Chapter 14
For any calorimeter within the range of its ordinary use, such a thermometer and radiation correction taken from one normal reading is approximately correct for any conditions with the same or a duplicate thermometer.
The percentage of moisture in the steam, corrected for thermometer error and radiation and the correction to be applied to the particular calorimeter used, would be determined as follows: Assume a gauge pressure in the trial to be 180 pounds and the thermometer reading to be 295 degrees. A normal reading, taken in the manner described, gives a value of T = 303 degrees; then, the percentage of moisture corrected for thermometer error and radiation is,
0.47(303 - 295) x = ---------------- 845.0
= 0.45 per cent.
The theoretical reading for dry steam will be,
1197.7 - 1150.4 - 0.47(t_{2} - 212) 0 = ------------------------------------ 845.0
t_{2} = 313 degrees.
The thermometer and radiation correction to be applied to the instrument used, therefore over the ordinary range of pressure is
Correction = 313 - 303 = 10 degrees
The chart may be used in the determination of the correct reading of moisture percentage and the permanent radiation correction for the instrument used without computation as follows: Assume the same trial pressure, feed temperature and normal reading as above. If the normal reading is found to be 303 degrees, the correction for thermometer and radiation will be the theoretical reading for dry steam as found from the chart, less this normal reading, or 10 degrees correction. The correct temperature for the trial in question is, therefore, 305 degrees. The moisture corresponding to this temperature and 180 pounds gauge pressure will be found from the chart to be 0.45 per cent.
There are many forms of throttling calorimeter, all of which work upon the same principle. The simplest one is probably that shown in Fig. 14. An extremely convenient and compact design is shown in Fig. 16. This calorimeter consists of two concentric metal cylinders screwed to a cap containing a thermometer well. The steam pressure is measured by a gauge placed in the supply pipe or other convenient location. Steam passes through the orifice A and expands to atmospheric pressure, its temperature at this pressure being measured by a thermometer placed in the cup C. To prevent as far as possible radiation losses, the annular space between the two cylinders is used as a jacket, steam being supplied to this space through the hole B.
The limits of moisture within which the throttling calorimeter will work are, at sea level, from 2.88 per cent at 50 pounds gauge pressure and 7.17 per cent moisture at 250 pounds pressure.
Separating Calorimeter--The separating calorimeter mechanically separates the entrained water from the steam and collects it in a reservoir, where its amount is either indicated by a gauge glass or is drained off and weighed. Fig. 17 shows a calorimeter of this type. The steam passes out of the calorimeter through an orifice of known size so that its total amount can be calculated or it can be weighed. A gauge is ordinarily provided with this type of calorimeter, which shows the pressure in its inner chamber and the flow of steam for a given period, this latter scale being graduated by trial.
The instrument, like a throttling calorimeter, should be well insulated to prevent losses from radiation.
While theoretically the separating calorimeter is not limited in capacity, it is well in cases where the percentage of moisture present in the steam is known to be high, to attach a throttling calorimeter to its exhaust. This, in effect, is the using of the separating calorimeter as a small separator between the sampling nozzle and the throttling instrument, and is necessary to insure the determination of the full percentage of moisture in the steam. The sum of the percentages shown by the two instruments is the moisture content of the steam.
The steam passing through a separating calorimeter may be calculated by Napier's formula, the size of the orifice being known. There are objections to such a calculation, however, in that it is difficult to accurately determine the areas of such small orifices. Further, small orifices have a tendency to become partly closed by sediment that may be carried by the steam. The more accurate method of determining the amount of steam passing through the instrument is as follows:
A hose should be attached to the separator outlet leading to a vessel of water on a platform scale graduated to 1/100 of a pound. The steam outlet should be connected to another vessel of water resting on a second scale. In each case, the weight of each vessel and its contents should be noted. When ready for an observation, the instrument should be blown out thoroughly so that there will be no water within the separator. The separator drip should then be closed and the steam hose inserted into the vessel of water at the same instant. When the separator has accumulated a sufficient quantity of water, the valve of the instrument should be closed and the hose removed from the vessel of water. The separator should be emptied into the vessel on its scale. The final weight of each vessel and its contents are to be noted and the differences between the final and original weights will represent the weight of moisture collected by the separator and the weight of steam from which the moisture has been taken. The proportion of moisture can then be calculated from the following formula:
100 w x = ----- (7) W - w
Where x = per cent moisture in steam, W = weight of steam condensed, w = weight of moisture as taken out by the separating calorimeter.
Sampling Nipple--The principle source of error in steam calorimeter determinations is the failure to obtain an average sample of the steam delivered by the boiler and it is extremely doubtful whether such a sample is ever obtained. The two governing features in the obtaining of such a sample are the type of sampling nozzle used and its location.
The American Society of Mechanical Engineers recommends a sampling nozzle 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. Many engineers object to the use of a perforated sampling nipple because it ordinarily indicates a higher percentage of moisture than is actually present in the steam. This is due to the fact that if the perforations come close to the inner surface of the pipe, the moisture, which in many instances clings to this surface, will flow into the calorimeter and cause a large error. Where a perforated nipple is used, in general it may be said that the perforations should be at least one inch from the inner pipe surface.
A sampling nipple, open at the inner end and unperforated, undoubtedly gives as accurate a measure as can be obtained of the moisture in the steam passing that end. It would appear that a satisfactory method of obtaining an average sample of the steam would result from the use of an open end unperforated nipple passing through a stuffing box which would allow the end to be placed at any point across the diameter of the steam pipe.
Incidental to a test of a 15,000 K. W. steam engine turbine unit, Mr. H. G. Stott and Mr. R. G. S. Pigott, finding no experimental data bearing on the subject of low pressure steam quality determinations, made a investigation of the subject and the sampling nozzle illustrated in Fig. 18 was developed. In speaking of sampling nozzles in the determination of the moisture content of low pressure steam, Mr. Pigott says, "the ordinary standard perforated pipe sampler is absolutely worthless in giving a true sample and it is vital that the sample be abstracted from the main without changing its direction or velocity until it is safely within the sample pipe and entirely isolated from the rest of the steam."
It would appear that the nozzle illustrated is undoubtedly the best that has been developed for use in the determination of the moisture content of steam, not only in the case of low, but also in high pressure steam.
Location of Sampling Nozzle--The calorimeter should be located as near as possible to the point from which the steam is taken and the sampling nipple should be placed in a section of the main pipe near the boiler and where there is no chance of moisture pocketing in the pipe. The American Society of Mechanical Engineers recommends that a sampling nipple, of which a description has been given, should be located in a vertical main, rising from the boiler with its closed end extending nearly across the pipe. Where non-return valves are used, or where there are horizontal connections leading from the boiler to a vertical outlet, water may collect at the lower end of the uptake pipe and be blown upward in a spray which will not be carried away by the steam owing to a lack of velocity. A sample taken from the lower part of this pipe will show a greater amount of moisture than a true sample. With goose-neck connections a small amount of water may collect on the bottom of the pipe near the upper end where the inclination is such that the tendency to flow backward is ordinarily counterbalanced by the flow of steam forward over its surface; but when the velocity momentarily decreases the water flows back to the lower end of the goose-neck and increases the moisture at that point, making it an undesirable location for sampling. In any case, it should be borne in mind that with low velocities the tendency is for drops of entrained water to settle to the bottom of the pipe, and to be temporarily broken up into spray whenever an abrupt bend or other disturbance is met.
Fig. 19 indicates certain locations of sampling nozzles from which erroneous results will be obtained, the reasons being obvious from a study of the cuts.
Before taking any calorimeter reading, steam should be allowed to flow through the instrument freely until it is thoroughly heated. The method of using a throttling calorimeter is evident from the description of the instrument given and the principle upon which it works.
SUPERHEATED STEAM
Superheated steam, as already stated, is steam the temperature of which exceeds that of saturated steam at the same pressure. It is produced by the addition of heat to saturated steam which has been removed from contact with the water from which it was generated. The properties of superheated steam approximate those of a perfect gas rather than of a vapor. Saturated steam cannot be superheated when it is in contact with water which is also heated, neither can superheated steam condense without first being reduced to the temperature of saturated steam. Just so long as its temperature is above that of saturated steam at a corresponding pressure it is superheated, and before condensation can take place that superheat must first be lost through radiation or some other means. Table 24[20] gives such properties of superheated steam for varying pressures as are necessary for use in ordinary engineering practice.
Specific Heat of Superheated Steam--The specific heat of superheated steam at atmospheric pressure and near saturation point was determined by Regnault, in 1862, who gives it the value of 0.48. Regnault's value was based on four series of experiments, all at atmospheric pressure and with about the same temperature range, the maximum of which was 231.1 degrees centigrade. For fifty years after Regnault's determination, this value was accepted and applied to higher pressures and temperatures as well as to the range of his experiments. More recent investigations have shown that the specific heat is not a constant and varies with both pressure and the temperature. A number of experiments have been made by various investigators and, up to the present, the most reliable appear to be those of Knoblauch and Jacob. Messrs. Marks and Davis have used the values as determined by Knoblauch and Jacob with slight modifications. The first consists in a varying of the curves at low pressures close to saturation because of thermodynamic evidence and in view of Regnault's determination at atmospheric pressure. The second modification is at high degrees of superheat to follow Holborn's and Henning's curve, which is accepted as authentic.
For the sake of convenience, the mean specific heat of superheated steam at various pressures and temperatures is given in tabulated form in Table 25. These values have been calculated from Marks and Davis Steam Tables by deducting from the total heat of one pound of steam at any pressure for any degree of superheat the total heat of one pound of saturated steam at the same pressure and dividing the difference by the number of degrees of superheat and, therefore, represent the average specific heat starting from that at saturation to the value at the particular pressure and temperature.[21] Expressed as a formula this calculation is represented by
H_{sup} - H_{sat} Sp. Ht. = ----------------- (8) S_{sup} - S_{sat}
Where H_{sup} = total heat of one pound of superheated steam at any pressure and temperature, H_{sat} = total heat of one pound of saturated steam at same pressure, S_{sup} = temperature of superheated steam taken, S_{sat} = temperature of saturated steam corresponding to the pressure taken.
TABLE 25
MEAN SPECIFIC HEAT OF SUPERHEATED STEAM CALCULATED FROM MARKS AND DAVIS TABLES _______________________________________________________________ |Gauge | | |Pressure | Degree of Superheat | | |_____________________________________________________| | | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____| | 50 | .518| .517| .514| .513| .511| .510| .508| .507| .505| | 60 | .528| .525| .523| .521| .519| .517| .515| .513| .512| | 70 | .536| .534| .531| .529| .527| .524| .522| .520| .518| | 80 | .544| .542| .539| .535| .532| .530| .528| .526| .524| | 90 | .553| .550| .546| .543| .539| .536| .534| .532| .529| | 100 | .562| .557| .553| .549| .544| .542| .539| .536| .533| | 110 | .570| .565| .560| .556| .552| .548| .545| .542| .539| | 120 | .578| .573| .567| .561| .557| .554| .550| .546| .543| | 130 | .586| .580| .574| .569| .564| .560| .555| .552| .548| | 140 | .594| .588| .581| .575| .570| .565| .561| .557| .553| | 150 | .604| .595| .587| .581| .576| .570| .566| .561| .557| | 160 | .612| .603| .596| .589| .582| .576| .571| .566| .562| | 170 | .620| .612| .603| .595| .588| .582| .576| .571| .566| | 180 | .628| .618| .610| .601| .593| .587| .581| .575| .570| | 190 | .638| .627| .617| .608| .599| .592| .585| .579| .574| | 200 | .648| .635| .624| .614| .605| .597| .590| .584| .578| | 210 | .656| .643| .631| .620| .611| .602| .595| .588| .583| | 220 | .664| .650| .637| .626| .616| .607| .600| .592| .586| | 230 | .672| .658| .644| .633| .622| .613| .605| .597| .591| | 240 | .684| .668| .653| .640| .629| .619| .610| .602| .595| | 250 | .692| .675| .659| .645| .633| .623| .614| .606| .599| |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____| |Gauge | | |Pressure | Degree of Superheat | | |-----------------------------------------------------| | | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 225 | 250 | |---------+-----+-----+-----+-----+-----+-----+-----+-----+-----| | 50 | .504| .503| .502| .501| .500| .500| .499| .497| .496| | 60 | .511| .509| .508| .507| .506| .504| .504| .502| .500| | 70 | .516| .515| .513| .512| .511| .510| .509| .506| .504| | 80 | .522| .520| .518| .516| .515| .514| .513| .511| .508| | 90 | .527| .525| .523| .521| .519| .518| .517| .514| .510| | 100 | .531| .529| .527| .525| .523| .522| .521| .517| .513| | 110 | .536| .534| .532| .529| .528| .526| .525| .520| .517| | 120 | .540| .537| .535| .533| .531| .529| .528| .523| .519| | 130 | .545| .542| .539| .537| .535| .533| .531| .527| .523| | 140 | .550| .547| .544| .541| .539| .536| .534| .530| .526| | 150 | .554| .550| .547| .544| .542| .539| .537| .533| .529| | 160 | .558| .554| .551| .548| .545| .543| .541| .536| .531| | 170 | .562| .558| .555| .552| .549| .546| .544| .538| .533| | 180 | .566| .561| .558| .555| .552| .549| .546| .540| .536| | 190 | .569| .565| .562| .558| .555| .552| .549| .543| .538| | 200 | .574| .569| .566| .562| .558| .555| .552| .546| .541| | 210 | .578| .573| .569| .565| .561| .558| .555| .549| .543| | 220 | .581| .577| .572| .568| .564| .561| .558| .551| .545| | 230 | .585| .580| .575| .572| .567| .564| .561| .554| .548| | 240 | .589| .584| .579| .575| .571| .567| .564| .556| .550| | 250 | .593| .587| .582| .577| .574| .570| .567| .559| .553| |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
Factor of Evaporation with Superheated Steam--When superheat is present in the steam during a boiler trial, where superheated steam tables are available, the formula for determining the factor of evaporation is that already given, (2),[22] namely,
H - h Factor of evaporation = ----- L
Here H = total heat in one pound of superheated steam from the table, h and L having the same values as in (2).
Where no such tables are available but the specific heat of superheat is known, the formula becomes:
H - h + Sp. Ht.(T - t) Factor of evaporation = ---------------------- L
Where H = total heat in one pound of saturated steam at pressure existing in trial, h = sensible heat above 32 degrees in one pound of water at the temperature entering the boiler, T = temperature of superheated steam as determined in the trial, t = temperature of saturated steam corresponding to the boiler pressure, Sp. Ht. = mean specific heat of superheated steam at the pressure and temperature as found in the trial, L = latent heat of one pound of saturated steam at atmospheric pressure.
Advantages of the Use of Superheated Steam--In considering the saving possible by the use of superheated steam, it is too often assumed that there is only a saving in the prime movers, a saving which is at least partially offset by an increase in the fuel consumption of the boilers generating steam. This misconception is due to the fact that the fuel consumption of the boiler is only considered in connection with a definite weight of steam. It is true that where such a definite weight is to be superheated, an added amount of fuel must be burned. With a properly designed superheater where the combined efficiency of the boiler and superheater will be at least as high as of a boiler alone, the approximate increase in coal consumption for producing a given weight of steam will be as follows:
_Superheat_ _Added Fuel_ _Degrees_ _Per Cent_ 25 1.59 50 3.07 75 4.38 100 5.69 150 8.19 200 10.58
These figures represent the added fuel necessary for superheating a definite weight of steam to the number of degrees as given. The standard basis, however, of boiler evaporation is one of heat units and, considered from such a standpoint, again providing the efficiency of the boiler and superheater is as high, as of a boiler alone, there is no additional fuel required to generate steam containing a definite number of heat units whether such units be due to superheat or saturation. That is, if 6 per cent more fuel is required to generate and superheat to 100 degrees, a definite weight of steam, over what would be required to produce the same weight of saturated steam, that steam when superheated, will contain 6 per cent more heat units above the fuel water temperature than if saturated. This holds true if the efficiency of the boiler and superheater combined is the same as of the boiler alone. As a matter of fact, the efficiency of a boiler and superheater, where the latter is properly designed and located, will be slightly higher for the same set of furnace conditions than would the efficiency of a boiler in which no superheater were installed. A superheater, properly placed within the boiler setting in such way that products of combustion for generating saturated steam are utilized as well for superheating that steam, will not in any way alter furnace conditions. With a given set of such furnace conditions for a given amount of coal burned, the fact that additional surface, whether as boiler heating or superheating surface, is placed in such a manner that the gases must sweep over it, will tend to lower the temperature of the exit gases. It is such a lowering of exit gas temperatures that is the ultimate indication of added efficiency. Though the amount of this added efficiency is difficult to determine by test, that there is an increase is unquestionable.
Where a properly designed superheater is installed in a boiler the heating surface of the boiler proper, in the generation of a definite number of heat units, is relieved of a portion of the work which would be required were these heat units delivered in saturated steam. Such a superheater needs practically no attention, is not subject to a large upkeep cost or depreciation, and performs its function without in any way interfering with the operation of the boiler. Its use, therefore from the standpoint of the boiler room, results in a saving in wear and tear due to the lower ratings at which the boiler may be run, or its use will lead to the possibility of obtaining the same number of boiler horse power from a smaller number of boilers, with the boiler heating surface doing exactly the same amount of work as if the superheaters were not installed. The saving due to the added boiler efficiency that will be obtained is obvious.