Oil Burning in Marine Boilers

OIL BURNING IN MARINE BOILERS Since the World War oil fuel burning for marine boilers has made rapid strides, and but for the question of cost would be more generally adopted. As an example of saving in boiler room staff, after the Cunarder "Aquitania" was converted to burn oil, her firemen were reduced to 42, as against 32o when coal-burning. Again in a coal-burning torpedo-boat destroyer of io,000i.h.p. the complement of stokers was 3o. In the latest class destroyers of 28,000s.h.p. and 1,400 tons, burning oil fuel alone, the stoker complement is 12. The advantages of oil fuel are as follows: (a) superior evaporative power per weight of fuel carried, giving increased radius of action—I lb. of oil with a heat value of 19,000 B.T.U. will evaporate 15 lb. of water up to and at 212°F., and I lb. of coal with a heat value of 14,500 B.T.U. will evaporate so lb. of water up to and at ; (b) ease of shipping into bunkers and putting into fires—only hoses and attachments are required; (c) less stokehold staff and bunker space required; (d) absence of coal dust and ashes—a big asset ; (e) no necessity to open furnace doors with consequent loss ; (f) proper regula tion of combustion, and capability of forcing the boiler; that is, obtaining more than the designed output.

Disadvantages of Liquid Fuel.

(a) Uncertainty of obtain ing supplies; (b) complication of piping and machinery; (c) pos sible leakage and danger from fire; (d) special appliances required for burning; (e) liquid and solid impurities found in the oil; (f) widespread contamination of water-ways.

Method of Burning.

Before entering the boiler furnace the oil has to be atomized, that is, split up into a fine spray, and for this purpose three methods are adopted; i.e., atomizing by means of steam, compressed air, and pressure. Owing to freedom from breakdown and reliability the last method is generally adopted in naval and marine practice. Many different forms of burners or sprayers are adopted. Fig. 9 shows a simple sprayer, in which the oil is forced round a series of helical threads upon the surface of a cone. By this means a whirling motion is imparted to the oil, the narrowed passage at the exit producing an increase in velocity. These factors greatly assist the spraying. The sprayers are fitted to distribution boxes on the furnace front. Fig. 10 shows a patent form of sprayer fitted to the furnace front of a cylindrical marine boiler. The pressure system of oil burning is very simple and is as follows : oil is drawn from the ship's tanks and strained ; this re moves all solid impurities which would choke the sprayers. The oil

is delivered from the oil pumps under pressure, an air vessel in the pipe-line assisting in maintaining a steady supply. The oil next passes to filters called cold filters and from there to heaters. This heating process may form particles of carbon in the oil, which par ticles are removed by passing it through filters fitted on the de livery side of the heater. From these filters, called hot filters, the oil passes under pressure to the distribution boxes on the boiler front, and from these through the sprayers, where it is atomized, to the furnace. For warships and fast passenger steamships oil is the ideal boiler fuel, but it is not considered that oil will become general in the mercantile marine owing to the question of cost. Many ships are now fitted to burn oil alone, coal alone or coal and oil combined.

Marine Auxiliaries.

The great increase in the number of auxiliary engines now fitted in naval and mercantile vessels calls for as much attention as do the main propelling engines. As regards design, the majority of marine auxiliaries follow accepted practice, but the following units call for special consideration.

Marine Condensers.

The modern method is to ensure high vacuum combined with reliability and that the weight of the appa ratus be as low as possible consistent with the work the condenser is called upon to perform. Separate air pumps are installed, one pumping out the feed water, and the other maintaining a high vacuum by pumping air from a cooler part of the condenser. Tube failure, through corrosion, is a frequent source of trouble, and the many experiments carried out with tube metal mixtures have not eliminated this.

Electrolytic Protection,

similar to that used to prevent boiler corrosion, has been adopted with good results. Metallic packing for the tubes, in place of the ordinary tape packing, has been tried with excellent results.

In connection with marine condensers a system has been devised known as the closed feed system. By this method the condensate is removed from the condenser and discharged to the boiler with out exposure to the atmosphere, thus preventing air mixing with the water and causing boiler corrosion. Another method coming into favour is the removal of the air in the feed water by mechani cal de-aeration, and this method seems to possess advantages over the removal of the air by chemical means.