PRINCIPLES OF NAVAL ENGINEERING 



can be said to be more important than any other, 

 the checkman's job is the one. 



One of the greatest difficulties in maintaining 

 the water level arises from the fact that the 

 boiler water swells and shrinks as the firing 

 rate is changed. As the firing rate is increased, 

 there is an increase in the volume of the boiler 

 water. This increase, which is known as swell, 

 occurs because there is an increase in the num- 

 ber and size of the steam bubbles in the water. 

 As the firing rate is decreased, there is a de- 

 crease in the volume of the water. This de- 

 crease, which is known as shrink, occurs be- 

 cause there are fewer steam bubbles and they 

 are of smaller size. Thus, for any given weight 

 of boiler water, the volume varies with the rate 

 of combustion. 



The problem of swell and shrink becomes 

 even more complex when we remember that the 

 evaporation rate also increases as the firing 

 rate increases and decreases as the firing rate 

 decreases. When the firing rate is increased, 

 therefore, the checkman must remember to 

 feed more water to the boiler, even though the 

 water level has already risen momentarily be- 

 cause of swell. On the other hand, the checkman 

 must remember to feed less water to the boiler 

 when the firing rate is decreased, even though 

 the water level has already dropped. Because 

 these actions may appear to be contrary to 

 common sense to a person who does not under- 

 stand the concept of swell and shrink, a good 

 deal of training is usually required before a 

 man can be considered qualified to stand a 

 check watch. 



The control of combustion involves the con- 

 trol of fuel and the control of combustion air. 

 There are three ways in which the firing rate 

 may be increased or decreased in order to meet 

 changes in steam demand: (1) by increasing or 

 decreasing the fuel pressure, (2) by increasing 

 or decreasing the number of burners in use, and 

 (3) by changing the size of the sprayer plates in 

 the atomizer assemblies. With every change, 

 the amount of combustion air supplied to the 

 boiler must also be changed in order to maintain 

 the proper relationship between fuel and com- 

 bustion air. The burnerman and the blowerman 

 must therefore work very closely together in 

 order to provide efficient combustion in the 

 boiler furnace. 



The burnerman cuts burners in and out and 

 adjusts the oil pressure as necessary to keep 

 the steam pressure at the required value. The 

 burnerman is guided by the steam drum pressure 



gage. Also, he watches the annunciator which 

 shows the signals going from the bridge to the 

 engineroom, and in this way he can tell what 

 steam demands are going to be made. 



On a double -furnace boiler, there are two 

 burnermen— one for the saturated side and one 

 for the superheater side. The burnerman on the 

 superheater side cuts burners in and out and 

 adjusts fuel pressure to keep the superheater 

 outlet temperature at the required value. The 

 burnerman on the superheater side is guided 

 by the distant-reading thermometer which indi- 

 cates the temperature of the steam at the super- 

 heater outlet. In addition, he must keep a close 

 check on the actions of the saturated-side 

 burnerman so that he will always know how 

 many burners are in use on the saturated side. 



When two boilers are furnishing steam to 

 the same engine, the burnermen of both boilers 

 must work together to see that the load is 

 equally divided between the two boilers. 



The blowerman is responsible for operating 

 the forced draft blowers that supply combustion 

 air to the boiler. Although the air pressure in 

 the double casings is affected by the number of 

 registers in use and by the extent to which each 

 register is open, it is chiefly determined by the 

 manner in which the forced draft blowers are 

 operated. The opening, setting, or adjusting of 

 the air registers is the burnerman's job; the 

 control of the forced draft blowers is the blower- 

 man's job. As may be apparent, the burnerman 

 and the blowerman must each know what the 

 other man is doing at all times. The blowerman 

 must always increase the air pressure before 

 the burnerman increases the rate of combus- 

 tion, and the burnerman must always decrease 

 the rate of combustion before the blowerman 

 decreases the air pressure. 



If a boiler is not being supplied with suffi- 

 cient air for combustion, everyone in the fire- 

 room will know about it immediately. The boiler 

 will begin to pant and vibrate, and the fireroom 

 force will receive complaints of "heavy black 

 smoke" from the bridge. If the boiler is being 

 supplied with too much air— that is, more excess 

 air than is required for efficient combustion— 

 the fireroom force may or may not know about 

 it immediately. White smoke coming from the 

 smokepipe is always an indication of large 

 amounts of excess air. However, a perfectly 

 clear smokepipe may be deceiving; it may mean 

 that the boiler is operating with only a small 

 amount of excess air, but it may also mean that 

 as much as 300 percent excess air is causing 



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