The types of generators on the market may be broadly divided into two, the hand-fed or non-automatic and the automatic. The makers of one condemn the other, but, as a matter of fact, both are good. The writer believes the hand-fed principle to be very good, but he always uses the automatic for residence work ; but would prefer the hand-fed for, say, village lighting, railway work, barracks, or wherever space and appearance count for nothing, and it is desired to actually see the gas in store, ready for use, each day. The automatic apparatus has (comparatively) a small gasholder, as, being automatic, the holder need only hold say one-fourth the evening's supply at once, remembering that as the store of gas diminishes, more is promptly made without attention.

It is obviously impossible to describe all makes of generators, but two may be taken, one of each kind, to show the principles involved and the detail a generator must have to afford ordinarily good results. Fig. 123 illustrates the Pictet apparatus, hand-fed (non-automatic). The gasholder is of the ordinary kind, as also is the washer, the part needing description being the generating tank. This is an upright closed tank, conical or taper at the bottom part, the lower end of the cone being fitted with a large full-way cock for the removal of the lime sludge. In the closed top of the tank is inserted a funnel-shaped feeding tube, down which the pieces of carbide are dropped by hand; and the lower end of this tube has its opening at an angle that does not readily admit of gas passing back up it. When the carbide has passed through the tube it falls upon a grid, or grated division, the object of this being to prevent the carbide falling into the sludge which collects in the conical bottom. Sludge (the lime left by the decomposed carbide) does not collect on the grid, as it is practically a cream of lime. When decomposition of the carbide is complete this falls through the grid quite freely. The objection to the carbide falling into the sludge is that this does not admit of water coming with sufficient freedom to the pieces, which, unless of quite small size, generate sufficient heat to form a crust of lime upon themselves. This results in a diminished gas supply, which is, too, of bad quality due to excessive heat during generation. The existence of the grid obviates this by allowing free contact between the water and carbide until all gas is evolved. As previously stated, this type of apparatus has to have a gasholder large enough to receive, at least, a volume of gas sufficient for the longest evening, and the greatest demand of one evening (though so simple an apparatus could be fed after dark without a light). This is a desirable thing in certain instances, but it necessitates a large gasholder which occupies some space, needs a larger hut, and is more expensive than a smaller one. This type of apparatus is said to work on the "carbide-to-water" principle, as distinct from other generators in which the carbide is stationary and the water flows to it.

Principles Of Generation 158

Fig. 123.

As an example of an automatic generating apparatus, Fig. 124 may be given. In this the gasholder is of the ordinary kind, but the holder-bell carries a drop-rod from its front edge as shown. A generating chamber and washer come on each side of the holder, supported on brackets attached to the holder tank. The brackets are not shown, nor are the generator and washer on the left side, these being duplicates of those shown on the right. As already stated, it is desirable that the generator parts be in duplicate, to work alternatively, and this provision here exists. Each washer is a plain cylindrical vessel, and it has to be explained that the water in these is replenished and kept at one constant level by a ball valve cistern which is attached to the back of the holder. This cistern is not shown, nor the pipes which connect it to the washers, as they would complicate the illustration. They can be easily understood to exist.

The action of the generator is as follows: - The carbide tray has its divisions about half filled with carbide, for the mass swells in decomposing. It is then slipped into the generating chamber, the front of which is closed by a compression lid. When the holder bell descends (due to the store of gas being used) the drop rod descends with it, and the arm from this rod presses on the lever which operates the water cock in the supply pipe to the generating chamber. Water at once flows in and wets some of the carbide, resulting in the immediate production of gas, which in turn raises the holder bell and releases the pressure on the lever of the water cock. The cock lever, being weighted at the other end, automatically closes the cock as soon as the arm rises from the lever. In the illustration the left - hand lever is depressed, causing water to be delivered to the carbide chamber on that side. Should the carbide be exhausted in that chamber the bell, on descending a little lower, will cause the right-hand generating chamber to come into use, and when this is noticed the left-hand one is known to be exhausted and can be recharged. On doing this the position of the arms on the drop rod are reversed (by means of set screws).

Principles Of Generation 159

Fig. 124.

It will be seen that the water of the washer is used for generation. The idea of this is to keep changing this water automatically, for otherwise it might get very foul. It will also be seen that the divisions of the carbide tray have holes (for the rising water to enter by) at different levels. This is to ensure that only a small quantity of carbide - one division - be operated on at once.

The Purification of Acetylene The washer that appears as part of a generating apparatus is a purifying vessel, as it arrests all condensible and soluble impurities,1 but it does not wholly suffice, as it cannot remove certain gaseous impurities. Phosphuretted hydrogen is the chief of these, and a substance that will arrest this suffices for the final purification. The material most largely used is chloride of lime - bleaching powder; but, although a good material, it caused some trouble at first owing to its being improperly used. It has to be diluted, so to speak, by the addition of some dry inert substance in powdered form, yet porous. Infusorial earth has served the purpose best, but a new and equally inexpensive finished purifying material has been introduced and is having the most favour at this moment. This material is known as "Puratylene," and is a specially prepared mixture of ordinary quicklime and calcium chloride, leaving the finished article like a friable pumice stone. An advantage possessed by this is the simple form the purifying vessel may take; for, instead of arranging the purifying agent (when in powdered form) in muslin bags on trays, or something equivalent to this, the vessel can have a plain interior and be merely filled with the substance named. The gas will pass freely through the mass, yet it cannot get through without intimate contact and satisfactory purification.