CHAPTER XXIII RULES FOR EFFICIENT OPERATION OF OIL FIRED BOILERS Since the advent of steam turbines the relative importance of the fireroom crew as a factor in economical operation has increased considerably compared with the engine-room crew. This is because the steam turbine, after it has once been properly set up, operates continuously at a fixed steam consumption for a given load and nothing can be done to improve its economy other than to keep the turbine, condenser and auxiliaries clean and in good operative condition. In the boiler room, on the other hand, continual watchfulness is necessary to keep the boilers operating at good efficiency, and the slightest laxity in attention to the various details results in a large waste of fuel. To assist the operators of oil-fired boilers in obtaining the best economy possible, the writers have prepared the following set of rules, which if carefully followed will bring the daily operating efficiency of the plant very close to test results: 1. Regulate Air to Suit Load. The regulation of the air supply is one of the most important things in the operation of oil-fired boilers. If there is not enough air, a great waste of fuel may occur as part of the atomized oil will simply pass up the chimney unburned. On the other hand, it is possible to waste just as much fuel by allowing too much air to enter the furnace as all of the extra air is heated up and passes out at the temperature of the chimney gases, carrying away with it an enormous amount of heat. To determine accurately the amount of air required for the best conditions it is necessary to analyze the flue gases. Many plants, however, are not provided with the apparatus necessary for this, and in such cases the air may be regulated with a fair degree of accuracy by an observation of the smoke discharged from the stack. For perfect combustion there should be no smoke, and if any smoke appears it means incomplete combustion and not enough air. If there is no smoke, however, it does not follow that the conditions are right, as no smoke may mean either just the right amount of air or a large excess of air. FIGS. 114-116.-Stacks of station A, Pacific Gas and Electric Company, San Francisco. This plant is one of the largest fuel oil operated power plants in the world. The first view shows improper combustion, the second proper combustion, and the third instances of proper and improper combustion in varying degrees. To properly regulate the air, therefore, if the boiler is operating with no smoke the damper should be gradually closed until light gray smoke just begins to appear; if then the damper is opened very slightly, this smoke will be barely perceptible and the conditions for the most economical operation will be obtained. 2. Prevent Air Leakage Through Setting.-Air leaks may be detected by holding a candle flame at the cracks in the boiler setting. If the flame is drawn in, it shows that there is an air leak which should be stopped up as far as possible. A good coat of fire-resisting paint over a brick setting is a good investment, for the bricks themselves are quite porous as can be demonstrated by dropping one in water and noting the air bubbles driven from it. There are two ways by which the air can be regulated, namely, by the damper at the outlet of the boiler or by the ash-pit doors. If the air is regulated by the ash-pit doors, the damper being left wide open, there will be a strong draft within the setting, tending to cause air to leak in through the cracks in the brickwork. The strong draft also tends to pull the gases through the setting by the shortest paths, so that some of the heating surface is not swept by the gases. If, on the other hand, the ash-pit doors are left wide open and the air is regulated by partly closing the damper, the draft inside the boiler setting is very slight so that the air leakage is reduced to a minimum. There is little force tending to change the direction of the flow of the gases so that they travel of their own momentum to the furthermost corners and fill out the setting completely, thus coming in contact with all the heating surface of the boiler. It is, therefore, much better to regulate the air by means of the damper than by means of the ash-pit doors. In the case of very light loads, however, it is best to use both the damper and the ash-pit doors because if the damper alone is used there may be a positive pressure produced in the upper part of the setting, causing gas and smoke to leak out into the fire room. 3. Analyze Flue Gases Frequently.-The exact position of the damper to suit different loads can only be determined by fluegas analysis. The CO2 should be kept as high as possible without producing CO. If it is found impossible to secure 132 or 14 per cent. of CO2 without a trace of CO, then there is something wrong with the furnace or the burners and an investigation should be made. The proper method of sampling exit gases should be used as it is quite possible through improper sampling to obtain very poor CO2 readings even though the furnace may be operating quite effectively. If correctly measured, the greater the CO2, up to a certain limit, the greater the efficiency. 4. Burner Must Be Suited to Furnace.-The function of the oil burner is to atomize the oil, and the most efficient burner is the burner that will atomize the oil with the least quantity of steam. Burners are of three general types-steam jets, mechanical-pressure jets and air jets. The air jet is largely used in metallurgical work while the mechanical jet, which delivers a conical flame, is used principally in marine work and to some extent in stationary practice. The steam atomization burner is used in stationary and locomotive practice almost universally. The choice of burners for central-station furnaces therefore lies between the steam jet and the mechanical, the shape and design of the furnace influencing the type selected. The furnace arrangement is the most important part of the boiler so far as economy of operation is concerned. If the air and oil are not properly mixed, it will be impossible to obtain proper combustion. It is important that the oil be completely burned before the cooling effect of the heating surfaces can operate to quench the fire. A large combustion chamber is of great advantage in an oil-burning furnace, as the larger the combustion chamber the more complete the combustion will be before the gases are cooled. 5. Keep Boilers Clean and Maintain Furnaces Properly.-Oil burners must be carefully watched as they are liable to become clogged with foreign matter or coated with carbon. This may cause the flame to shoot sideways, resulting in smoke and poor efficiency. Therefore a spare burner should always be kept on hand, so that as soon as any burner gives trouble it can be removed, taken apart and cleaned. All burners should be cleaned at regular intervals and adjusted so that the flame will not be projected directly against the boiler tubes or against the boiler setting, and so the flames from different burners will not interfere with one another. The checkerwork in the furnace floor must be carefully placed and maintained in good condition. The air openings often become slagged over or stopped up by pieces of brick breaking off. Unless they are cleaned out occasionally and kept open for their |