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RULES FOR S.I. UNITS The International System of UnitS, abbreviated as S.l. Units, is a unified and systematically constituted system of fundamental and derived units for international use. This system of units is now bemg used in many countries, and 1t is hoped that it will be used throughout the world very soon. In lndia, the Standard of Weights and Measures Act of 1956 (wide which we switched over to M.K.S. Units) has been revised to recognised all the S.l. Units in Science and Technology as well as industry and Commerce. Though there are many rules for the style and usage of S.I. Units, yet the following are important from the subject point of view: I.
For a number, having 5 or more d1gits, thedigits should be placed in groups of three. separated by space counting both to the left and right of the decimal point.
In a four digit number, the space is not required, unless the four digit number is used in a column of numbers with 5 or more d1gits .
Introduction to Steam Tables and Mollier Diagram l . Definition Steam IS the vapour form of wmer and is invisible when pure and dry. It does not obey the laws of perf~t gases, until it LS perfectly dry When the dry steam IS further heated. 1~ behaves more or less l i ~e a perfect gas. The steam is. generally. used as a working substance m the operation of steam engines and steam turbines
2. Formation or steam The fonnation of steam takes place, when we contmuously heat the water from any temperature 1n the following stages: I. The volume of water slightly 1ncreases with the increase in temperature. But th1s mcrease io;, generally. neglected for all types of calculations. 2. On further heattng. the temperature of water reaches the boiling po1nt. The boihng tempera- . ture of pure steam at normal atmosphenc pressure of 1.011 bar ( 1.013 x to' N/m 2 or 1.013 2 x 102 kN/m also known as I 0 13 x lOS kPa) is 100°C (373 K). But it mcreases with the • mcrease tn pressure. 3 After boiling point, the temperature remams constant. But the water starts evaporating and the volume of steam starts increasing. At this stage, the steam has some water particles in \USpension and it IS called wet steam. This process continues, till the whole water is converted into wet steam. 4. On further healing. the temperature of wet steam rema1ns constant and all the water particles 1n suspension are convened 1nto steam. At th1s stage, the steam is called dry saturated steam
5. On further heaung. the temperature of dry saturated steam starts mcreasmg and It obeys the taws of perfect gases. At this stage, the steam 1s called superheated steam. As a matter fact. the superheated steam IS used as a working substance in the operation of steam engtnes and steam turbines.
3. Properties of steam Thel followmg properties of steam are always needed for the calculations of its vanous parameters, wh1ch are requ1red 1n the operauon of steam engmes and steam turbines. I. Sptcific volunu! of steam ll1s the volume occupied by the steam per urut mass at a given temperature and pressure, lt is expressed 10 m3/kg and IS the rec1procal of the density of ~team. The spec1fic volume of steam increases w1th the increase 1n temperature and decreases with the mcrease 1n pressure. 2. Specific enthalpy of steam. It IS the total heat absorbed by the steam per unit mass from the freeLmg p01nt of water (0°C or 273 K) to the saturation temperature ( 100°C or 373 K) plus the heat absorbed Juring evaporauon. It is expressed 10 kJ/kg The specific enthalpy of steam increases with the increase 10 temperature and pressure
Introduction to Steam Tables and Mollier Diagram 1. Definition
Steam Ill the -.apour form of water and IS inviSible when pure and dry It does not obey the laws of perfect gases, until HIS perfectly dry. When the dry steam IS further heated, 11 behaves more or less like a perfect gas. The steam is, generally. used as a working substance 1n the operatton of steam engmes and steam turbines.
2. Formation of steam The formation of steam takes place, when we continuous!)' heat the water from any temperature 10 the follo\\ing stages: I . The volume of water shghtJy increases with the increase
temperature. But this incr-ease
is. generally, neglected for all types of calculatiOns. 2. On further heating. the temperature of water reaches the b01hng point. The bo1hng tempera- . Lure of pure steam at normal atmospheric pressure of I 013 bar ( 1.013 x I 05 N/m 2 or 1.013 x l0 2 kN/m 2 also known as 1.013 x I o' kPa) 1s IOOac (373 K). But it increases with the . mcrease 1n pressure. 3 After b01hng potnt, the temperature remains constant. But the water starts evaporating and the volume of steam starts mcreasing. At th1s ~tage. the steam has some water particles 1n suspensiOn and 1t IS called wet steam. Thts process continues, ttll the whole water IS converted mto wet steam. 4. On further heaung, the temperature of wet steam remams constant and all the water particles 1n suspension are converted into steam. At this stage, the steam is called dry saturated steam
5. On further heattng. the temperature of dry saturated steam starts Increasing and it obeys the laws of perfect gases. At this stage, the steam is called superheated steam. As a matter fact, the c;uperheated steam is used as a worlung substance in the operation of steam engines and steam turbtnes.
3. Properties of steam The I followmg properties of steam are always needed for the calculatiOns of its various parameters, wh1ch are required 10 the operation of steam engines and steam turbines. I.
of steam. lt IS the \'Oiume occupted by the steam per umt mass at a gtven temperature and pressure. lt is expressed tn m3/kg and is the reciprocal of the dens1ty of ,team The speci fie volume of steam mcreases with the increase 1n temperature and decreases w1th the 1ncrease tn pressure.
emhalpy of steam.. lt 1s the total heat absorbed by the steam per unit mass from the freeLing po1nt of water (O"C or 273 K) to the saturauon temperature ( IOOvc or 373 K) plu:o. the heat absorbed dunng evaporntton. It is expressed in kJ/kg. The spec1 fie enthalpy of :.team increases w1th the tncrease 1n temperature and pressure. (v)
3. Specific emropy of steam. II IS a theoretical value of heat energy, which can not be transfonned into mechanical work under the g1ven conditions of temperature or pressure. It is also called degree of disorder of the system. The most common tenn used is the change of entropy, which is mathematically given as: !ls
Heat supplied = Temperature of the system
It . expressed in k.J/kg K. The specitic cntrop) ol l>tcam decreases with an mcreal>e 1n temperature and pressure. 4. Steam tables
The vanous properties of steam (such asspec1fic volume, ~pec1lic enthalpy and specific entropy) of dry saturated steam and superheated steam vary with the vanationl> of temperature and pressure. These values were carefully determmed by observations and calculations first in F.P.S. system ~md were made available 10 a tabular form known as steam tables. Later on, these values were converted into M. K.S. units and then mto S.l. units. Due to conversion and rounding off the figures, there 1s a ~light difference in the figures quoted in different books. Even some of the authors have changed these values in different editions of the same book. However. 1n th1s steam tables the author has quoted standard figures, which are widely accepted and internationally recogmsed. There are rwo 1mportant steam tables. One of them is based in tenns of temperature and the other in terms of pressure. It is a general practice to give the followmg tables for some 1mportant values: I . Saturated water and steam (temperature) table
2. Saturated water and steam (pressure) table 3. Superheated steam table 4
Supercritical steam table
5. Saturated water and steam (temperature) table
It contains values of absolute pressure (in bar), ~pecafic volume (in m3/ kg), spec1fic enthalpy (i n k.J/kg) and specific entropy (in k.J/kg K) from 0°C tO 374.15°C (critical temperaLUre). A sample of this table is given below: Tempe- Ab~olute rarure in Pressure in bar
Specific volume in ml/kg Water
0.006 II 0.008 72 0.012 27
Specific enthalpy in kJ/kg
