DEPARTMENT OF COLLEGIATE AND TECHNICAL

EDUCATION

formation of steam

(UNIT-I) Session 5

COURSE OUTCOME:-Determine steam properties and

dryness fraction

APPLIED THERMAL ENGINEERING (V SEMESTER)

MECHANICAL ENGINEERING

Learning outcomes

At the end of this session -5 students will be able to understand

Construction ,working, explanation of Steam calorimeter Barrel calorimeter Separating Calorimeter

Throttling calorimeter Combined Separating and Throttling calorimeter

contents

Barrel calorimeter

Separating Calorimeter

Throttling calorimeter

Combined Separating and Throttling calorimeter

Steam calorimeter

• It is device used to measure dryness fraction of the wet steam.

• There are four methods of determining the dryness fraction of steam experimentally. They are

• Bucket or barrel calorimeter• Separating calorimeter• Throttling calorimeter• Combined separating and throttling calorimeter.•

Barrel calorimeter

• The Bucket/Barrel Calorimeter consists of a copper vessel which contains cold water. Thu upper vessel is insulated from the surrounding so as to prevent any heat transfer from or to the system.

• The top of the vessel is covered with wooden plate containing two holes. In one of the hole a thermometer is inserted to record the temperature and in another hole, steam supply pipe is inserted

• as shown in the below figure. The whole assembly is placed on the weighing bridge.

• In this calorimeter, steam from the main pipe of boiler enters into the calorimeter through nozzles. The steam coming in contact with water gets condensed and results in an increase in the mass and temperature of 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.

• let, ms= weight of steam condensed mw = weight of cold water in barrel Cpw= sp. heat of water

• x= dryness fraction of steam

•

• hfg = latent heat of the steam at pressure of steam P,

• t1, =initial temperature of water before mixing steam t2 = final temperature of steam after mixing of steam tsat = saturation the temperature of steam at

• ms[ x hfg1+ Cpw ( tsat-t2)]=mw Cpw(t2-t1)Therefore, Heat lost by steam = Heat gained by coal water From this equation, we can find the dryness fraction of steam

• ms[ x hfg1+ Cpw ( tsat-t2)]=mw Cpw(t2-t1)

Separating Calorimeter

• It is a simple calorimeter, used for determining the dryness fraction steam by separating the moisture content from the wet steam.

• It consists of two eccentric chambers, inner and outer chambers. They communicate with the help of opening at the top. The wet steam to be tested enters the inner chamber of separating calorimeter through a sampling tube controlled by a valve. In the inner chamber of calorimeter, perforated cups or baffles plates are arranged in such a manner as to separate the moisture from the steam. Hence, when steam enters the separating calorimeter, it undergoes a sudden reversal of direction of motion when it strikes the baffles plates. This causes the water particles which have greater inertia to separate from the wet steam. The water moves downwards and collected at

•

• the bottom portion of the inner chamber. The dry steam moves upwards and this steam is measured by condensing it.

• Let m= Mass of water collected in a particular time. M= mass of dry steam passing in the same time x = dryness fraction of wet steam

• Therefore, x= mass of dry steam =M/M+m

• mass of wet steam

Throttling calorimeter

In this, the principle of constant enthalpy expansion is adopted for the measurement of moisture in the wet steam is as shown in the below figure. In this calorimeter, steam from the main pipe of boiler enters into the calorimeter through controlled valves and moves into the well insulated expansion chamber in which the temperature is measured.

• During throttling, pressure of the steam decreases and volume increases. Neglecting the heat losses, total heat of the steam before and after the throttling remains same. Hence Steam becomes dry; therefore super heated steam is

obtained after throttling.

• Enthalpy before throttling = (hf1+ x hfg1)• Where, P1 = absolute pressure of steam before throttling

hf1 = sensible heat of steam corresponding to P1

• hfg1= latent heat of steam corresponding to P1and• x = dryness fraction of steam (at P1) entering the

throttling unit. Enthalpy after throttling (steam being superheated) = hg2+ Cp (tsup- tsat)

• Where, P2 pressure of steam, after throttling• hg2 = enthalpy of 1 kg of dry saturated steam

corresponding to P2 tsup = temperature of superheated steam after throttling

• tsat= temperature of saturated steam corresponding to pressure P2 Cp = specific heat of superheated steam at constant pressure.

• Enthalpy before throttling = Enthalpy after throttling hf1+ x hfg1= hg2+ Cp (tsup- tsat)

• hg2 + Cp tsup − tsat − hf1

-----------------------------------

• 𝑥 = hfg1

Combined Separating and Throttling calorimeter

line diagram of Combined Separating and Throttling

• Separating calorimeter gives approximate value because

of incomplete separation of moisture from wet steam where as throttling calorimeter can be used to find dryness fraction, only if superheated is obtained after throttling. Hence to overcome the limitations of these calorimeters and to obtain accurate results combined separating and throttling calorimeter is used.

•

When steam sample enters the separating unit, most of the water vapours are separated from the steam. Then dry steam passes throttling unit where the steam is superheated without change in enthalpy.

• calorimeter Let x1 = dryness fraction of steam sample in separating unit

• x= x1. x2

• x2 = dryness fraction of steam sample in throttling unit Therefore

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