Part 24

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.

Case 1--Horizontal pipe. Water flows at bottom. If perforations in nozzle are too near bottom of pipe, water piles against nozzle, flows into calorimeter and gives false reading. Case 2--If nozzle located too near junction of two horizontal runs, as at a, condensation from vertical pipe which collects at this point will be thrown against the nozzle by the velocity of the steam, resulting in a false reading. Nozzle should be located far enough above junction to be removed from water kept in motion by the steam velocity, as at b. Case 3--Condensation in bend will be held by velocity of the steam as shown. When velocity is diminished during firing intervals and the like moisture flows back against nozzle, a, and false reading is obtained. A true reading will be obtained at b provided condensation is not blown over on nozzle. Case 4--Where non-return valve is placed before a bend, condensation will collect on steam line side and water will be swept by steam velocity against nozzle and false readings result.]

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 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. 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), namely,