Where an oven or drying chamber is used continuously, it should be jacketed with slag wool or boiler composition, but for many purposes this is no advantage. As an example both ways, I will instance the drying of founders' cores where there is only one blow per day. The cores of an ordinary foundry can be dried by gas in a common sheet iron even in about half an hour; any accumulation of heat after that time would be useless, and a jacketed oven would be of no advantage.
For the disinfection of clothes in vagrant wards and hospitals for infectious diseases, on the contrary, a continued heat is necessary, and in this case the accumulation of reserve heat, which takes place slowly in a jacketed oven, becomes of value, as the gas can be turned low or out, and the ventilators closed, insuring a more complete disinfection with a much smaller gas consumption. Where an oven or heated chamber is much used for periods of over half an hour at once, a non-conducting casing pays well by reduced gas consumption.
For albumen and glue drying, leather enameling, tobacco drying, and purposes where a large space has to be very slightly and equally warmed when the weather is unfavorable, steam-pipes are generally used, but, not being always available, an exceedingly good arrangement may be made by placing at intervals in the room gas burners, of any construction, close to the floor, and surrounded with a sheet-iron cylinder, say 2 ft. or 3 ft. high. The top of these cylinders must be connected throughout with a fairly large flue, which will take the products of combustion from the whole, and this flue must be carried either horizontally, or with a slight rise, so as to utilize all the waste heat. The reason for having a number of stoves at intervals is that the heat in a flue will not carry, for any useful purpose, more than about 8 ft. or 10 ft., and a single stove would give an irregular temperature in any except a very small room. If all are not used at once, the flues of those not in use may be closed by a damper to prevent down draught. The use of hot water pipes heated by gas may also be occasionally advisable, but, unless for some special reason, it is much more economical to use coal or coke, as the bulk of water makes an exceedingly good regulator, and makes a fire practically as steady and reliable as gas, thus superseding the more costly fuel.
For one of my own purposes I need hot-water pipes, having very little variation in temperature night and day; and using coke for economy's sake, I get a regular temperature by heating a large quantity of water, about 200 gallons, with the fire, and inclosing this in a tank jacketed with slag wool. My circulating pipes run from this tank, and a practically steady temperature, night and day, can be obtained with the most irregular firing, and occasional extinction of the fire for several hours at once.
For the heating of liquids, the greatest economy is to be obtained from one single flame, of as high a temperature as can conveniently be obtained, and the flame must be in actual contact with the vessel to be heated. In jacketing vessels, to prevent draughts, care must be taken that the jackets do not cause currents of cold air to rise rapidly up the sides of the vessel, and so cool it. If this is the case, the use of a jacket, instead of being an economy, is a positive expense, and waste of heat. Many processes, such as making oil and turpentine varnishes, require a heat under instant control, and in these the use of gas is an important matter, as the loss and risk of fire are very serious elements of expense, more especially in small works where special and costly preparations for contingencies cannot be afforded. I have here a burner which, for its power, is, perhaps, the most compact and gives the highest temperature of any burner yet known, and it is easily made in almost any size; it has, I think, many special advantages. The use of gauze, which is its only weak point, is more than compensated for by the very high duties obtained in practice with it, owing to the compactness and concentration of the heat obtained. The following extract from my communication to the Gas Institute will give all particulars as to the constructive detail of this burner. Those who wish to go further into the matter will find the paper referred to in the publication of the Gas Institute for the current year, and also in the Journal of Gas Lighting, June 26, 1883, and the Review of Gas and Water Engineering, June 16, 1883.
"The first and most important part is the mixing chamber or tube, one end of which is supplied separately with gas and air, which at the other end are, or should be, delivered as a perfect mixture. It may be taken as a rule that this tube, if horizontal, should not be less in length than four and a half times or more than six times its diameter. It is a common practice to diminish or make conical-shaped tubes. All my experience goes to prove that, excepting a very trifling allowance for friction, the area of the smallest part of the tube rules the power, the value of the mixing-tube being no more than that of the smallest part. If the mixing-tube is upright, new sources of interference comes in; notably the varying specific gravity of the mixture. Except with one definite gas supply, the result is always more or less imperfect, and regular proportions cannot be obtained. This is now so well known that the upright form has been practically discarded for many years, and is now only used where the peculiar necessities of the case give some special advantage.
Fig. 1. SPECIAL HIGH POWER BURNER.
SHEWING ATTACHMENT B WHEN USED WITH A BLAST OF AIR
"The diameter of the mixing tube is a matter of importance, as it rules the quantity of gas which can be satisfactorily burnt in any arrangement. With large flames, given a certain size of gas-jet, the diameter of the mixing-tube should be not less than ten times as great. For instance, at 1 inch pressure, a jet having a bore of 1/8 inch will pass about 20 cubic feet of gas per hour. To burn this quantity of gas, a mixing tube is necessary 10/8 or 1¼ inch in diameter. By the first rule this tube must be in length equal to four and a half times its diameter, or 5-5/8 inches. It would appear that the mixing-tube, having 100 times the area of the gas jet, is out of all proportion to the size necessary for obtaining a mixture of one of gas to nine or ten of air; but it must be remembered that the gas is supplied under pressure. It is therefore evident that no mere calculation of areas can be taken, into account, unless the difference in pressure of the supply is also considered. A complete reversal of this law is shown in that ruling the construction of blowpipes, which I have already given in a previous paper on 'The Use and Construction of the Blowpipe.' In these the air supply, being under a heavier pressure, is much smaller in area than the gas inlet; and, to obtain maximum power, the air-jet requires to be enlarged in proportion to the gas pressure.
"Given a certain area of tube delivering a combustible mixture, the outlet for this mixture must be neither more nor less than the size of the tube. Taking an ordinary drilled tube, such as is commonly made, and of the dimensions before given--i. e., 1¼ inch bore--if the holes are drilled 1/8 inch in diameter the tube will supply 10 x 10 = 100 of these holes. In practice this rule may be modified.