How to Measure Dryness Fraction of Steam? (With Diagram) | Thermodynamics

In this article we will discuss about how to measure dryness fraction of steam.

Barrel Calorimeter:

To get the approximately dryness fraction of steam, a very simple ape called Barrel Calorimeter is used. This consists of a wooden barrel containing cold water.

Steam is taken from the pipe through which it is flowing and is condensed in the cold water. Temperatures of water before and after condensation are recorded. Knowing the quantities of steam and water, we can very easily calculate the dryness fraction steam. The calorimeter is shown in Fig. 10.19.

Basically, this calorimeter is used when we want to know the approximate dryness fraction of steam, or to have a rough idea about the dryness fraction. This calorimeter gives better results if the dryness fraction of steam is greater than 0.95.

Separating Calorimeter:

The separating calorimeter is quite simple in its action and it may be used for testing steam of practically any degree of wetness. 

The sample of steam to be tested enters the calorimeter at D and passes down through the central passage DF into the perforated metal cup H. The current of steam then reverses and flows as shown by the arrows in the jacket J surrounding the inner chamber C. From the jacket the steam escapes into the atmosphere through the small orifice at the bottom and exhaust pipe E.

The water in the steam is thrown out in the cup Hand collected in the chamber C. A gouge G has two scale on its dial, the inner one showing the pressure of the steam in the jacket while the outer one shows the weight of steam passing through the jacket for each pressure in ten minutes.

The graduations on the outer scale are determined by experiment with steam of different pressures, the escaping steam being condensed and weighted, but in using the instrument it is only necessary to take during a ten-minute test the average reading on the outer scale to obtain the weight of steam passing through the jacket.

The water separated from the steam is shown by means of the glass gauge U. A pointer P having a frictional grip of the glass tube is set to the water level at the beginning of the taste. The change in level of the separated water during a test is read off on the scale L. Which is graduated in fractions of mols.

Let W be the weight of steam passing through the jacket during a test and W is the weight of water separated in the same time, then x, the dryness fraction of the steam tested is x = W/(W + W).

The weight of steam passing through the jacket may also be readily determined by passing the escaping steam into a bucket of water where it is condensed, the increase in the weight of the bucket and its contents giving the value of W. In that case gauge G is not required, but it used its reading may be taken as a check.

This calorimeter will give better results when the dryness fraction of steam to be determined is above 0.95.

Throttling Calorimeter:

The throttling calorimeter was first introduced by Professor Peabody. Figure 10.21 shows Professor Carpenter’s modification of the Peabody throttling calorimeter.

P is the pipe carrying the steam to be tested. AB is the sampling tube through which the sample of steam flows to the vessels S which is the principle part of the calorimeter. V is the stop valve. The sample of steam enters the vessel S through a small orifice O and expands to a pressure a little above that of atmosphere into which it escapes freely through the exhaust pipe E.

The pressure of the steam in S is measured by means of the manometer or siphon gauge G which is a glass U-tube containing mercury. G is connected to S by 2a rubber tube T. Steam to the gauge may be cut-off by means of the stop cock C. A deep pocket K, containing cylinder oil, takes the thermometer R which indicates the temperature of the steam in S. The vessel S, the stop valve V, and the exposed part of the sampling tube AB must be carefully lagged to prevent undue loss of heat by radiation.

In using this instrument four observations have to be made:

(1) The pressure P₁ or temperature t₁ of the steam in the pipe P.

(2) The pressure of the steam in the vessel S as shown by the gauge G in inches/centimeters of mercury.

(3) Reading of thermometer R

(4) The height of the barometer in inches/centimeters of mercury.

The principle upon which the operation of the throttling calorimeter depends is that the total heat (enthalpy) in the steam after throttling is the same as the total heat (enthalpy) in the steam before throttling. This is given by the equation –

Since the pressure after throttling is in the neighbourhood of that of the atmosphere, and since the specific heat of steam at that pressure is little affected by the temperature, C_PV may be taken as 2 or 2.01 kJ/kg-K.

The operation of the throttling calorimeter depend on the steam being superheated after throttling and it will fail in its purpose when the steam is so wet before throttling that it is wet after throttling. This means that t₃ must be greater than t₂. Putting t₃ = t₂ the value of x may be determined by the throttling calorimeter must be greater than –

 

Basically the condition of steam after throttling must be known so that the unknown x-dryness fraction before throttling can be calculated. For this purpose the steam after throttling must be separated and its temperature should be more than the saturated temperature of steam corresponding to the pressure after throttling. To get such a result dryness fraction before throttling should be more than 0.9.

If the dryness fraction of steam is less than 0.9, then the steam will not be superheated after throttling and the dryness before throttling cannot be found.

In such a case, a combined separating and throttling calorimeter is used. In this, steam is first admitted to separating calorimeter where some portion of the water particles is removed and the quality of steam is improved to the required level for throttling calorimeter.