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Saving Waste Heat in Diesel Engines Ten Per Cent of Engine Power Can Be Recovered from Exhaust—Apply Engineering Methods—Steady Improvement—Commercially Practical ent day prime movers, and especially the oil engine, heat engines are machines of relatively low thermal efficiency. The steam engine and steam turbine cast away a large proportion of the available heat in the fuel into the condenser, while the in- ternal combustion engines does the same, though in a lesser degree in the exhaust gases and the cooling water. The turbine builders have improved matters somewhat by means of the re- generative condenser, while in the case of the oil engine, apart from such de- vices as exhaust turbo superchargers and the more or less restricted com- bined steam-oil engine of the Still class, the principal direction in which waste heat can be recovered and turned to good account lies in the ex- haust heat boiler. Such an arrangement is quite ob- vious and for that matter is by no means novel; it was applied many years ago by the Cockerill company of Seraing, Belgium, who have made a success of it in connection with blast furnace gas engines, some 10 per cent of the potential engine power being thus made available in the form of De the perfection of pres- By F. Johnstone Taylor where perishable cargo has to be kept warm or for heater coils in oil storage Spaces and in general the exhaust gas boiler for providing steam for gen- eral purposes and for steam driven auxiliaries and deck machinery is a sound practical proposition. © Some 30 per cent of the total heat in the fuel passes through the exhaust valves of an oil engine, though in the case of the two cycle engine the gases are appreciably cooler on account of their admixture with scavenge air. The average temperature in the for- mer case can be taken as 725 degrees Fahr. and in the latter at 525 degrees Fahr. Under average conditions with steam generated at say 90 pounds, the gases would be cooled to about 400 degrees and the proportions of avail- able heat in the steam would be 54 per cent in the case of four stroke en- gines and 37 per cent in the case of two stroke. Assuming in each case a fuel consumption of 0.4-pound per brake horsepower per hour, the four stroke engine can recover some 1800 B.t.u.’s and the two stroke engine about 1200. Converted in pounds at and from 212 de- grees Fahr. these figures become 1.22 and 0.84 respectively and these can be ee a ge electrical energy generated by a steam turbine. As these engines average 10,- 000 horsepower or so the added power 18 considerable, while the plant for producing it is relatively cheap. In the case of oil engined vessels, some make use of electrically driven auxiliaries, the power being economi- cally furnished by diesel driven gen- erators. At the same time steam is always useful on board ship and may be essential in many instances such as HUNUUULUNNTAUOUAUOONUYUOEEAIUU ODEO Diagram showing theamountofsteam per horsepower per hour it is possible to recover through use of waste heat boilers on diesel en- gi ed ships using the heat of exhaust gas as fuel. Also influence of boiler pressure on final steam yield s Q N a 2 K = : & S a AUTDNDUNVTVNU00000 0000000000000 taken as representative figures for both types of engines. According to Major Gregson, who deals with the subject very fully in a recent paper read before the Diese} Engine Users’ association of London, the following figures may be regarded as typical: Four Stroke Engine at Full Load Fuel consumption. 0.4 lbs. B. H. P/hr: Waste gas temperature T; = 740 Degs. F. Boiler pressure 160 lbs. MARINE REVIEwW—May, 1931 Corresponding temperature of steam = 870 Degs. F Suitable waste gas temperature leaving boiler, 870 Degs. F., plus say 70 Degs. F. = 440: -Degss: (Be = To. COs in WaSte. SABES cc sccsevcoacccssiee 6%34per cent .*.Corresponding excess air... «125 per cent Radiation, eté:,/ loss? (Say) cccdeccssocsccoons 5 per cent .:-Boiler efficiency........ 100 — 5 = 95 per cent Weight of gases per lb. of oil corresponding to 125 per cent excess air = 15.14 plus 17.7 =). 32,84 Ib; B.t.u.’s recovered by boiler per b.h.p. per hour = weight of gases per hour * tempera- ture drop degrees F. X specific heat x 0.95. Take specific heat between limits of tem- saturated perature worked to = 0.24. «B40 8/beheps/ hours =. 4 x Bo BA Se OTAGO — 440) X .24 X .95 = 900. 900 .?.Steam/b-h.p./hour —= —— = 0.93 Ib. from 966 and at 212 degrees F. Two Stroke Engine at Full Load Fuel consumption............c.cc00000 0.4 Ib./b.h.p./hr. Waste gas temperature at boiler = T, = 550 degrees Boiler working pressure.......... 35 lb. per sq. in. Corresponding saturated steam tempera- CUPE Seo csccisisecs ees sshaec ieee 280 degrees F. .*.Suitable waste gas temperature leaving boiler........ SS gs (S280. plus. 40 350 degrees F. COs In WASTE, GASES... icacccesecyasscvssocssee 5 per cent '.:.Corresponding excess aif............. 200 per cent Radiation 1Ogsiis..cscssscountss = (say) 7% per cent (larger boiler for given output than on four-stroke en- gine). « *s Boer CH ICIENCY, cccccccccscscucdstsceces cvs 921% per cent Weight of gases per lb. of oil = 15.14 plus 28.32 = 43.46 Ib. B.t.u.’s/b.h.p./hour recovered by boiler (same formula as for Case 1)—.4 X 48.46 x (560 == 850). 424. Ko 925 170. 770 .*,Steam/b.h.p./hour = ee = 0.8 lb. from and at 212 degrees F. The influence of boiler pressure on final steam yield will be particularly noted in the accompanying diagram: The above considerations assume of course that the engine is on full load. If it is on part load, the initial tem- perature T, is lowered and the rating has to be adjusted in accordance with the expression T:—T, Ve We s= 8 XK A T,—T, Mo ON, where S—the normal steam at full load in pounds from and at 212 degrees Fahr., s the steam at the lower load, T, and T, as before, V, and Vr the re- spective volumes assessed from CO2 figures and W, and Ws the respective fuel oil rates per brake horsepower per hour. It might be assumed that by adding a feed heater to the boilers that even more heat could be abstracted. That might be the case, but in practice this has been found an objectionable ar- rangement on account of the low tem- peratures inducing sweating and this moisture combining with the sulphur in the exhaust gases setting up trouble- some corrosion. 33