show the heating value of the gas by means of the following equation :

where H is the calorific value of one cubic foot of gas; W is the quantity in litres of water heated;

T is the difference of the temperature in degrees Centigrade of the inflowing, and of that of the outflowing, water;

G is the quantity in cubic feet of gas burned during the experiment.

W = 2 litres, T = 26.77° 8.77° 18°;

=104.65 calories.

A cubic foot of this gas would therefore have a calorific value of 104·65 calories.

It should be observed that this is a 'gross' value, which represents the total heat generated by the flame, including the whole of that of the hydrogen contained in the gas, which in the calorimeter is converted into water, and gives up its latent heat to the circulating water.

It is therefore of importance to ascertain the 'nett' calorific value of the gas used in such processes, which in many cases is 10 per cent. less than the gross value. The calorimeter gives a ready method of determining the difference between these gross and nett values, as we have only to measure the quantity of water condensed in the apparatus and collected in the small measuring glass. For every cubic centimeter of this water, an allowance of 0·6 calorie must be made. As the quantity of water produced is proportionally small, it is advisable to burn a large quantity of gas, say 2 to 3 cubic feet, for these determinations. Supposing 2 cubic feet of gas condensed 53 c.c. of water, we should ascertain the calorific value of the latent heat of the condensed water, thus:

which should be deducted from the gross value, leaving the nett calorific value equal to 88-75 calories per cubic foot.

Professor William Robinson, assisted by Mr. Alfred Hay, B.Sc., made with Junker's calorimeter a series of 130 calorimetric tests with Nottingham gas. The author was present and assisted during one test. All temperatures are given in Centigrade, the heating values in calories.

During the first and fourth day, samples of the gas were analysed by Professor Frank Clowes, D.Sc. By calculation from analysis the heating value of the gas was ascertained to be 164 calories gross, which fairly agrees with the above results.

Professor Robinson has also made some careful tests of Dowson gas, of which the following table gives the results :

The gas was taken from the main supply to the engine during a run. Recently, by using a Shaw gas governor, much less variation is shown in the water outlet temperature, and the calorific value of the gas is thus exactly determined during the gas engine test.

CHAPTER XXXIII

DOWSON GAS

THE economical conditions of obtaining power from coal or coke are daily becoming more understood, and to obtain an indicated horse-power hour from one pound of coal is a great achievement; but that it has been done is indisputable, the credit for which must to a great extent be given to Mr. J. Emerson Dowson, who has designed a plant for the manufacture of producer gas, very suitable for a gas engine, with which the above result has been obtained. Dowson gas is made by forcing a mixture of steam and air through a mass of red-hot fuel, when not only is the steam decomposed into its constituent gases, oxygen and hydrogen, but a sufficiently high temperature is maintained in the generator to carry on the process continuously and make the gas as it is required by the gas engine.

A complete and compact set of his plant is shown at figs. 191 and 192, capable of generating sufficient gas for 80 E.H.P.

It consists of the boiler A, which is fitted with superheating tubes, B air injector, C gas generator, D feeding hopper, E fire bars, F gas cooler, G waste pipe, H hydraulic box, I overflow, J sawdust scrubber, K coke scrubber inside tank of gas holder, L gas holder, M outlet from gas holder to engine, N N ashpit for generator, O automatic regulator to govern production of gas.

The process is as follows. A current of superheated steam passes continuously, by a pipe from the top of boiler, through the injector, which consists of a nozzle inserted in the mouth of a conical tube, open to the atmosphere.

The pressure of steam varies from 30 to 60 lbs. per square inch above the atmospheric pressure, according to the size of the apparatus used and the quantity of gas required. This pressure forces the mixture of steam and air upwards, through the incandescent fire in the generator. The steam is decomposed in presence of the incandescent carbon, and the hydrogen, being free, passes off. The oxygen from the steam, as well as that from the air, combines in the first instance with the carbon of the fuel to form carbonic acid. As this rises through

FIGS. 191, 192. - DOWSON GAS PLANT FOR 80 H.P. (EFFECTIVE)

the hot fuel it is reduced to carbonic oxide. The liberated oxygen combines again, partly with some of the carbonic oxide, to become carbonic acid, and partly with carbon to form carbonic oxide.

The gas used in the internal combustion engine should be clean and free from sulphur compounds, and it must be cooled to give a large amount of energy per unit volume in the cylinder. Hence, for such purposes, it is best to use anthracite coal, which, being nearly pure, does not contain sulphur nor yield much ammonia, tar, or other products which readily condense and foul the pipes and valves. Good anthracite is also suitable fuel for the generator, because it makes a dense fire, free from holes or passages, and it does not cake or yield much clinker. Ordinary gas coke in small pieces free from sulphur, which does not yield large quantities of clinker and which has been subjected to high temperatures in the retorts, is also used with good results. The gas is cooled by passing through the pipes F, then cleansed by passing through water in the hydraulic box H, then through the sawdust scrubber J on its way to the coke scrubber K inside the gas holder. The automatic regulator O regulates the supply of steam to the generator and, within certain limits, governs the production of gas, by the rise or fall of the gas holder, and this not only avoids waste of fuel, but renders the storage of much gas unnecessary, as the production of gas is very rapid.

Carbonic oxide is a very poisonous gas, and devoid of colour, whilst having great heating power; but Dowson gas has a characteristic though very slight smell, not readily detected, but, with the proper precautions of sound fittings the risk is reduced to a minimum.

Unfortunately for Mr. Dowson, gas engine makers have not paid sufficient attention to the construction of engines to deal with producer gas to the best advantage. It is beyond dispute that good results are obtained from producer gas, even when using coke. The cost of maintenance of a boiler has long been known and accepted as inevitable. We may also accept the cost of a gas-producer plant, for a gas engine, though the up-keep will be less.