149
In the other type of furnace, the outlet flue is in the front and over the hottest part of the fire, and the unburnt refuse being, as in the first type at the back, the fumes of this drying refuse are brought over the hottest part of the fire and are thoroughly heated, before reaching the flue.
This latter arrangement appears to me to be the more scientific of the two, for whereas, in the first case, the heated gases from the burning refuse are cooled in passing over the unconsumed refuse and thus enter the flue with a reduced temperature, in the second case the fumes from the unconsumed refuse pass over the hottest part of the fire and eventually pass out of the furnace thoroughly heated and innocuous.
Draught.
There is a diversity of opinion as to the relative advantages or otherwise of natural and of forced draught, but so far as I can ascertain the latter is considered the better by those who have had most experience.
The advantages of forced draught are that a higher temperature can be maintained, the rate of burning is accelerated, and the necessity for a very high chimney is obviated; on the other hand, there is a greater risk of dust, etc., escaping from the chimney and becoming a nuisance.
On behalf of natural draught it is urged that a sufficient heat is obtained to properly burn the refuse, with less wear and tear to the furnaces and a consequent lower annual outlay in repairs.
Under forced draught, fewer furnaces do the work so that there are less to repair; refuse can be burnt more quickly, and from personal inspection of both, I find that a greater steam pressure can be maintained in the boilers,
The force of the draught is usually equivalent to a pressure of about inch of water, but in Messrs. BEAVAN and DEAS' furnaces a pressure of 2 inches is maintained, hence the high consumption at Leyton (See below.)
Forced draught was specified for the proposed furnaces for Hongkong.
Flues.
The design and arrangement of the flues is an important factor in the subsequent successful working of the furnaces, for it is by them that the heated gases from the furnaces are conveyed and utilized, and in them that the dust is intercepted. They should be of such design as, for instance, to admit the fixing of boilers and the introduction of carcases to be cremated, though this is sometimes done in the furnace with small carcases of dogs, cats, &c. and with condemned meat.
The flues should also be so placed that the loss of heat by radiation is reduced to a minimum.
Fume Cremators and Dust Catchers.
In the old type of furnace with natural draught, it was generally found that unconsumed gases escaped from the chimney, creating a nuisance in the neighbourhood. To obviate this a second furnace, called a cremator, was constructed in the flues between the refuse furnace and the chimney, where the fumes passed over a special fire in order to render them innocuous. This arrangement is extravagant as it involves additional expenditure in fuel and attendance.
With a properly-designed furnace such additional expenditure should be unnecessary; it is, however, wise to introduce a cremator if it can be worked without additional expenditure, and this can generally be effected in connection with a "dust catcher."
A simple arrangement of the latter is shown in the design submitted by Messrs. GODDARD, MASSEY AND WARNER, and consists of a blank wall built across the flue, with a pit at the bottom, the flue being increased in height above the wall to give the required sectional area. The dust is intercepted by the wall and falls into the pit at the foot, while the draught is impeded and probably the temperature raised in consequence.
A very ingenious cremator and dust catcher has been recently erected at Edinburgh by the Horsfall Syndicate and a similar one is proposed by them for Hongkong. The flue ascends in a short spiral round the outside and to the top of a domed chamber; the gases enter this chamber through the dome and pass out to the chimney by a flue near the bottom. There is a depth of some three feet at the bottom of the domed chamber below the outlet flue which acts as a dust box. This cremator is said to throttle the heat and thus raise the temperature, and cremate the fumes. The draught beyond the cremator is dependant upon the height of the chimney, but can be regulated by dampers.
Chimney.
The usual height of chimney for furnaces with natural draught is about 180', this great height being rendered necessary to produce a draught in the furnaces and to obviate as far as possible the escape of dust, paper, etc. from the top. Where forced draught is used, one reason for the great height is removed, for the chimney is relieved of all duty in producing draught in the furnace itself and receives assistance for the draught required in the flues, and it should be possible to so design the flues as to intercept all paper and dust.
A lower chimney has the double advantage of costing less in initial outlay and upkeep and of being less liable to damage by typhoons.