« SebelumnyaLanjutkan »
By using the normal reading (tn) in equation (1) we have
Here X is the quality of steam indicated by the calorimeter when in reality the steam is dry. X, is the quality of the actual
A suggestion for a steam calorimeter attachment for determining normal reading of calorimeter. (See page 88.)
steam under test, as indicated by the same calorimeter. By subtracting one from the other we have the true moisture
Thus in the foregoing example, let us suppose the normal reading of the calorimeter thermometer was 290°F. Then the true moisture in the steam becomes
The steam therefore contains 1.57 per cent. moisture.
The Limitations of the Throttling Calorimeter.-A little consideration of the underlying principle of the throttling calorimeter brings to light a definite range of limitation to its usefulness. It will be remembered that this fundamental principle consists in liberating sufficient heat at the lower pressure not only to evaporate any moisture that may exist but to actually superheat the entire mixture. If there is not sufficient heat liberated, that is if too much water is held in suspension in the saturated steam, the steam at the lower pressure fails to become superheated and hence we have no means of measurement.
Thus if steam pass from 100 lb. absolute pressure per sq. in. to 30 lb. absolute pressure per sq. in., the total heat at the upper pressure is (X.L. + h.) which from the steam tables becomes (X,888 + 298.3) heat units and that at the lower pressure is H3, or 1163.9, if it be not at all superheated. Hence we have
This means that if there is a greater moisture content than 2.52 per cent. the steam calorimeter will fail to work because the mixture in the lower pressure space does not become superheated.
If instead of having the low pressure of 30 lb. absolute per square inch, the steam in the calorimeter had been throttled down to 14.7 lb. the value of Ha would have been 1150.4 instead of 1163.9 so that X. would become 0.9597 and the limit of the calorimeter in this case would be 4.03 per cent. of moisture.
Again if instead of steam at 100 lb. absolute pressure we had steam at 200 lb. and allowed the sample in the calorimeter to be throttled down to 14.7 lb. it may be found in the same way that the limit of the calorimeter is 5.66 per cent. of moisture. It is thus seen that the greater the difference in pressure between the high pressure and the low pressure in the calorimeter, the greater is the range of the calorimeter.
The Electric Calorimeter. It is now evident that if a definitely measurable quantity of heat could be added to the steam before it was allowed to expand, even very wet steam might be accurately measured by the throttling calorimeter. This is seen at once when we analyze the total heats involved. If E. be the heat units added to each pound of steam, then the total heat possessed by each pound of steam in the high pressure main
is (X.L. + h. + E.) heat units and since the heat in each pound of steam in the lower chamber is Hs, we have, since no heat escapes
In the Thomas electric meter an electrical mechanism has been invented whereby a series of small wires electrically heated impart a known quantity of electrical energy to the steam. This electrical energy dissipates itself as heat and since we can transfer electrical units into heat units and vice versa, a ready means is provided to assist the throttling calorimeter in doing its work by adding sufficient heat to widen the range of the throttling process. Thus, although the throttling calorimeter was found definitely limited as set forth above, let us investigate a case where the electrical calorimeter may be used. Let us assume the upper pressure to be 200 lb. per sq. in. and the lower pressure 15.0 lb. per sq. in. In this case there were electrically added exactly 40 B.t.u. of energy and the temperature of superheat t, was found to be 233.0°F., hence from the steam tables we find
The Separating Calorimeter.-In the separating calorimeter the moisture is mechanically separated from the steam. If we know the total amount of steam passing and also the weight of the water separated from the steam, it is of course an easy problem to compute the dryness of the steam. Thus, if W1 is the weight of water separated per hour in the calorimeter and W2 the weight of dry steam passing out of the calorimeter per hour, we have by inspection
Hence, if a separating calorimeter deposits 285 lb. of water per hour and if 10,000 lb. of dry saturated steam leave the calorimeter per hour, the dryness of the steam is
There are many principles upon which the separating calorimeter may operate. There are two forms, however, which are more usual than others. In one instance the steam mixture is given a rotary motion in its journey and consequently the water particles are thrown off by centrifugal force and collect in a drip below. In the other instance the stream flow receives a sudden reversal in direction. As dry steam easily performs this feat and water insists upon continuing its former direction of flow a separation is thus mechanically effected.
In this type of separating calorimeter the steam, with its moisture enters from the steam main at 6 and is forced to travel downward toward 3 at a high velocity. At 14, however, the direction is suddenly reversed upward toward 7 and later passed downward through 4 and out into the atmosphere at 8. When the sudden reversal takes place at 14, the moisture in the steam collects at 3 and its content is measured on the gage 12. The steam content, on the other hand, is calculated by means of Napier's formula as it passes through the orifice at 8 as illustrated in the text.
This type of instrument is not as accurate as the throttling type, as it does not get all the moisture out of the steam. When large quantities of moisture are present, however, it proves useful in taking out the bulk of the water or moisture while a throttling calorimeter connected in series later on accurately measures the remaining water content present. Thus by such a method of operation any degree of moisture present in steam is easily and accurately measured.
Correction for Steam Used by Calorimeter.-In a great many instances the total weight of steam passing per hour through the
steam main under test is of prime importance. Since most forms of calorimeter operate by diverting a portion of this steam out into the atmosphere, it becomes necessary to have some quick and ready means of computing the quantity of steam so diverted. Many years ago Napier deduced an approximate formula for the flow of steam into the atmosphere from a high pressure source. This formula is well within the degree of accuracy required for steam diverted through the calorimeter. If W is the pounds of steam flowing per second, p the pounds of pressure per square inch exerted by the steam in the main, and a the area of the orifice in square inches through which the steam passes, then
The Sampling Nipple.-The American Society of Mechanical Engineers recommends a sampling nipple made of one-half inch iron pipe closed at the inner end and the interior portion perforated with not less than twenty one-eighth inch holes equally distributed from end to end and preferably drilled in irregular or spiral rows with the first hole not less than one-half inch from the wall of the pipe. The failure to determine an average sample of the steam is the principal source of error in steam calorimeter determinations.
Conclusions on Moisture Measuring Apparatus.-Summing up the arguments of this chapter we see that for comparatively small quantities of moisture present in steam, the throttling calorimeter is the most accurate device for its quantitative determination. If, however, large quantities of moisture are present, two methods present themselves. Either we must first remove the major portion of the moisture by means of a separating calorimeter and later determine the remaining moisture content by means of the throttling calorimeter, or we must add a definite quantity of heat to the original steam supply by means of a device such as the Thomas Electric calorimeter and then determine with proper computation factors the moisture present by means of the throttling calorimeter.
As already shown the throttling calorimeter may be used up to moisture of 4 per cent. for steam at 100 lb. pressure and up to a little over 5 per cent. for steam at 200 lb. pressure. Most boilers deliver steam containing not more than 11⁄2 per cent. or 2 per cent. of moisture so that for nearly all ordinary work the throt