Specific properties of water and steam

Specific properties of water and steam

Definition

Specific properties are per kg. Examples are specific volume, specific enthalpy, specific entropy, specific internal energy and specific latent heat.

Symbol

These specific properties are represented by the small alphabet like v, h,s,u,cP etc.

Reference temperature for the enthalpy and entropy of water is 00C.

Enthalpy of water is taken as 0 (ZERO) at 00C.

Specific heat of ice = 2.1 kJ/kg0C, Specific heat of water = 4.18 kJ/kg0C and Specific heat of steam vapor = 2.3 kJ/kg0C

Latent heat of ice = 335 kJ/kg and Latent heat of vaporization = 2257 kJ/kg

Specific enthalpy

  1. Therefore, enthalpy of 1 kg of ice at 00C will become negative and will be –335kJ/kg. as the latent heat of ice is 335 kJ/kg
  2. Enthalpy of water at 0C = 0 kJ/kg
  3. Enthalpy of water say at 110C

=mcpdt = 1 x 4.18 x 11=45.98 kJ/kg

4.Enthalpy of water at 1000C

h= 1 x 4.18 x 100 = 418 kJ/kg

5. Enthalpy of water vapor at 1000C and at 1 atm pressure = 418 + LH

=418 + 2257 = 2675 kJ/kg

6. Enthalpy of water vapor, say, at 1190C at 1 atm pressure will be

h = 2675 + 1 Cp vapor dt

            = 2675 + 1x 2.3x 19

            = 2675 + 43.7= 2718.7 kJ/kg

DRYNESS FRACTION OF STEAM

It is represented by ‘x’. In a certain sample of steam and water, it is the ratio of mass of vapor to the mass of vapor and mass of liquid.

x = mass of vapor/(mass of vapor+mass of liquid)

Its value will lie between 0 and 1.

  1. When it is total liquid, dryness fraction is zero.
  2. When it is total vapor, dryness fraction is one.
  3. Enthalpy of wet steam

Wet steam with dryness fraction ‘x’

hx = (1—x)hf +x hv

Where hf is enthalpy of saturated water at its boiling point at a certain pressure

hv is the enthalpy of saturated vapor at its boiling point at a certain pressure

SPECIFIC VOLUME

(i) Specific volume of water= volume of 1 kg of water

= 0.001 m3/kg

(ii) Specific volume of wet steam vx =(1—x)vf + x vg

(iii) Specifc volume of super heated steam is found by using Charles law

vsuper / Tsuper = vsat/Tsat

SPECIFIC INTERNAL ENERGY

Internal energy of water of mass m,  U = mcvdt

For 1 kg,  u = 1 x cp x dt

Say  at 100C, u = 1 x 4.18 x 10 = 41.8 kJ/kg

Internal energy of steam

u = h-pv

SPECIFIC ENTROPY

(i) Entropy of water at 00C is taken as zero.

(ii) Entropy of water say at 100C

s = cp ln(T/273)= 4.18 ln(283/273) kJ/kg 0C

(iii) Entropy during phase change = sfg = hfg/Tsat

(iv) Entropy of wet steam sx = (1-x) sf + x sg

(v) Entropy of saturated steam s sat = sf + sfg = sv

(vi) Entropy of super heated steam = ssuper

ssuper= sv + cp vap ln(Tsuper / Tsat )

NOTE: ONLY SATURATED VALUES OF ENTHALPY, ENTROPY AND VOLUME OF LIQUID AND VAPOR ARE AVAILABLE FROM THE STEAM TABLES AS WELL AS FROM CHARTS.