Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGcan be said to be more important than any other,the checkman's job is the one.One of the greatest difficulties in maintainingthe water level arises from the fact that theboiler water swells and shrinks as the firingrate is changed. As the firing rate is increased,there is an increase in the volume of the boilerwater. This increase, which is known as swell,occurs because there is an increase in the numberand size of the steam bubbles in the water.As the firing rate is decreased, there is a decreasein the volume of the water. This decrease,which is known as shrink, occurs becausethere are fewer steam bubbles and theyare of smaller size. Thus, for any given weightof boiler water, the volume varies with the rateof combustion.The problem of swell and shrink becomeseven more complex when we remember that theevaporation rate also increases as the firingrate increases and decreases as the firing ratedecreases. When the firing rate is increased,therefore, the checkman must remember tofeed more water to the boiler, even though thewater level has already risen momentarily becauseof swell. On the other hand, the checkmanmust remember to feed less water to the boilerwhen the firing rate is decreased, even thoughthe water level has already dropped. Becausethese actions may appear to be contrary tocommon sense to a person who does not understandthe concept of swell and shrink, a gooddeal of training is usually required before aman can be considered qualified to stand acheck watch.The control of combustion involves the controlof fuel and the control of combustion air.There are three ways in which the firing ratemay be increased or decreased in order to meetchanges in steam demand: (1) by increasing ordecreasing the fuel pressure, (2) by increasingor decreasing the number of burners in use, and(3) by changing the size of the sprayer plates inthe atomizer assemblies. With every change,the amount of combustion air supplied to theboiler must also be changed in order to maintainthe proper relationship between fuel and combustionair. The burnerman and the blowermanmust therefore work very closely together inorder to provide efficient combustion in theboiler furnace.The burnerman cuts burners in and out andadjusts the oil pressure as necessary to keepthe steam pressure at the required value. Theburnerman is guided by the steam drum pressuregage. Also, he watches the annunciator whichshows the signals going from the bridge to theengineroom, and in this way he can tell whatsteam demands are going to be made.On a double -furnace boiler, there are twoburnermen— one for the saturated side and onefor the superheater side. The burnerman on thesuperheater side cuts burners in and out andadjusts fuel pressure to keep the superheateroutlet temperature at the required value. Theburnerman on the superheater side is guidedby the distant-reading thermometer which indicatesthe temperature of the steam at the superheateroutlet. In addition, he must keep a closecheck on the actions of the saturated-sideburnerman so that he will always know howmany burners are in use on the saturated side.When two boilers are furnishing steam tothe same engine, the burnermen of both boilersmust work together to see that the load isequally divided between the two boilers.The blowerman is responsible for operatingthe forced draft blowers that supply combustionair to the boiler. Although the air pressure inthe double casings is affected by the number ofregisters in use and by the extent to which eachregister is open, it is chiefly determined by themanner in which the forced draft blowers areoperated. The opening, setting, or adjusting ofthe air registers is the burnerman's job; thecontrol of the forced draft blowers is the blowerman'sjob. As may be apparent, the burnermanand the blowerman must each know what theother man is doing at all times. The blowermanmust always increase the air pressure beforethe burnerman increases the rate of combustion,and the burnerman must always decreasethe rate of combustion before the blowermandecreases the air pressure.If a boiler is not being supplied with sufficientair for combustion, everyone in the fireroomwill know about it immediately. The boilerwill begin to pant and vibrate, and the fireroomforce will receive complaints of "heavy blacksmoke" from the bridge. If the boiler is beingsupplied with too much air—that is, more excessair than is required for efficient combustion—the fireroom force may or may not know aboutit immediately. White smoke coming from thesmokepipe is always an indication of largeamounts of excess air. However, a perfectlyclear smokepipe may be deceiving; itmay meanthat the boiler is operating with only a smallamount of excess air, but it may also mean thatas much as 300 percent excess air is causing268