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The obtainable calorific value of this refuse as compared with coal is generally stated to be 1-10th,
but in Birmingham and Cambridge by a special arrangement of the boiler it is l-5th. As the primary
object generally speaking is the complete oxidation of the organic matter in the refuse, and the
combustion of the gases given off, it is found impracticable to place the boilers in the best position
for receiving the heat from the furnaces, as they would be in an ordinary engine with a coal fire.
(The old tins sell at 2/- per ton, and in Manchester are used for making oxide of iron paint.)
The same general plan is made use of in all the modern destructors that I have seen, the
differences being rather in detail than in principle.
The objects being :—
1. To expose the refuse to sufficient heat for a sufficiently long time to render it
innocuous by destroying all organic matter.
2. To expose the gases given off to a still greater heat, to render them innocuous.
3. To make use of the products of combustion.
4. To avoid nuisance in working.
5. To do the work economically.
The general plan of a dust destructor is as follows: The carts of refuse, which are fitted with
covers, are led up an inclined plane, the average gradient being 1 in 20, to a covered in tipping
platform about 20 ft. high, making the roadway leading to it about 300 ft., but natural elevations
are often taken advantage of, to save collar work in carting.
The refuse is received on the tipping platform during the day of about eight working hours ;
sufficient being deposited to feed the furnaces for twenty-four hours, but no prolonged storage of
refuse is allowed to take place.
From the tipping platform the refuse is placed in the cells in regular charges, without the
addition of any fuel, the cells or furnaces are arranged either in rows or back to back, and have an
inclination of about one in three from back to front, the raised upper end forming a fire brick
hearth, upon which the refuse is received, the front being fitted with fire bars and furnace doors.
The refuse having been placed on the hearth, where it is partially dried, is drawn down by
workmen by rakes on to the furnace on the firebars, and the clinkering is also done from the front.
The cells are usually worked in pairs, and the charging of the furnaces with fresh refuse is so
alternated that the " green gases " from the one furnace come in contact with the flames from the
alternate cell, which is then at its fiercest heat, and in a combustion chamber beyond, a constant
temperature of 1800°F. to 2000°F. is maintained, a temperature of not less than 1250 F. being
required to oxidise the gaseous products of combustion from the furnaces; the hot flames are then
conducted to the boiler, which is of the water tube type, and afterwards to a tall chimney in which
the temperature averages 300°F. The fumes from the chimuey should be quite inoffensive.
These high temperatures are obtained by means of a forced draught or by steam jets.
Without a forced draught the average temperature in the furnaces is 750°F., giving about
6-horse power per cell in winter, and about 3-horse per cell in summer. But with a forced draught, or
with steam jets, a temperature of 2000°F. is maintained in the furnaces and in the combustion
chamber, and 30 or more horse power per cell obtained from the boilers, generally speaking, one
pound of water being evaporated per pound of refuse, and 20 lb. of water so evaporated being
equivalent to one horse power. From this it will be seen that without a forced draught the high
temperatures required to oxidise the "green gases" cannot be obtained, and this was the chief
source of nuisance in the older forms of destructors, a prolonged exposure to a low temperature
not being equivalent to incandesence in a high temperature furnace. And, ton for ton consumed,
the cost of working a high temperature destructor is no more than working a slow combustion
destructor.
The average cost of constructing and working a destructor may now be considered. Though
influenced by numerous circumstances, the following figures are given:—
The average cost of constructing a destructor per cell (without site) £840
The average annual cost of working per cell £96
The average cost of destruction per ton £11½d.
The average labour per cell 1.17 men.
The average work done (8 hours shifts) 8 tons per man.
The average wages for stokers 80s. per week.
The average population dealt with 10,000 to 1 cell.
The average refuse dealt with, 24 hours 5 tons per cell.
(Beaman and Deas, 20 tons per cell).
The average height of chimney 168 feet.
The average cost of chimney to erect £6 3s. 4d. per ft.
The average time of leaving refuse in fire 1¼ hours.
The average refuse contains: 1/3 water (by weight), 1/3 combustibles, 1/3 incombustibles.