The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

Whether the engine is large or small, rough and ready methods of raising steam or heating water by passing the exhaust gases through boilers of ama- teur design or originally intended for coal firing are very little use, because the recovery of waste heat is a definite problem. The temperature of the ex- haust gases from engines is consider- ably below that encountered in direct fired boiler practice so that the bulk of the heat which is abstracted from the gases in a waste heat boiler must be conveyed to the water by a process of convection. In this case the heat transferred per unit of time and sur- face is relatively small and for a given duty a very large heating surface is essential. Thus the practical condi- tion of heat absorption by convection calls for a high gas velocity. In case of tubular boilers the smaller the diameter of the tube through which the hot gas is caused to pass, the closer are the gas mole- cules to the surface, the sooner will they reach that surface and the greater will be the number of con- tacts they will make with it in a given period of time. This constitutes the underlying principle of the Kirke waste heat boil- er which is extensively used in one form or another with large gas en- gines. It is for all practical purposes a locomotive type of boiler with a large number of long small bore tubes through which the gases are drawn at high velocity by means of a fan. Waste Heat Boiler Characteristics In the case of marine diesel engines the chief characteristics of a waste heat boiler are according to the Foster Wheeler Corp., who specialize in this class of equipment. (1) The boiler must be capable of absorbing heat from exhaust gases at very low temperature. (2) The boiler must be so designed as to permit of independent firing when the main engines are stopped. (3) It must be so designed that it will not be damaged by the hot gases should it contain no water and that it will not leak or show other signs of distress under intermittent operation of the engine. (4) It ought for preference to func- tion as a silencer and it must not ore- ate back pressure or be subject to the attack of corrosive gases. This company’s waste heat boiler embodies a special feature in the shape of the gilled tubes, which are steel tubes with cast iron protecting rings. These are claimed to provide ade- quate protection against any possible ill effects of corrosive elements in the exhaust gases and in addition to afford a very large heating surface in a con- fined space. This heating surface is contained in a cast iron housing and all the tube joints are outside it. The heating surface is arranged in two sec- tions which are connected to a hori- zontal steel drum fitted with the usual 34 boiler mountings. The feed water is delivered through the upper section in which it is heated before entering the drum and the steam is raised in the lower section through which the water circulates to and from the drum. The elements of the heating surface are horizontal. With the elements of each horizon- tal row closely spaced together, the gases flowing over the heating surface are successively restricted and ex- panded and this gives the necessary silencing effect. The oil burner at the base of the boiler maintains steam when the engine is not in use. There are no steam joints in contact with the gases, the elements being expanded into the tube sheet before connecting up the flange joints. While the soot blower is an essential part of the equipment, the boiler normally keeps quite clean by the blast of the waste gases. According to data furnished by the makers a boiler of this class working in conjunction with a 4000- horsepower M. A. N. engine (two cy- cle) evaporated 0.7 pounds of water per brake horsepower per hour. The Cochran boiler is in good re- pute in Great Britain as being one of the most efficient vertical boilers built and recently this firm has applied the general design to the raising of steam by the exhaust gases of marine diesel engines. This boiler is furnished with two upper nests of tubes for the waste heat recovery and a lower nest of tubes for oil firing. The latter are in direct communication with the furnace and all the tubes are secured between two common tube plates. In the upper nest there are, in one particular size of boiler which is standardized to meet a good many requirements, 311 114-inch outside diameter tubes and 41 stay tubes. In the middle section there are 297 tubes of the same size and 55 stay tubes, while those in the lower portion are 24-inch diameter. There are 102 of these and 29 stay tubes. With 1890 square feet of tube sur- face, these boilers are claimed to be capable of furnishing 0.80-pound of steam at 100 pounds per brake horsepower per hour of the engine. The Clarkson thimble tube boiler has been universally successful in con- nection with waste heat recovery and not only is it suitable for relatively large engines, but it can be well ap- plied to small semidiesel units on trawlers, ete., with advantage. Its essential features are an arrangement of inwardly projecting tubes of “thim- ble” form against which the gases im- pinge at high velocity. This creates the essential turbulence of the gases outside the tubes while within them there is a percussive circulation of the water which keeps them clear of de- posit at all times. In fact even with hard water it is claimed that a daily blow down and a periodical wash out serves to keep these boilers in good condition for an unlimited period, as the tubes rarely if ever leak or call for re-expanding. This is largely be- MARINE REVIEW—May, 1931 ‘rating, ete. cause there is a complete absence of expansion due to heat while the car. bon deposit on the tubes can be burned off when the boiler is shut down. A further development lies in the “duplex” type. This type of boiler was first installed in the motor traw- ler SAINT Martin LeGasseE in conjune- tion with a Polar engine of 700 horse- power. There are two separate thim- ble tube units enclosed in a common boiler shell, the overall height being 12 feet 2 inches and the diameter 4 feet 3 inches. The top unit is arranged to deal with the exhaust gases from the engine. They pass down through the nest of water filled tubes and re- turn through a central outlet pipe. The heating surface is 108 square feet and with an assumed gas temperature of 480 degrees Fahr. some 300 pounds of steam per hour is gener- ated at 75 pounds pressure. The bot- tom unit is designed for simultaneous or independent oil firing by means of a Wallsend Howden low pressure burn- er. It is arranged to fire vertically upward within a combustion chamber formed of refractory material and so arranged that the hot gases pass down through a nest of water filled thimble tubes. The heating surface is 180 square feet and it has an output of 800 pounds. of steam per hour. A good feature of the design is the well sub- merged crown of the lower unit which is halfway down the boiler as meas- ured from ‘the normal water level. If desired the top unit can be run dry as a silencer if no steam is wanted and the combined output is 1100 pounds of steam at 75 pounds. . The boiler has all the usual steam fittings. Another type is known as the Clarkson electric boiler and it is some- what novel. In place of the more usual oil burner for use in port, there are electric elements supplied with cur- rent from the ship’s sets. Practical Working Condition It is hardly necessary to point out that an exhaust gas boiler is bound to be a commercial proposition be- cause when the engines are running the heat supplied to it would other- wise pass away through the uptake. Normally such boilers could be used to the best advantage in supplying steam for the generator sets. Assum- ing a normal type of diesel generator to be using 0.5-pound of oil and de- veloping say 260 horsepower, that means 130 pounds of oil per hour or 1.4 tons per day, the cost of which can be definitely saved, which will mean in a diesel ship using 14 tons per day a reduction in overall fuel consump- tion of 10 per cent. If steam was re- quired for this or other purpose it can be put to further use. For instance on the M. V. Oraro, the steam from the exhaust gas boiler to the extent of 6000 pounds at 100 pounds pressure operates a 160-kilowatt generating set and then passes out at 5 pounds for accommodation heating, and evapo- ere sls Gala a ck a Me Lie ae Se ee TNT RE ee ee