Thermodynamic Data

1
Thermodynamic Data

• It is not possible to know the absolute value of
  or for a pure substance, but you can
  determine the change in or
  corresponding to a specified change of state
  (temperature, pressure, and phase).
• A common practice is to arbitrarily designate a
  reference state for a substance at which and
  are declared to be equal to zero, and then
  tabulate and/or for the substance
  relative to the reference state. For example,
• CO (g, 0?C, 1 atm)? CO (g,100?C, 1 atm)

reference state
We say the specific enthalpy of CO at 100?C
and 1 atm relative to CO at 0?C and 1 atm is 2919
J/mol.
2
Reference States and State Properties

• Some enthalpy tables report the reference states
  from the values of are based and some do
  not. It is not necessary to know the reference
  state to calculate for the transition from
  one state to another.
• from state 1 to state 2 equals
  regardless of the reference state upon which
  and were based
• Caution if different tables are used, you must
  make sure they have the same reference state
• This result is a consequence of the fact that
  (and ) are state properties, that is, their
  values depend only on the state of the species
  (temperature, pressure, state) and not on how the
  species reached its state

When a species passes from one state to another,
both and for the process are
independent of the path taken from the first
state to the second one.
3
Example
The following entries are taken from a data table
for saturated methyl chloride

• What reference state was used to generate the
  given enthalpies?
• Calculate and for the transition of
  saturated methyl chloride vapour from 50?F to
  0?F.
• What assumption did you make in solving question
  2 regarding the effect of pressure on specific
  enthalpy?

4
Steam Tables

• Tables located in the back of FR can be used to
  estimate U and H for liquid water and steam
  (water vapour) at any specified temperature and
  pressure.
• Recall the phase diagram for water

Vapour-liquid equilibrium (VLE) curve or
saturation line water may exist as saturated
water, saturated steam (vapour) or mixture of
both.
Subcooled liquid
superheated steam
5
Steam Tables

• Saturated Steam Tables data taken along the VLE
  curve or saturation line
• Table B.5 properties of saturated water and
  saturated steam as a function of temperature from
  0.01?C (triple point) to 102?C
• Table B.6 properties of saturated water and
  saturated steam as a function of pressure (same
  data as Table B.5 but over a much larger range of
  temperatures and pressures)
• Superheated Steam Table data taken from points
  below the VLE curve or saturation line
  vapour heated above its saturation
  temperature
• Table B.7 properties of superheated steam table
  at any temperature and pressure includes data
  for liquid water (data in the enclosed region),
  and saturated water and saturated steam

6
Notes on the Steam Tables

• Reference state for the tabulated thermodynamic
  data in the steam tables is liquid water at the
  triple point (0.01?C and 0.00611 bar) triple
  point is where all three phases of water can
  coexist
• Units are on a mass basis
• Heat of vapourization (evaporation) is the
  difference between vapour and liquid enthalpies
• Properties of liquid water are not a strong
  function of pressure at constant temperature,
  therefore since
• Volumetric properties of steam are tabulated.
  Dont use the ideal gas law.
• Remember

and
7
Steam Tables Interpolation

• Sometimes you need to an estimate of specific
  enthalpy, specific internal energy or specific
  volume at a temperature and pressure that is
  between tabulated values
• Use linear interpolation

use this equation to estimate y for an x between
x1 and x2
8
Example

• 1. Determine the vapour pressure, specific
  internal energy, and specific enthalpy of
  saturated steam at 133.5?C.
• Show that water at 400?C and 10 bar is
  superheated steam and determine its specific
  volume, specific internal energy, and specific
  enthalpy relative to liquid water at the triple
  point, and its dew point.
• Show and for superheated steam depend
  strongly on temperature and relatively slightly
  on pressure.

9
Example

• Steam at 10 bar absolute with 190?C of superheat
  is fed to a turbine at a rate of 2000 kg/h. The
  turbine operation is adiabatic, and the effluent
  is saturated steam at 1 bar. Calculate the work
  output of the turbine in kilowatts, neglecting
  kinetic and potential energy changes.