Gambar halaman

may take place and thus liberate the heat held in suspense in the fuel. The air about us is made up of about 20 per cent. oxygen and 80 per cent. nitrogen. The nitrogen is an inert, valueless ingredient that must pass into the furnace, absorb some of its heat and go out through the chimney, thus conducting away into the outer atmosphere some of the heat generated. The oxygen, however, upon coming in contact with the heated carbon, hydrogen and sulphur of the fuel, readily chemically reacts with them. Enormous quantities of heat are thus liberated, later to be absorbed by the water of the boiler, eventually to produce the steam delivered for the driving of the steam engine or the steam turbine. Furnace Operation. Since this series of articles is largely concerned with fuel oil practice, let us briefly outline the furnace operation for such practice. In a later chapter this will be taken up in more detail.

The Fuel Oil Burner and Its Function.-The fuel oil is sprayed into the furnace by means of an atomizer or burner which pulverizes the oil and delivers it in a gaseous vapor or in small globules at the hottest place in the furnace. Air is admitted from below and as soon as the temperature is raised to the ignition point chemical reaction takes place with the atomized fuel oil, and thus heat is generated. This heat is absorbed by the gases of the furnace and consequently their temperature is at once raised often times to 2300° or 2500°F. These furnace gases consist of the inert nitrogen that partly constituted the entering air, the carbon dioxide or carbon monoxide formed by the burning of the carbon, water vapor formed by the burning of the hydrogen, sulphur dioxide formed by the burning of the sulphur content, which latter ingredient is always small, and a considerable quantity of free oxygen depending on the amount of excess air admitted to the furnace.

The Path of the Furnace Gases. In their expanded condition, due to the absorption of such huge quantities of heat, the gases now travel upward. As they come in contact with the boiler drums or tubes through which water is circulating, the gases are, of course, cooled and the temperature of the water raised. In this maner the gases, having been chilled or lowered in temperature to 500 deg. or 600 deg. F., are finally passed up through the chimney, and steam generation within the boiler is accomplished.

The Economizer and Its Economic Value.-In some boiler installations a series of tubes through which cold water is passing,

is placed between the boiler and the chimney. The chimney gases are thus forced to give up still more of their heat. These outgoing chimney gases are consequently reduced still further in temperature.

Such a device as cited above, is known as an economizer. This reduction in the temperature of the out-going chimney gases reduces the draft of the chimney. Hence, the economizer is an economic success so long as the saving in feed-water heating is greater than the interest on the cost of the economizer installation and other apparatus necessary to produce artificial draft, plus the cost of maintenance of this additional apparatus.

Quantity of Air Required.-It has been observed that the entrance of air into the furnace is absolutely essential for furnace operations. Too much air, however, is detrimental, for more oxygen may be admitted than can be economically used by the fuel. Hence, too great an excess of air simply means the passage up through the chimney of excess gases which absorb heat only to convey it out into the atmosphere without performing a useful function. In successful boiler operation, therefore, some means must be provided, first to measure the draft; second, to test the ingredients of the outgoing gases; and third, to regulate the entrance of air into the furnace.

The Draft Gage and Its Principle of Operation.-A draft gage usually consists of a column of water placed in a U-tube. The

[graphic][merged small][merged small]

In order to exaggerate the readings of the draft in inches of water, the measuring tube rests on a slope of ten to one in this type of instrument, and thus readings to another decimal point are ascertained which would otherwise be impossible.

pressure in the chimney is less than the atmosphere without. Therefore, if one end of this tube is inserted into the chimney and the other rests under the atmospheric pressure without, the difference of water level thus obtained in the U-tube indicates the draft in inches of water. This may be converted into pounds

pressure (absolute) per square inch by applying the formulas previously set forth in the chapter on pressures.

Apparatus for Determining Ingredients of Out-Going Chimney Gases. For economic boiler operation the steam engineer should know the exact composition of the outgoing chimney gases. Since this is a matter of vast importance a later chapter will be given in which detailed discussions of methods involved and apparatus employed will be given. Suffice it to say at this point, however, that by means of such apparatus the engineer may determine whether the fuel is being properly consumed in the furnace and whether too little or too much air is being admitted into the furnace.

Draft Regulating Devices.-In fuel oil practice the proper supply of air may be determined to a nicety. Hence some means must be provided to regulate the air supply with the same precision. This is done by varying the amount of opening of either the ash pit doors or the boiler damper or both. If the air is regulated by partly closing the ash pit doors and leaving the damper wide open a strong draft may occur inside the boiler setting which tends to draw air in through the brick walls. As this is a detriment it is preferable to regulate the air by means of the damper.

The Chimney. After the gases have passed through and around the various heat absorbing tubes and drums employed in the modern steam boiler and economizer, they are shot up into the atmosphere through a lòng vertical passage. The structure housing this passage is known as a chimney. The height of the chimney and its area of cross-section through which the flue gases pass have an important bearing on the economic boiler or rather furnace operation.

In a general way, the reader now has a grasp of the fundamentals involved in modern furnace operation for the steam boiler. We shall next consider the container or shell for steam generation and its accessories.




ET us now consider some of the fundamental laws involved in heat transference, and then discuss the container or shell employed in steam generation together with the accessories that must accompany any high pressure steam generating unit to accomplish safe and efficient operation.

Going back once again to the homely tea-kettle for a simple illustration, we find that the container for the water and steam usually consists of a flat bottomed metallic vessel with free opening to allow the steam generated to escape to the atmosphere. There is also usually to be found an opening with a lid covering at the top where water may be passed in or the vessel cleaned at more or less irregular periods of operation in household economics.

FIG. 62.-The clean, clear cut appearance of the oil fired boiler


In the case of the steam boiler, however, vast improvements in physical configuration and construction become a necessity. Let us then examine some of these differences.

The Laws of Heat Involved in Steam Generation.-The transference of heat is found by experimental observation to take place in three separate and distinct ways-namely, by conduction, by radiation, and by convection.

On a wintry night if one stands in front of a blazing fireplace it is easy to find illustrations of these three methods of heat transference. Thus standing, one feels the heat radiating to his face in outward projections from the fire, for if an article such

as a solid screen, opaque to heat radiation, be placed between the face and the fire the sensation of heat on the face immediately disappears. If now from behind the screen one holds a metallic poker in the hot fire, it will not be long before the poker even at


FIG. 63.-Front view of new boiler installation.

In this view may be seen the pit and foundation setting for oil furnace in the new additional installation recently put in by the Pacific Gas and Electric Company, San Francisco. Note the clean trim appearance extending to the older fuel oil installation on the extreme left, which is quite characteristic of boiler rooms where oil is used as fuel.

the point behind the screen becomes so hot by conduction that it cannot comfortably be held in the hand. And finally should a sudden gust of wind blow down the chimney a hot gust of air

« SebelumnyaLanjutkan »