Where compressors of the rotary or turbine type are installed, there will be only water in expansion, but it is good practice to remove this, even though the air be re-heated, because the moisture will recondense in the conveyor pipe, and tend to choke the latter when small grained substances are being conveyed.
Types of Compressors. Reference has already been made to the power unit, and it is hardly within the scope of this work to describe in detail the various machines available. As, however, the economy of air conveying depends in a large measure on the cost of power, it is evident that the compressor should be of the most suitable type for the duty to be performed.
For small installations, single-stage reciprocating machines, driven directly by steam engines or by electric motors are, no doubt, the most suitable. In the case of large plants, using the air continuously in a number of air jets, where the load factor is high, it is certainly more important to install a two or three-stage compressor, owing to the greater economy of working. The larger capital expenditure will be compensated by the considerable saving of energy. As compared with single-stage compression to 100 lbs. gauge pressure, a saving of 20 per cent. can be effected by three-stage working, and with a constant load of from 75 per cent. to 100 per cent. of full load, a turbine or electrically driven rotary multiple-stage compressor is decidedly the best type to adopt.
In plants where exhaust steam can be used to advantage, as in large generating stations, a steam turbo-compressor, multiple-stage, exhausting to a feed water heater will show great economy, and the operating costs of a large plant of this type are very low compared with any other form of conveyor. This will be obvious when it is pointed out that maintenance costs on the conveyor are confined to renewals of bends and junctions in the pipe lines, and of flexible hose. There are no discharge valves or air locks to be kept vacuum tight, no filter strainers or sleeves to renew, and the power unit is not subjected to undue wear through extraneous matter entering the cylinders and scoring the walls or wearing the valves.
Compared with other forms of mechanical conveying, the pneumatic induction system is very low in maintenance costs, while the serious charges incurred in employing human labour are reduced to a minimum.
CHAPTER VII
STEAM JET CONVEYORS
A method of removing ashes from boiler furnaces which has been developed extensively in America is essentially a pneumatic system, although steam is the conveying medium instead of moving air. Steam is used because the apparatus is always in use on boiler plants, from which steam can be taken as conveying medium. No air compressor or other special plant is required. On the other hand, the simple use of a connection from the steam main is a matter of very little importance, and no check is ever made as to the amount of steam so used, hence 99 per cent. of the users consider that the steam jet costs practically nothing for “power” compared with a compressor which would have a certain sized motor connected, and could not escape attention as an additional power consumer.
Steam Consumption. Investigation into the actual consumption of steam jets would often give very startling results, especially after the plant had been in operation for some time and the nozzles had begun to cut and wear. As proof of the waste of steam possible in such a plant, it is interesting to note that Mr. David Brownlie, in a paper on Automatic Stokers,[1] gave results of actual tests made on steam jets as used in certain classes of stokers in which steam jets are allowed to blow down the hollow furnace bars. These tests showed that, whereas the makers estimated the steam consumption of the jets to be about 2 per cent. of the boiler output, the tests on 80 plants showed a consumption varying from 0·5 per cent. up to as much as 21·4 per cent. of the total output of boiler.
As further evidence of the waste of steam that can occur due to neglect of the cutting effect on the nozzles, one American firm has designed an ingenious warning or “tell-tale.” A small hole is drilled almost, but not quite, through the nozzle. While the nozzle retains its initial shape and size the apparatus acts normally, but as soon as the small amount of metal covering the end of the hole has worn away, the hole is exposed, and a certain amount of steam passes through it to a steam whistle which blows continuously until a new nozzle has been inserted in place of the one which is now worn so much as to make it uneconomical in steam consumption.
Provided that means are taken to prevent waste of steam due to worn nozzles, the steam jet conveyor is very serviceable and, being flexible and convenient, it is very useful for the purpose for which it has been developed.