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The Standard that Has Been Adopted.-To avoid inconsistencies and to develop some rational method of comparison, engineers have found it convenient and accurate to reduce all evaporative quantities of a boiler to a definite standard. In order to follow out this standardized comparison, all steam generating performances of boilers read as if the boiler took its feed water at 212°F. and atmospheric pressure, and converted it into dry saturated steam at 212°F. and atmospheric pressure, as set forth in the standard definition of the boiler horepower in the last chapter. It is clearly evident that no such theoretical
FIG. 49.-Platform scales and tanks for water measurement.
The boiler immediately to the right of the platform scales is under test. The tank below the platform scales into which the water is emptied after being weighed, is utilized to furnish all water for the boiler during the test. At the beginning of the test a hooked gage registers the height of the water in this tank, and at each hourly period thereafter sufficient water is weighed and emptied into it from the tanks above to maintain this exact level. By means of these data, properly taken, the factor of evaporation and the boiler horse-power are easily computed.
boiler has ever existed, yet this standard of comparison is found very convenient. Thus in any case of boiler performance, if M. represents such an equivalent or comparative standardized evaporation in lbs. of water per lb. of fuel, and M. the lb. of water actually evaporated in the boiler under conditions of test, we may now invent a factor to be known as the factor of evaporation, Fe, whereby such performances may be readily reduced:
In the same way, the equivalent evaporation of water per hour may be computed from the formula
Let us next analyze the factor of evaporation and see how we may actually compute its value for any given case. We have previously found that in the operation of the boiler, steam appears in three different conditions or qualities, namely in what is known as dry saturated, wet steam, or superheated steam. Let us then consider the valuation of the factor of evaporation for these three distinct instances.
Dry Saturated Steam.-In the case of dry saturated steam, the water enters the boiler already possessing a heat of liquid h corresponding to its entrance temperature which may be readily found in the steam tables. This water is next converted into dry saturated steam which has a total heat (H.) corresponding to the pressure at which the evaporation takes place. Consequently the actual heat which has been transferred from the boiler shell to the water is (H. - h) heat units. But to evaporate one pound of water at 212°F. into dry steam at 212°F. requires 970.4 heat units. Hence if Mu pounds of water are evaporated under test conditions, the number of pounds M. under standardized condi
Thus in the case of a boiler which takes its feed water at 101.8° F. and converts it into dry saturated steam at 180 lb. pressure per square inch, from the steam tables we find that H., is 1196.4 and hy is 69.8, hence the factor of evaporation is
In the case of wet steam all of the water entering the boiler is not converted into steam. As a consequence a certain portion of heat (h. h) is required to raise the temperature of the water from entrance temperature t, to the temperature of evaporation to and if only X. parts of a lb. are then evaporated into steam, only X.L. B.t.u. are required to accomplish this result. Hence, the total heat required per lb. of water so evaporated is
As a consequence the factor of evaporation in this case may from similar reasoning be expressed by the formula
(he + XeLe - hƒ)
As an instance showing the application of this formula let us assume that the boiler above mentioned did not evaporate the water into dry steam but that upon investigation it was found to contain 5 per cent. moisture. What now is its factor of evaporation? From the steam tables we find that he is 345.6, L. is 850.8 and h, is 69.8. Therefore the factor of evaporation is
Superheated Steam. In the third instance steam is not only evaporated to a dry saturated condition, but is finally sent from the boiler in a superheated condition. The steam tables are so arranged that we may find the heat necessary to raise the total heat of superheated steam when its pressure and temperature are known. Considering that the water entered the boiler at 32°F. let us then call H, the total heat of superheated steam. Since now the water entered the boiler with a heat of liquid equal to h, the actual heat entering each lb. of steam evaporated in the boiler under these conditions is (H, - h) heat units. Hence in this instance the factor of evaporation is likewise from similar reasoning computed by the formula:
H. - ht Hs 970.4
To follow up the same example as set forth in the preceding illustration let us assume that the steam is evaporated under the conditions hitherto mentioned, but that it appears superheated to the extent of 100°. Looking in the steam tables we find that the total heat H, of superheated steam at 180 lb. pressure and 100° superheat is 1254.3 and that the heat of liquid hy is 69.8, consequently the factor of evaporation is
To Compute the Boiler Horsepower. Since now by means of formula (2), we are enabled to compute the equivalent evaporation of Meh in pounds of water per hour that the boiler under test would evaporate were it taking its feed water at 212°F. and converting it into dry saturated steam at the same temperature, we can at once compute the horsepower of the boiler. Under such conditions of operation for every 34.5 lb. of water evapo
rated per hour, the boiler is developing one boiler horsepower. Hence to compute the boiler horsepower, we write the formula:
Thus if a boiler has an equivalent evaporation of 23,350 lb. of water per hour, its horsepower is found to be
We could of course develop an expression for the computation of boiler horsepower by taking into consideration the heat absorbed by the generation of steam per hour. For in our discussion in the previous chapter it was shown that one boiler horsepower is equivalent to the absorption of 33,479 heat units per hour. Hence, by computing the heat absorbed by the total pounds of steam generated per hour and dividing this by 33,479, we can compute boiler horsepower and arrive at the same answer as given in the above formula. It is better, however, for the beginner to follow fundamental definitions rather than attempt too many short cuts to gain quick results.
In conclusion the important relationship to bear in mind is the vast difference between the socalled mechanical horsepower and the boiler horsepower which was brought out in the previous discussion. With this relationship firmly fixed it must be remembered that equivalent evaporation is such an evaporation as would be brought about by taking in water into the boilers at 212°F. and evaporating it into dry saturated steam at 212°F. and atmospheric pressure. The formulas deduced above for equivalent evaporation and factor of evaporation enable us to do this.
HOW TO DETERMINE QUALITY OF STEAM
TEAM as used in engineering practice is said to be wet, dry saturated or superheated, depending upon the degree to which heat has been applied in its generation.
Wet Steam. As its name implies, wet steam is steam in which are suspended small globules or particles of water. Since such globules or particles of water indicate that insufficient heat has been applied, and consequently steam generation is imperfect, it is the function of all good boilers to generate steam as free from water as possible.
Although steam be generated dry or even superheated it may,
FIG. 50.-Thermometer inserted for however, after passing through superheat measurement. conducting pipes appear at the power generating unit in a wet condition. Hence the determination of moisture content and the heat loss due to its presence is an important one in steam engineering.
Let us assume X to be the proportion by weight of dry steam that exists in wet steam. Then the total heat represented in every pound of such wet steam at temperature t is
This is evident at once when we consider that to raise each pound of original water from 32°F. to the temperature t, it required ht, heat units. On the other hand since a proportion by weight