HEAT TREATMENT OF STEELS.
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Gas Ovens Substantially Reduce Costs.
In commercial chassis, hardening and annealing are of very great importance; indeed, the life of a lorry depends largely on the efficiency of the many joints and bearing surfaces that are heat-treated. Those concerned with the construction and maintenance of vehicles, therefore, are directly interested in the furnaces necessary for these operatiane, and consequently will examine closely, into any system for which great advantages are claimed. There have been unsatisfactory furnaces in the past, of which the dieappointing performances have prejudiced the minds of users against new systems, This prejudioe can be oeercome only by demonstration, and we had the pleasure recently of being present at the Diamond Foundry, Teuton, where the Davie Furnace Co. gave a demonstration to the Press of their " Revergen" furnace, operating on town's gas.
For this system is claimed very high thermal efficiency, with consequent great reduction in the gas bill. In the past an efficiency of 18 per cent, has been considered normal ; with the " Revergen" system an efficiency of 30.7 per cent, is claimed, and we are bound to say that in a three-fold demonstration, where all readings and calculations were checked by the journalists,present, Messrs. Davis substantiated what at first might seem an extravagant claim.
Rest is wasted in furnaces because the flue gases naturally come.away at the temperature of the oven, say, at 950 de
grees Cent. or 1,000 degrees Ceut. Merely to pass the flues under the floor of the furnace does not help matters, for heat will not pass from a colder body to a hotter one. The heat of these gases can only be used, then, by imparting it to i the ncoming gas or air, or both. In the " Revergen ' system the oven is arranged over two chambers, each filled with a. honeycomb structure of frebricke, and the recuperating action is 'brought about by arranging for a period of 20 minutes to 30 minutes that the incoming air shall pass no through one of these chequer bricks to a point where it mixes with the gas, whence the mixture, burning, passes to the far,side of the oven. Then the exhaust passes down the other chequertrick structure, to which it imparts practically the whole of its heat.
After tbe 20 or RD minutes, the bricks on the exhaust side will be very hot at the top, becoming gradually cooler towards the bottom ; the actual exhaust pipe, even after many home, is only warm to the hand. Then, by one movement of a lever, a four-way valve is operated so as to cause the incoming air to pass up through what was previously the exhaust chamber, thus reversing the dime(ion of the flames across the oven. Uader these conditions, the air takes up the 'heat of the honeycomb structure, and the exhaust heat is imparted to the chequer bricks in the other chamber. At. the end of another 20 minutes, the direction of the flow is reversed again, and so the process goes on, heat being retained in
the furnace that would otherwise have been wasted in the flues.
We actually saw three demonstrations of the annealing of steel billets, and were able to check. the figures, which are appended :—
Operation.—Annealing steel billets at 820 degrees Cent. Oven.-54 ins. by 32 ins. by 24 in.
Reyergen. .
Billets Charged.-20.
Weight Charged.-987 lb. Time Charged.—It, a.m. Meter, 4,058,400.
Time at 820 degrees Cent.-12.18 p.m. Meter, 4,039,750. Time taken.-1 hr. 15 mins. Gas used, 1,350 c.f. Flue temperature, 205 degrees Fahr.
Gas used per lb.-1.37 ref.
Gas used per ton.-3,069 c.f. Rate of beating lb. per hour.-790 lb.
Gas used to maintain :— For 1st, hour.-300 c.f., .304 c.f. per lb.
per hour. Flue temperature 120 degrees Fehr.
For 2nd heer.-250 c.f., .253 c.f. per lb. per hour. Flue temperature 120 degrees Fahr.
*Per 3rd hour.-325 c.f., .328 c.f. per lb. per hour. Fluetemperature 120 degrees Fahr.
Average 3 hours. 292 = .29i c.f. Aeerage calorific value of gas, 445 B.T.U.
Temperature in last hour • rising slightly, being 830 degrees Cent: at clase of test.