When the work exceeds the capacity of the mouth blowpipe, or when it is too continuous to be done with the mouth alone, a mechanical blower must be used, and the selection of this to suit the work required is a matter of considerable importance. The temperature of a given flame, the fuel combustion being equal, is greater in inverse proportion to its size. The smaller a flame becomes when the air blast is applied, the hotter it is, and the more work it will do, provided the air is not supplied in excessive quantity. Other things being equal, a high-pressure blast gives the most powerful flame, and the pressure of the air supplied is therefore a matter of serious importance. An average adult can, with an effort, give an air pressure in a blowpipe equal to about 36 in. of water pressure, or 1 1/2 lb. on the sq. in. The average pressure is, however, about half this, or rather less, the maximum being only obtained by a severe strain, which cannot be continued. A fan worked by the foot will give an air pressure equal to about 1/2 to 1 in. of water. A fan worked by power will give air at 1 to 5 in. of water pressure, depending on its speed and construction. An average smiths' bellows about 5 in. pressure. Small heavily-weighted circular bellows about 8 to 10 in. pressure.

Root's blower driven by power, 24 in. pressure. Fletcher's foot blower No. 2, 15 in. pressure. Fletcher's foot blower Nos. 3 and 5, 30 in. pressure. Fletcher's foot blower No. 4, 45 in. pressure. Cotton and Johnson's foot blower (variable), 5 to 20 in. pressure.

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The temperature of a blowpipe flame may be estimated from the above, being in close proportion to the pressure of air supplied, and it may be taken as a rough rule in brazing or hard soldering with gas, that, given an air pressure equal to 15 in. of water, a blowpipe, having an air jet of 1/8-in. bore, will braze work up to 1/2 lb. total weight. One with an air jet of 1/4-in. bore will braze up to about 2 lb. total weight, i.e. 2 brass weights, each 1 lb., could be securely brazed together with a blowpipe with 1/4-in. bore air jet, and supplied with air at a pressure equal to 15 in. of water, or 10 oz. on the sq. in. It will, of course, be remembered that the areas given are those of the air jet or point at which the blast leaves the blowpipe, and the area of the gas supply is that of the space between the air tube and the gas tube outside it. The area of taps and pipes to supply these must, of course, be larger, to prevent friction as much as possible. When anything like a high power is required, it is of the first necessity that any elastic or flexible tube used shall be perfectly smooth inside. A length of 6 or 8 ft. of india-rubber tube, with wire inside, will reduce a gas supply or a pressure of blast to about one half.

Practically this amounts to requiring apparatus double the size for the same work, and it therefore does not pay to use rough tubing. Applying the rule to other shapes of work, it may be taken that a blowpipe which will braze a block of 2 lb. total weight, when the work is supported on a good non-conductor, will braze brass plate up to -1/8 in. or 3/16 in. thick. Its capability of brazing iron is not so great, as iron does not take up the heat of the blowpipe so readily as brass does. When the blowpipe is supplemented by either a bed of burning coke or by a non-conducting jacket round the work, the power of any blowpipe may be extended almost without limit, as little of the actual work of heating the body of metal is done by the direct blowpipe flame.

In the construction of blowpipes for gas they should be so proportioned as to give the maximum effect for the minimum of fuel and blast. To do this the air pressure available must be an important factor. Speaking roughly, but still sufficiently near to make a correct rule to work by, a blowpipe requires 1 of gas to 8 of air. If the gas is supplied at a pressure equal to 1 in. of water, and the air at 8 time3 that pressure, the area of the gas and air pipes should be equal, to get the best effect. If the air supply is equal to 16 in. of water pressure, the gas pipe must be double the area of the air, and so on in proportion. Of course the air and gas supplies can be adjusted by taps easily, but in the first construction of a blowpipe for large work, this rule must be adhered to. Any departure from it reduces the power of the blowpipe, and ignorance of this simple rule has frequently caused failures which the makers of blowpipes have been unable to explain.

It is often an advantage to build up a blowpipe quickly for some special work, and the method and rules for construction are here given, bearing in mind always that a high-pressure blast gives the most compact and highest temperature flame, without having any actually greater quantity of heat in the flame produced.

At day, pressure = 10-10ths on the gas supply, a 1/2-in. pipe with a 1/2-in. bore tap will supply about 1 1/4 cub. ft. per minute, or 75 cub. ft. per hour. A 1-in. bore pipe and tap will supply about 5 cub. ft. per minute. About 25 cub. ft. of gas equals 1 lb. of coal in fuel value, and, therefore, a 1/2-in. gas pipe will supply at the rate of 1 lb. of coal, in a gaseous form, in 20 minutes. To burn this in a blowpipe, an air supply of 10 cub. ft. per minute is required, and given the available blast pressure the area of the air jet necessary is easily found.

For the construction of large blowpipes for special work, the stock fittings can generally be utilized, and an efficient blowpipe built up in a few minutes, as shown in Fig. 160. Nothing more is necessary than 3 short bits of tube, a T coupling and diminishing socket, or straight union. No taps are necessary on the blowpipe, if not at hand, as if an elastic tube is used the flame can be perfectly controlled by squeezing the tubes between the fingers, holding them in the same way as the reins are held in driving a horse. If a diminishing socket is not at hand, the end of the T-piece can be plugged up and the air tube fastened into this plug, and it will be a convenience if an elbow is put on the gas inlet close to the T, so as to turn the gas pipe in the same direction as the air pipe. In this form it makes a handy and convenient blowpipe.

For any except very small work, some mechanical blower is absolutely necessary. Those who do not care to go to the expense of any of the apparatus usually sold, can produce a good make-shift with one or two pairs of common house bellows. If an upholsterers' or sofa spring is placed between the handles so as to render the opening of the bellows automatic, the pressure of the foot on the top board will give a strong blast of air. This, although intermittent, acts very well for a large proportion of work, and a full-sized pair of house bellows will supply a blowpipe with an air jet of full 1/8 or 3/16 in. bore. A continuous blast, at all events for soldering and brazing, is not at all necessary, unless the maximum possible power is required. To obtain a continuous blast from this arrangement several ways may be adopted. It is of course necessary to have a reservoir, which is always under pressure, and some means must be adopted to prevent the air in the reservoir blowing back into the bellows, whilst they are being lifted between the strokes.

If a square tin or zinc vessel is made, with a sloping partition, shown at b (Fig. 161), the partition slightly open at the bottom, and the vessel half filled with water, the air when blown by the bellows through the pipe c, bubbles up through the water, which makes the bottom of the pipe c tight against the return of the air. As the air accumulates in the close part, it presses the water a under the partition to the other side, causing a difference in level, which exerts a continued pressure on the air pipe on the top. The deeper this vessel the heavier the air pressure which can be obtained, as this is ruled by the difference in level between the two water surfaces. This is the only means of getting a continuous pressure without a valve. The next easiest way is to get a second pair of bellows, plug up the hole underneath the inlet valve at the bottom, and in this plug insert a pipe leading from the first pair of bellows. The second pair then forms the reservoir, the air being taken from the nozzle to supply the blowpipe, and the necessary pressure must be obtained by weights on the top board or by a strong spiral spring resting on the top board.

The rule with house bellows is that they are made in a wholesale rough way, and very few are anything like air-tight. They should be carefully selected for the purpose by opening fully, stopping the nozzle with the finger, and pressing the handles heavily together. Many will be found to close almost as quickly with the nozzle stopped as with it open, and, of course, these are quite useless for the purpose.

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