all deliver into one common delivery 

 passage (E), the supply of air is practically continuous. As 

 the machine is driven in the same direction all the time, 

 it can be run at a relatively high speed, and is, therefore, very 

 suitable for electric motive power. The cylinders pass 



through an annular water-jacket (F), which serves to keep them 

 cool. Water is circulated through this chamber by a small 

 centrifugal pump. A small balance or pilot piston (Or) is 

 provided to minimise the side thrust 011 the piston. There 

 are no suction valves, the cylinders are filled with air at atmos- 

 pheric pressure at each stroke, instead of at a reduced pres- 

 sure due to the resistance of valve springs, thus increasing the 

 volumetric efficiency. The free air passes into the casing 

 through the passage (H). The piston has two ports arranged 

 on the top to correspond with two sets of ports in the head 

 of each connecting rod ; one set is directly on the top, the 

 other a little on one side. The delivery valves fitted to 'the 

 outer end of each cylinder are shown at (I) : the number of 

 such valves used for each cylinder varies with the size of the 

 machine. Following the course of the piston of one cylinder 

 commencing with the suction stroke, there is no partial 

 vacuum caused at the commencement of- the stroke, for the 

 little air left in the cylinder, being under compression, will 

 expand as soon as it has a chance. When the piston has 

 moved so far down that the hollow head of the connecting 

 rod comes abreast of the two ports (J) in the lower part of 

 the cylinder, free air is admitted to the cylinder through the 

 ports of the connecting rod and piston : at the bottom of the 

 stroke the piston covers the ports in the cylinder and the air 

 -enters direct from the inner chamber. On the compression 

 stroke no air can escape through the connecting rod head, 

 for this is now turned in the direction where there is no port 

 to correspond to that in the piston, while at the end of the 



